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MG MGB Technical - Oil on spark plug (Part 2)

This is a follow-on from my recent ‘Oil on spark plug’ thread. Whilst trying to sort out the oiling problem, another engine fault has arisen. I’ll start with a brief description of the work carried out so far:

Removed cylinder head and replaced all valve stem seals. Valves and seats were ground/recut/lapped at local engine shop.

Removed No.2 piston. The three parts of the oil ring were free to rotate together, but due to gumming were stuck together; the gaps were in line. Removed No.3 piston for comparison – it was in similar condition to No.2 except the gaps were not in line and the three parts were free to rotate independently. Removed all rings from both pistons and cleaned them, also cleaned the gummy deposits from the grooves that the rings fit into. Refitted the rings in their original positions, with compression ring gaps 90 deg apart and away from the engine thrust side. Refitted the pistons. Turned the engine a few revs by hand to check the pistons moved smoothly.

I then had second thoughts about the way I’d positioned the rings on No.3 piston, so removed it again. To my horror I found that the big end bearing was scored. Had a quick look at the crankshaft, which looked ok. I thought some grit may have fallen onto the crank. Put my close-up specs on and turned the crank, looking for any foreign bodies. Eventually noticed a fine scratch (about 3mm long, which could be felt by my fingernail) – this lined up with the scoring on the bearing. After some considerable thought I decided to get rid of the edge (high spot) of the scratch using a needle file. This took some time, as I was being ultra-careful not to mark the crank, and just remove the high spot.

I can only conclude that the damage to the crank happened when I inserted the piston – the big end bolts must have made contact with it, despite having taken care to line up the crank to avoid that happening. I fitted a new pair of big end shells (from Moss – unbranded – labelled ‘Made in Israel’). No problem fitting these. Refitted the piston (this time covering the big end bolts with a bit of rubber tubing). Then turned the crank a few revs, and removed the piston again to check the bearing for any marks – seemed ok. I then removed No.2 piston and inspected the big end bearing and crank, just in case there was any damage there.

Refitted the head and all the other stuff, new gaskets all round.

Before starting the engine I removed the plugs, disconnected the ignition and fuel pump, and turned the engine over on the starter until oil pressure was indicated. When I tried to start the engine it took a while for it to fire up and was very lumpy. I assumed this to be partly due to the oil which I’d wiped around the cylinders when refitting the pistons. After a few minutes it was running better, although with some misfiring. As the engine warmed up I found it still needed a bit of choke to run smoothly, so I increased the mixture on the carbs by one full turn (HIF4’s). This seemed to improve it. The gunson Gastester indicated 4% CO. I’m surprised I needed to make such a big adjustment – I wouldn’t have thought that the sort of work I’ve done on the engine would require any adjustment to the fuel mixture. Is a richer mix helping to mask some other fault?

Took the car on a short run. There’s definitely a lack of power, so there’s a problem with the engine that wasn’t there when I started this job. After a 15 minute run it was clear that the engine was getting hotter than it used to. This run included an uphill climb in 3rd but when I tried to accelerate it was very sluggish; I took it easy for the rest of the journey for fear of something nasty happening. When I lifted the bonnet the heat from the engine and radiator was quite noticeable, and when I left the car to tick over on the drive the cooling fan was running more than usual.

Observations so far:

1. After switching the engine off and lifting the bonnet I noticed a small amount of smoke coming out of the carb air inlets (at this stage I hadn’t refitted the air filters). I pulled off the crankcase breather hose and saw that the smoke was coming up from the crankcase.

2. While the engine was still hot I re-checked the valve clearances and noticed the crankshaft was difficult to rotate by hand (with spark plugs removed I can usually rotate it quite easily). The following day, when the engine was cold, the crankshaft would rotate easily.

3. There’s no problem with coolant circulation – after a few minutes all hoses and the radiator are warm.

4. I’ve rechecked compression (Gunson HiGauge) – all cylinders 175-180 (dry). Also checked manifold vacuum (Gunson LoGauge) – at 850rpm tickover it’s 16 to 17 in.hg. but does oscillate between 15 and 18. At first I thought this may be a sticking valve, but checked my other MGB and that’s the same.

5. I think I’ve seen more water droplets than usual coming out of the exhaust, but maybe this is because I’m working under the bright lights in my garage.

I think my next job will have to be to take a close look at the big end bearing which I fitted and see if there are any signs of binding or overheating. Any other ideas of what I might check next would be appreciated.




Brian Shaw

Brian,
I can't answer some of the questions, but I wonder if you have an air leak on the carb gaskets etc., or at the intake. Spraying carb clener at the jointd with the engine running may identify if there is a problem. My '73 MGB shows signs of lean running if I try to drive it with the air filters removed, so it might be a good idea to try a run with them mounted. (After you check the condition of that bearing.)

Ralph
L Poupard

Hi Brian

It might have been prudent to give the bore a glaze bust and fit new rings. Did you fit the compression rings the correct way up?

Having the ring gaps staggered is a red herring. I must admit I stagger the rings on assembly though, ! Rings rotate at approx 1 rpm per thousand engine rpm. The ring gaps will on occasion all line up, this causes no problems...not good on your gummed up oil rings though! The only way to keep a stagger would be to pin the rings as in a two stroke engine. The rings are pinned in two strokes to prevent the ring ends entering the exhaust ports or transfer ports. On assembly you must ensure the ring gaps do not line up with the exhaust valve cutouts or you may snap a ring if you are unlucky, same reason as two strokes!

Do you have K&N filters? If not, running without filters will cause the engine to run weak and overheat! As Ralph says above.

If you have a problem with a big end bearing it would most likely have destroyed itself very early after engine start.

I rather feel you are dancing to the engines tune rather than playing for the engine.

Peter
P Burgess

Stiffness, sluggishness and heat can all be associated i.e. the latter two with the first.

Did you recheck the timing? That can also cause the second two, particularly retarded.

One full turn of the mixture screws is a huge amount, did you check the mixture with the lifting pins i.e. are they both correct is one weak masking one rich as far as the Gastester goes? Did you recheck balance?
PaulH Solihull

Ralph - I should have mentioned that when I first started the engine and noticed the lumpy running and misfire, the air filters were on the car. I then adjusted the carbs 1 full turn richer which seemed to cure the misfire. I then removed the filters for better access to carbs and manifold.

Peter - I took careful note of how the rings were positioned as I dismantled them, and put them back the same way. I can understand it will run a bit hotter and leaner without the air filters (mine are original paper ones) but the heat from the engine bay when I lifted the bonnet after the 3 mile run was far worse than anything I've had on this car before, even after a hard drive on a hot day.

Paul - yes, checked the timing, but haven't rechecked the balance yet (spark plugs have always been about the same colour, except No.2 which oiled up recently). I'll put this on my list of things to check.
Brian Shaw

Update:

Progress on sorting this problem has been delayed due to roof repairs (house, not car).

I removed the sump and removed the big end cap and bearing shells which I replaced recently (No.3 cylinder). No sign of overheating there, or anywhere else on the crank. The new bearing shells have what I think is 'normal' light marking after their 20 minutes or so running. Checked them with Plastigauge which indicated 1.5 thou. Did the same check on No.2 big end which was also 1.5.

That's about as far as I've got so far. Relieved to find that there's nothing amiss down below.

A couple of the previous posts seem to suggest lean running could be the cause of the lack of power and engine overheating quickly. However, I did a Gunsons Gastester check which shows 4% CO - wouldn't this rule out the 'lean mixture' theory?
Brian Shaw

Just a reminder that a Gastester, which is shoved up the exhaust, will only indicate the average between the two carbs. If the carbs are unbalanced half the engine could well be running hot and the other half normal.
PaulH Solihull

Before you removed the pistons, did you remove the carbon at the top of the bore and was there much of a wear ridge, a big ridge can damge the rings on the way out. The compression rings have an up and down. The bevel on the inner circumference is up, as is any bevel on the outide of the top ring. Bevelled top rings are fitted if new rings have been fitted to un-machined bores, to stop them clouting the ridge.One assumes you aligned the oil holes in the big ends shells, and very importantly, put the pistons in their correct orientation, i.e., oil squirt on the thrust side of the bore.
Allan Reeling

Paul - I'll check the carbs balance next time I run the engine. First I'll be checking all the reassembly work I did around the carbs/manifold etc although I'm sure everything went back together properly with new gaskets all round.

Allan - Yes, I removed the small amount of carbon around the top of the bores before removing the pistons. The wear ridge was negligible (original engine, but low mileage 42k). I noted the ring bevel and took great care to put them back exactly as they were. The shell oil holes are correctly positioned (the shells are notched and will only fit one way). The piston tops are marked 'front', and the con-rods have not been seperated from them, so the squirt holes are still in their original orientation.
Brian Shaw

Not much action on this job recently, but I have been giving it a bit more thought.

The main thing that concerns me is the stiffness encountered when trying to turn the crank manually when the engine was hot (plugs removed). Definitely seems to be some resistance there that shouldn't be - maybe this would account for (a) having to adjust the carbs mixture 1 full turn richer to get it to run smoothly, and (b) the temperature of the block which seemed hotter than I think is normal for this car. Having now checked down below I'm sure there's nothing wrong in the crank area.

One incident (which at the time I thought insignificant) now gives me cause for concern. When I had No.3 piston out to check the rings my workbench wobbled a bit and the piston fell onto the floor. I had just got it ready to refit and it had the ring compressor fitted and tightened. It landed almost square on, with the ring compressor taking the impact and sliding a bit further down the piston. Close inspection showed a small nick to the lower side of the piston skirt area which I smoothed off with a craft knife. This damage was caused by the bottom edge of the ring compressor . As there was no visible sign of any other damage I continued with reassembly; I had no means of measuring or checking the dimensions of the piston, other than checking that it fitted in the cylinder bore with no obvious difficulty.

I'm undecided what to do next:

(a) Remove No.3 piston and have it checked, or just replace it anyway (a single Hepolite 18802 is about £30) and then reassemble and see what it runs like, or

(b) Run the engine again, this time monitoring the block temperature then check the stiffness of the crank when hot. Then do the same on my 'good' MGB to compare time/block temp info and crank stiffness.

Brian Shaw

Why not but run your "good" MGB first? You may find that it does the same as you have already experienced on this "troublesome" engine, or if not you will have a reference point at which to "aim". (I suspect the latter but you may be lucky!)
Michael Beswick

Yesterday I decided to remove the cylinder head, with the intention of removing the two pistons I'd had out for checking and cleaning a few weeks ago, just to re-check I've put everything back correctly.

I removed the carbs, spacers and heatshield. As I pulled the inlet manifold away from the cyl head a small amount of oil (about a teaspoonful) trickled out of the two inlet ports. When I removed the head I saw the top of each piston was covered in oil (see attached photo of No.3 piston - when I took this I'd cleaned up the others).

Checking the head itself, it looks as though oil has come down the inlet valve stems and into the combustion chambers (see second photo). The worst of the oily deposits is towards the rear (No.4 cyl) which figures, as the front of the car is raised slightly on axle stands. Light oily residue can been seen throughout the inlet ports, especially on and around the inlet valve stems and guides.

I had thoroughly cleaned the cyl head in October, just before the 3-mile run I did. After that run, I removed the plugs and peeked into the cylinders and saw no sign of any oil. Since then (10 weeks ago) the car has stood in my garage. I can only assume that oil has seeped through the valve stems whilst the car's been laid up, which surprises me. Should I investigate further, or should I ignore it?



Brian Shaw

Second photo (cylinder head)


Brian Shaw

There's something very wrong there, a teaspoonful in the inlet ports is a huge amount to me.

Are the valves sloppy in the guides? Or is stem or guide heavily grooved? I've heard of knurling the stem to create grooves to retain oil to lubricate the guides, maybe it has been way overdone. Unless the engine is at full throttle there is a significant partial vacuum in the combustion chamber on the induction stroke, and it's this that can suck oil through the guides. But only when it is running, and leaving it to idle a while like that then opening the throttle should create a good smokescreen.

But shouldn't the top of the guides be above the top face of the head, in fact significantly above any point at which oil could pool in the head? There should be very little oil lying in the head, it should run down the push-rod holes back to the sump. That only leaves oil that is running down the rockers and the top of the valve stem to run down the guides, and that should stop as soon as the engine stops.

Another source is oil sucked from the crankcase breather and put directly into the inlet ports, but that should affect all cylinders equally, depending on which inlet valves are open and which closed each time the engine come to a stop, and it should again smoke like billy-oh to have that amount lying on top of a piston. That's easily determined by disconnecting the breather for a while and sealing the engine-side - i.e. the PCV valve port or carb ports as appropriate and running it for a while to see what happens.

Another source can be oil sucked up past rings from the crankcase. Could you have undersized pistons for your bores?

PaulH Solihull

I'm with PaulH on this, it's very wrong. More so because you've fitted new oil seals on the head earlier. Were these the original O-ring type or the newer cups?
A teaspoon of oil coming out of the inlet port worries me also. The ports run flat or slightly down to the valve heads and if the oil is getting down the guides it would drain on to the top of the piston (vales open) or sit on the valve head (valve closed) to get that much out of the inlet port suggests the port had significantly more inside or the source is much neared the port. Has the head been flowed? I would start checking the impossible like looking for holes in the port walls at the push rod tubes. You could try running it with the breather disconnected (plug the tubes on the carbs) or via an oil catch tank to see how much is coming up from the crank case. This will help you decide if the pistons need further investigation/replacement. There again, as you've got the head off and the sump off just pull all the pistons, check the bores and recondition the engine as required....

The overheating could be as a result of sucking in loads of oil causing a very weak mixture hence the need to add one turn of the carb adjuster.
Fix the oil problem and then move on if other symptoms persist.

Hope this helps a little...

MGmike
M McAndrew

Paul and Mike-

Thanks for your suggestions. Just a bit of background info on the car:

First registered 1981, I bought it in 1986 from its first owner (genuine mileage 15k) and it has been a reliable runner for the last 25 years; mileage is now 42k. Until August this year the engine has had no work done on it except for routine servicing and adjustments.

In August I noticed oil on No.2 spark plug, also oil on top of the piston. The other three appeared normal. Following this, I have done the following work:

1. Subjected the engine to the ‘Redex’ treatment (in case oil rings were gummed/sticking)
2. Removed the head. I got my local engine shop to recut the valve seats and grind/lap the valves (slight pitting to a couple of exhaust valves). They also reground the valve stem ends to remove rocker wear (rockers needed slight re-profiling to suit)
3. Fitted new valve stem oil seals (original o-ring type)
4. Removed No.2 piston, removed its rings and removed all traces of gumming (the 3 parts of the oil ring were tending to stick together)
5. Removed No.3 piston to see how it compared with No.2 (less gumming but cleaned it up anyway)
6. Fitted new No.3 big end bearing shell (original was damaged by me)

Since doing the above work, the engine has been run only once: for about 30 minutes (20 min warming up on driveway, then out for 3-mile run). Following this run, spark plugs were still clean but engine very hot and crank almost impossible to turn by hand.

I checked the valves for sloppiness in the guides when I removed them a few weeks ago and they seemed to be ok, although I had no means of measuring the clearance. I intend to remove the inlet valves again and I will look at this more closely.

I think your explanation of how oil gets down the guides is spot-on, so I guess the front of the car being raised has not influenced the seepage of oil as I had assumed. I have occasionally turned the crank by hand, so different inlet valves have been left open at different times.

I have just spent a bit more time looking at the head, and oil has definitely got into the combustion chamber via the inlet valves. The oil appears to have flowed out from around the inlet valve.

Apart from the recent work described above, the engine is as it left the factory, so the pistons are standard originals and the bores are good and still showing honing (I can detect a slight ridge near the top of the bore which seems reasonable for its mileage).

I have some ‘improved’ (cup-type) oil seals, so in the next day or two I’ll remove the inlet valves, check for play in the guides, and fit the new seals.

Attached photo shows combustion chambers 1 and 4.




Brian Shaw

Did you check the piston ring end gaps before you fitted them to the pistons? If not, you may have too little or no end gap. That will break the rings and cause oil to be burned at an alarming rate. RAY
rjm RAY

Brian, very oily!!
I'm still surprised at the amount of oil getting down the guides ( if in fact it is). The clearance between the valve stem and guide is only about .001 and so is very difficult to measure. However, check the valves have no noticeable sideway movement when holding the valve head and wiggling sideways. Fit the new seals, refit the head and run it with the breather pipes disconnected. That way you'll prove the oil it not from the crank case. If you get loads of oily smoke from the crankcase pipe, it's off with the head again and strip the pistons. Any ridge at the top of the bore mean a rebore but if it's only mild a new set of cords rings will be a good compromise.
One last comment on the pic's... One of the chambers seems to have a crack from the plug hole to the inlet valve! Hope that's just a trick of the light??
MGmike
M McAndrew

Significant oil behing the inlet valve is unlikely to be as a result of stem/guide wear or even bad or no oil seals. Either someone has ground through the inlet tract to a push rod hole or there's a crack. There is considerable oil flow through the rocker cover but never enough to drain down a valve stem, that amount of oil can only be getting behind a valve through a crack in the head, a crack in a guide (they are cast iron!)or a hole. Check the head closely!!!
Allan Reeling

Puzzling with that kind of history.

Rather than oil I think I'd be dribbling paraffin (rather than petrol) down the valve stems and pooling it round where the top of the guides protrudes above the head surface, while the head is off, to see if there is any route in that way. As the oil seems to move between 2, 3 and 4 at various times I'm still thinking this is the way in.
PaulH Solihull

Ray - I didn't check the piston ring gaps, I re-fitted the original ones which gave good readings on my Gunsons compression gauge.

MIke - Today I removed the inlet valves and checked for sloppiness in the guides. I set up a dial gauge (see photo) and cleaned all oil from the stem and inside the guide. Sideways play is 0.0025" which is within spec (see S.Strange's 'The Book').
The 'crack' you can see in the photo is actually a bit of rough casting - I've cleaned that area up and had a close look.

Allan - I'll be cleaning the head up thoroughly tomorrow and give it a good inspection.

Paul - why is paraffin better than petrol? (it's about the only thing I haven't got at the moment!) I've got methylated spirit, white spirit, petrol, light machine oil.

Removing the inlet valves has confirmed they were all correctly installed and the stem seals correctly positioned.

Just a further thought on the 5ml or so of oil that trickled out of the inlet ports when I removed the inlet manifold - could this be oil from the carb dampers? I just had a look at the carbs which have been on the bench for a couple of days, sat upright and tilted back at about the same angle as they would normally be on the car, and some oil from each damper has accumulated at the outlet ports (just by the butterfly). I last topped the damper oil up just before the 30-min engine run. The oil level in each damper is now about a half-inch lower.


Brian Shaw

Brian

When you top up the dashpots on the carbs how far up do you top them up? The oil should only be just above the piston, item 23 in the diagram, and not over the top of the cylinder it sits in. I have seen drawings in the past that have suggested the oil is filled above the top of the cylinder (or should that be piston). If you're over filling them this could be the source of the oil.

Bob



R.A Davis

Bob - I've always topped up as recommended in the MGB Driver's Handbook: " .... top up the reservoir until the oil level is 1/2 inch above the top of the hollow piston rod." This has always seemed like overkill to me, because this level of oil results in a full 1.5" of resistance when I re-insert the damper assembly.
Brian Shaw

The attached shows dashpot fill level, from factory manual


Roger T

I only said paraffin for safety reasons, I don't have any so would use petrol, but I'm not recommending it!

As far as damper oil level goes it depends on the carb type as to what the manuals say. HSs show it below the top of the oil reservoir as in Roger's image, whereas HIFs show it above (top right in the attached image). To me what is above is going to be lost, so I do my HIFs the same as my HSs i.e. below, and don't have to top them up from one years end to the next. Unscrew the plastic cap, lift it up, and lower it again. If you can feel the resistance of the oil before the plastic cap reaches the cover you have enough. How much before shows you how much of a 'reserve' you have, the oil only needs to reach slightly above the bottom of the damper piston to do its job, anything above that is overkill as Brian says. I have seen a claim that oil above the top is essential to lubricate the sliding contact between the outside of the reservoir and the piston cover, but HSs didn't need it, and my HIFs haven't had it for 90k or so and have suffered no ill-effects.

As to it being the source of the oil it's an interesting thought. Whilst the oil container on HSs is blind-drilled so oil cannot drain away (what is below the top of the reservoir anyway) it seems that HIFs are through-drilled then plugged. If this plug is faulty then all the oil could drain into the combustion chambers. After all this it may just be a case of making sure the oil level is below the top of the reservoir!



PaulH Solihull

I'll make sure I stick to a lower damper oil level now. I would sometimes top them up between routine services to maintain the level shown in the handbook.

I've just finished cleaning up the cylinder head and there are no signs of any cracks or other faults. I'll pour a bit of paraffin or something similar to form a small pool around the inlet valve guides, to check for leaks at the guide/head interface, and leave it to soak overnight.
Brian Shaw

Brian,
I see the tappet chest cover on the bench in the background of your picture. Whilst off the car you should consider modifying this in line with the info on PaulH's site.
http://www.mgb-stuff.org.uk/enginetext.htm

MGmike
M McAndrew

The paraffin test has been done and has proved the valve guides are ok. I have re-fitted the inlet valves without the o-ring seals, and fitted the 'improved' type that fits over the top of the guide (part number 12B2104).

Removed No.2 and 3 pistons again to check them. Crank, big end shells, pistons and rings all seemed ok, with no evidence of binding or overheating.

I have stated previously that the ridge near the top of the cylinders is negligible; I have just checked this, and cylinder bore wear is 1 thou.

I measured the ring gaps when fitted into the cylinders:

Top compression ring 27 thou
2nd compression ring 25 thou
(workshop manual specifies 12 to 22 thou for both)

Ring to piston groove clearance is 3.5 thou
(workshop manual specifies 1.5 to 3.5 thou)

I've re-fitted the rings the same way round as I found them - top comp ring has it top inside edge bevelled, 2nd comp ring has its lower outer edge stepped - is this correct?

Can't think of anything else to check, so I'll put it all back together when Moss delivers some more gaskets. Next, I'll give the engine a run with the crankcase breather disconnected.

Brian Shaw

"I've re-fitted the rings the same way round as I found them ". Offsetting the ring gaps as well?
Roger T

Brian, given the ring gaps and having gone this far I would spend the £40 on a new set of rings and run a simple glace buster down the bores.
With judicious use of plenty of rags you will be able to keep the grit out of the important places.

Best of luck

MGmike
M McAndrew

Hi Brian,

I've just had a read through your woes, and what I noticed is that you say you have a stiff crsanf and you have replaced the bearing shell on #3. Maybe somehow you have an undersize shell, which would explain the tightness.

No ideas on the oil, though.

Best of luck

Herb
Herb Adler

Hi Brian,

I've just had a read through your woes, and what I noticed is that you say you have a stiff crank and you have replaced the bearing shell on #3. Maybe somehow you have an undersize shell, which would explain the tightness.

No ideas on the oil, though.

Best of luck

Herb
Herb Adler

Roger - I've offset the ring gaps as per w/shop manual (I'm not sure how important this is, they soon move around when the engine runs).

Mike - I can see your point, but consider the following:

1. Cylinder compression figures are all good
2. Doesn't burn oil (never seen any blue smoke) and I've never had to top up the oil level between services
3. I only expect to do another 4 or 5,000 miles in this car before I sell it (I'm ashamed to admit it, but it took me 13 years to do the last 5k!)

I suspect the ring gaps were probably out of spec when the engine was built.

Herb - I agree with you. When I found that the crank was very difficult to turn (I blistered my fingers trying to turn it a full rev) I decided I had to remove the sump again and check that No.3 bearing, also everything else I'd done. The shell showed no sign of binding or overheating, and a Plastigauge showed the clearance to be 1.5 thou - the same as No.2 which I also checked for comparison. I checked the shell thickness with a digital vernier and it's the same as the original shell.

Thanks guys for your suggestions. Stuck indoors at the moment, if I open the garage this wind will blow the door off!
Brian Shaw

I was just about to re-fit the cylinder head when I heard something rattling inside the casting. After a bit of fiddling I managed to get it out - a lump of casting debris in the coolant passage. It has probably been in there since the head was manufactured. It measures 25 x 15 x 8mm. Not sure if this is responsible for the apparent overheating last time I ran the engine. I'm surprised it hasn't caused a problem in the past, but maybe the size and weight of it has limited its circulation.


Brian Shaw

I've found similar, though considerably smaller dimensioned debris. Depending how that may have been moving around I would think it capable of affecting cooling, if only in a localised area of the head.
Roger T

Checking the cylinder head for any more casting debris, I removed the thermostat to find it is damaged (see photo). The left-hand spring securing bracket has detached from the top (the bracket appears to have pulled away from the top of the stat, and the rivet has sheared). I had removed this a few weeks ago when I took the head to the engine shop to have the valves ground, I'm sure it wasn't like this then. Checked it in a pan of boiling water - it still seems to open and close ok but probably not at the correct temperature. I'll fit a new 88 degree one.


Brian Shaw

Where did that come from? Looks like something Brunel would have used :o)
PaulH Solihull

That's an original Smiths thermostat. It looks exactly like the one that I removed from my '67 some 40 years ago. I wish that I had kept it. You could actually take it apart and replace individual components. RAY
rjm RAY

Ray - the stat is original equipment c.1980. It's a Western Thomson and comes apart quite easily. The only broken part is a sheared rivet which is 3/32" dia. brass.
Brian Shaw

Brian, I wish that they still made them like that. Imagine, a thermostat that is serviceable. RAY
rjm RAY

ENGINE OVERHEATING

The overheating I’d observed was probably caused by the carbs being out of balance (as suggested by PaulH above). I set these up again from scratch. I clamped a thermocouple to the cylinder head to monitor engine temperature during warm-up and running (I checked my ‘good’ MGB first for comparison). Both engines produced similar time/temp data.

OIL ON SPARK PLUG

I’ve run the engine this week for a couple of hours or so in total, including a 3-mile run. It looks like my original problem of oil on No.2 plug has gone away – plugs have stayed clean and I can’t see any oil on top of the pistons. I need to have another go at setting the mixture as I noted acceleration was slightly off. I set it using a Gunson Gastester to get 3.5% CO, but this was difficult with the Gunson which seems to be inconsistent – three attempts gave a different reading each time.

I’m not sure what caused the plug to become oily, but the following seem likely:

1. Valve stem oil seal (new seals fitted)
2. The three parts of the piston oil control ring were gummed together, with the two ring gaps in line (all rings removed and cleaned; piston grooves cleaned)

I’ve eliminated the crankcase breather being a source of the oil. I diverted it to a clear plastic bottle (and plugged the carb ports) to catch whatever came out of it while I was setting up the carbs. I saw a small amount of oily smoke and the bottle caught a few drops of water and no oil (ambient temp in the garage was just above freezing, 90% humidity).

Since I lowered the carb damper oil level, no oil has got through to the jet/outlet port area.

Just one more short run and another go at setting the mixture, and I’ll get it to the MOT station.
Brian Shaw

If you are on good terms with the MOT station, why not use their gas analyser to set the CO-far more accurate than Gunsons. You might want to do it after it has passed the MOT emmisions though.....(Can't remember the age of your car and hence the need or not for "proper" gas test)
Michael Beswick

Having used emission test equipment since '68, I can say with certainty that they are very delicate and quite fickle to work with. The calibration will go out without any notice and I have had several expensive professional models fail in the middle of a test. If your local MOT testing station will let you use their equipment for a few moments, it will go a long way to narrowing down just where your problem lies. RAY
rjm RAY

"I set it using a Gunson Gastester to get 3.5% CO"

You can't do that with twin carbs and a single exhaust. You have to get the carbs balanced first, both for air-flow and mixture, and the mixture right according to the lifting pins. If you do then need to meet an emissions limit you can use an analyser, but you must only ever adjust both carbs by the same amount in the same direction or you will mess up the balance and have to start again.

I've found the Gunsons very iffy, the reading on mine changes according to how I tilt it, and gives a higher reading if sitting on warm tarmac compared to up on a saw-bench! By comparing the readings when my V8 comes back from the MOT I know how to position mine, but these days I just weaken both carbs by a quarter turn before the MOT and richen them again by the same amount afterwards ...

Looks like excessive oil in the carb dampers may have been your problem all along.
PaulH Solihull

Paul - before I used the Gunson Gastester I balanced the carbs, following the full tuning procedure as per workshop manual. I'd taken the carbs off and removed the floats, jets, needle valve, etc, to clean out the bits of rubber debris. Reassembling on the bench allowed me to easily set the jets to the recommended starting point - level with the bridge, then adjust 2 turns clockwise. Then with engine idling I used the Gunson Carbalancer to balance airflow. The Leyland manual tuning procedure doesn't mention lifting pins, although I've seen reference to this somewhere else.

I did another Gunsons CO check this morning, but only warmed the engine up in the garage for about 15 minutes, plus 30 seconds at 2500 rpm. Reading was 5.2%. Weakened the mixture to get a reading of 3.2% then went for a short drive, after which it was just over 4%. Turned the mixture down a touch more to get about 3.2%. Drove it again. Engine seemed to perform well.

I also turn the carbs down by a quarter turn just before an MOT. My recent MOTs have given the following CO readings (my own Gunson reading in brackets):

1.8 (1.8)
1.8 (2.5)
3.3 (2.3)
3.6 (2.5)
1.2 (2.2)
2.1 (1.8)
Brian Shaw

Brian,

One attachment here, second in following post. Respectively left and right hand columns of single page. Whole page image too large to attach.

These accompanies the carb diagram in my post of 31 December. Both that image and these are from my (Australian) Leyland manual C1972, covering cars here to that period ie up to 18GG.

Lifting pin discussion top of image no. 2.


Roger T

Brian,

Second post.

I'm assuming the principle is the same for the earlier and later SUs. If not, someone will correct the measures or whatever necessary.


Roger T

The Leyland Manual only describes using the lifting pins for HS carbs as HIFs were used when emissions limits had to be met, hence the fine adjustment is by using the exhaust gas analyser However given the erratic readings of the Gunsons it is the same procedure for best running as for HSs i.e. turn the jet screw for highest revs is the coarse adjustment and the lifting pin the fine adjustment. It does take practice to judge what the engine is doing as you lift the pin, bad enough on the HS but almost subliminal on the HIF. You can train your ear by alternately weakening and richening until you *can* hear it on the lifting pin, and the correct position will be about mid-way between the two. The more you practice the finer you will get it. Incidentally the lifting pin goes up quite a long way before it starts lifting the piston, you can feel the extra weight, but may need to do it with the air-cleaner off to see it happening, again until you get used to it.
PaulH Solihull

Roger / PaulH -
Thanks for the info, I'll do a bit more fine tuning when the weather improves.
Brian Shaw

The car passed its MOT today. I set the carb jets to give a reading of 3.2% CO on the Gunson. MOT emissions result: 3.6% CO, 1028ppm HC (maximum allowed is 1200ppm).

So far, there's no sign of oil on the plugs or pistons, so the original problem may be resolved.

I'm concerned about the high HC. Previous MOT tests for the last 20 years for this car show HC values from 826 to 1132, so it's an ongoing problem. I've checked the vacuum advance on the distributor (my other MG failed its MOT because of a leaky unit a few years ago). I'm not sure where to start looking now - the carbs are balanced and 3.6% CO doesn't seem too lean. Dwell angle is 55 deg, timing 9 deg BTDC.

Brian Shaw

Hi Btrian

Don't forget as long as a car passes the co at idle the HCs are retested at higher rpms, this was brought in to help with A series metros notoriously high HCs at idle.

Rubber bumper Bs tend to have high HC readings when the vac is connected direct to the inlet manifold.

The cos and HCs yo-yo depending on the idle rpm,especially when the fan cuts in.

Peter
Peter Burgess Tuning

As Peter says, it's all a matter of balance. When I used to setup and test cars for emissions, I would tinker with the timing as well as the idle mixture setting. In those days, the final result was all that mattered, so I could advance or retard the timing to get the required readings on the analyzer. After the car had passed the emissions test, I would reset the timing and fuel mixture to where the engine ran best. Even when working on cabin cruisers, with a single or dual engine setup, I would set the timing when the craft was under full load on the open water. After this, I would lock down the distributor in place. RAY
rjm RAY

"Don't forget as long as a car passes the co at idle the HCs are retested at higher rpms, this was brought in to help with A series metros notoriously high HCs at idle.

"Rubber bumper Bs tend to have high HC readings when the vac is connected direct to the inlet manifold."

Interesting, but with the higher revs for the HC test i.e. throttle open any more than a smidgen the vacuum signal will be the same from both carb and manifold sources anyway. Also with higher vacuum the engine becomes more efficient (faster idle with the same throttle opening) so I'd have expected that to result in lower emissions.
PaulH Solihull

There is so much advance at idle the engine runs like a bag of s***! The fuel picks up and burns more efficiently at a raised idle speed say 3000 rpm and the excess advance has less effect.

Peter
Peter Burgess Tuning

So you are saying all UK MGBs built after September 76 run like a bag of s***? Not what I've found, which is that it's easier to get those through emissions and run well, than my carb vacuum, which has to be weakened bya quarter turn before the test to pass and back again afterwards to run decently. Carb-vacuum MGBs certainly run more efficiently as the throttle starts to open, I can see that on a vacuum gauge, and that is with both vacuum and centrifugal advances increasing initially, vacuum tailing off again as the throttle is opened wider, but centrifugal more than compensating for that. At idle adding manifold advance makes the engine run faster, which can only be because it is more efficient. The reason North America went to manifold advance in 1971, and the UK in 1976, was to reduce emissions at idle.
PaulH Solihull

We will have to agree to differ, we have always found the rubber bumper Bs run best without loads of idle vacuum advance ( which is non existant advance once the throttle is depressed spritedly) but using a distributor with more centrifugal advance from idle but less at high rpms. From memory you dont believe in rolling road tuning and measurements so it has to be seat of the pants comparisons?

Peter
Peter Burgess Tuning

I've never had a car on a rolling road, heard tales of engines that aren't used to having the knackers revved off them when they aren't used to it being damaged, and I'm not interested in squeezing every last ounce out of them anyway. But the main point is that off-idle the two vacuum sources give the same results on a given distributor anyway - http://www.mgb-stuff.org.uk/ignitiontext.htm#vac
PaulH Solihull

Are we not talking about idle for the emissions test initially then higher rpms if the HCs are too high at idle yet the CO% is a pass?

I have never revved an egine more than the owner wants. if things dont sound right I wont even put them on the rollers. I do not like things going wrong!
Only a handful of problems in 25 years. Mostly self(owner)induce on home prepared race engines.

Our new rolling road mainly measures using energy storage flywheel effect which only takes around 10 seconds for a power run.

I attach a pic of a run on a standard MGB Chrome Bumper car. The run took 7.5 seconds in 4th gear. That included a rest of 1.5 seconds or so before I accelerated ( allows equlibrium before we record).


Peter


Peter Burgess Tuning

This thread was discussed between 26/10/2011 and 11/02/2012

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