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MG MGA - exhaust note on a 1500

The car starts first time every time, (a little choke is used). It idles fairly good initially, and it runs quite well. After a 15, 20 minute drive, when the engine is fairly hot, ( the temperature never exceeds 190F), the exhaust on idle becomes very erratic and develops a very uneven note. Could this be a valve sticking in its guide due too excess heat or perhaps a faulty guide. Everything else is in perfect order. Can someone advise what the problem might be.

Frank
F Camilleri

Frank, I think a sticking valve should cause an audible clicking noise due to the tappet clearance, also should affect the engine pick-up from low rpm. Have you noticed any of these?
Art Pearse

Art, I'll be honest with you, I didn't pay any attention to any clicking noise from the tappets, nor noticed anything abnornal from the engine. The car seems to ran OK, it's only at idle with a hot engine that the exhaust sounds terrible. Of course this doesn't mean that what you mention is non-existant. I shall have to listen carefully and pay particular attention to both these malfunctions the next time I come back from a drive.

Frank

F Camilleri

Frank,
It's my understanding that the B series motors never had a good idle note. Pesonally, I think its related to the siamesed ports in the cylinder head. That PUFF, PUFF, PUFF idle kinds of adds to their charm!

GTF
G T Foster

"The car starts first time every time, (a little choke is used)" leads me to believe that the mixture is set too rich, since a cold engine should need a lot of choke to start first time (even for Malta!). Once the thing is fully up to operating temp, it will show as too rich and give lumpy idle just as leaving the choke ON would.
Check mixture when fully warm - I'd guess you are a couple of flats rich.

FRM
FR Millmore

Yep, check the mixture. Could be just one carb is rich.

Ken
k v morton

GTF, I'm not convinced on what you are saying. A friend of mine, in fact he is my neighbour, also owns a 1500 mga. His exhaust note is ever so sweet. No puffing are far as I can tell. It seems to me that what FRM is telling me is more logical and makes more sense. In any case, thank you both for your input on the matter. I shall follow FRM's advice, at the same time keeping my fingers crossed.

Frank

F Camilleri

Frank

Have you fitted a new distributor recently or played around with the timing etc? Some time ago I fitted a replacement distributor that turned out to have a totally incorrect advance profile for the MGA. I dynamically timed it at 3500rpm and 32 degrees. Unfortunately this resulted in a hopeless lower end performance, poor starting, rough idling and dreadful exhaust note. Not realising it was the distributor, I thought I had blown some of the baffles in the exhaust box and bought a new one, all to no avail.

If not, at the very least you may want to check the points gap. Rule one thing out at a time.

Just a thought.

Steve
Steve Gyles

Steve, I did replace the distributor some time back. I had bought it from Jeff Schlemmer of Advance Distributors in USA. It's a rebuilt original item which was supposedly set with the proper advance curve as per mga specs. How can I know if the distributor is the culprit. Is their a way I can check it out? As for the points gap, this was set spot on at 60 degrees using a dwell angle meter. I doubt if the gap has deviated from this setting, but I will check it again tomorrow just to make sure. Also to mention that the plugs in use are NGK B6ES and they are new, as are all the HT leads. To be quite honest I am at a loss.

Frank
F Camilleri

Frank, its sometimes worth comparing the gap set for 60 degrees dwell with the workshop manual gap of .014" to .016". If the actual gap is much different to this then it can be indicative of something not quite right. Normally its due to wear in the spindle somewhere but this should not be the case with your distributor.
J H Cole

Frank,
It has been mentioned that maybe your carbs (or one) are set too rich. The opposite may also be a cause for rough running, ie, too lean/weak. I had this when the car was hot - vaporisation of the fuel, and eventually the car would stop. It would then take many minutes before it would restart. Richening the setting cured it.
Maybe try a couple of flats up on the jet nuts?
Peter.
P. Tilbury

Since it idles better initially it doesn't seem like it would be the distributor. Like a few others have suggested, the carbs may be set a little rich. Its been my experience that the B series engines with SU carbs will idle differently when cold or hot and differently dependant on whether it is 30°F or 90°. Generally leaner and faster idle the lower the ambient temperature is and slower and "lumpier" at high temperatures, especially in an MGA if it has been idling, say in traffic on a hot day and the engine compartment temp. rises.
Thats why SU made the attempt at jet minor self adjustment with the bi-metallic spring on the HIFs in an attempt to meet tightening emission regulations. You may want to pull the spark plugs out to check their colour and your mixture. One test at idle and another test at highway speed after a quick shut down and pull over (in a safe area to do so!) This will tell you whether you are rich or not and whether you have the correct needles. I had the wrong needles in my MGA 1500 initially, and while it idled nice, it was very lean at high speed. If I set the carbs for high speed, then it was rich at idle. With the correct needles now, it will idle beautifully from about 50 - 85 deg. F. Colder than that it idles nicely, but at a slightly higher RPM. Hotter than that and stuck in traffic it will start to become lumpy, but I just ignore it. It always "readjusts" itself when things cool down. :>)

Ralph
Ralph

Frank

I do not pretend to be an expert on this, so I stand by to be crucified.

The dwell angle is the angle of rotation of the distributor shaft when the point are closed. The smaller the points gap, the larger the dwell angle. Wear in the points mechanism (normal) causes the points gap to reduce (unlike spark plugs where the gap increases), increasing the dwell angle. One of the effects of this is that the coil gradually runs hotter, decreasing in efficiency and perhaps causing rough running, particularly noticeable at low rpms. The fact that you notice the problem after the car has warmed up would suggest this is the case with your car.

When did you last check your dwell angle and/or your points gap.

In my humble opinion this may be the source of your problem.

Steve
Steve Gyles

Worn throttle shafts? You can't get a good idle if they are.
John DeWolf

Frank,

Be sure you don't have a jet that is sticking a little after you release the choke. When you push the choke knob in also push the jets up with your finger. It they move then you have a sticking jet. It will run rich and idle with a lope once warmed up.

Jim

Jim Ferguson

John DeWolf - No John definately no worn throttle shafts. Shafts have both beeen replaced after carb bodies were re-bushed not very long ago. All other inputs will be closely followed one by one. Steve, you also mention the coil getting hotter. Well my coil gets so hot after a run that it becomes untouchable. Is this telling me something perhaps. Thank you all, I shall come back with whatever I find, if indeed I find anything.

Frank
F Camilleri

Coil too hot to touch is a no-no, sign of a coil on its last legs about to expire. Use an ohm meter to measure resistance of the primary winding (between the side posts). It should be about 3.2 ohms (slightly lower for a high performance unit). If you find 1.6 ohms its the wrong coil, intended for use with ballasted ignition system (drawing too much current at full system voltage).
Barney Gaylord

OK, following the advice given above this is what I found.

Spark plugs all sooty black, with their gaps at approx .027". The points on the distributor were slightly open,.017". Carburettor piston dampers had hardly any oil in them. Checked the jets after the choke is pushed back all the way in, and found no problems here. So here's the action taken: screwed the mixture nuts on the carbs 2 flats up to lean the mixture a little, then cleaned the plugs re-gapped them and fitted them back in place. The points gap was manually re-set to .016". And, the last thing was putting a little oil inside the carbs hollow pistons for damping, oil used SAE 20.
I haven't tried the car on the road yet. I'm hoping to go for a short spin this morning, weather permitting, rain is forecast today.
Barney thanks for you input. I tried about 3 or 4 different coils, all Lucas but used. Not had any problems with any of them except the excessive heat issue. When I measured the resistance of the primary winding, I did not come across any of them with a resistance of 3.2ohms. They all show 5ohms or higher. Is this too high for my coil. Should I go out and buy a new 12volt coil?

Frank
F Camilleri

Just got back from a nice drive. The sky is all cloudy and grey but the rain stayed away. This is what has resulted after the fiddling I did yesterday to try to eliminate the rough idling and uneven exhaust note.
The car ran perfectly with no noticeable malfunctions. When I returned to my garage after the drive, I left the engine running for a while, and I noticed that the idling speed had increased a little, about 3 to 5 hundred revs. I adjusted the idling to just under 1000 revs. Removed the plugs and to my delight the black soot had all disappered, they are now very light brown. It seems that I was running a little rich before, as Peter Tilbury said, and turning the mixture nuts up a couple of flats cured this small problem. The exhaust note has remained exactly the same as it was on initial start up before the drive.
That said, comparing my exhaust note to that on my mate's car (MGA 1500) mine is not as nice and even as his. Remebering when I replaced the camshaft on the car, the cam sprocket keyway was nearly double the size the thickness of the woodruff key. The PO had wedged the excess gap with tiny pieces of brass strips. I found that the timing marks on the two sprockets did not line up exactly as they should, leading me to believe that I may be one or two teeth out. My question is this: would this have any effect on the running of the engine and its overall performance. I noticed that on second gear mostly, and possibly third gear, the engine doesn't pick up speed as much as I think it should. Any comments on this.

Frank
F Camilleri

Frank, if the cam sprocket keyway slot is 2X the width of the keyway, how do you know which side to pack it? The valve timing could be off quite a bit. Sounds like you should buy a new sprocket.
Art Pearse

1/32-inch offset of the cam key equals 3 degrees change of cam timing timing, The cam key is 1/8" (4/32") thick. If the key way was 1/4" wide, equal wear (1/16") on both sides, and all the packing went on one side, it would change cam timing 6 degrees, which is 12 degrees at the crankshaft. That would indeed make it run much different.

Retarding the cam up to 6 degrees gives more torque at top end, like a race engine, with some loss of low end torque. The trick there is holding the intake valve open longer/later for better cylinder filling at high speed. Advancing the cam up to 6 degrees gives more torque at lower speed, good for trailer towing around town, but with some loss of power at top end.

Coil with 5-ohms primary resistance is a bit unusual. That would likely have lower peak voltage output. Maybe suspect your ohm meter needs to be re-calibrated. But if it is correct, then yes you could likely use a new coil. Take the ohm meter with you to the auto parts store and check the new coil before you buy. Do NOT buy the 1.6-ohm coil.
Barney Gaylord

Barney as always your info is very detailed and easy to follow. From your information I have decided to replace both the camshaft sprocket and also the coil. I have one question on the sprocket if I may. Will the new sprocket come with the timing mark punched on it? If not, do I just measure the position of the punch mark on the old sprocket, and punch the new one exactly in the same position?

Frank
F Camilleri

Frank

I was advised to change both sprockets and the chain, not just the one sprocket. Mine came punched.

Steve
Steve Gyles

Thanks Steve, I guess it makes sense to replace both sprockets and chain. I think I'll take your advice and do just that. As a matter of interest where did you buy the items from?

Frank
F Camilleri

The WSM gives the procedure for checking valve timing without taking anything apart. If it is off, then you get to fiddle, but I recommend you learn how to set up a cam correctly if you do need to take anything apart. OE parts were subject to enough deviation to make a difference, repro or reground parts are much more suspect. I've seen factory marks off by 10 degrees, no marks, two sets of (factory!) marks. It is possible that your "worn" parts were actually modified by the PO to make it come out correctly. Relying on the factory marks is iffy at best, especially for an inexperienced amateur, and more so if the work is done with the engine in the car - it's just too difficult to judge the lineups by eye with your head upside down and at an angle. Always do the WSM check after this work.

Everyone who has driven a bunch of the same sort of car will know that most are OK, a few are dogs, and if you are really lucky you get one that goes like the wind. I've come to the conclusion that this is most often the one with the "best" cam timing. I took a TR Spitfire apart that had the cam set 10 late at the factory (and marked at that setting - these have a switchable sprocket that is punched after it is in the correct position), plus 4 deg wear; simply replacing the chain and timing correctly gave about a 15% increase in both power and fuel mileage - nothing else on the engine was changed.

I time the cam 2 deg advanced with a new chain; this is excellent as is, but the chain bed-in will put it dead on after a few thousand miles, where it will be stable for years.

FRM
FR Millmore

How to install timing chain and sprockets when there is no punch mark, see here: http://mgaguru.com/mgtech/engine/cm105.htm
Barney Gaylord

MGA's were never good "idlers", nor was the exhaust note very "sporty"!
Barry Bahnisch

That's a sweeping statement Barry, I love the way my bog standard 1500 sounds and idles!
Lindsay Sampford

Frank,

I have recently replaced my timing chain and both sprockets. Upon disassembly I found that the cam was about three teeth off in alignment. You can see the punch marks on the new parts (Moss) in the attached photo.

In following Barney’s detailed instructions, I ended up loosening up the entire cylinder head since I also wanted to replace the damaged aluminum rocker pedestals. I also found that it was not possible to loosen the rocker arm adjusting screws enough to move all of the rockers aside. It would have been possible to move all of the rockers aside if the cam could be rotated but I had already removed the sprockets and timing chain.

In the end pulling the head turned out to be a good thing as a magnaflux (sic?) test found three cracks. Now thanks to Art Pearse I have a good candidate for a replacement head. Art was kind enough to pack up an extra head and drive across the border to NY to ship me the head. For his time and effort he certainly didn’t make anything for the small amount I paid. Many thanks Art.

As you can see from the attached photo, the dots are lined up but the crank is slightly past TDC. In other words the dots are about a half a sprocket tooth off alignment when the crank is at TDC. Should I look for an off-set key, spend a bunch of money on an adjustable cam sprocket or not worry about it?

John


jjb Backman

I found interesting crank sprocket damage when I had my engine apart a year ago. Never did find the missing bit.

Steve

Steve Gyles

Looks to me like you may have missed position by one tooth. If you disassemble, rotate cam sprocket one tooth the the right (clockwise), put the chain back on and line up the punch marks again, it may come out about right.

One tooth on the chain is 9 degrees of cam rotation or 18 degrees of crank rotation. That's a lot of error if you get it wrong. The engine will still run, but not so good. If you think there is a few degrees of error in location of a keyway, the only way to know for sure is to stick a degree wheel on the crankshaft and measure vale lift points with a dial indicator.
Barney Gaylord

I sent in an order today for both the crank and the cam sprockets. A new timing chain has also been ordered. Hopefully, I will come back on this forum on the subject after I've replaced the old ones. Thanks to all for your help. Special tks to you Barney and when the time comes to fit the parts I will no doubt refer to your site for the timing procedure.

Frank
F Camilleri

With a standard cam, set the #1 intake clearance to .060" and it should just open at TDC. This from the WSM.
Art Pearse

Lindsay, Good on you for going into bat for MGA's. They are great little cars, I think that I may have been the first person in Oz (or anywhere?) to restore one to factory specs (in the 1970's) and have been an advocate for them before and since (I have owned six in all, still have my restored 1600 fhc and roadster). Great fun to drive (took mine to the UK in 1996 for a series of events), much better roadholding than an MGB but nowhere near the torque (oops!).
Barry Bahnisch

A question for Steve Giles.
Steve you say that when you replaced both sprockets and chain, the sprockets came already punched. Is this standard procedure when buying these items for a stock cam. Where did you buy yours from?

Frank
F Camilleri

Frank

I never really thought about it at the time. I just ordered the bits from Bob West and the sprockets arrived punched. I don't even remember being asked what cam I had. Does it make a difference? I would have though not. But what do I know? Not my strong topic.

Steve
Steve Gyles

OK Steve, thanks. I ordered my sprockets from Moss UK. I believe they are on the way already, so it's pointless to give them a ring to ask. In any case I'm not really bothered too much, Barney Gaylord has a very detailed article on how to do cam timing using sprockets without punch marks. My mind's at rest.

Frank
F Camilleri

This thread was discussed between 04/11/2010 and 14/11/2010

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