MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Power loss when hot

I have a restored 1960 Frogeye, but with circa 1977 rebuilt Marina 1.3 A-plus engine (screw-on canister oil filter, pointing up). This car/engine combination has been running since May 2009 and has just covered 500 miles of pretty much shortish proving-runs.
Its fitted with a profesionally rebuilt unleaded 1275 midget head and fully re-furbed twin 1 1/4 SU HS2 of the biased needle type, standard needle AAN, tuned to the best of my ability (no rolling road yet). Ignition is standard Luas45D and Luacs standard coil, and resistive leads, static timed only (but no run on or pre detonantion). Exhaust is a Moss-sourced stainless "lcb" which has meant using a two piece heat shield , although a heat blanket does cover the whole of the manifold. Fuel delivery is from an electric / electronic pump mounted at the rear. Being a complete restoration, the fuel tank and all lines(inc small see-through in-line fuel filter)are new. It starts well (although very sensitive to choke on warming up ) and runs well on the shorter proving runs.
The problem is that the engine looses power when it gets "hot" - typically after a completely faultless run at moderate speeds (40/50/60 mph) on open roads for about 20 minutes at English summer ambient temp (24 degrees C), encountering town stop-start driving or moderate hill climbs sends the indicated temp to just-above 190 dgrees F. First indication of trouble is hesitation/ flat spot on very-very light throttle pedal, then progressively increasing hesitation under load with miss-fires (and occaisionally back-fires under full pedal)until the engine will not pull at all and we come to an embarrasing stop. The engine will still idle at this point(and it will still start) but it wont pull-away.
Wait for a while and it will run normally again, until the heat rises and the same performance starts again (journeys home completed in a series of stages!).
Ive checked other threads on the site, but none quite fitted my problem. Anybody brave enough to tell me where to start??
L S C Brown

Is the back fire you mention at the carbs or the tail pipe?

Trevor Jessie

Fuel, start there.

For some reason your carb float chambers aren't re-filling fast enough.

The temp rise ( car isn't overheating is it?) is just a coincidence, the time it takes to empty the float chambers is the same time it takes to reach that temp.

It could be a blockage or pump failure, good luck.

AndyB
Andy Borris

After that...

Possible that you're running slightly weak and that hot inlet air is pushing your engine over the edge (weaker still).

You could check this by going say 2 flats richer. If no difference, put it back.

I wouldn't bet against an HT problem: coil, capacitor, rotor arm(!) can all exhibit variable characteristics at high temp.

Also - don't discount inlet manifold: leaks or cracks could open with temperature... check by spraying WD40 when hot and engine should speed up. (This is easily done at 40 mph; but your wife/gf probably won't want to be under the bonnet at higher speeds. If on your own, try it at hot idle.)

Finally - if you're still looking for ideas: how about a water hose / head gasket / etc leak that sprays the HT with vapour when the engine's hot? Frankly, that's my favourite cause since it's 5 secs to tighten a jubilee clip. (Mind you, on the std Aseries config this would have to be head or the hose on the heater post at engine rear.)

A
Anthony Cutler

Once you check out all those suggestions... then you might need to listen for a sticking valve. You'll hear it when the engine cools and it breaks free... POINK.
Trevor Jessie

I have 5 thoughts

1. fuel line to close to a heat source

2. the carbs are set to lean

3. Wrong spark plug heat range...my need a colder plug

4. valves have lost there proper gap.

5. The coil...Yeah I know, but it might be worth putting in a known good coil just for kicks and grins, its easy/super fast and you can take it out of the mix right away as a possiable odd duck culpret.

the part im locking onto is the problem starts when the engine is at full operating temp.
Prop


""First indication of trouble is hesitation/ flat spot on very-very light throttle pedal""

Classic symptoms of weak mixture as is the need for exact choke requirements.

Try richening it up a full turn and then see how things respond from there.
Robert (Bob) Midget Turbo

Well there you go...lots of good ideas and more to come...Might consider just rebuilding it agian. LOL

What I do in these situations...is devolop a plan by looking at the 3 componets electrical, fuel and compression....what are the top 10 things that cause this type of problem under each of those 3 headings and work from the bottom up starting at 10.

BUT (This is the most important part) remember this is a Midget we are talking about. So toss that list into the tolit then flush before you even pull the bonnet lever. You want to look for the most stupidist least obvious thing imaginable and theres your problem. AKA. loose spark plugs.

No short cuts...You have to make the list 1st and flush it.

Prop
Prop

why no rolling road yet?
can you not find one within a halfhour's drive?
David (davidDOTsmithAT stonesDOTcom)

why should you do a roling road if the car is std setup and not even run in yet?

just a proper tune to make sure it runs right
then run it in proper (no more short runs until you've put on at least 1000km of long runs)

puting it on a RR now wil only stress things that should not be stressed yet
Onno Könemann

Thanks guys -its really helpful knowing you're out there on a real-time basis ready with ideas and a good sense of humour. I will start the list,honest, Prop, and at the same time look at some of the concensus in this thread (weak mixture). In answer to Trevor, by the way, the back-fire is from the exhaust. I've already replaced a poor rotor arm that stopped me dead in early testing, so I know these things (when non-genuine) cant be trusted(the joke is that back in the late 80's I was the Aftermarket Marketing Manager for Lucas ignition products here in Birmingham!)
I'll let you know how I get on - but don't hesitate to add if someting else occurs.
Les Brown
L S C Brown

Weak mixture could of course be caused by a low fuel level in the float chamber if the fuel pump fails to keep up. As Andy Boris suggests. Maybe your fuel filler cap isn't venting properly? Or maybe you have an air leak on the hose between the tank and the fuel pump such that it draws air in when the pump is operating, and fails to give a good fuel supply. This won't necessarily leak fuel out, and can be difficult to detect as on tickover, when you have the bonnet up to check, there will be sufficient flow and everything will appear to be OK.
Guy Weller

If it was backfiring through the exhaust, then a lean mixture seems less likely. Usually lean mixtures backfire via the carbs. I'm going out on a limb and guessing a valve train issue. Loosen the tappets by a few thousands and see if it changes anything. If it dies, pop the bonnet remove the rocker cover and see is any of the valves are hanging open esp the exhaust.

I've never personally seen this on an A-series, but I had a similar issue on an old Toyota.

However, Check all the simple stuff first.
Trevor Jessie

Les - Had a very similar problem with my 1275 - after much frustration turned out to be the condenser ( new 50 miles previously) having eventually got to the bottom of the problem and having lost a degree of faith in modern condensers built and fitted an ignition amplifier from Maplin. not missed a beat since!
David
David Hunt

I'll echo David and mention once again that the condenser has been the root of problems for several of us, to the point that some of us wags now suggest "check the condenser" for any number of unrelated problems.

I ran into this issue a few years ago, and checked everything BUT the condenser, assuming that nothing could go wrong with something that simple. Wrongola - when I finally swapped out for a new one, the problem was completely solved.

Not long after that, I converted to a pertronix unit, thus avoiding points and condenser. Like David said - "Not missed a beat since!"

Best of luck,

-:G:-
Gryf Ketcherside

When it starts to misfire, what does the tachometer do?
A wildly flickering needle would indicate an electrical problem.
A relatively steady, although dropping, needle would suggest a fuel problem (although this could include vale problems)

Ignore of course if yours still uses the tach with a mechanical drive rather than the electronic type.

Guy
Guy Weller

I'd ditch the heat blanket. I'd check the grade of plugs. +

Whereabouts in the West Midlands?
Daniel Thirteen-Twelve

Les,

I had identical symptoms - rise in temperature then a misfire under load - with my problem a few year's ago, which proved to be sticking exhaust valves in new bronze guides supplied by a very well-known A-Series tuner. Replaced every part of the fuel system and swapped most of the ignition system for known good components. Each change seemed to improve matters, but never cured them.

Has the head been fitted with valve stem oil seals on both inlet and exhaust? Old thorny one this, but with bronze guides, it is usually better without seals on the exhaust guides. They can be removed in situ, I watched Peter Burgess do it, but very tricky!!

What cam and ignition timing are you using? Whatever cam it is, try opening up the exhaust tappet clearances by 0.003" more than the standard for the cam - gives more seat time, thus more valve cooling and less likely to 'nip' - also from Peter Burgess. Also get it set up on a rolling road to make sure that the mixture is correct throughout the rev and power range.

Finally is the water pump to head by-pass still in place?

In my case fuel and ignition were both correct, and I had the 'new' guides replaced with Peter Burgess's own and apart for a little bit of initial trouble, which is when the exhaust seals were removed and the tappet clearance opened up for about 500 miles, I have had no problems with sticking valves since then (~4/5 years ago).

Richard
Richard Wale

Some answers (and questions): I've checked that I've got a vented cap Guy, although the fuel level in the see-through in-line fuel filter puzzles me. When we come to a stop it seems low, but on switching the ignition back on there is no audible ticking from the pump(as there always is on cold start-up)but as I said before the engine still starts and idles. As for the tacho,that's a good point: I had some donor electronic innards put into my old mechanical head by a local specialist here in Redditch - its fine when the engine is at steady speed but any acceleration sends it crazy. I haven't noticed what happens in the "failure" mode (I'm usually looking for a safe place to stop, looking as though I meant to stop there!), although I will try and remember next time.
Why should I ditch the heat blanket, Daniel - this was my first line of defence against cooking the carbs and I was going to wrap even more things in it? We're near Alvechurch by the way.
Richard, you are talking about those bits that I left to the professionals (Halesowen Engines) so I dont know what the unleaded conversion involved in detail althogh new valves were fitted as well as the guides - I will check, but it would have been a few moths ago.The cam was the original cam from the Marina 1.3 as was the sintered rocker gear, so nothing special (only the cam bearings replaced). The water pump by pass is still there - why do you ask? In my 2 years of restoration I haven't come across Peter Burgess - should I have? (I've dealt with Peter May on a few things though - eg clutch release roller bearing). Finally I will take all your advice on the condensor. Now enough writing for the moment - I really ought to lift the bonnet.

Les
L S C Brown

Try bypassing/remove the in -line fuel filter it is not unknown for them to cause fuel starvation type symptoms as you describe.
good luck
Jon
Jon Warren

I would agree that in line filters can restrict the fuel flow rate, although I don't think that the presence of a large air bubble in there is anything to worry about - it is commonplace and doesn't itself seem to impeed delivery.

Not sure if anyone has mentioned it but maybe the valve clearances are too tight - this can cause sticking valves with symptoms similar to what you describe.

If the rev counter needle is going wild it maybe suggests something irregular about the ignition side of things.
Guy Weller

I'll try taking the filter off-line. Looking at it again today the level was very low, and I did see some air bubbles entering the exit tube of the filter. Switching ignition on did replenish the level a bit ,but to less than half full. After a warming up run (short, but temp above 190 on the dial) richened up the mixture by 3 flats on each and then a bit of fiddling with ballancing. Piston lifting does now give slight increase in revs, but idles a bit rough. Noticed the coil was very hot during this tuning (too hot to keep hand on )can't recall if this is normal? If its fine tomorrow I'll risk a longer run and see how the fuelling changes have affected things.
Les
L S C Brown

Running a hot coil will indeed give the symptoms you describe, so its worth checking the voltage getting to the battery. Use this link to go through the tests in detail http://www.1978mgmidget.com/Lucas_Generator_and_Control_Box_Tests.pdf
f pollock

you should know Peter Burgess he is a regular at this BBS and well known fot his head work (cylinderhead that is) ;P

The blanket does not remove the heat it relocates it.
To the cilinder head mostly giving al kinds of nasty ness.
And the manifolds tend to crack more when they cant disipate their heat normaly
And the heat from an non tuned engine with lcb should not give a problem anny way

coil should not be that hot!
Onno Könemann

Thanks Onno, and Mr Pollock for the info(I have fitted an alternator during the rebuild, so voltage is internally regualted)but the coil may be related to my problem.I hate to say it, but when checking the distributor today, the points gap did look a bit thin (ie too little dwell time on the coil)- this was only set 500 miles ago and I had assumed it was OK.(what a pig access is to the 45D on the A plus - everthing is in the way!)
Head re torqued and tappets re gapped, plus new coil. I'll save fitting new plugs until final fuelling setting. Dreading a test run
Les
L S C Brown

I don't like a heat blanket because it traps heat. I'm not a big fan of heat shield either. You need to let the hot air out of the engine bay, bearing in mind the LCB puts more heat into it than the cast iron manifold(pop the bonnet at night at take a look).

However, since it's not that easy or cheap to do a neat job of letting more air of the engine bay and IF you have a fuel vapourisation problem (I'm not convinced you do - sounds more like a bad coil) the most effective solution is to put a thermal barrier coating on the inlet manifold - something by Camcoat. That usually does the job and so I've not know anyone need to put a thermal barrier coating on the carb float bowls.

What you could do on your test drive is take a garden water spray thingy (or just a couple of bottles of wate) and drive the car to the problem occurs. Next, pour the water onto the inlet manifold and carb float chambers and re-start the engine. If the problem goes away you have a fuel heat soak problem.
Daniel Thirteen-Twelve

I dont know that I like that idea danial...yes it would work, but just below the intake and carbs is the header and the thin side of the block....

CRACK!!! And I dont mean cocain.

Thats just my opinion

Prop
Prop

OK thanks- I may cut down on the blanket, so it simply becomes a single piece heat shield. Thankfully no one pulled me up on my dwell-time issue: should be shorter rise-time on thinking about it!

Les
L S C Brown

I had a poor running condition and hot running after my last rebuild. Retorquing the head bolts at about 1,000 miles made all the difference. This was with a Payen paper gasket so OEM gasket retorquing instructions may not hold.
Glenn Mallory

I have a similar problem with my 1968 Midget Mk III. After getting nicely warmed up (15-20 minutes driving) when I slow down, the engine sputters and then just stops, and we roll to a halt. The engine will not then restart until a wait of 20 minutes. I have had replacement dynamo and coil fitted but this problem has not gone away. Any ideas?
MD Hurn

In this case you have a clear 20 minute window in which to do some diagnostic tests. Has the coil got 12v across it? Do the spark plugs all spark when you turn it over attempting a restart? Check the timing - has it gone astray?

Thinking aloud, could it be that the float chambers are full of crud and blocking up? Or the needle valves sticking? ie you drive until the float chambers are empty and then have to wait 20 minutes for them to refill? Or perhaps the float needles stick and then the floats drop down again when it cools? I wouldn't expect that to happen on both together, but one may have failed in the past and now the 2nd one is doing it too? Just a thought.

Anyhow first off I'd check the electrics when it's stalled to work out why it won't restart. If everything checks out ok then look at the fuel system.

Dynamo - I'm don't see how that's involved. If the battery has enough charge to turn the starter then it's got plenty of charge to run the fuel pump and create a spark. As with all faults you've got to be systematic and eliminate things one by one.

Ant
Ant Allen

The business with the dynamo - where fitted - is that unless the voltage is correctly regulated the coil can receive too much current and will overheat and break down. After a cooling off period of 20 mins or so the car will restart as normal till it gets hot again and the cycle is repeated. Check the voltage going to the battery at 3000 rpm.
f pollock

So it could be a regulator box fault (or incorrect setup) rather than the dynamo itself. That makes sense now.

Ant
Ant Allen

Just to close out my problem; latest test run was successful! I think I should have been more systematic though, as a number of changes were made (new coil and condenser, re gapping the points and tappets,and a tweak to richer fuelling)My feeling is that the coil/condenser/points combination was the probably cause - the tacho now has a steady needle! I'll now treat the car to a rolling-road session to complete the set up.
As a rider for MD Hurn and as per Ant's suggestion, on my early testing-runs had the same problem - cause was the rotor arm (testing gave spark off the king-lead from the coil, but nothing off the end of the plug leads)Replaced with a reputable part
Good luck

Les
L S C Brown

Les
Peter Burgess is a name worth checking out for rolling road sessions, I've only ever heard good things and I was very pleased with the service I had.

Other rolling roads are available!!

Cheers Carl
C Bintcliffe

Les as you are in W.Mids, there is only one rolling road to consider, Aldon in Brierley Hill. They do a brilliant job and are very used to the vagaries of Spridgets. Mention MASC to Alan the proprietor, and he will knock something off!!
R Cole

This thread was discussed between 10/08/2009 and 27/08/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.