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MG Midget and Sprite Technical - summer problem part 2

My standard 948 get starvation problems when hot.
If I give full throttle for > 5 min first the exhaust gets a harsher tone, than the motor begins to hesitate 2 times per second. The speed goes from 140 km to 95 (full throttle). It gets worse for a minute if I give half throttle. But 3 minutes later it can go 140 km again (for 2 or 3 minutes). After such a bad run I inspected the interior of the combustion chamber wit a scope; it is white. White valves white pistons. White sparkplug my conclusion to hot to lean when hot.

What did I try and control already?

Little but significant effect:
Open the heater!
Richer needles EB instead of GG on the H1 (1 1/8) carburettors make it a little bit better. I use K & N filters.

Possible but insignificant effect.
Colder Sparkplugs, BP7ES instead of BP 6 ES
Peter Burgess valve stems 15 thou valve clearance instead of 12


No effect:
Change the mechanical fuelpump
Pulling the choke when it happens or trying to prevent it.
Cleaning tank and fuelpump filters, opening the air hole in the tank filler cap
Other coil, distributor cap distributor.
Insulating fuel lines against vapour lock. (it helps against vapour lock)
I controlled the vacuum advance: Not perfect the advance is about 32 degrees at 5000 rpm. It should be 34 at 3500 rpm.
Colder thermostat


Any suggestions are most welcome as I struggle with this for 7 years…. And summers are hot these days

Flip



Flip Brühl 948 frog 59

Flip, Put some household plastic over the fillerneck instead of the cap. Pierce a hole or 2 in the plastic and run again.

What you discribed could very well be caused by a vacuum in the tank. The rubber grommet inside the cap can crack and/or expand so its over the vent holes. Took me a long time to discover.

Other possibility is a vapourlock but that would cause a more or less stop.

Bas

Did you see the cheap smooth gearbox on Marktplaats?
Bas Timmermans

sounds to me like its running lean...Id say look at all the usaual suspects....

leaking at the manifold, the intake and exhaust manifold flanges are the same thickiness...one not thicker then the other, butterfly shafts arent worn in the bushings, butterflys are alinged correctly....good gaskets on the float chambers, good gaskets at the filter housing and the face of the carb throat, flange on the throat is flat, rubber blocks between the head and the carb are ("crack free"...gez how common is that problem) no open vacume ports, vacume ports with screws in them are tightly sealed on carbs and intake manifold...vacume lines are in good shape no holes or cracks, float bowls tight and secure that attach to side of carbs, the angle of the float bowls is square and level....and of course my favorite that I keep battleing...just to much dam* heat in the engine bay in the middle of july and august....need some fender heat vents, the vacume advance on the dissy is working properly...ect.ect.

Basically look for anything that air can find away into the combustion chamber ....the filter should be the only area where air should enter your engine


dont forget to use some WD40 and spray over the engine and listen for an increase in the enigines rpm(running faster)

prop
Prop

Bas

I use a filler cap with key now, there is a big hole! Thank you for marktplaats. I did send an e mail

Flip

Prop,
I did nont cheque all your suggestions. I replacesd the butterfly spindles and bearings, controlled the vacuum lines. controlled the advance. What does the WD 40?

Flip
Flip Brühl 948 frog 59

If you spray the WD40 blocks the intake of air at any leaking point for a while and you should notice. WD40 is comonly used as its not flamable.

Bas
Bas Timmermans

Flip, I assume your fuel pump is cam driven - mounted on the side of the engine block. I offer as another suggestion a cause I have seen in the past - that is the cam wear on the pump lobe of the cam means your fuel pump doesn't pump enough fuel under high demand. If its this changing the pump won't show any improvement - you could improvise an electric pump to check it out. Or maybe measure the fuel delivery from the pump when you crank the engine over - suirt some into a jar for 2 minutes - and compare with someone elses vehicle?

Good luck anyway
John Barber

Flip To overcome that problem I mounted a blanking plate at the block and an electrick pump at the back. If you are somewhere and your pump is faulty, what are the chanches of finding an new mechanical or a new electrical pump? That question was input for my disicion.

Bas
Bas Timmermans

Im guessing if the lobe on the camshaft where worn you could not just add a block to the fuel arm to build it up as its still only going to move the fuel arm the same distance as the size of the cam lobe...

or can you, as the distance of the fuel arm is being moved the same distance with or with out a block...but having the block will actually push it further back.....

does that make any scnce....

which one would be correct???


AS to what is WD 40....here is a link....it also can be sprayed around suspect air leaks on engines to pin point the problem.

example ...if you suspect air leak at corner of back intake manifold gasket area,,,spray it in that area with the engine running ... if the engine pickes up speed....you found your problem

http://www.wd40.com/products/wd-40

prop
Prop

Flip,

Do you have any rubber hose between the pump and the tank? I had an intermittent problem like yours that was caused by the suction collapsing the rubber hose. A metal line solved it.

Charley
C R Huff

At one time I had similar symptoms which always occurred in warm weather if I took a particular long incline at full throttle. It had all of the impressions of being caused by an inadequate fuel supply not keeping up with the engine.
In the end it turned out to be a valve problem, cured simply by increasing the valve clearances by a couple of thou.

Guy
Guy Weller

Flip,

Two questions for you; do you have an oil cooler installed, and what oil pressure are you seeing when you start to loose speed? Second, what coolant temperature's are you seeing when this happens?

While my summer operation was never to your extreme, I installed an oil cooler on my MK-III about a dozen years ago, and this helped to improve my summer operation.

Regards,

Larry C. '69 midget
Larry C.

Guy; I increased the valve clearance from 12 to 15 thou,

Larry; No oil cooler, water is 165 oF, oil pressure > 55 psi,

Prop: WD 40 gave no improvements or change in the smooth running idle.

I noticed a few misfires yesterday when it happened again, and lost the vinyl hood (strong crosswind 140 km/h...)

Flip
Flip Brühl 948 frog 59

Flip, I am sorry to hear that didn't work.

But worth a try at least - you might have been lucky!

Guy
Guy Weller

well thats not all bad!!!

good news is you know its not.......

running lean from extra air getting in
Its not the valves being out of adjustment
Once you try bas's Idea for checking out the fuel pump you will be able to cross that off.....sometimes you just got to write down a plan of elimination and work it thur .....startig with the most obviuous and working your way backwards.....I dont know why, but for me its always the last thing on the list, or close to it...


prop
Prop

Prop,
The worst thing is when it is the first thing on the list. Only you don't check it properly and go through another 20 possibilities before eventually, in desperation returning to re-check #1. At that point you find that you replaced a faulty rotor arm with a new one which didn't cure the problem because it was faulty straight out of the box.
Guy Weller

That is true lol.....

but on the odd (and rare) occasion that it was the 1st thing....I never want to belive it and end up discounting it till I get to the end several hours later, and look back....thats always good for an old fashioned kick in the paants...good thing our knees dont bend or work in that direction....lol.

prop
Prop

This thread was discussed between 11/08/2008 and 15/08/2008

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