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MG Midget and Sprite Technical - Strange idling problem

For over 10 years I have used a normal closed circuit breather system on my Mark 1. It has worked fine until recently. Now, when the engine is hot it sometimes won't idle properly. Sometimes it is too fast and uneven, and right now it won't idle at all and also sounds uneven, as if I have twin carbs that are out of sync (but I have a single HIF44). I suspected the breather valve, and tried removing it from the housing and sealing the port to the manifold. It then idled correctly, although hunted a bit which I put down to the Kent 276 camshaft. Put the valve back in and it won't idle again. The diaphragm looks perfect - no holes, flexible. The valve and spring seem to work OK. Last week I replaced the throttle spindle which was slightly worn but not seriously so.

Any ideas folks? If you want more info on my engine etc just go to my profile.
L B Rose

Obviously... its that part you suspected

but it does sound like a vacume leak of some type around the carb or manifold
Prop and the Blackhole Midget

It does sound vacuum related - replacing a worn throttle spindle will have increased manifold vacuum at idle.

I was thinking it may be distributor vacuum advance - assuming that you have one - but the effect you get when blocking off the breather port would tend to rule that out.
Dave O'Neill 2

Often down to the breather injesting blobs of oil mist which then fuels the idle mixture to run too fast and out of control. Does it idle better when cold - is it smokey on the overrun or 'run on' when hot?
Fergus

Hmm this could be interesting (where's Lawrence when you need him :-)

If you want the simple fix.. remove the valve and vent the crankcase via a filter to atmosphere.
Otherwise, here goes....
1, check you have no Carb/manifold air leaks after you refitted the carb (this has the effect of reducing manifold vacuum and therefore the valve may not close fully)
2, remove the inlet pipe to the valve and partially block it to mimic applying vacuum to the crankcase. Does the idle improve eg does the valve close? (you may need to almost completely block the inlet pipe). If not replace the valve. Other test are; the valve has a small hole in the lid to allow air to the outside of the diaphragm when it's being sucked in, make sure this is clear. Remove the valve and make sure you can get it to close when the inlet is blocked (you might need to suck hard (ooo errr) or rig up a vac pump of some sort).
3, Once you confirm the valve will actually close under vacuum, check all the pipework to the crankcase for damage/splits.
4, Remove the oil filler and partially block the filler. If no improvement, check how much crankcase pressure is generated and/or rebuild engine! If all okay fit a new filler cap but make sure you get the correct one and it fits snugly.
5, see simple fix.....

Best of...
MGmike
M McAndrew

4a check dipstick seal and/or replace seal before rebuilding engine :-)
M McAndrew

Thanks Mike, I'll try all this when I next get it warmed up. Probably tomorrow.

Les
L B Rose

Another item to check are the nuts to the intake / exhaust manifold to the cylinder head as well as carburetor mounting to intake manifold.

These loosened up over the years on my 79 B and created a vacuum leak.

Cheers
Gary
79 MGB
gary hansen

This thread was discussed between 31/10/2016 and 06/11/2016

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