Vacuum Leak | MG MGB Technical

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MG MGB Technical - Vacuum Leak

1977 with Stromberg carb, emissions removed. While searching for a slight oil leak, Stromberg O ring was suspect and was replaced, I discovered a crack in my exhaust manifold. Short story is I took it to two different reliable welders, neither was able to get the weld to take. I did manage to find a almost new one that had been cleaned and bead blasted. Whenever I replace anything on the engine I take the time to detail and paint with the appropriate materials. Painted manifold, EGR and heat shield before putting everything back together. Started car with manual choke, idle speed was very hard to control actually kept rising and a distinct "hissing" was coming either from the EGR or rear of Stromberg near the manifold. Car was running extremely hot to the point the newly painted EGR valve and heat shield blistered. Never had that happen before. Could hardly shut car down it was idling so fast. I did not use any sealant on the new EGR valve gasket. I did use sealant on the new carb gasket that attached directly to the Stromberg. Does the second carb gasket go behind the plate that attaches to the manifold, which is where I put it with no sealant, or does it go behind the induction heater that attaches to the plate after the plate has been attached to the manifold? Any thoughts on this problem would be appreciated.

JCH Hibbard

Almost all sealants, on the market today, won't hold up to gasoline. Hylomar, made by Permatex, is one of the few exceptions. Since you have desmogged your engine, getting rid of the troublesome EGR valve would be a good idea. A block off plate can be made out of some thin scrap metal. Make certain that the surface of your replacement exhaust manifold isn't warped. These units operated at very high temperatures and were prone to warping. When an EGR valves fails, the engine will never idle properly. A heavy duty, high temperature resistant gasket is a necessity for use at the EGR port as it is recurculating extremely hot exhaust gases from the exhaust system back into the intake side of the engine. I've been bypassing or deleting these dreaded devices since '68. RAY

rjm RAY

You really need to go over everything you did, as it seems certain that one of those steps isn't right.

Paul Hunt

Thanks Ray and Paul for your input. Most certainly something is not right and the leak is definitely occurring near the back of the carb or the EGR. The only thing I am not 100% certain of is the gasket placement for the carb assembly. Most certainly one goes one the carb itself with sealant, the second has to either go on the back of the induction heater to the plate or on the back of the plate to the manifold with no sealant which is where I put mine. anyway, it will all come apart this weekend.

JCH Hibbard

Three gaskets,one between plate and manifold, one between plate and induction block, and one between induction block and carburettor.

Hartley Mayers

Thanks Hartley,

hoping to get busy on this this weekend. Going to check for trueness on the flat services of the exhaust manifold and replace the gaskets again. Will probably try Ray's suggestion regarding the EGR valve.

JCH Hibbard

Put everything back together this weekend.Checked everything includuing the trueness of the new manifold at the EGR and CARB positions. All checked out nicely. Blocked off EGR as recommended by Ray as all emissions have been removed. Reassembled everything according to specs,no more air leak,car statrs beautifully,but.....after running a few minutes the idle speed continues to rise and is not managable.Cant get below 2000 rpms.
Tried adjusting carb stetting but no luck. Need help.

JCH Hibbard

Have you tried locating where the leak might be coming from? This would, to me, be the first step to take as it allows you to focus on a particular item or area. With the engine running, spray some carb cleaner or WD-40 around the various joining areas of the head to manifold, manifold to carb, and any items bolted onto the intake manifold. When the sound of the engine smooths, for a few seconds, it indicates an area where the liquid is making a temporary seal. Focus on what might not be correct in that area. But, be aware that the carb cleaner can damage the rubber/neoprene diaphragm of the Z-S carb if the spray is directed into the throat of the carb.

Speaking of diaphragms, when was the last time you compared the one in your Z-S to a new one? I have seen several older ones with cracks, pin holes, and/or the material of the diaphragm either too stiff or too loose. If the diaphragm has not been replaced recently, it might be worth doing so--assuming there is a good quality replacement available from any of the suppliers.

Les

Les Bengtson

A vacuum leak by itself isn't the cause of not being able to reduce the idle speed to stalling point. It *will* cause a high idle for a given idle screw setting, but you should still be able to stall the engine by unscrewing the idle screw(s).

The butterfly should be capable of being fully closed, which means that no fuel can get into the engine, even if air can via a leak at a manifold joint or plug, for example. The only exception to this is if the throttle spindles and bushes are so worn that some air/fuel mixture can sneak past the fully closed butterfly.

Other than that something is holding the butterfly open, or it's poppet valve is stuck open (if it has one). In the former case either the butterfly is misaligned in the throat and cannot fully close it off, or something else is holding it open. This could be incorrect adjustment of the throttle cable i.e. insuffient free play, idle screw, fast idle screw or auto choke mechanism, or some other mechanical defect affecting the throttle spindle rotation.

You need to disconnect the throttle cable, back the idle screw right off, remove the autochoke assembly or back the fast idle screw right off, then see if you can press the butterfly fully closed. If you can't, and everything has been disconnected from the throttle spindle, then the problem is internal.

Paul Hunt

Thanks Les and Paul, unfortunately found another problem I suspect is contributing to the leak. The induction heater behind the Zenith Carb has a piece missing. I will have to try to find a replacement before I can proceed. I do have a new diaphragm that I can put in but for now the induction heater is the main problem.

JCH Hibbard

If the induction heater has a piece missing, that is the most probable cause of your air lead. However, did you spray some light oil around the various openings as I suggested? I suspect that, if it is the induction heater, it would be possible to put a layer of duct tape around the joint areas which would provide a temporary seal for the heater joints, thus allowing you to confirm that is the primary source of the leak.

Les

Les Bengtson

Replaced the induction heater,reinstalled everything no air leaks but now I have a gas leak coming from the automatic choke assembly.

JCH Hibbard

I thought you had a Stromberg with a manual choke, but either way it sounds like you need to rebuild the carb or at least fix the fuel leak.

Charley

C R Huff

Yes it is a manual, the drip is coming from the bottom of the choke housing.

JCH Hibbard

This thread was discussed between 17/08/2013 and 26/09/2013

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