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MG MGB Technical - Number 3 cylinder not firing

Number three cylinder in my supercharged MGB is not firing. There appears to be a good spark from distributor. I have switched around spark plug wires and plugs and still #3 will not fire. Spark plug is whitish in color. Checked compression: #1 117, #2 115, #3 109, #4 110. Also, I checked valve clearances and hey are OK. Any thoughts on what I should start looking at to diagnose the problem? Thanks.
C.E. Sampson

I had the same problem which was caused by a intake manifold leak. The center cylinders run slightly weaker than the outer cylinders (in theory) because of the siamesed exhaust port between the two. Hence, problems commonly show up in either cylinder two or three.

Les
Les Bengtson

Thanks Les. I will try spraying a little carb cleaner near the manifold to see if that changes RPM. Interestingly, I have found that cylinder is firing at higher rpm - 2000 or so and up. But, not at idle. What does this mean?
C.E. Sampson

Richer mixture off-idle compensating enough for whatever is causing the weakness on that cylinder?
PaulH Solihull

If you're running a Moss supercharger, be aware that there were gasket problems between the intake manifold and the supercharger. The gasket is very thin and is prone to blowing out underneath the blower. It's difficult to diagnose, since the affected area is out of sight. I had the same problem, a few years back, and wound up making my own gasket from some heavy duty Neopreme gasket material that I picked up at my local NAPA dealer. It turned out that the bottom portion of the gasket was blown right out of the space between the intake manifold and the blower itself. The original gasket was literally paper thin and not up to the task of sealing the two surfaces, especially when under pressure. RAY
rjm RAY

I had the same problem as Ray, and its not easy to see. When this gasket goes it leaks out under boost but is a vacuum leak under normal driving and behaves the same as a leaky manifold gasket. I made a much stronger gasket and for once put a good sealer both surfaces. Have done 35000 miles since with no problem. The nut on the blow off valve can work loose over time as well but that just needs a nip up. Your compression figures are a bit low, it may pay to try a different gauge cause they are even, just low. Good luck. Denis
D M HILL

Thanks for your advice. I believe the problem is the manifold gasket. There is a blip in rpm's when I direct a short spray of carb cleaner at the center of the manifold near the third nut. I did use yellow adhesive but it is certainly acting like a vacuum leak. I am going to remove the SC and replace the gasket. Chris
C.E. Sampson

This thread was discussed between 19/11/2011 and 24/11/2011

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