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MG MGB Technical - #1 CYL. Not firing....Help

I started getting a miss and popping rough at low RPM above 2500 smoother ......this is a 73 MGB/GT less most of the smog stuff....I pulled plug wires and found that the #1 had NO effect when pulled and replaced several times like the others which caused a sudden roughness. Tried a new rotor, Dist. Cap ,new plugs and plug lead for #1 all with no effect. I turn out the lights and watched for sparks...nothing....I see no tracks in Cap.....I feel the spark in #1 when pulling the lead and replacing.


Where should I start looking ...spark is getting the plug .......is this a stuck valve kinda thing....anybody have any other tests I should try??????????.....Tom
Thomas Koch

First don't set too much store by the lack of effect pulling number 1 has - these engines tend to idle on 2 cylinders only.

Do a compression test on the engine.
Chris at Octarine Services

Well I did a Comp. Test ..and the results are

#1..130LB
#2..135LB
#3..140LB
#4..150LB

Chris what does this tell you ..?

Tom
Thomas Koch

What does the plug look like, is it wet with fuel?
Stan Best

They are consistent with a low compression (8.0 to 1) engine but the variation across the cylinders is greater than 10% which indicates a problem.

I assume those are "dry" readings - put a teaspoon of oil in each cylinder in turn and repeat the test.

Significantly higher readings indicate that the rings are the problem, no or little increase in the readings point to bad seating valves.
Chris at Octarine Services

The plugs are all coming out dry no oil or gas build up .....I did a Dry Comp. Test I'll try the wet test..
I pulled the valve cover and on valve #1 I can't seem to find any point where I get gap of any size.....all other valves get a .013 gap at some point while turning the motor from crank bolt from underneath......I need to adjust that first valve I guess.....Tom
Thomas Koch

Yes you do! You also need to consider that the first valve is an exhaust and may be suffering from valve seat recession if the head has not been fitted with hardened exhaust seats.
Chris at Octarine Services

That seems to nail the problem. The popping in the exhaust will be fuel/air mix being pumped out past the leaking exhaust valve.
Stan Best

Today I adjusted the valves and then ran another compression test and it helped a great deal. #1 was up the 140# and it runs much smoother. Thanks for the help...I did hook up my compressor w/100lbs and could hear no hissing at either carbs or exhaust
Thomas Koch

This thread was discussed between 09/08/2013 and 14/08/2013

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