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MG MGB Technical - engine miss

I wrote a few weeks ago about a 74 that has a miss in the #2 cylinder. I have replaced the intake gasket, but there is still a miss. All cylinders show the same compression, and there is spark at the plug. Does anyone have any suggestions on what the problem could be.
Could it have something to do with the distributor vacum advance unit.

Your help is appreciated

Rick
rick baranyi

Rick-
Let's check the simple stuff first.

Check the condition of the distributor cap. Look for a heavy build up on the contacts both inside and outside. Clean both them and the contacts on both ends of the spark plug leads with CRC QD Electronic Cleaner.

Another thing that you should check is: how old are your speark plug leads? The one for #2 cylinder may be leaking current. Try thid test: Park the car in the garage and leave the hood open. When it's dark, go into the garage with a spray bottle full of water and fire up the engine. The darker it is, the better. Spray the water in a fine, heavy mist into the engine compartment. If you see a blue flash, then you'll know that a spark plug lead is leaking.
Stephen Strange

It could be something do with the relationship between cam lobe and points for No.2, but then perhaps not if you have a spark at the plug. However presumably it has to be laying on the block for you to see it, and plugs find it harder to fire under compression than when not under compression. This means there could be a marginal insulation problem somewhere that is breaking down only when the voltage rises higher to fire the plug under compression.

Could be a plug, have you swapped the plugs round? Do this first and try again. What is the condition of the non-firing plug? With a new set of plugs does the problem happen straight away? Or only come on after a while?

If after swapping plugs it is still No.2 it could also be the cap or a lead, so turn the distributor 45 degrees and move all the plug leads to the new correct plugs and try again. If the problem stays with No.2 it is something to do with the cylinder. If it has moved with the lead/cap/cam lobe position then swap the leads round. If it doesn't move with the lead it must be the cap or cam lobe.

One other faint possibility is the rotor. Normally these either work or not i.e. affect all cylinders, but if the points phasing is incorrect i.e. the relationship between the time the points open and the position of the rotor to the cap contact, or you have the wrong rotor i.e. it doesn't have enough contact width or it is in the wrong place, it could just be affecting one cap contact more than others. Ironically vacuum advance could well be a factor here, as changing vacuum advance moves the relationship between rotor and cap contact. If you have manifold vacuum disconnect it (and plug the manifold port, you may have to increase the idle speed to stop it stalling, adjust both carbs by the same amount in the same direction) and see if that makes a difference. If you have carb or no vacuum advance connected then try sucking as hard as you can on the pipe from the distributor and see what difference that makes. It should cause the engine to speed up, but does it cause a misfire?
PaulH Solihull

Paul,

When you suggest turning the distributor 45 degrees to see if the problem follows, it sounds like you are thinking of your V-8. Wouldn't that be 90 degrees for a 4 cyl?

Charley
C R Huff

Sorry yes, 90 degrees :o)
PaulH Solihull

This thread was discussed between 13/09/2010 and 15/09/2010

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