85 mercruiser I/O cylinder 3 not firing

85 mercruiser cylinder #3 is not firing... found this out by pulling off the spark plug wires 1 at at time everyone except #3 shut down the engine when #3 was pulled the engine still ran. there is spark because I took a spark tester and put to a good ground and saw and also heard the spark, there is gas because when I remove #3 plug there is a faint smell of gas on it and it is wet with gas, also when I put a new plug it it still looks brand new after running the motor (tried another plug from a working cylinder also tried a different wire from a working cylinder). now onto the other things I did to rule out any other issues , Timing has been checked and it's fine, compression = 145-150 on all cylinders even #3, New plugs, New wires, New points, New cap, New rotor top came with tune up kit. I am lost at this point hopefully someone has seen this or know an answer....

motor info-
1985 mercruiser 140
s/n- 06920736

Re: 85 mercruiser I/O cylinder 3 not firing

Not much stuff left to check, except the distributor cam. It could be worn to the point where it does not open the points when it should. With the distributor cap and rotor off place the center wire from the coil near the block than have someone crank over the engine, you should get 4 good spark from each revolution of the distributor shaft. If not your dwell might be way off, so double check with a dwell meter. Good luck.

Re: 85 mercruiser I/O cylinder 3 not firing

He said there's a spark in #3. With the other 3 firing, not sure what the problem would be. You have compression, fuel, spark. Are you getting a spark if you have the plug itself connected to the wire?

Re: 85 mercruiser I/O cylinder 3 not firing

Oops missed that:facepalm:. Only thing left is to pull the valve cover and see if you have a broken spring or sticking valve that did not show up in the compression test.

Re: 85 mercruiser I/O cylinder 3 not firing

Yes there is spark with the plug in the cable grounded to the block... Thats why we did the compression test expecting to see bad results on cylinder 3 but nope all 4 cylinders gave 145-150 did compression test 2 times to verify..

Re: 85 mercruiser I/O cylinder 3 not firing

Ok so today I installed electronic ignition and it is still doing the same thing Cylinder 3 just does now want to fire I also took of the valve cover there are no stuck springs or valves I am completely lost here :facepalm::facepalm:

Re: 85 mercruiser I/O cylinder 3 not firing

Is the firing order right (1-3-4-2) and in the right direction (CW from #1 on the cap)?

Re: 85 mercruiser I/O cylinder 3 not firing

yes that is the firing order and the in the right direction

Re: 85 mercruiser I/O cylinder 3 not firing

You may have a vacuum leak, spray some WD-40, or Brake Clean or something like that around the base of the carb while the engine is running and see if the engine changes.

Re: 85 mercruiser I/O cylinder 3 not firing

Ok i will try that would that cause only 1 cylinder not to fire though? and do you happen to know what each cylinder does?

Re: 85 mercruiser I/O cylinder 3 not firing

what type of plug is in there.try swapping a plug with another cylinder.

Re: 85 mercruiser I/O cylinder 3 not firing

did the plug swap with a different cylinder and even tried a brand new plug

Re: 85 mercruiser I/O cylinder 3 not firing

When you had the valve cover off did you crank the engine and verify that all valves were actually moving up and down? Long shoy but who knows good luck Bill

Re: 85 mercruiser I/O cylinder 3 not firing

when you had the valve cover off did you also check to see if the rockers were excessively loose on that cylinder?

Re: 85 mercruiser I/O cylinder 3 not firing

With the engine running spray some carb cleaner along the manifold where it meets the block.This gasket will fail and cause a lean condition in one cylinder.