Cylinder not firing

[cramsaka]

3.0 L 4 Cyl GM engine number 4 cylinder is not firing. Compression checks OK and spark at plug when pulled and earthed. Rough idle is experienced. Have done a vacuum leak test using carby cleaner spray, no obvious leaks around manifold and carby. Any suggestions to check further would be appriceated. Cheers

 

[JustJason]

Re: Cylinder not firing

Welcome to iboats...

When you say you have spark at plug when earthed, are you hooking the spark plug wire up to a spark board tool? The spark needs to be able to jump a 7/16ths gap and be hot and blue in color.
Other than that, you can use a multimeter to ohm out the plug wire to make sure there are no breaks in it or to high of a resistance. Pull and look at the inside and out of the distributor cap, the the rotor, and the spark plug. Plugs do go bad and are sometimes bad brand new out of the box. You can try swapping the #3 plug and #4 plug to see if the problem changes cylinders or not.

 

[Bt Doctur]

Re: Cylinder not firing

Does #4 plug look cleaner than the rest? Does the miss go away after a while?
Could be ingesting water from a leaking riser .

 

[cramsaka]

Re: Cylinder not firing

Thanks for replies. Spark plug thread was coated in oil and electrode was black but not sooty. Is it possible that a valve is not returning/seating quick enough. Is it possible that i do have a vacuum leak that i havent found. Sprayed some fogging oil into the carby and will start the engine again tomorrow. Not a motor mechanic and only learning lots of info from these forums.

 

[bigskiohio]

Re: Cylinder not firing

a bad plug will not fire under compression pressure.

 

[cramsaka]

Re: Cylinder not firing

Resprayed carby cleaner around manifold especially underneath and found a vacuum leak. Took manifold off after receiving a new gasket and found that the original gasket was different to the new one.(not enough meat around the ports!) Engine now runs like a dream idleing at around 650RPM. Will put a shunt on the distributor and confirm timing is correct. Finally get to take the boat out!

 

[mkast]

Re: Cylinder not firing

The spark needs to be able to jump a 7/16ths gap and be hot and blue in color.

7/16th = 0.4375 inch.
What kind of coil are you using?

 

[JustJason]

Re: Cylinder not firing

Factory ignition coils. And I use stevens intruments adjustable spark gap boards to test for proper spark. 7/16ths gap on the board is pretty much a universal minimum for testing spark on anything.