yammy firing on 3 cylinders only

[Brigs]

i have a 4-stroke 4cylinder 50hp yamaha that idles rough. i have another of the same engine that does not have this problem. i have exchanged all attachments electrical and otherwise from one block to another and the problem (rough idle) persists. the following was done <br />compression checks - 210 psi on all cylinders<br />carburettors synchronised<br />power pack, plugs ok<br />checked ring gap ok, deglazed, and the # 3 cylinder still does not work. any ideas? thanks<br /><br />Brigs

 

[jim dozier]

Re: yammy firing on 3 cylinders only

Back to basics. You have compression-OK. Check for strong blue spark on all cylinders. If you have spark, check the timing. If there is spark at the right time with compression the problem had to fuel mixture, too rich or too lean. Try to isolate which cylinder(s) aren't firing by pulling plug wires (best to ground the loose wire) one at a time. Squirt some gas in the carb throats of the offending cylinder and see if it picks up, if so gas isn't getting to where it needs to be. Try disconnecting the fuel line and see if it picks up as the carb gas level goes down, if so perhaps a stuck open float or something. <br /><br />If you know you don't have spark, get a manual and test the individual ignition components for the offending cylinder instead of replacing everything.