1972 evinrude 100hp v4 runs fine at idle but doesnt give full power

[tate456]

hi all, i have a 1972 evinrude v4 100hp. This spring i decided to pull the carbs and clean them as last fall it seems to run a little rough. i pulled the bowls and sprayed everything out reassembled and the boat started much easier. HOWEVER now when i attempt to go full power it seems as though i have about half power boat used to do around 40 now does around 20. just will not fully rev out. It has a fresh tank of gas and 2 cycle oil mixed appropriately, fresh ngk spark plugs, and i have ensured that the throttle linkage is tight. i just cant think of anything else i would have hurt in the process. Hopefully you guys can throw me some ideas ...anything helps!

 

[emdsapmgr]

Possible that your engine is only firing on 3 cyls. Connect a timing light to each plugwire and see if all 4 are sending fire to each plug. I'd change out those NGK's for a set of Champion plugs. OMC's always run best with Champions. I'd put a set of QL77JC4's in it. You can find some excellent ignition troubleshooting info at this website: cdielectronics.com (better than that from the original factory service manual.)

 

[tate456]

will connect a timing light or some sort of spark tester to check, for spark on each cylinder i suppose it is possible i have a bad plug out of the box, those plugs you recommended dont look like what i pulled out of the motor when i bought it ...it had the flat style plugs so thats just what i always put back in ..they are cross compatible i take it ?

 

[racerone]

Test for spark.------You must see spark jump a gap of 7/16" or more.---Test compression too.

 

[emdsapmgr]

Your 72 did have an optional plug: the Champion, the L76V. That was a gapless plug. The basic plug should be QL77JC4.

 

[tate456]

yep the ngks that are in are the cross reference of the champion gapless.

 

[boobie]

Do a cylinder drop test on it. You'll know right away which cylinder is giving you the problems.

 

[tate456]

just unplug one wire at a time until it doesnt make a difference?

 

[jimmbo]

You say it runs fine at idle? doesn't shake a lot? If so, I suspect a carb issue, like a gummed up main jet or passage. When you reinstalled the carbs, did you verify the linkages between them and the sync between the carbs and the ignition? As for the surface gap vs. regular plugs... I doubt using conventional plugs would cause your problem unless the gap was way way off. I have had very good luck with the surface gap type, I swear by them, others swear at them.