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90 Yamaha 40 rough idle (need help!)

Lit549

New member
I bought this boat last year,it's ALWAYS idled rough.I rebuild carbs even though they were spotless clean.I used Yamaha rebuild kit.STILL idles rough.Compression is perfect,new fuel pump and hose,I've tried different premix ratios ,everything from 50:1 to 100:1,same thing,idles rough.Its at a boat mechanic now and he's tried everything,STILL idles rough.Its a 1990 Yamaha cv40eld twin cylinder ,2 carb ..Any ideas??It runs great at higher speeds,just idles rough and hesitates just a little while taking off..
 
Sounds like a an issue with low speed settings or timing. Those C40e were pretty bullet proof however, they were designed to be commercial engines that were loud and never idled "clean". DO NOT THIN FUEL BELOW OR ABOVE THE MANU SPEC! (Dont mean to yell but always follow protocol) Check the spark and make sure you are getting good spark that jumps at least 9mm. If that's good go over the fuel system and set idle screws at 1.25 turns off bottom. When you rebuilt the carbs did you make sure the floats were level to spec? After going over the fuel system check the spark with a light and make sure she is firing at the correct time. I just went through this with a rectifier. The #2 and #3 cyls were firing on each other's time. It was the rectifier that while charging was messing the rest of the ign system up.
 
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