My 470 loves to go fast, hates to idle

[natemoore]

I start the engine on muffs, set the throttle to about 1500 rpm to let it warm up, then jump out of the boat to attend to other things, only to jump back into the boat because the rpm are increasing to an uncomfortable level...OR...I set the throttle to about 1500 rpm, jump out to do stuff, then jump back in because the rpm is slowly decreasing to the point that the engine conks out. Out on the water, I'm continually raising the doghouse and tweaking the rpm adjustment screw. Too fast. Too slow. This has been going on for three or four years.

Today on the water, it didn't want to idle at all. I had to start the engine with half throttle (throttle only), slam it back to neutral, then immediately into forward. Once past that idle zone, the engine runs very well.

I rebuilt this carb a couple/three years ago when I rebuilt the engine. Followed the procedures in the manual as closely as I know how. I don't know what the problem could be. Timing is right on. The throttle linkage isn't slipping or wandering and has no slop in it.

Other times, it idles as smoothly as my Tundra. My engine seems to like 91 octane pure gasoline. The last tank was 87 octane 10% ethanol, and I noticed this decrease in performance. Couldn't get on plane unless my passenger went to the bow, whereas before with the pure 91, it was no problem.

Fellow 470 owners, do any of you have this problem?

 

[stonyloam]

Same thing happened to me this summer, idle got.... Well sort of wierd. Sometimes OK, sometimes high or low, the finally would not idle at all. Come to find out that the screw holding the fast idle cam was loose and had started to back out. The idle adjust screw resting on the loose cam caused the idle to be all over the place. Finally the screw backed out far enough the cam fell off of the shoulder and the idle screw did not hit at all, so the engine would quit when it was returned to neutral. Retightned the screw and all was well again. Talking about #22 & 23 here: https://www.mercruiserparts.com/Show...70%2D3%2E7L%29. Just something to check. Good luck.

Oh yeah, throttle only with just a tiny bit of throtle was OK (750 rpm all day) would only die when returned to neutral.

 

[natemoore]

Same thing happened to me this summer, idle got.... Well sort of wierd. Sometimes OK, sometimes high or low, the finally would not idle at all. Come to find out that the screw holding the fast idle cam was loose and had started to back out. The idle adjust screw resting on the loose cam caused the idle to be all over the place. Finally the screw backed out far enough the cam fell off of the shoulder and the idle screw did not hit at all, so the engine would quit when it was returned to neutral. Retightned the screw and all was well again. Talking about #22 & 23 here: https://www.mercruiserparts.com/Show...70%2D3%2E7L%29. Just something to check. Good luck.

Oh yeah, throttle only with just a tiny bit of throtle was OK (750 rpm all day) would only die when returned to neutral.

Thanks, I'll check that. Also, probably need to go back through the idle mixture adjusting procedures (#26 & 27 in diagram).