Raising Idle RPM?

Did the link 'n sync and motor (1995 175 hp carb'd powerhead) is running like a top. However, my idle in neutral dropped to 600 rpm (500 in gear). The manual suggested advancing idle timing from 6 deg. ATDC to 4 deg. ATDC to increase idle speed if it's too low. I did that and picked up 100 rpm, but it isn't quite enough. I want to get to about 900 rpm in neutral, 700-800 rpm in gear. The idle timing doohickey won't let me advance timing any farther.

I can run all day if I bump the rpm's in gear to 800, but extended idle time in gear at the current setting (600) causes the motor to start stumbling and stalling after awhile.

Everything seems to be positioned perfectly- butterflies are closed at idle, timing advance and carb opening positions are well sync'd, control cables are new and work smoothly. I tried playing with the trunnion nut on the throttle cable, but that screws up other positioning and the the high idle lever stops functioning.

Any tips?

Re: Raising Idle RPM?

are you saying the idle stop screw is bottomed out. and you can't turn it in any farther. does it have the rubber tip on it? i have in the past had to put a longer bolt in there.

Re: Raising Idle RPM?

Hi-

My motor doesn't have an idle stop screw. It isn't as simple as that. The carburetor cam is sync'd with the timing advance lever, if that's what you mean, and the rubber roller on the cam is new. I'm out of town and don't have a manual (or the motor) in front of me, so bear with me if I screw up some of the terminology.

Re: Raising Idle RPM?

i don't have that manual either so. will investigate.

Re: Raising Idle RPM?

Thank you!

In the end I may have to "finesse" the opening of the carb butterflies with the action of the timing advance (either start timing advance slightly before opening the carbs, or vise versa), but I don't know which way to go with that, or if that's even the answer.

Re: Raising Idle RPM?

If the motor is healthy and everything is correct that motor will idle perfectly all day at 500 rpm in gear. Generally speaking if there's an issue with the idle adjustment, we can't adjust it low enough.

What spark plugs are you using?
What grade fuel?
Are the carbs clean?

That motor loading up at idle is a classic sign of carb passage issues. Forcing the idle higher may lead to shifting issues and / or eventual gear damage.

Re: Raising Idle RPM?

Thanks, dhadley,

I'm using the Champions recommended for the motor and 87 octane. I'd say the carbs are clean as evidenced by the fact that it idles very well and will only start to stumble after 15 minutes or so at that low idle speed, which, btw, is below spec for the motor. As I mentioned, I can run indefinitely at 800rpm with no issue.

I'm only trying to get the idle speed up to spec, which should not cause any LU/shifting issues. And, it isn't worth a complete carb teardown to get there. Given the amount of non-adjustable plastic BRP used in the various linkages, cams and other moving parts, I think it's very likely that normal wear will introduce some degree of variability into engines that old.

I'm not looking to jury rig anything- the motor is in good shape and I keep it that way.