Fast Idle Speed

[ScottieScott]

I have a 1985 Mercruiser 170, and the normal idle speed is around 900-1000 rpm's. If I adjust the idle downward, it is really rough or outright dies. I have the timing set at 4 degrees before tdc.

New Pertronix I electronic ignition, flamethrower coil. new distributor cap, new rotor, new plugs, new wires.

I don't have a problem with the idle, it doesn't sound too fast to me, but if I am not mistaken, specs for optimum performance is 600-800 rpm's.

Could it be that my rpm guage is off?

It really seems 'normal' if you listen to it, not too fast. Is this a problem?

Thanks,
Scott

 

[alldodge]

Doesn't sound to fast, idle rpm for the engine is set with it in gear and in the water. Once your in gear I would guess it's around the correct rpm

 

[ScottieScott]

Thanks for the quick response - I will check it this weekend.

 

[alldodge]

Looked up your engine for so added info. Timing set at 4 BTDC and idle at 650-700 RPM in forward gear.

Now with that if your compression is below 150 this can require the idle to be a slight bit higher to keep it running

 

[Starcraft5834]

Ive got a 1987 3.0 . 1000 rpm idle is fine.... I tried to tweak mine down to recommendation too,,, struggles a bit there,,,given age of engine, gonna leave it at 1000.....yours is likely similar.... every engine given its age and condition is a bit different,,,,1000's fine....lowering it to much on an older engine can cause to many stalls and fiddling around in the water. If it sounds right at 900-1000, it is...

 

[stonyloam]

Rough idle can indicate a wealk cylinder. Do a few of things: get a dwell/tach meter and check the idle against your boat tach. With the engine idling and just a little rough, pull the plug wires one at a time to see if you have a cylinder that does not drop the rpm when you pull the wire, which would indicate a weak cylinder. Do a compression check. check the adjustment of the idle screw in the base of the Mercarb. If you do find a weak cylinder, pull the plug and check the color of the plug insulator. Brown or tan is normal, white indicates coolant intrusion. You have a 470 so ya gotta stay on top of it PM wise or you will be sorry. Good luck.

 

[JerryIrons]

Scott,

Your gauge could very well be off, get a timing gun, or dwell meter that measures rpms as well, then you can be sure. Most of my gauges are off, I think they are the originals. ( I have the same engine, except 1986) I think low rough idle like that could also be caused by a carburetor that needs to be cleaned, when was the last time it was done? Gas is newer? High idle can be rough on the outdrive gears when shifting. Idle rpm measurement is supposed to be done in the water, in gear, and it does lower some when you drop it in the water compared to on muffs. (at least mine does)

 

[Volphin]

Don't forget about the idle mixture on the carb as well. Richen it up a bit.

 

[ScottieScott]

Thanks for all the responses. I have done a compression test, and all are between 160 and 168 (#4 was 168). I just changed the plugs but the old ones were brown. I don't think I have a bad or weak cylinder. I have a feeling that my problem is I am trying to hit the target on the driveway.

I will check in in the water in gear this Saturday.