engine revs itself when cold

[valocol]

I have a 2001 125 saltwater and over the winter when running the engine and temperatures are not too bad (few degrees above freezing) the engine seems to run fine when you start it first but when its perticularly cold and temps drop below freezing when i start the engine and set the throttle to around 1500 rpm it seeems to rev itself up and down in waves from 15oo to 2000/2500 and will continue to do this for quite some time. Is this purely the cold weather and the engine taking time to heat up or is there some other issue. Plugs are only 6 months old with 20/30 hrs on them.. Any ideas would be great

 

[crem1]

Re: engine revs itself when cold

Sounds like a fuel supply issue

 

[Chris1956]

Re: engine revs itself when cold

I did not find it on the parts list, however, some Mercs have an air valve that increases the air to the carbs, and therefore the speed of the motor, when the engine is cold. You might look for a hose running from a valve in the block to the carbs. If this is the issue, perhaps the thermostat is faulty as well.

 

[ENSIGN]

Re: engine revs itself when cold

I believe that cold start valve is located at the rear of the cylinder head.operating in very cold water will keep the valve from closing all the way and therefor keep changing the idle speed.you may have to just live with it.

 

[Faztbullet]

Re: engine revs itself when cold

The 125 does not have a thermal air valve. I have run across a problem similar to this on a customers "guide" boat at Dale Hollow lake 2 years ago and it would only do it if air temp was in the "teens". It was a 3 cylinder 90 Merc he used for float-n-fly trips, and after several "calls" to figure this out it was found to be icing in carbs, after engine idled for about 10 minutes problem went away, also when planing off it would miss real quick a couple of times do to moisture in intake. Found out later from customer that it would not do it as bad if engine was tilted up(full trim not tilt) and he went to a different station not selling 10% blended fuel...