Misfire

BogMonster · 28 March 2006, 21:54

The Johnson 115 on my RIB has a slight misfire. It only happens when idling, but if you knock the motor back to idle for any length of time at all, it misfires on 1 cylinder slightly, and smokes a bit too, open the throttle a bit and it quickly clears and runs perfectly at anything over about 2000rpm with no smoke at all. Doesn't matter if cold or hot, same thing.

I don't know a lot about 2 stroke engines, and particularly oil injection ones, is this a common thing? The 2 stroke lawnmowers Dad used to own usually ran like a bucket of bolts at the best of times and the strimmer I have - also 2 stroke - is equally temperamental!

Is it likely to be just a bad plug, or something to be more concerned about? As I say it is fine once you open the throttle a little. I'm going to get a new set of plugs for it anyway as they look to have been in there rather a long time, but I just wondered if there is anything I should check.

Ta

Stephen

Swifty · 28 March 2006, 22:44

I had a Mercury that would idle and cough/backfire occaisionally. Throttle up and it ran sweet. Adjusted the idle mixture to a richer setting and it cured it. If it is a carb model, mark where the idle mixture screw is (so you can put it back if it doesn't work) and screw it all the way in - not tight, just so it tightens up a bit, then unscrew it 1½ turns. Tweak it slowly then let it idle for a bit and see if it is ok. Adjust it in/out for a sweet smooth idle.

Shouldn't let it idle with too weak a mixture as it can overheat cum self destruct with too little oil. As in life, rich is better than lean!

Andy_Rs600 · 29 March 2006, 09:54

The Mariner 50 4-stroke we runat work do this as well. I'm told running in cold water may cause the engine to need a slightly richer mixture, and the water here is about 3 degrees! (Don't know how reliable said advice is though)

BogMonster · 29 March 2006, 10:12

Quote:

Originally Posted by Andy_Rs600

The Mariner 50 4-stroke we runat work do this as well. I'm told running in cold water may cause the engine to need a slightly richer mixture, and the water here is about 3 degrees! (Don't know how reliable said advice is though)

That would make sense, as the sea temp here is quite cold. It would also explain why it is quite grumpy starting when it hasn't been used for a couple of days - I had a job getting it going yesterday and it was noticeably cooler ambient temp than it has been since I got the boat. Once it has been run, it fires up straight away.

My main concern is that it isn't a sign of an impending conking-out, so if that is the only reason then I guess it is nothing to be concerned about. I'll read the book of words and see if you can fiddle the mixture at all.

Thanks