2-Stroke Outboard intermittent coughing

[Guest]

I have a Mercury 9.9HP 2-stroke powering a trailer sailer. It really is a bit too powerful for the boat, and this may be causing a couple of problems we have with it...

1) The engine starts well, but after ten minutes running at half revs or so, the engine coughs once, with a slight knocking / backfiring sound, the revs drop for a few seconds, then the engine recovers and runs OK again. Only once did it stall completely, and afterwards it started again OK. Could this be fuel starvation or contamination? It's worth mentioning that at the beginning of the season we had to have the engine stripped by a professional as it stalled when the revs went above half power. It turned out there was a piece of light grey 'paint' in the carb. The rubber fuel line is light grey. Do these sometimes break down inside, dropping bits into the engine?

2) I think this is unrelated. When the engine has ticked over for a couple of minutes, and I rev it up to get going again, there is a lot of blue-grey smoke from the exhaust for half a minute or so. I guess this is due to it being a 2-stroke, but it seems a bit excessive. We've been careful to put in the correct mixture (1:100). Any ideas?

3) This may not be a problem, but the tell-tale water running out of the back of the engine is only slightly warmer than the lake we are in. Is this normal, or could a water passage be blocked somewhere, causing the engine to overheat and cough etc. as in 1) above?

Thanks,

Adrian.

Ady

 

[jfm]

Re: some guesses

1. Very likely to be fuel. Carb adjustment or duff carb float valve or crud in a carb, or something like that. The only other thing it could be is electrics, difficult to guess, possible intermittent short on the HT side (can you hear a cracking noise as an arc forms at the shorting location?

2. Again, suggests carb problem

3. This is normal. If there was a blockage, the tell tale would be non-existant or very hot. It is normal for the temp of the tell tale to be just luke warm.

To fix the carbs you need a dealer who really knows how. Where in the country are you located and which merc dealer do you normally use? On the other hand, it's quite easy to strip the carbs down, check the float valve operation, clean it all out, and put back together provided you do not adjust any of the positions of the adjustable jets (most jets in these are fixed, fortunately)

JFM

 

[ToMo]

Have experienced similar and it turned out to be the 2/ mix being slightly over oiled

ToMo

 

[longjohnsilver]

Re: some guesses

Taking the carb off and cleaning it shouldn't be too difficult. Make sure you clean the jets - poke an appropriately sizes piece of wire through them because even though they may look ok I've had same problems and this has cured it.

 

[Guest]

Re: more guesses

The engine being overpowered would not cause it to cough - it might rip the transom off, sink or other things. Slightly overpowered is ok.

We had coughing on a mercury 15hp due to a manky choke spring. Lashing wd40 all about made it return nicely.

100:1 is fine, and it isn't that sensitive. I don't think the tube disintegrate, at least, not for lots of years.

 

[Guest]

Re: some guesses

Thanks JFM,

We live in Northumberland and had used Powerhouse Marine in Newcastle, but found they didn't clean out the carb properly last time there was a blockage. We're looking for a good service place and would appreciate a reccomendation.

Ady