90HP 2 stroke coughs

[CharlieH]

I have 2 x 90A 2 strokes. Both have new plugs and fuel filters, both run off the same fuel tank. Both start fine and run well. However, when warm and at idle one engine occasionally coughs and after a few minutes stalls. It will start again no problem but will do the same. If I then go into gear and run then no problem. Both have an idle speed of 700rpm when warm. Any ideas please?

[boscoe99]

What you are describing sounds like what is referred to as a lean sneeze.

The carburetors may need to be adjusted and reset to factory specifications. Or, maybe the idle mixture circuit might just need to be set to add a bit more fuel. Are you familiar with carburetors?

The damn things run very, very lean now, thanks to big brother the EPA.

[CharlieH]

Thanks for your post. I'm not that familiar with carbs are they easy to adjust or do I call out an engineer?

[robert graham]

Or, carb/carbs on that motor may need to be removed, cleaned, Linc/sync as per factory specs...a good Yamaha dealer could do it for you....

[CharlieH]

The problem is where I live is a long way from a Yamaha dealer and I only have a yard trailer so it expensive just getting the boat there so I try and do everything I can.

[boscoe99]

What model year is the motor? 2002 maybe?

You don't sound like you are in the US given the use of the word "engineer" for what we would call a mechanic. If you don't have an engineer near you or if it is difficult to take the motor in then you will be on your own.

At the risk of taking some heat I will tell you how to richen up the idle mixture on what I believe to be your motor. There are three carburetors correct? Each one has an idle mixture screw. Turn each screw out (counter clockwise) 1/8th of a turn. Maybe 1/16th of a turn if you can do it. Then, see how the motor runs. It should get better. If it does, but the problem does not disappear altogether, then turn each screw out another 1/16th or 1/8th of a turn.

Oh, before you do anything, take an indelible marker and mark each screw head at the 12 o'clock position so that you can get the screws back to their existing position if you have to. See the illustration below:

[CharlieH]

Thanks for this I will give it a try tomorrow and revert. Yes you are correct I live in Cornwall in England right near Lands End. The engines are 2005. The RIB is a commercial dive boat so we can still use 2 strokes in UK. I have also seen Chevron Techron and Quicksilver Power Tune could help - any thoughts on these products? Your advice is much appreciated thank you.

[boscoe99]

Hey, you are in Doc Martin country. Ever heard of him or Port Wenn? I am due to get my series 6 DVD's this evening. Hate to see the show end. Rumor has it this is the last year of production.

A 2005 Yamaha might have what we call EPA (environmental protection agency) plugs over the idle mixture screws to try and keep people from fixing their motors. EPA over here would just as soon that folks got so sick of using an overly lean two stroke motor that they would stop using it altogether.

You can drill a small hole in the plug being careful not to go through too far and bugger the mixture screw. Then turn a disposable small screw into the hole and pull the plug out with a pair of pliers.

[robert graham]

Might also be a good idea to drain each carb with the hex head brass drain screws at bottom/side of float chambers....put a clean white rag under carb being drained to check for water/debris coming out of float chambers...and rag will catch drain plugs if you drop one....good luck!...

[CharlieH]

So you watch Doc Martin - I won't spoil the end of this series for you. Port Isaac (Port Wenn is a fictitious harbour), where it is filmed, is about 20 miles away nearer to civilisation from where I live, so you can guess the kind of facilities I have to deal with. Our local general engineer is about as useful as PC Joe Penhale! Will try your advice today and also drain the carbs. Thanks again.

[99yam40]

have these motors run well in the past?
If so and now are just starting to give you this problem , it is probably best to pull carbs and clean the passages instead of just adjusting the mixture screws. Will need to follow link and sync procedure in service manual when reinstalled.

As said be very careful drilling that disc as there is not much clearance between it and the screw and you can damage the screw and cause you to buy a new carb if you break off the screw if you cannot get it out or damage the seating area

clean carbs set to factory specs should run well.

Do these motors have a choke or are they prime start enrichment models?
Could have a problem with that system if they have it and the motor runs fine after warm up. But you did not say if they run well after warmed up, so that is just a thought I had

[CharlieH]

OK. My carbs don’t have the screw caps on so I did as you mentioned gave all three an anticlockwise 1/16 turn. The red pen marks that I put at the top are a great indicator. I then drained all the carbs through a white cloth and the fuel looked fine – I have external water separator filters. Started the motor and let it get warm then idled for 30 minutes – purred like a kitten! Fingers crossed it has sorted the problem.

[boscoe99]

Glad to hear that the motor is running better now.

PC Penhale's are everywhere. We have a few over on this side of the pond as well.

By the way, the idle mixture screw initial setting for your motor's carburetor is 1 & 1/4 turn out (counterclockwise) from the full in position, plus or minus a quarter turn. So, as you can tell you have a bit of leeway if they need to be turned out a bit more.

If you had lost the starting position (which you have marked) you would turn the screw in until it was lightly seated. Be careful not to over tighten it. Then, back it out 1 & 1/4 turn to start with. Proceed from there if needed.

[throrope]

boscoe99 - Thanks, not even the service manual references 1-1/4 turn from seated.