6hp 2 stoke, bogs down and dies at low revs/throttle

[mikedefieslife]

I spent the last couple of hours taking apart and cleaning the carb on my Yamaha 6hp 2 stroke.

The engine was running perfectly until a month or so ago. Moving the throttle to the start position the engine might turn over a couple of times before dying. Giving it some choke, will see it roar into life at what sounds like high rpm, but if the choke isn't shut off almost instantly, it will bog down and die.

Once running and operated at above level the start position, it runs fine. If the throttle is reduced too much, perhaps to slow down or to change gear to neutral or reverse, it will bog down and cut out.

I thought this was a carb issue, but the carb actually seemed squeaky clean. No debris or build up or varnishing of the float bowl, nice clean clear main jet (#100), all the other orifices seemed nice and clean too.

The spark plugs are new from May, and the fuel is fresh, and in any case has been changed many times since the problems started.

Any suggestions as to the problem, or perhaps ideas for further diagnosis?

[99yam40]

since choking helps, I would think the low speed jets/ passages (pilot/idle)have a problem.
have you made sure it has good compression?

[mikedefieslife]

The pilot is the ones with the adjustable screw right? At least I believe it is from my 2t motorcycle days. I removed the screw and was able to get cleaner to come out of the other side of the hole. It was too small to actually shove anything in there.

Not got the tools to check compression (I live on a boat with a diesel engine), but I'm guessing it's fine as there is no loss of power. It will still propel an 2.6m inflatable with two adults in it to 12knts.

[TownsendsFJR1300]

What year and model # engine?

Does sound like a clogged LS jet (if equipped).

Did the engine sit with old, un-treated ethanol fuel for that month?



For instance in the below carb, did you remove the needle jet, part #15 and # 7 and check ALL orifices that they are clear?:
https://www.boats.net/catalog/yamaha...lhy/carburetor




.

[mikedefieslife]

The year doesn't really matter with these. The 6hp 2 stroke used the same carb and jets from the when it was first introduced in the mid 80s right up until it was discontinued in the 2000s.

I seem to have it working now

I originally missed checking jet number 7 in the diagram below as there was a cap hiding it. Cleaning that, and a bit of throttle and idle adjustment, and I have it running well. Next step is to get the tilt release working properly, but that is another thread.

https://www.boats.net/catalog/yamaha...shy/carburetor

Interestingly, the main jet (15 in the diagram) on mine is #100 every diagram I've seen shows #98. Not that it is the problem since it's had that jet since I've had it. I wonder if the previous owner when heavier on the 2t oil than the recommended 100:1 hence the larger jet.

[99yam40]

having a wrong jet, could lead to problems.
always post a complete model # when asking about a motor.

a previous owner of my C40TLRX drilled out the main jets.
when I finally figured out that was done after a long time of a poor running motor, I replaced with new factory sized ones and the motor runs a lot better.
unless you can ask the person why they did that and if it helped, you will never know if it was a good idea

[TownsendsFJR1300]

Posting the model # for those not familiar with your engine makes it much easier for those trying to help..

I had to bring up several years till I found that carb I posted and the JETS THAT WERE LIKLEY your issue.. (And were)..


Re the main jet, how do the spark plugs look?

[mikedefieslife]

A few of you have said always post a complete model number.

I didn't and don't know the model number since there are no markings on the engine. You know what old outboards are like. The serial numbers and other stuff are the first things to wear away.

All I know is that it is a 6hp model 2-stroke probably somewhere between the 90s - and whenever they stopped making them. I did have a thread last year where we try to identify it.