Yamaha 8hp 2 stroke running rough & stalling

Hi a newbie to the group,

We have a Yamaha 2hp 2 stroke Model 8MWHS. Purchased used about 5 years ago I think it must be at least 15 years old

During the past few years the engine started running rough and stalling at times. We changed the plugs and it made a world of difference.
We removed the fuel filter and replaced. Also installed another inline fuel filter. Pulled the carb off and inspected for dirt (It was spotless). Drained the fuel tank and filled with fresh fuel. All oil is measured with a measuring container before pouring into the tank.

But about a year ago the new plugs did not make any difference and we had the Carb removed and stripped down and cleaned in Barra de Navidad, Western Mexico. I watched the mechanic strip it down and he was extremely careful about making sure everything was very clean when he stripped it down.

It made a difference for a few days but then back to sometimes running rough and stalling and then weeks of good service with it running like a champ then times of rough and stalling. Its been like this for the past 18 months

Now we are close to a good OB mechanic here in La Paz and he took it for a spin last week. You guessed it the darn thing ran perfectly. I took it out this morning and it stalled at medium speed. It started back up again but there are times it is hard to start after it stalls. At times it runs rough without stalling

It appears to me like it might be a fuel related problem like a dirty carb.

Or would going to a slightly hotter plug make any difference(B6HS-10 instead of a B7HS-1))?

Is there a rebuild kit available with new jets for the carb on this old engine?
If so can you please provide a part number for this?

Any suggestions would be greatly appreciated

Thank You

Chuck
Jacaranda
La Paz, Mx

[boats.net]

Sounds like need to remove carb and see if dirty. I wouldn't change heat range on plugs. I don't have a parts listing for that model number but have a 8MSHS which is a 1994 and probably same as yours. The carb kit(number 16 in diagram) doesn't come with jets or fuel pump diagrams.
Yamaha Outboard Parts 1994 8MSHS REPAIR KIT Diagram
Yamaha Outboard Parts 1994 8MSHS CARBURETOR Diagram

Thank You for the reply I mis-stated the model number it should read: 8MSHS

I have removed the carb and will give it to the local OB mechanic for inspection and cleaning.

I pulled the plugs out for inspection and they appear pefectly clean and no sign of fouling. Less than 30 hours on them

Will report back the results

Kind regards

Chuck

Just a follow up to my earlier post. I took your advice, pulled the carb off and passed it on to the local OB guru Sea Otter Jimmy here in La Paz. He returned it yesterday morning and said it was filthy and actually brought some of the gunk to show me. I re-installed it yesterday evening and a quick run around the marina seems to run perfect. Time will tell though.

I understand that once the oil is mixed in the fuel it has a fairly short life span so we always mix up only what goes into the small 3-gallon tank. When leaving the boat for more than a month we run the engine dry. In addition the fuel is always filtered before it is put in the tank and I have 2 inline filters between the tank and the carb.

Ok with that said....

How do we prevent this problem in the future?

Is it possible to use a carb cleaner once a year or when I notice problems pointing to a dirty carb? If so can you recommend one?

Thank you again for your time and suggestions!!

Chuck
Jacaranda
La Paz, Mx

[boats.net]

Using a 10 micron fuel/water separator filter helps. You might try draining carb by loosening the drain screw on bootom of carb bowl when putting up for storage to make sure there is no gas left in carbs at all.

[dorf]

My local Marina/Boat/Repair Shop has 2-3 mechanics and mostly all they do all summer is carburetor work. Cleaning out gunk and deposits.

They swear by boaters using Evinrude/Johnson 2+4 Fuel Conditioner all year long.

I used to use Stabil and they say No, No, No.!!!

Anyway I store allot of gas, all year long, for various boat, skis, generators and have absoutley no problems with stale gas. Gas gets rotated every 7-8-9 months.

Cheers !!

well another update

I filtered all the fuel in the tank and cleaned the tank out before fltering the fuel again before putting it back in the tank. Ran the dink around for about 30 minutes. The dingy stalled a couple of times during this time and before the second stalling started to run rough.

I called the mechanic an he said it might be a faulty kill switch. He mentioned that when I first spoke with him about the problems a few days ago. Suggested I remove the white wire from the kill switch. I did that and ran the dink again for about 20 minutes. It ran perfectly without stalling or running rough.

But we have found in the past that the engine could run for days perfectly and then start acting up again.

Before I try replacing the kill switch I want to run it again tomorrow for another hour and get your comments on this replacement.

What do you think could it be the kill switch ?

Maybe it was a couple of problems, dirty carb and a faulty kill switch??

Your comments are greatly appreciated

Thank You

Chuck

[dorf]

For what it is worth...

I had a 50HP Evinrude 2 stroke that did basically the same thing. I messed witrh the carbs forever,(mine were clean) cleaning ,sychronizing the air flow etc. I finally changed the CDI Unit...problem solved.

I'm not thinking it is the kill switch ,but ????????

For about $150.00 you couod eliminate that possibility with a new CDI.

Hope you solve the problem !!!

Cheers !!!

We have run the engine for about 3 hours with and without the kill switch attached. Now it seems to run perfectly with no difference between either one. Of course our life story!

We are headed offshore to some remote islands for the next month but will let the group know the outcome once we get back in wifi range.

Thanks for all the suggestions

Happy Holidays

Chuck
Jacaranda
La Paz, Mx
www.jacarandajourney.com