20hp 1996 two stroke problem

[Juzatheman]

Hi, from Finland.
I have big issues with my 20 DERO engine it just wont keep running it kind of back fires from exhaust side and stops if i give it more rpm s it wont die when back fires but go on its knees for a while and just when it get rpm s back up thump and back .

I have talked local yamaha service here in Finland and have done evrything they have told me and any of these havent change the problem.

New nkg sparkplugs three sets for sure
Carburators cleaned new gaskets and needels adjusted
Did check the flaps and new gaskets there
New fuel and hoses and tank
New lower seals for the crankshaft

It feals like ignition timing jumps around and sparks at the wrong time...

I dont have a lot experience of these 2 stroke outboard engines, but i have been car mechanic over 20 years and counting ..

I think it could be the cdi ignition box but im not sure and it costs about 600$ here so i just dont want throw more money away just to try it out..

Any info would be great. Thanks adv. Juza

[TownsendsFJR1300]

You might give this company a call / e-mail and see if they can help:

YAMAHA : Carmo Electronics, The place for parts or electronics for your Motorbike Quad Scooter Car or Jetski

[Juzatheman]

This morning i did check the cylinder leak test and it pass good allso the compression test was good both cylinders over 7 kilos. So mechanicly engine is good.

[99yam40]

I would test the inputs to the CDI and the output as the service manual says.

will need the proper test equipment

[Juzatheman]

Just going to.. i print the test guide and start to measure.

[Juzatheman]

Well i was middle of doing the ignition check list with a multimeter and i had a pico oscilloscope connected to the sparkplug wires and it just bang after bang hardly get it up an running and the secondery ignition wave form jumping around and didnt get good picture what the hell is going on... so i end up adjusting the pilot screws and all of sudden the engine stop coughing and start to idle better..
And before this i have turn the pilot adjusment from other end to another and it have never fix or even a little make the problem less.. but know like i lightning stroke it cure the back fire issue.... and the carburators have been in the yamaha service ultrasonic was or clean up what ever it is called.. didnt help any i my self have torn those appart and mechanicly cleaned and flushed with carb cleaner and nothing...

[TownsendsFJR1300]

Glad its running!

I have to wonder if all the jets were actually removed and visually checked.

You may have finally pushed a small piece of crap out..

[Juzatheman]

Did't have time to take it on the lake yet. Did a once more barrel run test this morning cold engine and same difficulty to get it running cough after cough 😈

[TownsendsFJR1300]

This is the closest thing I could find for your engine(here in the states)

1996 20MLHU Yamaha Outboard CARBURETOR Diagram and Parts

Do you have a manual actual butterfly for a choke or something else?


I know you posted the carbs were cleaned... Did they (or you) actually pull all the jets out and visually inspect to make sure they were all clear? Just dunking don't count...