Quick: 70HP 2-stroke Misfire at part and WOT

[keaton85]

The motor is a late 80s early 90s? 70HP Yamaha 2-stroke CDI 3cyl.

I just went through the carbs, fuel pump and did a compression test. 120PSI on all cylinders and one stuck needle on the bottom carb (fixed now).

The issue I'm having is at part throttle and WOT she stumbles all over itself and won't increase RPMs. It ran great on the first run of the season but after a tank of fuel it started with this issue. It was taken out of commission about two years ago for this same issue, so it's a long term issue that was not caused from my doing.

I believe it's the plug wires though, why? because I get a nice shock if I'm touching the lower wire while it's running AND once the motor has warmed up for a while I can usually get it to run WOT without issue. Suspecting that the wires dry out once the motor comes up to temp...

What do you guys think?

 

[keaton85]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

Update: Took the carbs out and found a paint flake from the oil feed lines in the bottom carb. Along with some other sediment in the other two carbs. Nothing major but after a quick once over the motor runs fine now.

BUT, another issue I found after more inspection. The lighting/charging windings on the stator are shot. A small section of the windings is corroded away. Still runs fine without but I will have to rewind that coil at some point soon. Will the CDI be ok without that coil? (the two green leads)

 

[keaton85]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

And again, after a day of running around it's back to sputtering and not wanting to go WOT and now it's extremely hard to get into forward or reverse..

So frustrated...

 

[shackle101]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

You may need to rebuild the carbs and make sure there adjusted right. I clean mine with a ultra sonic cleaner, but that's kinda overkill. Do you have a service manual if not I'd get one.

 

[keaton85]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

I should get an ultra sonic at some point...

I'm going to pull them again and check to see if any of the floats are leaking. That would change jetting on the select carb.

These carbs are mint though, look brand new!

How much does the idle needle effect WOT performance on these?

 

[shackle101]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

Carbs not being adjusted correctly can cause all kinds of odd things including RPM issues. (and can ruin a cylinder quickly) Mine recommend 1 and 1/8th turns out, but your model may and most likely will differ. Also are you sure the needles are not worn? or the plunger attached the the floats both would be a good things to check. The Service manual will tell you exactly how to adjust them.

Really this could be simple as a blockage in the fuel lines or fuel filter as well. I spent hours working on my motor to find out it was a simple blockage before lol. That instance it was causing the motor to reduce RPM's after I raised the throttle about mid way. Then it would stick at lower RPM not reaching high/max RPM.

I know the service manuals can be a bit pricey I got mine form here cheap though: http://www.recrepairinfo.com/ Had to email to get the correct official one though.

 

[keaton85]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

I'm going to pull the carbs again and just soak them and reclean. I can pump fuel through the lines without issues and sprayed them straight through. Maybe the fuel pump? idono!

The other item I need to check is the charge coil, if this has shorted windings then that might give a week spark under load? OR would that cause issues without load as well?

I have to rewind the lighting coils anyway, that I might as well rewind the charge coil while I'm in there.

Thanks shackle101 for your insight! I did download a PDF manual but it's kinda old haha...

 

[keaton85]

Re: Quick: 70HP 2-stroke Misfire at part and WOT

I just went through the carbs again, also found the top cylinder to be running lean since the plug was very gray and the piston was clean!!! still perfect compression...also did the fallowing:

Set float levels to 14mm according to the 13mm-15mm spec.
Removed the o-rings on the needle valve seat as they had been eaten away. Still seals perfect, wondering if those o-rings need to be installed?

I'm still having an issue here, I can go a little past idle in gear and then it stumbles all over itself.

BUT if I run it for a bit a little past idle motoring out to the mooring field it will just come out of it and have full throttle with no issues, and act fine all the way through the power range.

Any help for the Yamaha Gurus?