missfire on yamaha 140 v4

[wet webby]

My 1990 yam 140 v4 seems to have a missfire..I have replaced the plugs,problem then seemed to get better BUT there is still a miss on piston 4.
I have switched the plugs and the ht cables but no different.
When you pull the lead of that cylinder the tone change is much less noticable than all the others.
Can anyone shed some light on my problem?

Thanks

Gary

[Nos4r2]

Drain that carb and clean the jets, and compression test the motor.

[IBWET]

Originally Posted by wet webby

My 1990 yam 140 v4 seems to have a missfire..I have replaced the plugs,problem then seemed to get better BUT there is still a miss on piston 4.
I have switched the plugs and the ht cables but no different.
When you pull the lead of that cylinder the tone change is much less noticable than all the others.
Can anyone shed some light on my problem?

Thanks

Gary

Thing to remember is if it's two stroke and the problem turns out to be a fuelling issue, that cylinder has not only not been getting fuelled correctly ,it's also not been getting oiled. ..I would not run it anymore before I got to the bottom of it .

[Pete7]

I thought Yamahas injected the oil directly into the crank case rather than mix it with the fuel ?

Pete

[Stoo]

Originally Posted by Pete7

I thought Yamahas injected the oil directly into the crank case rather than mix it with the fuel ?

Pete

My 150 2 stroke pumped the oil up through some wee tubes to each cylinder. It's a sure-fire system. Well, except for the two times I blew the entire power head to smitherines...

[JSP]

Originally Posted by Pete7

I thought Yamahas injected the oil directly into the crank case rather than mix it with the fuel ?

Pete

Is that when the engine puts it's own 2 stroke in from a separate tank?

[Poly]

JSP, Most fairly recent 2 strokes did that. But most mixed the fuel and oil before it goes through the carb and thats how the oil gets into the cylinder, if you don't get fuel you also don't get oil and bang! Yamaha's (or certainly some of them) don't mix the fuel and oil the same as the others - but squirts oil into the engine directly (thats the limit of my knowledge!). This has the advantage of reducing problems with the carbs gumming up with oil and the bang of the fuel doesn't get delivered... ...on the otherhand as Stoo found out if the pump fails it still goes bang.

[Pete7]

Neck on the line but I think pre injection 2 strokes, that:

Mariner, Mercury, Evinrude and Johnson mixed the oil and fuel together before the carb.

Yamaha and Suzuki injected it seperately.

And Hondas were just slow

Pete

[Nos4r2]

Originally Posted by Pete7

Neck on the line but I think pre injection 2 strokes, that:

Mariner, Mercury, Evinrude and Johnson mixed the oil and fuel together before the carb.

Yamaha and Suzuki injected it seperately.

And Hondas were just slow

Pete

IIRC, you're right.

Not sure I'll ever trust a Suzuki autolube system since some of the spectacular failures they had on bike engines in the 70's due to poor routing/hoses though.