sounds like three cylinders only when throttle opened up

[dbelch]

Took the '86 out for a much needed battery charge ride, ran about half a tank through it at highway speeds.  Always runs like a top.  On the way home I pulled out to pass and noticed that the engine ran rough only when the throttle was opened up full bore...settled right back down when it was back to highway speeds again.  Still had half a tank, but tried the reserve...no change.  Electronical or fuel related?  Only when opened up, otherwise dandy.  Thoughts?   

[Yamifj1200]

Took the '86 out for a much needed battery charge ride, ran about half a tank through it at highway speeds.  Always runs like a top.  On the way home I pulled out to pass and noticed that the engine ran rough only when the throttle was opened up full bore...settled right back down when it was back to highway speeds again.  Still had half a tank, but tried the reserve...no change.  Electronical or fuel related?  Only when opened up, otherwise dandy.  Thoughts?  

I wonder if you are having fuel starvation issue due the gas cap flapper not operating properly? If it runs properly at lower rpms that's what I'd be looking at.. IIRC the problem would happen at about a half of a tank of fuel..

http://www.fjowners.com/index.php?topic=2024.0


Eric M

[andyb]

What do the plugs say?  It should be readily apparent.

[dbelch]

What do the plugs say?  It should be readily apparent.

Thanks for the input..I put a fresh set of plugs in, checked coil conections and it ran the same.  Filled the tank up, did a full throttle run through the gears.  At about 5000 rpm it was like a turbo-charger suddenly kicked in, my arms are still 2" longer!  Seems fine now, it was like something worked its way through the fuel? system.  I'll take a look at the fuel cap though, I take it that may be a venting problem?

[yamaha fj rider]

Could have been a kinked fuel line or when was the last time you changed your fuel filter? Hope this helps?

KURT

[andyb]

Could be fueling, but I had a similar problem early in my FJ's ownership.  I had a marginal plug connection that was not allowing enough juice to the plug, so under heavy use it lacked the oomph to fire the plug.  That plug was very black and smelled of gas when I checked it after a WOT run; had it been a lack of fuel it would have been quite white instead.

And yes, nearly dislocated my shoulders on the power when i figured it out.  

[yamaha fj rider]

Yes a bad spark plug boot or connection could be the culprit.

Kurt