Running nice in the winter - running terrible in the summer

[borcherta]

Hello again forum!


I have an 1993 Yamaha Waverunner VXR 700, it recently got new SBT cylinder/pistons and a new Mikuni Carb.


Here in south-central at the golf we had around 95 degree every day, 6 weeks ago.
And six weeks ago the jet was running terrible, I always find it hard to explain the diagnose: its a mix of bogging down and running bad.
Even after many little adjustes to the carb high/low screws, it never really ran nicely. It starts right up and idles nice though.


I left the ski sitting untouched in the yard for 6 weeks.




So now yesterday we were out again and suddenly the ski was running nice and very smooth.
I still have the feeling it didnt have full power, but no bogging no hestitation, all much better than expected.




But ... how can that be? And whats the cure?
Yesterday we had maybe 70 degrees, so thats a huge difference to 95.
Other than that: nothing changed and no one touched the jet.


Any thoughts?




Thanks!
.axel

[jeffg426690]

Group K has a great article on carb tuning and what temperature variation does. A 25 degree swing is a big difference in terms of air intake. Riding a two stroke yr round sometimes requires some adjustments for good air/fuel ratio.

[steve45]

A 25° temperature change really isn't that much. I think you may have a bad spark plug or bad wire.

[Myself]

Check your waterbox. The stock ones are fiberglass in the older skis. I commonly find cracks in them or broken baffles blocking the exhaust flow.

[32DegH2O]

You are running rich...change the jetting. Yes, 25 degrees makes a huge difference...not to mention humidity influences. 95 degrees can hold a lot more humidity! Colder air has more density thus needs more gas which is why it runs better as temps go down.

[steve45]

Our high & low temperatures often differ 40° F on the same day. Are you going to tune or ride?