Soft lean seize ....Should I run it ?

[Freakie]

Hi guys

A few of us did a long range river dash a few days ago....full flooded winter river upstream 70 miles with 2 refuel stops... Ski ran very well but the last 10 minutes we were WFO and after running flawlessly (for years) my ski just died. Weather was reaaly cold...8 degrees celcuis. I tried to restart it, but cranking was very slow to the point I suspected my battery was dead....however I waited 2 minutes and cranked again and she fired up and she ran fine until we got to the pick up point. Did a few bursty speed runs around a few bouys and she behaved perfectly..... Anyway, this all felt like a soft lean seize so i did a compression test....front 160 psi back 174 psi.....Ski is running fine on the trailer. Head is a gidled ADA..motor is 781 Jetmaniac....

Question now is....should I just run it, or will it lose more compression quickly. The rivers are running crazy rapids and we want to ride the weekend before they subside hence my question. I have turned up the high speeds about 3/8 s of a turn as a safety. Would I still risk blowing up the motor or should it be okish to run like this.. This run will be mostly blippy and not WFO.
Appreciate your insights. Also...does the JM 781 have a rebore capability or is it last piston size already. tks very much

 

[JC-SuperJet]

It will seize again, probably at the worst possible moment. It's a vicious downward cycle.

The Metal Pixie Dust builds up inside the Ring Grooves. Rings need to float and rotate freely at all times.

Any Crocodiles riding with you Guys? Have a Rescue Ski to tow you back to the Dock?

 

[fastgtfairlane]

If you want to save money down the road, go ahead and remove and tear it down for inspection. Run it and risk it really coming apart and taking out the cylinder or cases then you'll be starting all over. I had the same thing happen to me and ended up needing a crank. Engine ran fine, still had power. Pulled the head and the rear piston was all burned up. Bearing started letting go. I had around 15psi difference as well.

 

[Quinc]

richen her up a hair and send it. But check compression after your next ride and see if it changes.

 

[waxhead]

If it was just a lean seizure it never would have pulled on to the high speed jet. It was probally over propped and got to much heat into it. Putting more fuel in will reduce the power and thus the heat which helps make it last

 

[Freakie]

Thanks for your replies guys, much appreciated. All very valid insights. Running rapids has it's risks which is mostly why I'm still on the fence.....failing at the worst of times could end me up losing my rickter entirely. If I could "test" it in a safe environment I would definately run it and check compression every 5-10 minutes.

I think I will forgo todays river run and opt for a testing session at our closest lake. Must be getting old

 

[wmazz]

If it was just a lean seizure it never would have pulled on to the high speed jet. It was probally over propped and got to much heat into it. Putting more fuel in will reduce the power and thus the heat which helps make it last

++1

Along time ago, I thought most seizures were lean seizures, and
no matter how I tried to correct or compensate for for a lean fuel
supply @wot, my fixes were ineffective.

Load and ignoring basic engine building practices are a common
cause.


Bill M.

H.K. wrote an article back in ~1987 that suggested our fuel pumps
would lean out at WOT on modified engines unless you increased
the needle and seat diameter. Theoretically possible, but it was not
a problem in practice. I sold a lot of N&S because of that fear.