Ski cuts out at WOT. Rear plug is charcoal black

SXIPro

JM781 Big Bore
Little history: Ski has always run decent, but was having intermittent ignition problems so I got a coil from John at JSS last week because I was sick of cutting back plug wires.. Boat ran great last week. Was out today, ski was ripping just fine. Came in, refueled, went out rode for like 20 minutes, took off on a WOT run and the ski just quit. Got it started and cruised back to shore. It would not hold idle. Came in, pulled plugs they looked a ittle crappy, put in new plugs. Went back out ran fine for a while and started doing the same thing. It just shuts off on WOT runs and then when I restart it it stumbles and idles crappy. Came back in and front plug still looks pretty much new, rear plug looks like burnt toast. WTF is going on??

Oh, stock dual 38s, MSD enhancer, B-pipe, ADA head if that matters. (all these things have been on the boat for last 3 years.
 

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SXIPro

JM781 Big Bore
So, it gets worse. I do not have a compression tester, but I put my thumb over the front plug hole and turn it over and it blasts my thumb off the hole. Do the same thing to the the rear hole and it just 'puffs'. ARGGHHHHHH!!!
 
I would pull the head but I like to look inside all the time. That gray stuff looks like piston material... how does it feel...
 
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SXIPro

JM781 Big Bore
If you are asking about the gray stuff it doesn't feel gritty or anything it just looks burnt. It is freaky...even though the ski seems to have dropped a ton of compression in the back cylinder, when it runs, it runs pretty good until it warms up, that's when it cuts out at WOT and then gets sketchy to start and hold idle again. I'll pull the head tonight. I am praying it is something simple like a blown o-ring in the head, but I may be in for a new top end...this sucks. Riding season just started up here. :frown:
 

SXIPro

JM781 Big Bore
if it puffs that's probably cause no compression in that cylinder, go buy a comp gauge

Yeah, I'll probably have a nice piston paperweight. Excellent. I'll find out tonight. Anyone know what may be the root cause? I've been doing some reading and a lot is pointing to crap in my carbs as leaning to a lean seizure.:dunno:
 
You won't know till you take the cylinder off. Take pics. There are many reasons for motors to die... The damage will usually point you in the right direction.
 

SXIPro

JM781 Big Bore
how do you like that Samuel Adams??

That particular one is not one of my favorites. It came in a summer 'variety pack'. I'm drinking a Sam's summer ale right now, and it's pretty good. A little light for my taste, but after looking at my motor, I'd drink turpentine right now if I were out of beer.
 

SXIPro

JM781 Big Bore
You have some nice deto in the rear cylinder.
You open up the carbs yet?
You recently change the prop?

No, I need to crack the carbs. Probalby do that tomorrow. A lot of folks think I may have a clogged internal filter or something like that which leaned this thing out.

Prop has been on over a season. Why do you ask?
 
I didnt think kehins had internal fuel filters... My 92 750 I pulled apart last night to clean (which is bone stock NOTHING HAS EVER BEEN DONE TO IT) Single carb did NOT have an internal fuel filter.. Most of the ski companies did not change anything from year to year... Is it Kehins or Mikunis....
What about a reed breaking in the rear cylinder...
Did you also check the rear crank seal to see if its still intact?
All o rings on the head still in tact?
Burn looks good on the front, dougbt it was fuel... you premix?
If rebuilt what kind of pistons were used?
Where are the scoring marks on the cylinder or is it just top of the exhaust side where the chunk of piston is missing?
Is the ring intact or did the ring catch the exhaust port.
Is there any porting work done?

Lots of questions to ask till you find your answer... Settle for less and you get to rebuild again which sucks...
 
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No, I need to crack the carbs. Probalby do that tomorrow. A lot of folks think I may have a clogged internal filter or something like that which leaned this thing out.

Prop has been on over a season. Why do you ask?

He probably ask about your prop because if you got the wrong pitch it would run the piss outta your poor motor if it was not enough prop..
 

Scorn800

Ride for life
Location
North NJ
No, I need to crack the carbs. Probalby do that tomorrow. A lot of folks think I may have a clogged internal filter or something like that which leaned this thing out.

Prop has been on over a season. Why do you ask?

If your running hooker/dynfly. They put a more of a load on the motor. What could have been perfect tuning on old prop could be alittle lean with new prop or it could just be crap in carbs. Boats always run great lean till they blow up.
 

SXIPro

JM781 Big Bore
If your running hooker/dynfly. They put a more of a load on the motor. What could have been perfect tuning on old prop could be alittle lean with new prop or it could just be crap in carbs. Boats always run great lean till they blow up.

Ahhh. Interesting. It is a hooker, but I ran a full year on it in 2009. Previous prop was a Solas Concord 13/17 3mm cutback.
 
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