Help with identifying parts/issue

I bought this ski a few weeks ago from a guy who didnt know much about it. He said the engine rips and it was ready to ride. Upon inspection I found a good size rock jammed in the prop. I bought it without starting it(also missing battery)

I pulled the pump apart and got the rock out, cleaned up the prop and reassembled. I put a new battery in and about half tank of new fuel and tried to start it with no luck. Put some new plugs in and it fired up and slowly cleaned out and became more crisp.

Took it to the water and it felt great for about 45 seconds and then died. I reached under and pulled a decent size stick from the prop. Took off again but this time it barely got out of the water and then slowly cleared out and got mid range power. From this point on it had only mid range throttle response. Low end would barely get up to plane and if I tried to grab throttle from the mid it would fall on its face.

Got home and pulled the carbs which I think are blackjack 38mm(hoping someone can confirm or deny) both high and low speed jets says 100 on them. Carbs were clean inside.

Compression tested next but with the R&D intake manifold unbolted. Didn't 20200722_220219.jpg20200726_104305.jpg20200727_194447.jpg20200727_194602.jpg20200727_193902.jpg20200728_201702.jpg20200802_112440.jpg20200802_112423.jpg20200802_112447.jpg20200722_183859.jpgthink about it at the time but realized later that would probably not produce accurate compression readings. 100psi on both. Pulled the top end apart and everything looks decent (to me)

I ordered a new girdle head from Thrust with 35cc domes, new 82mm pro x pistons and new gaskets (Current bore is at 81.5mm)

I dont think compression was my issue so I'm turning attention back to the carbs while waiting on parts. Based off other friends jetting it seems the high speed 100 is too small. Previous owner swears this setup was dialed and the ski ran perfect 1 year ago but has sat since then. I have very minimal experience with any of this and still trying to learn what parts I have here or what could be causing problems. Any input on jetting, other concerns or help identifying parts would be much appreciate.
 

DylanS

Gorilla Smasher
Location
Lebanon Pa
I would check compression on a good battery with the throttle wide open again, 100 psi is low. If you look up “mikuni super BN carb manual” on google they have a really good write up on the basics of tuning this style carb and trouble shooting. What exactly was it doing when it was acting up? Was it a rich hesitation that gradually cleared up but still had no power up top? Gotta put as much info out there so some of the forum gurus can work their magic lol. And who knows if the ski was ever “dialed in”, unless you really trust the guy!
Good luck man!
 
I’d rebuild the carbs, my ski had the same issue no bottom ran great 3-5k rpm and no top end almost wanting to die at WOT. Rebuild the carbs and new plugs bam just like new.

P.S. since you got a new head want to sell that twist?
 
I’d rebuild the carbs, my ski had the same issue no bottom ran great 3-5k rpm and no top end almost wanting to die at WOT. Rebuild the carbs and new plugs bam just like new.

P.S. since you got a new head want to sell that twist?


Yea I'll sell the twist head. My cell is 916 605 6288

I bought some 82mm pro x pistons and had the cylinders bored to match. The machinists said I had too much cylinder to piston clearance for my last pistons, a little over double what it should be. I just finished putting the top end back together and I'm getting 113psi on both cylinders. Leads me to believe my cheapo comp guage is very inaccurate. I'm borrowing a snap on tomorrow to check again. Going to step up the main jet to a 135 and go test it out.
 

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I would check compression on a good battery with the throttle wide open again, 100 psi is low. If you look up “mikuni super BN carb manual” on google they have a really good write up on the basics of tuning this style carb and trouble shooting. What exactly was it doing when it was acting up? Was it a rich hesitation that gradually cleared up but still had no power up top? Gotta put as much info out there so some of the forum gurus can work their magic lol. And who knows if the ski was ever “dialed in”, unless you really trust the guy!
Good luck man!

Yea it seemed like rich hesitation that gradually cleared up and then would sometimes kick and take off but still no top end. Felt like 1 cylinder didn't have spark but would somwtimes come alive. When checking spark I did find the top of the msd boot had a small slice that shocked me through the top of the boot. Figured if it didnt have anything to arc to it would still give reliable spark. I have new boot kit on the way.
 
A friend of mine has this same pipe setup and was asking me what jet I planned on running. I didn't know there was supposed to be a jet there, and when I checked it I just found the barbed fitting. He thinks this is likely what was causing my issue, putting too much water into the exhaust maybe? It seemed like when I first put the ski into the water it ran normal and then slowly got worse the longer it ran. If I parked the ski and let it sit for a bit and tried again it would start out ok and then slowly get to the point of not even getting up to plane. Does this sound like it could be the culprit? Where can I buy this jet by itself? Thanks for the help guys, I really appreciate it!
 

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Is it getting hot? Mine was getting hot loosing power from a used pipe I bought and that fitting had a small hole 1/16” and putting barely any water into the pipe.
 
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