Very strange jetmaniac 735 engine issues

mxhead

mxhead

Location
ontario
Looking for some help. I'm at a point where I'm ready to give it up for the rest of the season. Done nothing but spend thousands of dollars on this thing and countless hours wrenching on it. Iv rebuilt the motor. Brand new gaskets, compression test, leak down test, you name it. It just won't run right. If someone is willing to give there insight I would appreciate it, or if someone is willing to get it running with me there time is worth something.

Quick explanation what's going on:
-It has "ran away" on me twice now out of the water
-Barley idles and runs like crap on the stand, even sounds like garbage
-when in the water in barley gets on plane. You maintain the same throttle and all of a sudden lights up and out of the water it goes. Let off throttle and back to a bog.

All I changed was flame arrestors and went from a 115g pop off springs to a 95g as per Chris from jetmaniac recommendations for the new filters.

Ignition is oem with msd boots. MSD enhancer on its way.

Attached are some pictures of my carb and stator. Is the timing right? Looks like it's stock timing and isn't advanced or retarded any.

Reeds are in good shape. One was starting to curl a bit so i removed it and flipped it to make it "seal" again.

I did another leak down test with flywheel off and you can see the small leak on the front crank seal. Will that SMALL of a leak cause this issue. Funny that the mag side piston did score a bit before rebuild. But having a very hard time thinking that is my issue now.

The motor was a 718 jetmaniac build. It's now bored over fresh with 83mm(735cc) piston.

Help...
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Mike W

Mike W

Infidel
Location
North Florida
Have you contacted Chris about these issues? He is great at talking you through these kind of issues.

Having said that, I would thouroughly go through the carbs as a starting point.
 
mxhead

mxhead

Location
ontario
Mike W said:
Have you contacted Chris about these issues? He is great at talking you through these kind of issues.

Having said that, I would thouroughly go through the carbs as a starting point.
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Been in touch with Chris through out the whole issue. Unfortunately he's a very busy man. I'm out of town right now at a ride weekend and stuck sitting on the beach

The carbs have brand new rebuild kits in them with zero hours on it. This issue was happening before the rebuild.

I don't know if the crank seal leak would be causing my issue or not

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Mike W

Mike W

Infidel
Location
North Florida
Pressure test your carbs. Make sure your needle and seats are sealing well. I chased my tail a long time just to find out my slave carb had a bad leak. (See picture) You just need to rule out the carbs before you can go forward.

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mxhead

mxhead

Location
ontario
holjen77 said:
Mag piston has that steam cleaned look you get when you have water leaking in from the intake gasket.
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You are right on the money. We had a leaking base gasket.. But that's fixed and this issue is still here

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Last edited:
jetx

jetx

Location
Midland, Ontario, Canada
Which gas tank are you running? Had a fellow rider with a thrust tank have some of the rubber gasket for fuel cap fall into the tank and lodged itself in fuel lines causing a restriction of the fuel and making it run like poop, andhad sinilar symptoms to yours.

Also where those new crank seals? Brand?
 
mxhead

mxhead

Location
ontario
Little update:

Last night I swapped out the entire electrical system for a 61x set up that we have for a spare. And problem is still there so we can rule out electrical.

I'm running a cheap cart tank that the trixstar hull is molded for. I really wish I could run a different tank, but without cutting the carbon fiber i have to run this one.

Crank seals have not been replaced. Those are the seals that jet maniac Installed when the motor was built for the previous owner.

Gasket and o rings have been replaced. Leak down and compression tests have been done many times now to check for leaks. Last night the leak down test dropped a pound or 2 in over an hour. Very very small leak around crank seal. And that's at 10-15psi. What pressure does the crank case build?

I have a set of spare 38s here. Those are getting put on next to rule out carbs. If that doesn't fix it. Looks like.im buying a new motor and enjoying the rest of the summer!

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Freakie

Freakie

Location
Cape Town
Do your leakdown test again......in my experience if the cases cant hold a steady pressure of 10 pounds.....even dropping a pound or two......that's a big problem......rotate the crank while it's under pressure and see if the leak worsens.....I had a similar situation where I could not imagine the small leak I had between the case halves caused a complete runaway out of the water.....case could not hold 10 psi but settled at 8 psi after 20 mins....fixed the leak with some polyurethane on the outside.....just to sure that was the issue.....bingo....ran 100 % again
 
mxhead

mxhead

Location
ontario
gary carnovale said:
thanks for the help, we ended up running with lower pop off, -10 from your number. no more runaway engine and bog is gone.
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Thanks for the help Gary, glad this headache is finally fixed. See you in a couple weeks to do some riding this time!

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