Super Jet Detonation

[Balduccimoto]

Ive been chasing this problem for a bit now, has happened on two different motors (1st was 781 2nd was 771) and the 2nd time was tuned by a local mechanic. Ive been getting pre-detonation on both holes and it looks like the edges of the piston is chewed. With the 2nd motor I thought everything was fine since I had put around 50 gallons of gas through it with no issue. On Sunday the front cyl had melted on the exhaust port side and the ring caught the exhaust port. Right now it's getting bored .5 over with new pistons and new 37cc domes as opposed to the 36cc domes that were in there before.

Every now and then I get what feels like a fuel delivery problem and it seems to occur more on reserve (I assume because the fuel pick up starves from fuel sloshing). It feels as though the ski bogs down b/c lack of fuel and after a few seconds will resume normally.

I have checked the vent, the fuel lines, all 3 fuel filters internal + external, carbs clean
I assume the static timing on the stator plate should be correct,
I pressurized the fuel system and it holds.
I blew into the primer to make sure it's not sucking air through there.
The OEM pick up tubes are in good shape.
I pulled apart the fuel pump and the diapghrams are good (I rebuilt with genuine mikuni over winter).

Jetting is:

771 cc Big Bore
145 Main
75 Pilot
Unsure N/S + Pop off (set by mechanic who is familiar with the engine)
1/2 Turn High
1 Turn Low

Motor Set Up:

771cc Big Bore
Dual 38 Mikuni
OEM Flame Arrestor 1 Screen Removed
Factory B Pipe (limited chamber)
Jetworks FCV
#Zero Lightened Flywheel
MSD Enhancer
Hooker 10/16
Solas Mag Pump
Bored 85mm nozzle


I have been using 93 octane from the same mobil station. I am really starting to suspect their fuel is old or something awry with their gas. Is this a possibility? Seems like a busy station. I just really need some fresh ideas I don't want to be doing any more motors, i've been having motors self destruct for reasons out of my hands and bc of detonation, really tired of it :banghead:

 

[Balduccimoto]

Forgot to mention, I have been doing WOT for 5 second bursts. When it blew I was doing full throttle while varying throttle position. (wot, let off to 1/2, wot, let off to 1/4)

 

[BruceSki]

Whats your oil ratio?

 

[Vumad]

With the 771 you listed jetting but not carbs, and on the 781 you listed carbs but not jetting.

Are you running a petcock. Can you have an air leak letting it lean out at high speeds. Are you checking your plugs for signs of leaning or pre-ignition prior to these problems. Is your reserve pickup close enough to suck to the bottom of the tank.

This is not something I am very familiar with, but I can help brainstorm until someone with more experience chimes in.

You should ride with a full tank. Low fuel can cause momentary leaning which can cause damage. I like to keep my tank above 3/4 on primary and 1/2 on reserve. Anything below 1/3 is for cruising back to get more fuel. Air in the fuel lines on a tweaked motor will kill a motor.

 

[Balduccimoto]

Sorry I may have made my first post a bit confusing since there's alot of info to be had.. I am mixing 40:1 yamalube. It's very difficult to keep the fuel topped off for me. The break near me is a bit away and when the ski starts bogging heavily I know to switch to reserve and head back to ramp. I sometimes get to swim it back to ramp a few 100ft, im a lucky SOB in that regard haha. When the melt down happened, it was with a fresh tank of gas and nearly 10 min into riding.

Both motors ran the same dual 38 mikuni with choke plates deleted. The 781 had the following jetting:

781cc
135 Main
75 Pilot
47 psi pop off (Stock N/S whatever that is)
If I can remember correctly.. 1 1/2 High 1 low?

the current 771
145 Main
75 Pilot
Unknown popoff n/s
1/2 high
1 low

 

[ryan]

If it was a set up issue it would of never made it 50 hours and then blow up.somthing is wrong somewhere. I would lose the fuel valve for starters.
The needles are 2.0s with an 80 gram spring.
We have many of this set up running with no issues.

 

[Balduccimoto]

I took apart the fuel selector and that's clean and checked it holds pressure. Maybe the gas not up to par? I been looking to put a stock motor back in this is crazy.

 

[BruceSki]

You've gotta have something else going on. I'm running the same motor for the last year with no issues and I can hold it wide open for a minute straight. I've even got a smaller main than you @142.5 and a 9/15 hooker so its revving higher.

 

[Balduccimoto]

I know I believe any motor can wot all day if set properly with no issues. Obviously sonething is going on but I can't find it. I'm stumped and need ideas thus why I'm asking here. I keep doing 360 and condemning the gas being a. Not really 93 or b. Old. I'm getting annoyed with this issue now, I'm ready to just gut everything and start fresh

 

[waxhead]

2008 and above have a more restrictive water box if you have not moddified the water box the pressure drop across your stinger is not enough and you will deto your pistons. You need to cut the water box open and use a hole saw on it

 

[BruceSki]

Wow, now thats a new one. never heard that before.

I have a RN box in mine with a baffle removed cause it was loose and rattling.

 

[waxhead]

Took me a few pistons to figure it out my self

 

[ryan]

Kind of weird that it would run so long and then burn both pistons due to a waterbox that was in there for the 1st 49 1/2 hours with no issues.hows that work ?

 

[ryan]

Sorry gallons not hours.

 

[Balduccimoto]

Waxhead, what made you look inside the box? I'm sure a highly restrictive exhaust system could cause deto; the exhaust gas wouldn't escape and cylinder temps would rise but I would think it would have to be such a restriction that would actually make the ski feel like a clogged catalytic converter. Regardless, I'm gonna look into it but it seems like a try and see ordeal.

Ryan this thing is detonating for a long time now. It just turned out sunday was the day I'd be WOT, detonation started and continued until it gave me some nice carnage. There was detonation surrounding both pistons. Why didn't they both burn up same time? Well on sunday it decided to blow the front cyl all bc maybe the air temp, maybe water temp was a bit warmer, maybe there was a air pocket in the line and maybe the gas a little old. Anything is just enough to get the cylinder temp a bit high and start detonating at exponential rates. Its kinda hard to tell when the motor gonna go when I'm already on the brink of deto.

 

[txsuperjetfreak96]

if the stock waterbox is setup for a 701, when these manufacturers went to 1100 and 1200cc motors, they needed bigger waterboxes, and or outlets, maybe im talking outta my arse :dunno:

 

[schicks]

do you know what temp you cylinders are running at? do you have an infared temp gun? they are cheap now.

 

[ryan]

Check your fuel. There is no reason for that setup to be burning pistons. It is way big on the main jet size .

 

[waxhead]

I have built a few expansion chambers for other things in my time. I know the result of to small a stinger it keeps the heat on the piston. By having a restrictive water box you decrease the pressure drop across the stinger, It was not until i did it then I read the bit by yamaha to say they had changed the water box to make it quieter as well in the 2008 and later models. 800 seadoos used to do the same thing until you changed the water box. As soon as I did it I could run flat out with no issues.
A restrictive water box will not act like a four stroke it will still flow enough to let out the gases but not enough to let out the heat and it keeps it in the chamber. Where it snacks on the edge of your piston

 

[Balduccimoto]

do you know what temp you cylinders are running at? do you have an infared temp gun? they are cheap now.

Right before deto I felt the water coming out of the pissers and it didnt feel hot.

Waxhead, do you have any pictures of the inside of the waterbox? I think I may just get a AM waterbox, I was thinking about getting one anyway. I did look, and they did change the waterbox for 2008.