SX/SXi/SXi Pro **** Need Experienced rider/mechanics help****

[Kryan]

A few years ago I purchased a 1993 Kawasaki 750 sx, shortly after I purchased it the engine went in it. With the little time i spent on it I absolutely fell in love with the sport, being a former motocross racer this is the closest related sport i have found thats easier on the body.

I put a Small Block Technology Engine in it hoping to solve the problem and be an easy fix, that was not the case. Ever since the engine was installed I keep popping water boxes.

The jet ski runs fine on the stand hooked up to the hose, my mechanic ran it through the proper warm up procedures when breaking in a new engine. But as soon as you put the ski in the water and start it; then comes the loud back fire sound and the water box explodes in a couple different places. It's almost like the little bit of back pressure from being in the water is causing it to back fire.

I really have no Idea what the problem is and how to solve it. All I know is that I have put two water boxes in it thinking i had a bad one, and have about $3,000 in something i rode maybe four times. I have not ridden it in two years, its spring, and I really want to get out there. Couple guys down the road just got there hands on a couple older skis, and are gearing up to get riding..

Any one with any advice as to where I can purchase another water box ( Fiberglass Vs. Aluminum ) (aftermarket vs. Stock). And what could be causing this to happen, I would really appreciate it.

 

[Vumad]

Fiberglass is weaker than aluminum. It will break the 1st time you have a backfire. Only buy aluminum or stainless.

Stick with stock, you can buy them cheaper.

You're blowing water boxes because of the backfire. That is not a waterbox issue, that is a motor issue. If it is a stock waterbox, and stock pipe, it most certainly is from a tuning issue.

The backfire is due to unburnt fuel burning. You need to take a couple steps just to start...

Remove the pipe. Make sure it is clean. Clean out any oil etc deposits on the inside of the pipe. Flush the cooling chamber very well. Insure any water injection holes inside the exhaust (not sure if there are any) are clear. Oil etc build up in the exhaust gives fuel to explode, and clogged cooling will cause over heating which can contribute to the problem.

Put in a new set of plugs and run the ski on the hose for a while. You can start by idling it in the water or on the trailer. Check the plugs. If they are black, you might be running too rich, dumping lots of unburnt fuel into your exhaust to later explode. Plugs should be dark brown. Think of them as marshmellows, if they need to go back on the fire (white or light brown in places), you are too lean, if they are burnt (black), you are too rich. If they are wet with fuel, you are way too rich. Make sure you are running the right plugs. The number is the heat. I *THINK* the stock 750 runs BR8ES, but confirm this before you change your plugs.

Check your carb tuning. I was indicating this in the above paragraph. If you have an all stock motor, then you should have factory jetting. Check your reeds and etc, although, I think the only intake problem would be carb tuning. I don't believe gaskets or reeds or any of that would cause this.

Check your ignition and stator. Missing ignition cycles will allow fuel to go through unburned, fuel for the backfire later. Random ignition could cause combustion while the exhaust ports are open, carrying that combustion right to your box.

A backfire is caused by unburnt fuel burning in the intake or exhaust. Somewhere, past the motor, you have fuel burning when it should not be. You need to eliminate that excess fuel, and eliminate the ignition.

 

[Vumad]

Lean can also cause the backfire but I would venture to guess you are more lean than rich.

You can't just swap a motor and bolt everything on, and have everything run perfect...

although, it probably should...

start by checking your timing. Maybe you reinstalled your flywheel and electronics incorrectly.

 

[kx454]

Had an older Yamaha do this at our shop, ended up being a bad cdi

 

[swannysxr]

Stator is in a fixed position on flywheel cover, so that eliminates being installed incorrectly. Most likely in cdi as Kx454 said

 

[Kryan]

Thanks for all the information above, very helpful!!! A mechanic I was just talking to said also that it could be a bad CDI.. Will make sure jetting is all correct and rule tuning as a problem first.

 

[Vumad]

Stator is in a fixed position on flywheel cover, so that eliminates being installed incorrectly. Most likely in cdi as Kx454 said

I put timing advancement kit on my 750. The late models had 6* advanced timing, and I had the 1st year of that advancement, but didn't know. So I had 12* advanced over stock. Granted, it melted my pistons, no backfiring, but just because it's fixed doesn't mean it's right. He could have a damaged woodruff key, flywheel or crank shaft, has some half-assed modification or incorrectly used timing advance, or so forth.

Just because a bolt can only go in one way, doesn't mean people can't jack it up.

 

[swannysxr]

I put timing advancement kit on my 750. The late models had 6* advanced timing, and I had the 1st year of that advancement, but didn't know. So I had 12* advanced over stock. Granted, it melted my pistons, no backfiring, but just because it's fixed doesn't mean it's right. He could have a damaged woodruff key, flywheel or crank shaft, has some half-assed modification or incorrectly used timing advance, or so forth.

Just because a bolt can only go in one way, doesn't mean people can't jack it up.

There was no mention of timing advance installed, but I no what you mean.

 

[john zigler]

I have had this trouble with 750's, and found a sheared flywheel key. ( many times back in the day)

Pull the flywheel, and inspect before buying any parts.....

Be sure you have the correct flywheel, and it is properly installed / torqued if you do find a sheared key.

 

[Vumad]

There was no mention of timing advance installed, but I no what you mean.

The only way to test the CDI (practically speaking) is to install one you know works. Inspecting all the other parts is free.

I don't think any of the electrical issues are the problem, I completely agree that it's the CDI (if it's electrical and not carburation), but always start with the cheap and free before throwing parts at it.

Inspect, test, replace. That order, always.

 

[swannysxr]

I have had this trouble with 750's, and found a sheared flywheel key. ( many times back in the day)

Pull the flywheel, and inspect before buying any parts.....

Be sure you have the correct flywheel, and it is properly installed / torqued if you do find a sheared key.

That thought came to mind , but he stated it's a new SBT motor and he was having same problem with old motor.

 

[Kryan]

This is actually a new issue. I replaced the engine with a SBT motor because it seized up on me one day while I was riding. The issue with it backfiring and popping the water boxes is something that has been happening since the engine swap. Keep in mind though to that it only happens when the machine is in the water. This does not happen when it is ran on the stand hooked up to a hose. Thanks for all the input its much appreciated!!

I am in the process of looking for a new water box any suggestions? Looking for something aluminum, something to go with the stock exhaust, and something that can just be installed with not modifications other then jetting if needed.

 

[Vumad]

Did you install the box backwards maybe? Not familiar with that box or if that would cause such an issue.

 

[Kryan]

I haven't been doing the work on the ski a close friend/mechanic/fellow jet ski rider has. Not sure, I would think he would know which way it would go. He was owned numerous jet ski/super jets and preforms all work and modifications to his own machines.

 

[Balduccimoto]

I think the waterbox can only go one way on those. I hope you didn't put another fiberglass box in there, the later models came with aluminum box and bolt right in place.

Start with pulling the current plug to see if it's running rich or lean. If that looks OK, try new spark plugs. If it still happens, I would check the key on the flywheel since that was tampered with when swapping the engine and check that there is no timing advance plate. If that all looks OK then I would swap a known good CDI in. It sounds to me the timing is off/screwy.