1997 SeaDoo Challenger won’t transition off idle

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Nextboat1

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Hello,
I’ve spent a lot of time reading through the forums, but have not been able to solve my problem. I have a 1997 Challenger with a rebuilt engine purchased from sbt. It will start and idle fine but when I try to give it throttle it quickly bogs and dies or occasionally backfires through the exhaust and then dies. It does this both in and out of the water. I tried the fuel selector in the on position and in the reserve position with no change. Along with the engine the fuel lines and filter were replaced, the fuel selector was replaced, the pick up was removed and cleaned, the spark plug wires were trimmed back, a new rotary valve was installed, and it was converted to premix. It is currently running 32:1 as per the break in instructions. Any help or suggestions will be greatly appreciated!
 
Did you rebuild the carbs with only genuine Mikuni kits and needle and seats?

Did you get the correct rotary valve and time it correctly?
 
I have genuine mikuni kits on order. I will check that the rotary valve is timed correctly when I pull the carbs.
 
Got my rebuild kits this week only to find that they are not genuine mikuni kits. Where can I find genuine mikuni kits along with genuine needles and seats? I think my problem lies in the accelerator pump, but would like to do a total rebuild on the carbs. I noticed prior to removing the carbs that when throttle is given I do not see a shot of fuel in either carb. Thanks!
 
You can get the parts from OSD seadoo or the kits from Amazon are genuine, they will be "sold by Mikuni".
Mikuni Super BN Rebuild Kit MK-BN38/44 SPR
by Mikuni
$42.68
 
I pulled the carbs apart last night and got them cleaned up. I was a bit disturbed by what I found in the mag side carb.

After the rebuild I checked the pop off and was at 42 and 44 lbs. I set the low speed screws 1 turn out and the high speed screws 0 per the manual. I’m going to try to get the carbs back on this week. Hopefully this solves my problem!


IMG_0767.JPG
 
I would clean out your fuel tank and replace the fuel hoses and filters for cheap insurance.
 
Finally got everything put back together this afternoon. I verified that the rotary valve timing was correct while I had the carbs off. Checked for two solid streams of fuel from the accelerator pump, good there. Hit the button, fired it up, and no change. It still backfires when you hit the throttle. I tried disconnecting the red wire from the rectifier after reading some other posts, no change.

Fuel lines have been replaced. Fuel filter is new. Tank has been cleaned. Fuel valve is new. Spark plugs are new and gapped correctly. Plug wires have been trimmed. I’m at my wits end with this thing! I’m not sure where to look next. I appreciate all the advice so far.
 
At what rpm does it start spitting and sputtering? The boats have a neutral safety switch that limits rpm to 3500 when in neutral. See if the engine will start when it's in forward or reverse. If it does start, the neutral safety switch is bad. The MPEM thinks the boat is in neutral and will not rev.
 
I thought the same thing. It will not start in gear. It will beep once when brought back to neutral and then it will fire right up. It will backfire and big as soon as any throttle is given. It’s idling around 3k out of the water and 1500 in the water. I haven’t verified the in water reading since the carb rebuild.
 
I originally thought it backfired just through the exhaust, however after working with it this weekend i noticed that it is backfiring through both. While I was checking the streams from the accelerator pump I had the air box off. I started it once this way and could see it back fire through the carbs. I could hear it through both. That was the first time I had started it with out the air box on.

The engine is a newly rebuilt engine from SBT.
 
Backfire through the carbs points to a timing issue. If you are sure the RV alignment is correct and the flywheel key hasn't stripped, I would test with a timing light per the shop manual. It may be possible that with the rebuilt crank, the timing is off.
 
I had the same thought and just read through the procedure in the manual. I also pulled the plugs and preformed the test outlined in the manual to check for a twisted crank. I didn’t notice the pistons moving in the same direction at any point. Could it be twisted such a small amount that I don’t notice it and still cause a problem like this?

I never removed the magneto flywheel from the hub flywheel. I left them together when I pulled it from the old engine, so I don’t believe the problem lies there. I installed a new key in the crank, slid the hub flywheel on over the key, and tightened to spec. Could it have sheared the key that easily?

If the timing does prove to be off would you assume a crank that is twisted or an issue with the flywheel/key? I guess either way the engine would have to come back out to verify.

Thanks!
 
I doubt there is an issue with the flywheel or key since you replaced it. My thoughts were, maybe the crank is not perfectly aligned. I don't know to what extent SBT disassembles cranks during rebuild. . My thought was, if the mag side end is disassembled from the web, it may be possible that it isn't aligned the same as it was from factory and might require re-timed
 
If it did require a timing adjustment would it have to adjusted with a programmer connected to the mpem, or is there a mechanical adjustment?
 
Maybe check the rotary valve clearance. Scoring or too much gap will cause rough running engine and back firing.
 
The valve and cover are new. I made sure everything was very clean during assembly. I checked the clearance per the manual and it was within tolerance. Thanks for the idea though.
 
I had similar problem with my 787. It turned out to be the flywheel / key / crank. I discovered that I had play in the flywheel placed on the crank. Was able to visually position correctly at an end of the play. Lots of lock tight and torqueing the fly wheel back on the crank put me back on the water.
 
It’s been awhile, but I finally got a chance to work on the boat again. I followed procedure outlined in the manual for checking timing with a light and it was spot on which I was not expecting.

I decided to go ahead and check the compression and found the pto cylinder was at 155 and the mag was 115. I believe I have found the problem! I tried calling sbt but they are closed for thanksgiving, just my luck.
 
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