Help! Sea doo 657x backfiring

[1995seadoo]

Hello, I have a 1995 sea doo spx with the 657x motor. It’s backfiring out the exhaust and carbs like a machine gun. Here’s the story: Just bought it for $500, seller said it ran rough. Get there and the second you rev it, it backfires. I take it home and find out he’s running 20:1 (oil injection deleted). Check the spark and it’s extremely weak. Drain the old fuel and put new spark plugs in, and she’s running beautifully(for a few minutes) Run it a little bit longer (on the trailer with water hooked up) and it starts backfiring again. This time only backfiring at high rpm’s, mainly the rev limiter. Called a local sea doo repair shop, they said it’s 100% normal for a 657x to backfire when the engine isn’t on a load and what’s happening is the engine is cutting the spark at the rev limiter and it’s causing a build up of fuel which causes a backfire. It makes sense to me but why has this never happened to me before with my other skis? I’ve had 2 other 657x’s and never had this problem, so the fact he’s saying it’s completely “normal” sounds a bit weird to me. So far what I have done to try to fix this: New gas (40:1 ams oil dominator), new Ngk Br8es plugs, rotary valve timing checked, carbs cleaned/rebuilt with oem kit, checked flywheel for sheared key (looked fine), compression is around 110 per cylinder but my crappy harbor freight tester reads low, I also checked the rectifier, was told you check the volts of the battery as you rev it and as long as the volts are 15 or lower your good, mine stayed in the 12 range so i’m assuming it’s fine. Looked for any potential air leaks, found none. Tried adjusting carbs in all different ways, nothing. Although my rotary valve and rotary valve cover do have some scratches in them, the scratches on the cover do slightly catch your finger nail. I heard this is bad but causes bogging in the water, not backfiring. The head of the motor has an SBT sticker on it, and from what I heard that doesn’t help the situation. I’ve never tested it in the water yet. I seemed to have tried anything and everything that could possibly go wrong and everything seems to check out. What other things am I missing? Is there a way I can attach a video of how it’s running?

 

[mikidymac]

There is no reason to bounce the engine off the rev limiter out of the water.
It could backfire if you are holding it wide open out of the water which is not a good thing to do.
I would water test it.
The voltage should not be at 12V running at 4500 rpm, it should be 13.5-14.5V.
A bad voltage regulator can cause backfires and strage running for sure.

 

[1995seadoo]

There is no reason to bounce the engine off the rev limiter out of the water.
It could backfire if you are holding it wide open out of the water which is not a good thing to do.
I would water test it.
The voltage should not be at 12V running at 4500 rpm, it should be 13.5-14.5V.
A bad voltage regulator can cause backfires and strage running for sure.

I understand you definitely shouldn’t bounce the motor off the rev limiter with no load, but it’s how I have to test if it’s still backfiring or not. And it’s not only at the rev limiter, maybe a few thousand rpms below the limiter as well. When you pull them out of the water i’ve always held them at the limiter for a few seconds to blow the water out before winter storage, and i’ve never had them backfire so i’m certain this isn’t normal. Thanks, I will water test it.

 

[1995seadoo]

There is no reason to bounce the engine off the rev limiter out of the water.
It could backfire if you are holding it wide open out of the water which is not a good thing to do.
I would water test it.
The voltage should not be at 12V running at 4500 rpm, it should be 13.5-14.5V.
A bad voltage regulator can cause backfires and strage running for sure.

Okay so I water tested it today and this is what happened: First off, I can’t believe how much fuel these things burn, I went through 6 gallons in less than an hour! But she’s not doing good… When you try to accelerate it just falls flat on its face. It will stall if you hold it wide open at idle speeds. You have to slowly increase the throttle in order to get up to speed, but once it gets on plane, it runs flawlessly. I got it up to 50mph no problem. I noticed when it got past its bogging point (if that makes sense) it would start bouncing off the limiter as it was trying to get up to speed. Does this mean my wear ring is toast? Also, does the bogging have to do with my rotary valve cover being all scratched up? Or do I need a simple carb adjustment? I thought it might’ve been running lean, so I added some choke to assist it. It stalled. So maybe it’s to rich? Here are my current carb settings: High speed is at 0 and low speed is at 1 1/2 turns out, running at 40:1 fuel/oil. As for the backfiring issue, I believe it’s still happening but only at the rev limiter. I think I can feel the pop through the seat, but you can’t hear it. Maybe i’m just imagining it, but I feel like it’s still backfiring. I’m not really concerned about that anymore, because if it’s running good it shouldn’t ever touch the rev limiter right? Incase you haven’t noticed, i’m a kid, new to the jetski world. I’ve only worked on dirtbikes haha. I only bought this specific ski because my dad bought the same model brand new back in 1995, and he said this is the ski to have. Any help is appreciated!

 

[burtshaver2021]

I would say all of the above, if the cases are all scratched up I would consider looking for a new set of cases, also with what sounds like a fair bit of damage to the cases and rotary valve cover I would have a look that the rotary valve is in time and turning tightly with the motor, bouncing of the rev limiter indicates driveshaft is worn or wear ring or impeller, the bogging could very well be the carb, run a temporary line to the tank to eliminate most other fuel issues except the carb.

 

[1995seadoo]

I would say all of the above, if the cases are all scratched up I would consider looking for a new set of cases, also with what sounds like a fair bit of damage to the cases and rotary valve cover I would have a look that the rotary valve is in time and turning tightly with the motor, bouncing of the rev limiter indicates driveshaft is worn or wear ring or impeller, the bogging could very well be the carb, run a temporary line to the tank to eliminate most other fuel issues except the carb.

Thanks, I already timed the rotary valve so i’m sure it’s good, as for the scratches, the case (i’m assuming you mean the motor itself) surface had no scratches. Just the rotary valve and rotary valve cover were all scratched up. But if it was the scratches causing my issues, wouldn’t it always run bad no matter what? Because like I said, around half throttle and higher is good, once it gets up to speed.

 

[burtshaver2021]

Thanks, I already timed the rotary valve so i’m sure it’s good, as for the scratches, the case (i’m assuming you mean the motor itself) surface had no scratches. Just the rotary valve and rotary valve cover were all scratched up. But if it was the scratches causing my issues, wouldn’t it always run bad no matter what? Because like I said, around half throttle and higher is good, once it gets up to speed.

Oh, I thought the case was bad to, in that case have you measured the clearance between rotary valve and the cover with the O ring out? Not sure if it would make it run poorly but it does cause hard starting and if it’s way out of spec might as well have it cleaned up. If you have it apart, maybe just double check the rotary valve timing just to be sure but sounds like most issues might be fuel related then

 

[1995seadoo]

Oh, I thought the case was bad to, in that case have you measured the clearance between rotary valve and the cover with the O ring out? Not sure if it would make it run poorly but it does cause hard starting and if it’s way out of spec might as well have it cleaned up. If you have it apart, maybe just double check the rotary valve timing just to be sure but sounds like most issues might be fuel related then

So I think I figured it out. As we all know, how it runs on the trailer is so much different then how it runs in the water. So I decided to run it on the trailer to see what it did. If it’s idling, and you smash the throttle wide open, it bogs then dies. I took a better look at the oem rebuild kit I had, and just realized it’s sorta a universal rebuild kit, including different parts for different type of carbs. I did some research and learned about pop off, and how that works inside the carbs. Turns out, it’s really important to have the correct spring inside the carb, you know, the spring that pushes up on the little arm, that lifts the needle. In the kit, it comes with 2 different spring lengths. The longer spring will require more crank case pressure to open the needle, making the pop off point really delayed. I tore the carbs back off, and sure enough, I had the wrong springs in. Not only were the wrong, one carb had a long spring, and one carb had a short spring I read each carb needs to be within a few psi of each other, and having 2 different springs throws that way off. So I put the right springs in it, and fired it up. At idle, if you smash the throttle, it revs right up with very little hesitation. So as for running on the trailer, I’d say it’s great. So I hope it will also run good in the water. No matter what I still need that wear ring. I took a look, and it’s bad. Like 1/4 inch gap between the wear ring and the impeller. I’m assuming that’s really bad. Thanks again for your help.

 

[burtshaver2021]

Fingers crossed that it runs good on the water. Yes, you definitely need a new wear ring