97 Seadoo XP 787 blowback/backfiring issues

[remix919]

My 97 Seadoo XP 787 was having some blowback issues at all RPM ranges and backfire at high RPM's while running on the hose. It will run in the water, but will bog down and cut off unless I increase throttle very slowly, and even with throttle down, I'm not getting anywhere near full power. It also seems to have some custom carb setup with dual Mikuni 44's (anyone know exactly what carb setup this is?) and some special rotary valve cover by watercraft magic (apparently company doesn't exist anymore?). I checked compression and both cylinder are right at about 150psi. I'm getting good spark on both plugs, very nice and bright blue when I touch the plug to the cylinder head. I rebuilt the carbs and replaced the needle and seat with genuine Mikuni carb rebuild kits but that doesn't seem to have helped at all other than I now see even more fuel spraying out the carbs. I checked charging voltage at the battery while the ski was running on the hose and revving it up to around 5k rpms and it holds pretty steady between 13-13.2v. So my next step was to pull the rotary valve and cover to check timing and assess the condition of the parts, they look ok to me but was hoping someone with more experience could give me better insight. The cover had minor wear marks but nothing that would catch my fingernail. Rotary valve looks a bit beat up especially the side facing the engine and I plan to get a new one but would that cause all these crazy problems? At this point I'm kind of at a loss at what would cause these issues?

Also, can I use some light grit sandpaper like maybe 3000 to sand some of those deposits off the valve?

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[etemplet]

Did you take a reading on the rotary valve clearance? Put a straight edge and use a feeler gauge to see if the valve landing surface is warped. I"ve seen a couple of pretty messed up surfaces and the engine still running good. While you have it apart make sure th timing is as xlose as you can get it.

Your problem is likely in the carburetors. Sounds like you need to open the low speed jets up 1/4 turn and see if the hesitation improves. Good Luck !

 

[remix919]

Did you take a reading on the rotary valve clearance? Put a straight edge and use a feeler gauge to see if the valve landing surface is warped. I"ve seen a couple of pretty messed up surfaces and the engine still running good. While you have it apart make sure th timing is as xlose as you can get it.

Your problem is likely in the carburetors. Sounds like you need to open the low speed jets up 1/4 turn and see if the hesitation improves. Good Luck !

You're probably right, I had posted in another forum and a member had told me I'm running "Buckshot" performance carbs, when I had rebuilt the carbs I had put everything genuine Mikuni back in them including the short silver spring that came in the kit, perhaps I should have used the longer silver spring (2 came in the kit, both shiny silver). I'm going to try swapping the springs out so I have a higher pop off pressure and see if it runs better like that. I also already ordered another rotary valve but due to the fact I seem to have a custom valve cover, i'm not sure where I can either get a replacement cover or my cover re-machined?

 

[Ckrawiec]

You're probably right, I had posted in another forum and a member had told me I'm running "Buckshot" performance carbs, when I had rebuilt the carbs I had put everything genuine Mikuni back in them including the short silver spring that came in the kit, perhaps I should have used the longer silver spring (2 came in the kit, both shiny silver). I'm going to try swapping the springs out so I have a higher pop off pressure and see if it runs better like that. I also already ordered another rotary valve but due to the fact I seem to have a custom valve cover, i'm not sure where I can either get a replacement cover or my cover re-machined?

I am NOT an expert but have read extensively on this topic......I believe the '97 - 787 engines with the dual carb (STOCK Mikuni) setup used the 80 gram black springs. If you have an alternative carb setup...I have no clue other than possibly trying to get your existing carb situation setup as close to the stock Mikuni spec's - maybe?????

 

[etemplet]

I'm going to follow you up on this for sure. I have a sooped up 787 XP (1996) I'll be working on this winter. It has a lot of performance parts along with the Buckshot Carbs. I haven't looked into it very much but I read about a guy that had so much trouble with the Buckshots that he went back to stock. LOL We'll see how I make out.... LOL Here are a few pics.

 

[remix919]

I'm going to follow you up on this for sure. I have a sooped up 787 XP (1996) I'll be working on this winter. It has a lot of performance parts along with the Buckshot Carbs. I haven't looked into it very much but I read about a guy that had so much trouble with the Buckshots that he went back to stock. LOL We'll see how I make out.... LOL Here are a few pics.

Very nice! lol, any chance you've already taken the carbs apart? Curious what grams springs you have in yours.

 

[etemplet]

No I haven't done anything to this ski at all. I'm not good at figuring out which springs either. I think you need a tester for that. This ski is likely not set up like yours due to all the performance changes. It has a Factory Pipe which itself would require jetting changes.