1996 Sea-Doo newly rebuilt 787 having carb problems

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Big Fish

Big Fish

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Just installed a newly rebuilt Sea-Doo GTX 787 in my ski. Ran premix oil for first tank. Ran great initially then after 1/2 a tank the ski won’t go over 3500 RPMs and just sputters along. I rebuilt the carbs, cleaned the rave valves and put fresh 93 octane gas in. Still same problem HELP. What’s next
 
You’ll have better luck with this in the regular 2-stroke section rather than the performance section...

What is it doing at 3500? Bogging? Surging? Have you tried to give it some choke when it happens?
 
At 3500 it just slowly puts along but it sputters a bit almost like it’s going to backfire. When you snap the choke it launches really well for 2 seconds then back down to puttering at 3500. I assumed because of that it means I have a gas / carb problem. That’s why I cleaned the carbs and changed the fuel filters but no change. It starts and idles perfectly. I have never touched the low speed and high speed jets to adjust but do you think that could be the next step?
 
You shouldn’t have to adjust the screws at all. You have either an air leak somewhere in the fuel system, or you have some plugged up passages in the carb still. To rule out the air leak, take the fuel line off of the reserve nipple on the sending unit and run it directly to the carb inlet bypassing the filter and separator. If it runs better like this, you know you’ve got a leak somewhere in that system. If not, I think you’re going to have to dig back into the carbs and make sure they’re absolutely clean.
 
Not for nothing but....The collective wisdom of these forums pretty much dictate that ALL carb rebuilds should only be done with OEM mikuni carb kits.....assuming that maybe that is/was not the case.....bite the bullet and go OEM into the carbs. Did you rebuild/replace the carbs with the new engine or just bolted the old carbs back on???? Air leaks, pinched gasket?????

I too am battling a "carb" problem on my '96 GTX.....runs great on the trailer, won't behave on the water (under load). I used SBT carb kits.....looked and fit fine....but in the carb world....TINY TINY clearences really matter and the OEM kits seem to eliminate "carb" issues.

I'm going back to Mikuni kits for the carb rebuild process (AGAIN !!!! - hahaha).
 
All, thanks for the input all. I did use Mikuni rebuild kits and originallly I did just bolt the old carbs back on. It ran great for about 2 hours of running. I was running a lot of premix oil in the gas as instructed for breakin. After 2 hours or so I started having the current issue. So I guess the air leak could have happened during that 2 hours. I did install an inline fuel filter so maybe there is a leak somewhere at the filter. I will have to try just replacing the gas line with the filter and or bypass the pet cock? Not sure where that is.
 
Given it was running great then began to run lean as indicated by applying choke, I'd say one carburetor is plugged with debris in the tiny passages of the low speed circuit. You really have to clean them well, I always use compressed air to blow the passages out well, a BUNCH of times, not just once. Then I clean them again 2x more. Sometimes it's not easy to get the junk out of this passage. That might be the reason the previous engine failed.
 
Sportster-2001-951C-Stock said:
Given it was running great then began to run lean as indicated by applying choke, I'd say one carburetor is plugged with debris in the tiny passages of the low speed circuit. You really have to clean them well, I always use compressed air to blow the passages out well, a BUNCH of times, not just once. Then I clean them again 2x more. Sometimes it's not easy to get the junk out of this passage. That might be the reason the previous engine failed.
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I really thought I did a thorough job cleaning the carbs but I think I’m not 100% sure where those passages are otherwise I would have made sure. I tried viewing a carb rebuild thread on this and I’m still not 100% sure where those passages are. Also I did not remove the jets and clean in there during the rebuild. Is it possible the blockage is in with the jets? Again originally it was running good then this happened
 
Big Fish said:
I really thought I did a thorough job cleaning the carbs but I think I’m not 100% sure where those passages are otherwise I would have made sure. I tried viewing a carb rebuild thread on this and I’m still not 100% sure where those passages are. Also I did not remove the jets and clean in there during the rebuild. Is it possible the blockage is in with the jets? Again originally it was running good then this happened
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These carbs are particularly stubborn in terms of cleaning, it's my hope to add your name onto the long list of "victims" if this does turn out to be the case.

I like to work over an old white bed sheet so I can see the schmutsa come flying out onto the white cloth. In the below images, this circuit in particular is highlighted in red and green. As you can see, part of the cleaning proces to blow the junk out and clear the passageway will involve removing the low speed jet and the low speed idle mixture adjuster. Feel free to hold your finger over either and/or both ends of the passage while blowing through with high pressure air and carburetor aerosol solvent multiple times, back and forth in both directions and all combinations to wash away the corrosion debris. If you see a big wad of goo come flying out onto your pretty white bed sheet, this is a good thing. Keep going till there is no more and all orifices are unclogged.Mikuni off-idle Transition ports.jpgMikuni Cross Section.jpg
 
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Sportster-2001-951C-Stock said:
These carbs are particularly stubborn in terms of cleaning, it's my hope to add your name onto the long list of "victims" if this does turn out to be the case.

I like to work over an old white bed sheet so I can see the schmutsa come flying out onto the white cloth. In the below images, this circuit in particular is highlighted in red and green. As you can see, part of the cleaning proces to blow the junk out and clear the passageway will involve removing the low speed jet and the low speed idle mixture adjuster. Feel free to hold your finder over either and/or both ends of the passage while blowing through with high pressure air and carburetor aerosol solvent multiple times, back and forth in both directions and all combinations to wash away the corrosion debris. If you see a big wad of goo come flying out onto your pretty white bed sheet, this is a good thing. Keep going till there is no more and all orifices are unclogged.View attachment 41521View attachment 41522
Click to expand...
Wow thanks these pics look very clear now to me. I definately sprayed carb cleaner in there but definately didn’t check to see if it was flowing out anywhere. Once I get the carbs off would you also suggest I remove the hi and low speed jets and clean in there as well before reinstalling? I understand the procedure for that is to mark the turns to fully closed before removing so I can set them back the same # of turns when reinstalling. Does that make sense to do also?
 
You definitely will do a much better job of cleaning out the debris by removing the low speed jet. Removing the high speed jet also isn't a bad idea, however, the high speed circuit rarely is the problem due to it's large diameter, corrosion debris has difficulty stacking up into a clog.

Count the number of turns to fully closed, this might be 1 1/2 turns for instance. Closed means lightly yet firmly seated using finger pressure, not cranked down hard with a large screwdriver. The mixture adjuster screw specs you must locate in the documentation specific to your model and engine are the number of turns open from the fully closed position.

About 70% or more of the total fuel consumed, is sucked by engine vacuum through the low speed circuit of the carburetor.
 
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Sportster-2001-951C-Stock said:
Just FWIW, if you were to fail at restoring your carburetor, you wouldn't be the 1st to exeprience this. There are scores of others who've finally replaced their original Mikuni's with spanking new and resolved their running issue in spades.
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Any suggestions on where to buy replacement carbs if it gets to that point, do you buy in pairs and is there anyway to know which one is the issue so you only buy one? I hope I. D.o.n.t get there because I know they must be expensive. This is a 23 year old ski so likely cost more than the ski is worth.
 
You can buy brand new carbs from OSD, or you can buy cleaned and refurbished sets on eBay... They actually aren’t as bad as you would think. Refurbs go for about $250 for the set, and new carbs run $420 for the pair.
 
Sportster-2001-951C-Stock said:
Just FWIW, if you were to fail at restoring your carburetor, you wouldn't be the 1st to exeprience this. There are scores of others who've finally replaced their original Mikuni's with spanking new and resolved their running issue in spades.
Click to expand...
This weekend I hope to pull the ski and dig the carbs out again. I saved all the original Mikuni parts so I go all the way backwards to the original parts and make sure everything is squeaky clean. The ski ran great last season after only cleaning the carbs, roughly in the yard just spraying carb cleaner through them. I had hoped that doing full rebuilds this past winter on the bench would have been bulletproof: but alas.....no dice. She starts and runs fine on the trailer but bogs on the water with throttle. I’ll get her sorted out, along with 12V low message and a water pressure regulator leak ( just found).
 
I know it’s a pain to keep having to go through them... I have eight of these carbs between my two skis and the boat, and of all of them, I had one set that I just couldn’t get to act right. I wound up giving up on them and buying a refurb set and I’ve had no issues since.
 
Big Fish said:
Any suggestions on where to buy replacement carbs if it gets to that point, do you buy in pairs and is there anyway to know which one is the issue so you only buy one? I hope I. D.o.n.t get there because I know they must be expensive. This is a 23 year old ski so likely cost more than the ski is worth.
Click to expand...

Yes, OSD. But you had the engine running great, so I have very high confidence you can get them cleaned out. Also, make sure there are no fuel line air leaks, if the fuel pump is sucking air from a leak instead of fuel, the engine will hesitate.
 
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