1996 GTX Backwards Firing after Rebuild?

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BoaterCycle123

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Hello All,

Long time reader first time Poster! I have a 1996 Seadoo GTX 787 that had a rusted crank when I bought it. I have completely stripped it down, used a separate parts engine for the new crank and mixed match different pieces. Jugs were bored to 82.25mm. RAVE valves were rebuilt. New Pistons and Rings as well as a gasket kit. The build is complete, but when trying to fire it, it looks like the engine is trying to fire backwards. I only assume this from watching the grease nipple on the PTO. I have tripled checked the Rotary Timing, but feel like it may still be something there? There was a nice pretty mark on the case of where to line up the plate. I confirmed that this was the correct spot by lining up a protractor from the bottom of the MAG side intake. It has been lined up. Is there somewhere else I may have goofed the timing? I should be okay to switch the magneto gear, correct? The timing isnt specific to Crankshaft on those?

Other reason I think it is firing backwards is because I am sure it blew the starter after trying to start it. It makes a horrendously sound clank when trying to turn over.

Any help would be appreciated. I am pretty confident that it isnt rotary timing. But I've been wrong before. I'm extremely mechanically inclined, so nothing is typically out of my ability.
 
The mark in the cases isn’t the correct mark.

You can print the degree wheel from the manual then follow the manual procedure exactly. The MAG piston has to be at TDC.
 
As mentioned above, degree wheel and by the service manual, TDC on pto cylinder, etc. Miki mispoke, not TDC on the mag cylinder.

I'll add, may want to check the rotary valve clearance, refer to the SM, recommend the solder method. I'm doing a second 787 rebuild and both were way out of spec.
 
GGuillot said:
As mentioned above, degree wheel and by the service manual, TDC on pto cylinder, etc. Miki mispoke, not TDC on the mag cylinder.

I'll add, may want to check the rotary valve clearance, refer to the SM, recommend the solder method. I'm doing a second 787 rebuild and both were way out of spec.
Click to expand...
Miki was correct, TDC Mag piston
 
I'll check those tonight, I cut out the paper rotary wheel and was checking it per a video I saw. It lined up with the mark. I cannot find a degree wheel cutout in the SM. Does anyone have a link for that?

And SM calls out TDC on Mag cylinder. I'll check clearance. That's the only thing i did not check. Is it easy?

Also, I checked another 787 sitting on the same trailer, which runs great so far (no water test yet on it) and it is at the same degree. It fires without trying to go the opposite way.
 
BoaterCycle123 said:
I'll check those tonight, I cut out the paper rotary wheel and was checking it per a video I saw. It lined up with the mark. I cannot find a degree wheel cutout in the SM. Does anyone have a link for that?

And SM calls out TDC on Mag cylinder. I'll check clearance. That's the only thing i did not check. Is it easy?

Also, I checked another 787 sitting on the same trailer, which runs great so far (no water test yet on it) and it is at the same degree. It fires without trying to go the opposite way.
Click to expand...

You mentioned a 'horrendous clank' when starting. Not trying to scare you, but the rotary valve shaft gear may have stripped. It's been known to happen.

This would explain the backfire issue if the timing is dead on like you say. The other thing I'd still check is the RV clearance.
 
It's most likely the ignition timing is out. Did you remove the magneto cup from the flywheel at any time during the rebuild?

Chester
 
Chester said:
It's most likely the ignition timing is out. Did you remove the magneto cup from the flywheel at any time during the rebuild?

Chester
Click to expand...
I sure did. I thought the key way lined that up. How is that supposed to be reinstalled? This sounds like an issue I have not covered...
 
It's not about how you put the flywheel on the crankshaft, it's about how you put the cup on the flywheel. The end of one of the protrusions on the cup (rotor) must line up with the hole in the flywheel for proper ignition timing. If you use the proper puller, you never have to remove the mag cup from the flywheel.

Chester
 
Thanks Chester. I did not remove the cup from the flywheel. I did use a proper puller to take that off. So I like to think I'm safe there. However, the flywheel and mag cup may be from the parts engine. Would that cause a timing issue? I just used the flywheel that had better teeth.
 
BoaterCycle123 said:
Thanks Chester. I did not remove the cup from the flywheel. I did use a proper puller to take that off. So I like to think I'm safe there. However, the flywheel and mag cup may be from the parts engine. Would that cause a timing issue? I just used the flywheel that had better teeth.
Click to expand...

On the flywheel cup there's two opposing magnets on the outside, they're like 1" x 1/4". The small hole on the flywheel itself is what you're verifying, one of the magnets directly near it, either one is fine.


I'd remove the mag cover and double check, you should be able to do that with the engine still in the ski. May need to get the exhaust pipe out the way and support the engine to accomplish.

The only other thing that could cause a timing issue is the rotary valve timing being off or the shaft gear stripped. Another thing that could cause issues is rotary valve clearance being out of spec, it causes scavenging between cylinders. I'd at least check it, fairly easy to accomplish, couple pieces of solder, tighten the cover down then remove and measure the solder thickness.

If you haven't already, I'd double check RV timing with a degree wheel, fairly cheap to purchase online.
 
Last edited:
GGuillot said:
On the flywheel cup there's two opposing magnets on the outside, they're like 1" x 1/4". The small hole on the flywheel itself is what you're verifying, one of the magnets directly near it, either one is fine.


I'd remove the mag cover and double check, you should be able to do that with the engine still in the ski. May need to get the exhaust pipe out the way and support the engine to accomplish.

The only other thing that could cause a timing issue is the rotary valve timing being off or the shaft gear stripped. Another thing that could cause issues is rotary valve clearance being out of spec, it causes scavenging between cylinders. I'd at least check it, fairly easy to accomplish, couple pieces of solder, tighten the cover down then remove and measure the solder thickness.

If you haven't already, I'd double check RV timing with a degree wheel, fairly cheap to purchase online.
Click to expand...

Thanks G.

I will be digging into all of that tonight. I will post tonight if more questions pop up!
 
Solved!!!

You guys were right. The parts engine we bought had the Mag cup in the wrong place!! It was fixed and started right away! Good call by you guys!

Just needs some Carb tuning. I know it's hard to run these out of water due to temps and prop spinning faster with little resistance. And I am sure there is a thread for this. Can someone post the link or tell me best way to tune the dual carbs on these?

Thanks Again guys!!!
 
If it’s stock there is no tuning. Stock Seadoo’s run great with stock settings. If it doesn’t something isn’t right.
 
I'm not sure this hasn't been messed with before! I need to know the best way to set the screws at stock. When starting, it currently runs at 3500 rpm, where the non rebuilt one only runs 2200ish. And when throttling the rebuilt one, there is no change in RPMs. That tells me something is off.
 
BoaterCycle123 said:
I'm not sure this hasn't been messed with before! I need to know the best way to set the screws at stock. When starting, it currently runs at 3500 rpm, where the non rebuilt one only runs 2200ish. And when throttling the rebuilt one, there is no change in RPMs. That tells me something is off.
Click to expand...
Does this help?

06e2f6caa76a06d9626bf7fb324a36a1.jpg
 
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