2000 GTX top end rebulid -failure

[wlong1957]

After rebuilding the top of my 2000 GTX it would not start.
My compression guages are not the best. But the PTO cylinder was improved and the mag cylinder unchanged. They both held pressure as well.

I ran premix down the supply hose to the carbs. Also verified the pump was pulsing when cranking. There appears to be spark at the new plugs. All I get is intermittant back fires. Loud enough to sound like a shotgun going off. And it will blow apart the rubber hose on the exhaust.

Question could the crank shaft be damaged throwing timing off? I am about done with this 23 year old seadoo. I paid $1000 in 2010 and cant justify a rebuild SBT or similar engine. Plus the time

Any advice would be appreciated.

 

[tobytd]

Check mag cup position

 

[Ckrawiec]

After rebuilding the top of my 2000 GTX it would not start.
My compression guages are not the best. But the PTO cylinder was improved and the mag cylinder unchanged. They both held pressure as well.

I ran premix down the supply hose to the carbs. Also verified the pump was pulsing when cranking. There appears to be spark at the new plugs. All I get is intermittant back fires. Loud enough to sound like a shotgun going off. And it will blow apart the rubber hose on the exhaust.

Question could the crank shaft be damaged throwing timing off? I am about done with this 23 year old seadoo. I paid $1000 in 2010 and cant justify a rebuild SBT or similar engine. Plus the time

Any advice would be appreciated.

A backfire to me could mean the exhaust can't get out the way it's supposed to....I "think" that could mean the exhaust valve is not opened/opening in sync with the combustion process.....since these engines use a rotary valve plate for intake/exhaust....I'd check that part of the system.

OLD 2 stroke seedoos are a labor of love...AND always frustrating. I have a '96 that drives me crazy but I can't get rid of it....I am so freakin stubborn that I WILL that MF to run....lots of time wasted - YES.....but when she does run, she runs! It is my favorite ski though.....

 

[wlong1957]

Check mag cup position

Could you let me know how to check the mag cup position

Thanks

 

[wlong1957]

A backfire to me could mean the exhaust can't get out the way it's supposed to....I "think" that could mean the exhaust valve is not opened/opening in sync with the combustion process.....since these engines use a rotary valve plate for intake/exhaust....I'd check that part of the system.

OLD 2 stroke seedoos are a labor of love...AND always frustrating. I have a '96 that drives me crazy but I can't get rid of it....I am so freakin stubborn that I WILL that MF to run....lots of time wasted - YES.....but when she does run, she runs! It is my favorite ski though.....

Could you let me know how to check the exhaust valve

I am a novice. Felt accomplished replacig pistons and rings and gaskets. Any advice is appreciated

 

[tobytd]

2000 GTX has a 951 engine. No rotary valve. No valves. Two-strokes exhaust when the piston travels below the exhaust port.
carbs must be right. sounds like it may not be delivering fuel correctly. Leaky needle seats? Excess fuel in crankcase?
might also check reed valves.

 

[wlong1957]

2000 GTX has a 951 engine. No rotary valve. No valves. Two-strokes exhaust when the piston travels below the exhaust port.
carbs must be right. sounds like it may not be delivering fuel correctly. Leaky needle seats? Excess fuel in crankcase?
might also check reed valves.

I put a cap full of premix in each cylinder no fire. I put premix in the carb throat no fire. But after trying to start has a huge backfire and blew off the exhaust rubber hose.. Also disconnected the fuel line and put a funnel on the line to carb. disconnected the pulse line on the front of the mag carb. It pumped while cranking. The rave valves help when applying vacuum to each valve and release when letting off vacuum.

If it was a carb issue wouldnt it at least sputterred.

 

[tobytd]

Rave valves are moved by pressure not vacuum. One way check valve turns crankcase pulse onto pressure and supplies rave solenoid

 

[Ckrawiec]

2000 GTX has a 951 engine. No rotary valve. No valves. Two-strokes exhaust when the piston travels below the exhaust port.
carbs must be right. sounds like it may not be delivering fuel correctly. Leaky needle seats? Excess fuel in crankcase?
might also check reed valves.

I always screw up on WHAT engine is in these things.....I'm stuck in the 90's....nevermind me. Thanks for the enlightenment!

 

[tobytd]

Post #1 : PTO cyl improved, MAG unchanged. Did the ‘rebuild’ include NEW oversized pistons and rings?
Fresh bore, hone and port chamfer?

 

[etemplet]

You are guessing if you didn't check spark. a few things to look at.... Are your wires connected properly? Did you install new spark plugs? You could have flooded the crap out of the engine...that's easy to do when pouring in premix (that's why I use the taboo starting fluid ) . When you did the top end did you check your carbs (pop off ?) Did you determine why you needed a top end? You may have a carb issue especially if you haven't rebuilt them in 10 years? You can also have fuel delivery problem. Simple checks.

Personally I don't think the two strokes are a labor of love or a pain in the butt at all. You give them what they need and they'll reward you with reliable service and they are twice as cheap and more fun to me than the heavy 4-strokes. Buy a set of these things and check your spark. Good Luck

 

[wlong1957]

Post #1 : PTO cyl improved, MAG unchanged. Did the ‘rebuild’ include NEW oversized pistons and rings?
Fresh bore, hone and port chamfer?

I put in new standard 88mm pistons,rings and gaskets. The original pistons were chipped with the rings broken.

The original cylinders were smooth with no damage.so I left them alone. I put the pistons on the rods with new bearings that came in the kit. The new pistons felt like they were making good compression when oiled.

My compression guage showed and increase in compression compared to what it was initially. But then my guage broke. I tried ether spray and it didnt do anything.

Gas premix poured into the plug holes with new plugs did nothing

I verified that there is spark to each plug when cranking.

With gas and starting fluid after multiple turnovers all I get is a loud backfire which blew off the hose to the exhaust.

I took off the rave valves and the rings wre intact as far as I could see.

Ill post pics of the original pistons.

I appreciate your input.

Thanks

 

[wlong1957]

You are guessing if you didn't check spark. a few things to look at.... Are your wires connected properly? Did you install new spark plugs? You could have flooded the crap out of the engine...that's easy to do when pouring in premix (that's why I use the taboo starting fluid ) . When you did the top end did you check your carbs (pop off ?) Did you determine why you needed a top end? You may have a carb issue especially if you haven't rebuilt them in 10 years? You can also have fuel delivery problem. Simple checks.

Personally I don't think the two strokes are a labor of love or a pain in the butt at all. You give them what they need and they'll reward you with reliable service and they are twice as cheap and more fun to me than the heavy 4-strokes. Buy a set of these things and check your spark. Good Luck


View attachment 61201

You are guessing if you didn't check spark. a few things to look at.... Are your wires connected properly? Did you install new spark plugs? You could have flooded the crap out of the engine...that's easy to do when pouring in premix (that's why I use the taboo starting fluid ) . When you did the top end did you check your carbs (pop off ?) Did you determine why you needed a top end? You may have a carb issue especially if you haven't rebuilt them in 10 years? You can also have fuel delivery problem. Simple checks.

Personally I don't think the two strokes are a labor of love or a pain in the butt at all. You give them what they need and they'll reward you with reliable service and they are twice as cheap and more fun to me than the heavy 4-strokes. Buy a set of these things and check your spark. Good Luck


View attachment 61201

I Verified spark with each plug close to groud when turning over. I could see the spark on the plug. Each wire was labled Mag and PTO so as not to mix up. Installed brand new ngk br8es plugs. Tried to start with starting fluid first, then a cap of premix in each cylinder, then premix into each carb throatt, and finally premix again. No Luck. Then I disconnected the fuel line to the carbs. Attached new fuel hose and fed with premix to a funnel. Still no start.

Didnt check carb pop off as I am unsure what that is. Pistons definitely need replacement. See the attached pics.

 

[tobytd]

Piston damage is from LEAN fuel mixture. WILL happen again without fuel system rebuild.
Look for Mikidymac’s carb overhaul sticky.

>forums<. >2 stroke Seadoo PWC forum <

 

[wlong1957]

I plan on getting new compression guage before the weekend and recheck each piston. If good then do you think my no start is a fuel /carb issue?

Shouldn't it at least sputtered or fired up with gas premix in each cylinder or starting spray?

 

[tobytd]

Auto Zone etc will ‘rent’ a gage.

 

[mikidymac]

1. I can tell you that with that piston damage and not boring the cylinders oversized and new oversized pistons you are never going to get good enough compression numbers to run correctly. Can't just slap in new stock sized pistons on worn out cylinders.
2. 951's with used crankshafts typically do not survive new top-ends only.
3. Your piston damage was from running lean caused by bad carbs and fuel system. No point in doing a top end without fixing the cause.
4. Most backfires on these are caused by too much fuel in the exhaust from flooding, leaking needles and seats.

Sorry, but you are going to have to dump a lot more money into this thing to fix it correctly.

 

[wlong1957]

Thanks for the reply.

I bought a new compression gauge yesterday. Ill test compression this weekend. If poor then I'll just disassemble and part it out as it is 23 years old now and look for a newer model. I

 

[etemplet]

Compression isn't as important as the bore size and condition of the cylinders. New pistons and rings are gonna bump compression up unless you have a serious problem.

 

[Poormanracing]

Did you lube everything good when you put back together?alot of times after a rebuild these puke and bang thru the exhaust if you go nuts with the oil during reassembly.you could be blowing the flame out trying to start it and all that unburnt fuel or vapors are going into exhaust.

 

[wlong1957]

s

1. I can tell you that with that piston damage and not boring the cylinders oversized and new oversized pistons you are never going to get good enough compression numbers to run correctly. Can't just slap in new stock sized pistons on worn out cylinders.
2. 951's with used crankshafts typically do not survive new top-ends only.
3. Your piston damage was from running lean caused by bad carbs and fuel system. No point in doing a top end without fixing the cause.
4. Most backfires on these are caused by too much fuel in the exhaust from flooding, leaking needles and seats.

Sorry, but you are going to have to dump a lot more money into this thing to fix it correctly.

After buying a new gauge set. There is minimal compression with new pistons and rings in the old cylinder. So now I figure I have 3 choices
1. Sell or give away for parts.
2 replace with a rebuilt SBT engine if available
3. remove engine and rebuild top and bottom completely


Overall this was a good learning experience. t But I'll have to think it over about which option to take. As all we really need is a ski to just ride around. Doesn't have to go fast. Just fast enough to keep the gnats off our face

Thanks to all for the input

 

[etemplet]

s


After buying a new gauge set. There is minimal compression with new pistons and rings in the old cylinder. So now I figure I have 3 choices
1. Sell or give away for parts.
2 replace with a rebuilt SBT engine if available
3. remove engine and rebuild top and bottom completely


Overall this was a good learning experience. t But I'll have to think it over about which option to take. As all we really need is a ski to just ride around. Doesn't have to go fast. Just fast enough to keep the gnats off our face

Thanks to all for the input

What size are the pistons? Stock bore?

 

[wlong1957]

Yes. Standard 88mm. I was hoping since that the cylinders appeared in damaged that all that was needed was pistons and rings

 

[etemplet]

Well you can measure the cylinders and determine if they are within tolerance. I suspect they'll be worn out though. I always go up (.5mm) using WSM pistons. It is easier to clean the bores up completely with the larger size. I've done top ends on 951s with the engine in the ski. Not easy but doable and you don't have to take so much apart.

Removing the engine makes it much easier to work on and certainly you can do more checks. Good luck whatever you choose.