Low compression on both engines 787 speedster, only 1 with problems.

[lithium2g]

Speedster 1998 Rotax 787. Both original AFAIK.
Hey lads I havn't had my seadoo out for 6 months. There is no such thing as winter where I live just been busy. Engines fired up on the garden hose no problem. Took it out and no problems starting in the water. But after idling out and running low rpm for a minute, upon throttling i realized L side engine wasnt running at all. She wouldnt fire at this point. When cranking it gave some different noises, sounded like back firing. Im not really sure. It started intermittently on the trailer with some more noises sounding like backfire and would shut off.
So im thinking its dirty fuel in the carbs and we need to do a carb rebuild/clean. I've read mikeydac's guide several times in the last few years and i was about to buy the rebuild kits and the gear...i havn't taken the carbs off yet tho. IM prettty nervous attending to this task
Before i went down this path firstly I figured I'd test the engine compression just to rule out engine issues. I don't have the skill, will or the equipment to do engine rebuild here in Australia so I would be calling time on the girl if rebuild is required.
I bought a compression tester today. And watched a video of someone doing this on the tubes. On the dodgy L side engine im only getting 135PSI
So I try it on the good working R side engine both mag and pto is 135PSI also. Im thinking both engines are rooted?? Or my tester is bad. Or my testing method is bad.
When I had this boat professionally serviced 3 years ago they were all reportedly 160PSI.

I read about doing WOT (wide open throttle) when doing compression test, but I don't know what this means.
is that having the throttle maxed out when cranking the engine?

Any advice on how I should proceed?

I've installed/rebuilt/replaced drive shafts, jet pumps and reverse cable all thanks to this forum and its wisdom. Hopefully I can get through of this one.
Thanks

 

[Sandman251]

The compression of your cylinders is fine. I would be looking in a different direction for the problem. Don't let anybody tell you that it's due for a rebuild at 135 PSI.

 

[Poormanracing]

Throw the compression tester away.to many variables that's why sea doo doesn't list any compression numbers.on that motor you can pull the rave valves and look right at the pistons and cyl walls.look for scuffing.or to a leak down on the compression stroke.looking at the rave valve it looks like it is starting to drool unburnt oil.i would pull the raves and have a look.

 

[Poormanracing]

Speedster 1998 Rotax 787. Both original AFAIK.
Hey lads I havn't had my seadoo out for 6 months. There is no such thing as winter where I live just been busy. Engines fired up on the garden hose no problem. Took it out and no problems starting in the water. But after idling out and running low rpm for a minute, upon throttling i realized L side engine wasnt running at all. She wouldnt fire at this point. When cranking it gave some different noises, sounded like back firing. Im not really sure. It started intermittently on the trailer with some more noises sounding like backfire and would shut off.
So im thinking its dirty fuel in the carbs and we need to do a carb rebuild/clean. I've read mikeydac's guide several times in the last few years and i was about to buy the rebuild kits and the gear...i havn't taken the carbs off yet tho. IM prettty nervous attending to this task
Before i went down this path firstly I figured I'd test the engine compression just to rule out engine issues. I don't have the skill, will or the equipment to do engine rebuild here in Australia so I would be calling time on the girl if rebuild is required.
I bought a compression tester today. And watched a video of someone doing this on the tubes. On the dodgy L side engine im only getting 135PSI
So I try it on the good working R side engine both mag and pto is 135PSI also. Im thinking both engines are rooted?? Or my tester is bad. Or my testing method is bad.
When I had this boat professionally serviced 3 years ago they were all reportedly 160PSI.

I read about doing WOT (wide open throttle) when doing compression test, but I don't know what this means.
is that having the throttle maxed out when cranking the engine?

Any advice on how I should proceed?

I've installed/rebuilt/replaced drive shafts, jet pumps and reverse cable all thanks to this forum and its wisdom. Hopefully I can get through of this one.
Thanks


View attachment 60989

Also ground those plug wires.you are asking for trouble leaving them dangling on the cyl head like that.it will spark and if you got fuel and oil comming out of spark plug hole it will go boom

 

[Sunsport]

Even if your gauge isn't accurate, the fact that all 4 are reading the same tells you that you are likely in good shape there. I would start with the carbs - they very likely could be the root cause of your issue with does sound like fuel related - starts on the trailer and runs so so with no load, but not fueling well enough to run with load in the water.

 

[lithium2g]

Its definitely a bit confusing, I've seen several posts before people talking about "oh man your compression is <140, definitly need to look at a top end rebuild soon".
I'll take your word for it for sure tho.

Also ground those plug wires.you are asking for trouble leaving them dangling on the cyl he

ad like that.it will spark and if you got fuel and oil comming out of spark plug hole it will go boom

My bad, i learned of this afterwards.

I would start with the carbs

Yeah im pretty sure this is the issue. I've got a genuine carby rebuild kit coming so hopefully i'll find success.
Cheers lads.

 

[Poormanracing]

Even if your gauge isn't accurate, the fact that all 4 are reading the same tells you that you are likely in good shape there. I would start with the carbs - they very likely could be the root cause of your issue with does sound like fuel related - starts on the trailer and runs so so with no load, but not fueling well enough to run with load in the water.

In the 90s I have had every type of gauge and have had 951 cyl scuffed to all hell and every gauge would read the same all around.ever since then I throw a leak down on it bring cyl passed exhaust port put a strap wrench on pto and check it that way.every engine builder and long time mechanics have inches of dust on there compression gauges.and that's why sea doo only gives leak down numbers.and on the 787 and 951 pop off the rave and see what cyl and skirts look like.if the skirts are scuffed it won't pull enough fuel thru the intake nor blow enough thru the transfers to fire off.hence all of the drool coming out of the rave bellows.

 

[Sunsport]

In the 90s I have had every type of gauge and have had 951 cyl scuffed to all hell and every gauge would read the same all around.ever since then I throw a leak down on it bring cyl passed exhaust port put a strap wrench on pto and check it that way.every engine builder and long time mechanics have inches of dust on there compression gauges.and that's why sea doo only gives leak down numbers.and on the 787 and 951 pop off the rave and see what cyl and skirts look like.if the skirts are scuffed it won't pull enough fuel thru the intake nor blow enough thru the transfers to fire off.hence all of the drool coming out of the rave bellows.

Sounds plausible. I guess in my experience which doesn't sound to be as much as yours, with my 787s and 951s that had issues, with my compression tester, usually one cylinder read much lower or even zero so it has been clear there has been a problem/damage. Anytime, I've had good numbers/even across the cylinders, I have found my engines to be good.

 

[Pedec]

That a cheap compression tester do not trust it. Get a good compression tester it does help diagnose some problems. They are all even so I would guess it ok.

 

[mikidymac]

A decent compression tester should be in every ski mechanicals tool box. It is the quickest way to find a cylinder or piston issue.

580-787 seadoo’s are perfect compression at 150 psi and 120 psi they are done. 951 is perfect at 130 and done at 110.

Ground the plug wires and hold the throttle Wide Open when cranking with a good battery and stop when the needle stops climbing.

Do not add oil to check compression like an automobile.

Don’t trust a junk compression tester.

If all cylinders are dead even but low suspect a bad compression tester before tearing apart the engine.

 

[lithium2g]

hey lads, turns out it was the spark plug needing replacing. feels bad.

 

[petec]

Speedster 1998 Rotax 787. Both original AFAIK.
Hey lads I havn't had my seadoo out for 6 months. There is no such thing as winter where I live just been busy. Engines fired up on the garden hose no problem. Took it out and no problems starting in the water. But after idling out and running low rpm for a minute, upon throttling i realized L side engine wasnt running at all. She wouldnt fire at this point. When cranking it gave some different noises, sounded like back firing. Im not really sure. It started intermittently on the trailer with some more noises sounding like backfire and would shut off.
So im thinking its dirty fuel in the carbs and we need to do a carb rebuild/clean. I've read mikeydac's guide several times in the last few years and i was about to buy the rebuild kits and the gear...i havn't taken the carbs off yet tho. IM prettty nervous attending to this task
Before i went down this path firstly I figured I'd test the engine compression just to rule out engine issues. I don't have the skill, will or the equipment to do engine rebuild here in Australia so I would be calling time on the girl if rebuild is required.
I bought a compression tester today. And watched a video of someone doing this on the tubes. On the dodgy L side engine im only getting 135PSI
So I try it on the good working R side engine both mag and pto is 135PSI also. Im thinking both engines are rooted?? Or my tester is bad. Or my testing method is bad.
When I had this boat professionally serviced 3 years ago they were all reportedly 160PSI.

I read about doing WOT (wide open throttle) when doing compression test, but I don't know what this means.
is that having the throttle maxed out when cranking the engine?

Any advice on how I should proceed?

I've installed/rebuilt/replaced drive shafts, jet pumps and reverse cable all thanks to this forum and its wisdom. Hopefully I can get through of this one.
Thanks


View attachment 60989

My right motor compression is 111 and it's running rough left motor had new positions and rings and is around where your is now. And runs good starts fine carb rebuild is pretty easy top end also. But I'm no mechanic lol. I'm wondering if my compression ISS what's wrong with mine?

 

[petec]

The compression of your cylinders is fine. I would be looking in a different direction for the problem. Don't let anybody tell you that it's due for a rebuild at 135 PSI.

What about 111 -115 area?

 

[petec]

hey lads, turns out it was the spark plug needing replacing. feels bad.

Guess I'll try that too

 

[petec]

A decent compression tester should be in every ski mechanicals tool box. It is the quickest way to find a cylinder or piston issue.

580-787 seadoo’s are perfect compression at 150 psi and 120 psi they are done. 951 is perfect at 130 and done at 110.

Ground the plug wires and hold the throttle Wide Open when cranking with a good battery and stop when the needle stops climbing.

Do not add oil to check compression like an automobile.

Don’t trust a junk compression tester.

If all cylinders are dead even but low suspect a bad compression tester before tearing apart the engine.

My 717 new piston and rings is around 135? And runs good. But one motor is around 111-120 or so both cylinders? I also have a cheap compression tester off eBay?

 

[mikidymac]

My 717 new piston and rings is around 135? And runs good. But one motor is around 111-120 or so both cylinders? I also have a cheap compression tester off eBay?

I would check with a good gauge.
It is a good sign that both cylinders are the same so your gauge could just be running low.

On an accurate gauge 150 psi is perfect and below 120 it's done.

 

[Poormanracing]

On the 800 and 951 pull the rave valves off and look inside the cyl and check the skirts for scuffing.i am not a believer of checking a motors health by a compression test.thats why sea doo never posted a compression number to there motors.a leak down with piston rings above exhaust port will tell you what's up.imo

 

[petec]

I would check with a good gauge.
It is a good sign that both cylinders are the same so your gauge could just be running low.

On an accurate gauge 150 psi is perfect and below 120 it's done.

120 I put in new plugs running starting much better

 

[mikidymac]

120 I put in new plugs running starting much better

If it was me I would keep running it.

 

[petec]

If it was me I would keep running it.

Why would you keep running it? Thanks.

 

[mikidymac]

Why would you keep running it? Thanks.

120 is the minimum but he said it is running good and his gauge accuracy is in question. If it continues to run fine then it is likely his gauge is reading about 15 psi low.