96 xp cranks but does not fire

[myclubsoda]

Hello evryone,

I just bought a 96 xp with the 787 engine. The fellow I bought it from informed me that the motor was overhauled a year and a half ago. I can see new gaskets through-out the engine so I believe him.
When I Made an initial inspection I found the 7.5 amp fuse in the coil box blown. The short turned out to be in the vts module, this due to water ingress through the boot of the cable. So, I have the vts system disconnected at this time.
Here is my situation, engine will crank and seems it wants to start but won't. At times it will momentarily fire (1-2 sec). If I let it sit for about 30 sec or so and then crank, it will fire and run for about 5 seconds. When it fires There is some intermittent pops, perhaps a lean mixture, then it either stalls or runs at very low idle like its not getting enough fuel and finally stalls.
Here is what I know, I have fuel in the tank, my filler cap is loose, my in-line filter is clear and full of fuel.
I am new to the world of water craft and any help would be greatly appreciated. Thanks,

 

[seadoosnipe]

Rotary valve....

With the informtion in your post and it's newly rebuilt engine, I'd be pointing my finger at the rotary valve. If it's one or two teeth off, this would be the result.
Did the guy you got it from say whether or not it ever ran smoothly after the rebuild? The running for severl minutes after it cools from trying to start, is that it's running off the fuel and oil that was dumped and cooled from the last time it was trying to start.
I'd hate to see you have to do it, but I'd be checking the rotary valve. You'' have to remove the carbs to get to it. Then, you have to bring the mag piston up to TDC, then use a degree wheel to align the valve plate to it's proper position. It's not really that hard. But for me, it's a lot easier because mine is in a boat. More room for me to get in the boat and work on it.

I was looking for some information on your Mikuni carbs and whether or not you have the accelerator pump set up. There is a possibility that this can cause the same type problem. But I think it may be a bit remote. Because it's easier to check the carbs for proper fuel delivery, I'd make sure that the carbs were working correctly before diving into the rotary valve. But if you have any popping or pinging in the motor, that's probably going to be the problem.

 

[myclubsoda]

The story goes like this, He bought the ski with the engine already overhauled. He used it for the season, if there is such a thing here in CA, and stored it. Keep in mind that the temperature here during the winter is about 40 degrees. When he stored it, it would not idle ,however, he had off idle performance. He thougt his issue was with the blown 7.5 fuse. I found out that it's not, that fuse powers the vts. I noticed that the vts module has a wire that runs to the grey box that houses the engine module. I don't know whether this wire supplys some kind of signal that is needed.

 

[myclubsoda]

Here is an update,


motor will turn over and fire, however, it feels like its running on one cylinder.
It will rev up, at this point it feels okay.
At idle it back fires through the exhaust. Also, the mag side cylinder is hot to the touch and the pto side cylinder is not hot at all, this after I had it running a few minutes. I didn't have a compression guage with me today so I did the following.
As I cranked the motor I put my finger on the plug holes. The pto side cylinder had strong compression and suction on the other hand the mag side cylinder had very little compression and no suction. I was able to overcome the compression with my finger. Could this still be a rotory valve timing or perhaps a top end issue. Any help would be appreciated, thanks.

 

[seadoosnipe]

Either or....

It can still be either or, but does sound more like an issue with the top end, piston rings.
But if the rotary valve is not where it's suppose to be, then when the piston travels down on it's stroke, where the valve should be open, it could be closed, or partial, that would keep the piston from being able to suck in any fuel or air to build pressure.
The rotary vavle is kinda like your intake valve on a car. It the intake valve doesn't open, it's not going to get any air or fuel. There fore, no compression.
You said the cylinder seemed dead due to the heat. Have you tried to test the plug for fire? You can remove the plug, and lay it against the motor, with a "DRY" piece of wood or an electrically insulted tool, and turn the engine over, see if there is spark.
One more thing. Even if there is no compression on that cyliner, it would still start. Compression truly only affects the engines' torque. Say, if you had 75 pounds on one piston and 40 on another, the ski would still start. But as soon as you put some torque foot pounds on it, then engine would die. There is no way for the engine to make any power. There's no compression for that to happen. But out of water and no resistance on the shaft, it would still run.
I'm still leaning on the rotary valve. Check the plug for spark like above, then let me know what happened. And remember, just because your not building pressure in that cylinder, it may not mean the rings are bad. Not till you know that rotary is on TDC.......:cheers:

 

[robin savell lloyd]

Sound like top end is shot I have seen 2 cycles over the past 20 + years with low compression just not able to start. 4 cycles seem to need less to start. look into low compression and it's cause before moving forward. good luck. robin

 

[seadoosnipe]

Not always true...

Under normal circumstances Robin, this would be true. But if you read from the beginning of the post, you'll see where he is getting some "pop" back in his ignition.
If the rotary valve is not in time, that can lead to the "pop" or backfire conditions that he seems to be having.
The other thing, you won't get a true and correct compression test if that valve fails to open on the intake/comression storke like it is suppose to. That's why I think it's in the rotarty valve. Hell, for that matter, the rotary valve my may have lost it's lock on the shaft and is skipping teeth as it turns.
But if it is, then he'll also be able to see the rings, because that would require a break down.
I'd say check the rotary, if it's out of position, put it back. Then if after a while, he's faced with the same running conditions, then I'd point my finger at the gear to crankshaft connection............

 

[myclubsoda]

Thanks guys,

I do have spark on both cylinders. Its hard to start, however, when it starts it looks like its running on one cylinder. When I accelerate, it pops (backfires)
once the revs get going it seems okay. One time it really back fired hard when I started to crank it. I will check the rotory valve timing to start. I will post my findings, Thanks Again.

Gary

 

[robin savell lloyd]

Under normal circumstances Robin, this would be true. But if you read from the beginning of the post, you'll see where he is getting some "pop" back in his ignition.
If the rotary valve is not in time, that can lead to the "pop" or backfire conditions that he seems to be having.
The other thing, you won't get a true and correct compression test if that valve fails to open on the intake/comression storke like it is suppose to. That's why I think it's in the rotarty valve. Hell, for that matter, the rotary valve my may have lost it's lock on the shaft and is skipping teeth as it turns.
But if it is, then he'll also be able to see the rings, because that would require a break down.
I'd say check the rotary, if it's out of position, put it back. Then if after a while, he's faced with the same running conditions, then I'd point my finger at the gear to crankshaft connection............

This is all new to me rotory valve so I can be wrong. I would thank that being that it has compression on one cylinder it should on the other even if it had to get a litle air from exhaust and compress it. I know it sounds stupid but when I can not find logic I will take the stupid and try to make it seem logical.:ack:

 

[seadoosnipe]

Good point...

No, that's actually a pretty good point. I did think of that, but then, looking at my own rotary, with the degree of the valve, there is some point when one of the chambers is closed and one open, then just a small amount of time when neither are open. It's not much, but I'm wondering if it is enough to keep his intake stroke from being able to suck in any air.
What your saying has a lot of merit though.
He is backfiring and in the manual, that is a result in the troubleshooing guide to the rotary being out of time. I do know, that when the compression is bad, you can still get a motor to start, but it will die as soon as you put a bit of shaft torque on it. He does say, there is popping and backfiring.....
So, I think it's the rotary, but I wouldn't discount the fact that he may have a dead cylinder too.............good point Robin.

 

[myclubsoda]

Update on xp

I checked the timing, it was okay. I went ahead and removed the top end and found the mag piston rings broken and the cylinder liner damaged. It looks like the engine ran hot and deformed the piston.
My question is, how much can I Bore the cylinder before I need to replace it?
The piston that came out is 1.0mm over. Any other suggestions that may be relevent to this job would be greatly appreciated.

Gary

 

[robin savell lloyd]

You need to find cause of damage or it will hapen again photos of piston, cylinder and head will help us give you an idea what hapened. good luck Robin

 

[Dennis]

Bad Compression

Hi,
I think everyone has had a valid point. I'd like to add something that hasn't been brought up yet. A SeaDoo 2 cyl. carb engine that has 1 cyl. with little or no compression may not start at all or run very little because of this reason......
the carb fuel pump uses pulse from one of the cylinders to operate. If the pulse line for the fuel pump is going to the cylinder that has little or no compression there will little or no fuel pump operation.
Low compression = low pulse = low pump operation = little or no fuel in carb/carbs = little or no running. Thanks, DAWG

 

[robin savell lloyd]

that is a good point dawg i have never thaught of it like that I have two questions for you dawg are the crank cases sealed from each other? one side for each cylinder on a 2 cylinder? thanks robin:hurray:

 

[seadoosnipe]

Very good point Dawg!....

Yes, since the carbs work on pulse from the MAG piston, then if that piston is low or a complete loss of compression, then the carbs won't get any fuel. Very good point!......and yes Robin..........the cylinders are seperated at the rotary valve chamber.

 

[Dennis]

Hard Startin'

Hi ROBIN,
Yes, each clylinder is seperate from the other. That is why there are internal crankshaft seals. They also keep the rotary gear oil from entering the crankcase area. I simplify it in my mind(dangerous place) by looking at each cylinder as a seperate engine.
DAWG

 

[Dennis]

Sorry Seadoosnipe

Sorry Louis,
We were both whacking on the keyboard at the same time again!!!!!!!!
DAWG

 

[seadoosnipe]

But.....

But I bet with each beer, his engineering mind is trying to figure a way for internal porting to work, so the pto carb finds it's own pulse for fuel!....:rofl:

 

[myclubsoda]

I will take some pictures and post them. In the meantime, what do you guys think about boring over 1.0mm. What about the rave valves, would they have to be cut as well? Thanks for all your help!

 

[seadoosnipe]

....probably...

You probably will have to shave if you bore over. But consider that you can get a new under end, cylinders and pistons, for about $400. That's for both!....what's the boring job gonna cost?

 

[myclubsoda]

pictures of the pistons

Here are some pictures of the mag side piston. The plan was to bore out the holes to 1.5mm. The cost of the boring job is about $120. plus the cost of the pistons. I believe I need to shave my rave valves as well; can anyone confirm this? And if I do, do I shave the valves on the half moon side.
seadoosnipe mentioned I can get a new under end, cylinders and pistons for about $400. What is an under end? seadoosnipe, if you read this, can you elaborate a bit more? The name and phone to the place I can purchase these items would be nice as well. Thanks

Gary

 

[robin savell lloyd]

more photos please of cylinder and botom of head.:cheers: :coolgleam: :cheers:

 

[seadoosnipe]

Upper end.....

Sorry Gary....that was a typo....I meant upper end. The upper end rebuild is the cylinders, pistons and rings.
Sorry, I usually proof read after I type but just got to busy. I type about 75 wpm, so it's like I'm talking in my head as I write to ya'll........sorry.
That piston looks pretty nasty. What does the other one look like? It looks like an extreme case of "detonation"...........here's a link, you'll notice your piston near the bottom of the page....http://www.theultralightplace.com/pistons.htm

 

[myclubsoda]

Thanks seadoosnipe

The other piston is okay. It does have some pitting on the top, but the rings are okay. I will post more pictures tomorrow. Any feed back you can give on the rave valves concerning shaving them after boring the sleeves? Thanks

Gary.....by the way thanks for the link

 

[seadoosnipe]

Sorry Gary....

Sorry Gary....I forgot to answer that one for you.
Yes, you'll have to machine your RAVE too. But if your boring 1.5mm, then you'll only machine the RAVE .75mm.