Slow to build RPMs, and needs to unload pump to go past 5200

[TLH101]

Suggestions for tuning this thing: '97 GTX 787.
Has a very slow climb in RPMs, runs up to 5200rpm and no more. If I release the throttle and hammer it quick, or hit a small wake to unload the pump, jumps right up to max RPM. If I let it drop below 5K, have to trigger it quick to rpm back.
Carbs were rebuilt with Back to OEM w/ OSD kits. Pop-off set at 30psi (lowered from 40 to get a transition). Carb low speeds set at 1 turn out, and tried every position to 2 turns at 1/8 turn increments. Hi set at 0 on Mag, 1/2 on PTO. opened both highs an additional 1/4 turn
Changed rectifier.
Tried RAVEs set at top and bottom.
New spark plugs, gapped correctly.
Bypassed fuel valve.
Tried 2 different sets of carbs.
Checked to make sure correct impeller.
Compression 145 both cylinders.
Pressure tested engine.
What am I missing?????

 

[mikidymac]

Does your ski have the smooth or slotted rave valves?

 

[TLH101]

Smooth.

 

[etemplet]

What's your pop-off setting (I found 36-38psi to work best on both my 787 GTXs)? Is the needle lever level with the top of the carb body (not sure the correct name for that ) ? Looking into the carb barrel, are the three holes spraying nice streams when tested? Also the larger hole just to the left of the 3 holes should flow nicely.

The 97 was known to have transition issues and my wife's had those very problems you are having. I did replace the rave valves with slotted. Here is a tecnical bulletin on the problem. One of the members here shared it sometime ago so don't thank me.

 

[AKnarrowback]

Yes, I keep that TSB on hand and send it out for the 97 gtx 5k problem.

I ordered the rave housings but used a fine grit disk in a 4 inch grinder to slot my rave valves. Not the most high tech machine work, but I took my time and polished things after "milling" the grooves and the old gtx has only hung up at 5k one time in the two years since I did the mod. It used to do it ALL THE TIME on every ride.

I did a jb weld job on the original housings to fill the gap the new housings have cast in place. I only ran them for 15 minutes to make sure the different design would make a performance difference before ordering the factory units.

It's worth doing the parts upgrade.

The way to get it above 5k was to let off then nail it where it would go to full rpm's. But a gradual increase would hang it up, a gradual decrease from above 5k would also lock it in at 5k. Really irritating.

 

[GGuillot]

Suggestions for tuning this thing: '97 GTX 787.
Has a very slow climb in RPMs, runs up to 5200rpm and no more. If I release the throttle and hammer it quick, or hit a small wake to unload the pump, jumps right up to max RPM. If I let it drop below 5K, have to trigger it quick to rpm back.
Carbs were rebuilt with Back to OEM w/ OSD kits. Pop-off set at 30psi (lowered from 40 to get a transition). Carb low speeds set at 1 turn out, and tried every position to 2 turns at 1/8 turn increments. Hi set at 0 on Mag, 1/2 on PTO. opened both highs an additional 1/4 turn
Changed rectifier.
Tried RAVEs set at top and bottom.
New spark plugs, gapped correctly.
Bypassed fuel valve.
Tried 2 different sets of carbs.
Checked to make sure correct impeller.
Compression 145 both cylinders.
Pressure tested engine.
What am I missing?????

I'd check timing...

 

[etemplet]

I would ride the ski in circles and create waves then jump them to unload the impeller and it would hook up. LOL The shit you gotta do sometimes. LOL

 

[TLH101]

Thanks guys, I'll take a look at the RAVEs.

 

[TLH101]

I'd check timing...

I was wondering about that, as engine came from a different ski, so mpem timing could be out. Is there any way to check and set timing without a BUDs?

 

[GGuillot]

I was wondering about that, as engine came from a different ski, so mpem timing could be out. Is there any way to check and set timing without a BUDs?

You'd need a buds or candoo pro to fine tune it electronically in the MPEM, but it's only a few degrees either advanced or retard. Mechanically, at the Rotary Valve you can ck it. Then you can ck it with a timing light. You'd find TDC on the MAG cylinder, mark the PTO flywheel to reference. The service manual explains how to do this. When you fire it up and shoot the timing light at it you'll see where it is.

 

[mikidymac]

You should never have to mess with the timing in the computer. Set the timing marks on the stator and the rotary valve and that’s it.
Honestly it sounds like your rave valves.

 

[GGuillot]

You should never have to mess with the timing in the computer.

He replaced the RAVES. Why would someone not at least want to ck it? Each MPEM can be matched to the engine or crankshaft and he's using an engine from a different ski. Aside from mechanically setting it at the rotary valve and TDC, etc. the MPEM can electronically compensate as much as 3 degrees in either direction.

 

[mikidymac]

He replaced the RAVES. Why would someone not at least want to ck it? Each MPEM can be matched to the engine or crankshaft and he's using an engine from a different ski. Aside from mechanically setting it at the rotary valve and TDC, etc. the MPEM can electronically compensate as much as 3 degrees in either direction.

Never heard of ever having to mess with the MPEM timing but I guess it could be possible. Like you said worth a check.

 

[TLH101]

Never heard of ever having to mess with the MPEM timing but I guess it could be possible. Like you said worth a check.

Check the factory service manual.

 

[mikidymac]

Check the factory service manual.

Yes, I know it's in the manual but like I said, I have never heard of having to mess with it. Not saying you won't just saying I haven't seen it be an issue.

I'm sure @Chester would have a better idea than me.

 

[etemplet]

At 30 PSI pop-off at my elevation (sea level) the carbs are richf. I cannot open the high speeds at all as it causes a loss of RPM at WOT. Carbs are still a tad rich with my pop-off at 36-38 ( both are the same it has just been a while since I set them).. The (2) GTXs 96/97 were my first skis and I started off at 23psi pop-off. and worked my way up learning as I went. As I went up the transition performance improved. My wife's ski topped out at 37mph unless the impeller got unloaded the she took off. in the END, I replaced the springs with stock and went with new genuine mikuni needles and the skis ran much better My wife's was now healed of it's issues and ran very well. After I did the TSB upgrades her 97 was way faster and better performer than my 96. Also with regard to carb settings you may need to take into consideration your height above seal level and density altitude, I surely would. Do any shops nearby recommend re-jetting?

I'd move the pop-off back up using stock springs and do some plug checking throttle chops. From the wrestling I did with my carbs (I had two 787 GTX skis) I'd have no problem dropping down a main jet size just to check. As mentioned... I"m at sea level and I"m about the drop a main jet size on my ski to see what it does.

I've never messed with the timing either... I'll have to read up on that. LOL I doubt seriously you have an issue with that but it's worth checking if you run out of ideas. Good Luck !!

 

[TLH101]

At 30 PSI pop-off at my elevation (sea level) the carbs are richf. I cannot open the high speeds at all as it causes a loss of RPM at WOT. Carbs are still a tad rich with my pop-off at 36-38 ( both are the same it has just been a while since I set them).. The (2) GTXs 96/97 were my first skis and I started off at 23psi pop-off. and worked my way up learning as I went. As I went up the transition performance improved. My wife's ski topped out at 37mph unless the impeller got unloaded the she took off. in the END, I replaced the springs with stock and went with new genuine mikuni needles and the skis ran much better My wife's was now healed of it's issues and ran very well. After I did the TSB upgrades her 97 was way faster and better performer than my 96. Also with regard to carb settings you may need to take into consideration your height above seal level and density altitude, I surely would. Do any shops nearby recommend re-jetting?

I'd move the pop-off back up using stock springs and do some plug checking throttle chops. From the wrestling I did with my carbs (I had two 787 GTX skis) I'd have no problem dropping down a main jet size just to check. As mentioned... I"m at sea level and I"m about the drop a main jet size on my ski to see what it does.

I've never messed with the timing either... I'll have to read up on that. LOL I doubt seriously you have an issue with that but it's worth checking if you run out of ideas. Good Luck !!

I'm at 4400 ft altitude.

 

[jts32]

He replaced the RAVES.

I don't see anything that says the RAVEs have been replaced, only that he tried adjusting the caps, which has very little affect.

Had a '97 GTX with the same symptoms. Replaced the RAVEs and housings with the redesigned parts and never had a problem after

 

[TLH101]

I don't see anything that says the RAVEs have been replaced, only that he tried adjusting the caps, which has very little affect.

Had a '97 GTX with the same symptoms. Replaced the RAVEs and housings with the redesigned parts and never had a problem after

The original engine was swapped with a good used engine, due to leaking crank seals. The engine I put in worked fine a '96 XP, and it has driven me nuts, as to why it would not run in the GTX. I used the raves that were in the engine I put in the ski. They are not slotted. I am going to pull the slotted raves out of original engine and install them to see what happens. Wish me luck.

 

[jts32]

Be careful that the new engine hasn't been bored out and the old engine has standard boring. Anything over a +.25mm over bore requires the RAVE valves be trimmed to prevent the valves from contacting the pistons. Hate to see you put in standard RAVEs on a bored engine and have clearance issues.

 

[TLH101]

Already thought about that. The engine I put in is 1mm over, so I will have to trim the raves.

 

[etemplet]

I'm at 4400 ft altitude.

Exactly and add a hot day and the engine will perform like it is at 6000' or more density altitude. That's a lot less air which equals a lot more fuel.

 

[TLH101]

Well, turns out the engine had the slotted RAVEs, but one housing was the wrong one, with the notch. Replace with correct housing, and the transition cleared up fine. Now will idle, starts good, hot or cold, and transitions great. Right up to 5200 RPM, then need to let off throttle for just a slit second and back on. Runs up to 6400 RPM just like normal. Sometimes a small wave will unload the pump and will pickup RPMs. Pulling the choke doers nothing to gain RPMs. Both spark plugs are burning a nice even chocolate brown.
I don't know where else to look now.

 

[etemplet]

What is your pop-off setting? I think you might be a little rich because a a low pop off setting or the nozzles in the barrel might not be clear. 6400RPM is about 500RPM from optimum. I'd drop a jet size on the mains for that altitude. Good Luck !

 

[TLH101]

I am at 4400ft. 6400-6460 is as many rpm as any Sea Doo, with a stock impeller, will turn here. Especially in 100 degree air. Not rich, plugs are burning perfect.