no bottom end torque on 97 SPX

[poissonn]

I've had this issue the past season and rebuilt the carbs through winter as well as a bunch of other things to make sure the ski was ready for this year...Guess I was wrong. The engine almost want to die when there is too much load on it, let's say when you do a 360* and keep it WOT to get out of the hole, it doesn't have the punch it used to have to get out of the water, it just loses RPMs....makes things a lot less fun!

- 1997 SPX with 96xp electronics.
- Compression is 135 psi in both cylinders.
- ±52mph GPS top speed with 17/22 skat trak swirl impeller
- carbs rebuilt and cleaned with ±28psi pop-off and stock settings.
- All 3 raves are flush.
- Water is leaking between the two black halves of the water regulator but there is no hole in the diaphragm.
- Carbs are not adjusted perfectly, as it sometimes die if I punch it after idling for a while but it was this way even before.

Anything I'm missing there? Is the 15psi difference from a rebuilt engine cause that much loss of torque?

 

[1983]

Compression is too low, check it again with another gauge.

spark plug boots ground
fuel selector off
throttle wide open
both spark plugs out
test compression

report back readings with good known gauge. 135 is too low

show picture of water reg, where it has water

 

[Mekanix]

Yup 135 thats where your problem lies.

Is it also overheating and scoring the cylinders?
Or is it running lean.

 

[manolocross]

I don't think 135 is low...a brand new SBT ENGINE has 140 psi compression. 110 to 120 is low. But it should run well at 135.

 

[Coastiejoe]

The 135 doesn't bother me to much. Have seen many run well at that PSI.

Were the carbs rebuilt with OEM parts? Needle and Seats?

 

[Matt Braley]

Repair the leaky water regulator. I think that's where your performance problem is. An even 135psi on both holes will still do although getting tired.

 

[wabash]

I have the same issue on a 94 GTX 657. Which doesn't have RAVES nor a water regulator. Running 150 psi compression. Waiting to see what you find. I went thru my fuel system and found leaks on the vent side which prevented any build up of pressure in the tank. Haven't had it on water yet to see if that fixed it. Also a weak spark will cause this. Put in new plugs and put on new plug boots and trimmed the ends of the plug wires so the new boots had clean wire to grab on to. The old ones didn't have the best contact in the boot looked like they'd been on there a while maybe original. Like I said don't know the results and I won't for another week or so. Anxious to see if you find anything.

 

[Matt Braley]

I have the same issue on a 94 GTX 657. Which doesn't have RAVES nor a water regulator. Running 150 psi compression. Waiting to see what you find. I went thru my fuel system and found leaks on the vent side which prevented any build up of pressure in the tank. Haven't had it on water yet to see if that fixed it. Also a weak spark will cause this. Put in new plugs and put on new plug boots and trimmed the ends of the plug wires so the new boots had clean wire to grab on to. The old ones didn't have the best contact in the boot looked like they'd been on there a while maybe original. Like I said don't know the results and I won't for another week or so. Anxious to see if you find anything.

He is gonna find that his water regulator bellows is compromised. You have a different ski and a different problem. The fuel tanks do not get pressurized at all. Air must be let in to let the fuel out.

 

[wabash]

He is gonna find that his water regulator bellows is compromised. You have a different ski and a different problem. The fuel tanks do not get pressurized at all. Air must be let in to let the fuel out.

The vent system is not a breather type system it has a check valve to let air in and a relief valve to let vapors out at 1.5 psi. It will allow pressurization up to 1.5 psi. Pressure comes from the vapor pressure of the gasoline. Some say without this there may be issues with fuel feed to the carbs. It's hard to imagine but that's what the word is. Need a tight fuel system including the vent. I suspected I'd find a plugged water injection elbow but no they were clean.

 

[Bills86e]

Clean
My guess is the RAVES are staying open.

During operation, it is the spring inside the cap that holds the valve closed, as the pressure of the exhaust increases it overcomes the tension of the spring and opens the valve. BRP makes the XPS (Castrol) for rave power valve engines.

I suggest you read the "how-to" section pictoralized article by RAMPAGE

Bills86e

 

[wabash]

Out of curiosity I decided to check compression between both my 94 GTXs. The "newer" one with around 15-20 hrs on its full rebuild has 140psi on both holes. The older one with around 100hrs on its full rebuild has 160psi on both holes. I wasn't worried about 140 at first but seeing 160 on the other one it makes me wonder. Now both holes are 140 so I don't think there's a failure of any kind it may be either not broke in yet or the cylinders were machined a little off however I did have them checked by the same machinist that machined the cylinders on the older one. So I'm not sure why it's at 140. Anyone chime in on compression? The OP states his is at 135. I have one at 140 with the same symptom. Could it be low compression?

 

[Mekanix]

Not sure why sbt's are so low to start off with but it does make sense since people say they reduce compression on purpose to increase longevity.

Each time I've rebuilt an engine according to the specs in the manual (as in using the right parts and gaskets....) I've always seen 155-165 psi after the first few starts. (I'm at sea level maybe that makes a difference) I've found that the volume in the head did differ in each build though.
The only things that cause a difference in compression besides the build specs are excessive carbon deposits, worn rings allowing more ring gap and allowing leakage. Or weak ring pressure causing blow by. And severe mechanical issues like pinned rings from debris or aluminum deposits on cylinders and scoring.

Wabash, if your head was ever shaven to repair it, that could account for the extra compression.


Found this on good ol Wikipedia.
A compression reading of 150 psi at sea level significantly less 6000 feet, the expected reading would be 150 psi x .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.

 

[wabash]

Hmmm. I did just discover too that I have two different casting numbers on the cylinders between the two skis. The older one registering 160 psi I know has the original cylinders just remachined for oversize pistons and rings. The one showing 140 I did an exchange for the cylinders. What if I got a hold of 657x cylinders by mistake? I started a new thread as to not high jack this one as I am now off topic. It's titled 657 vs 657x cylinders so please reply there.

 

[1983]

Compression is too low.

Water regulator bellows pierced

 

[wabash]

If it helps any I ended up finding out my compression as too low. For other reasons than what you have but still the same effect. So if fixing your water reg doesn't get you all the way back to where you want then you'll prob need to address the low compression.