GTI with SBT engine & low compression ?

[65Taurus]

I placed an SBT engine in a 98 GTI this past August. My experience with SBT was great and I thought that the machine was running very well. We only managed to ride for about 15 hours before our northeastern US watercraft season has pretty much ended. I thought that as part of the winterization process, I would measure and record the compression each season.

This is where my surprise came. I measured the Mag cylinder first and got 140 PSI, then measured the PTO cylinder and only got 115 PSI. I assumed I must be doing something wrong so I decided to measure it several times under different conditions and got the following.

Engine Cold: .......Mag: 135 PSI ........PTO: 110 PSI

Engine Warm: ......Mag: 140 PSI .......PTO: 115 PSI

Warm with ..........Mag: 145 PSI .......PTO: 123 PSI
Sea-Doo Lube
sprayed in
Cylinders

Warm again .........Mag: 138 PSI .......PTO: 115 PSI
With Lube
burned off

I cannot vouch for the accuracy of my gauge, but the difference remained fairly consistent. I never checked the compression prior to this time, but the lower compression in the PTO cylinder correlates with my noticing that right from the start, whenever I checked or replaced the sparkplugs, the PTO plug was always darker.

Could this be the result of an engine assembly defect? Since I am not very hard on this machine (very little WOT) might the compression still improve on the low cylinder with additional ride time or is it sure to be broken in by 15 hours?

Typically when a question like this is asked, one of the first replies then asks about what did the original engine fail from. Well, I learned a lot with the purchase of this machine. The original engine failed a few hours of riding time after I purchased the machine. When a dealer friend and I tore the engine down we found a seemingly classic water ingestion induced crank bearing failure with bearing debris pitting on the head and pistons. The previous owner was apparently less than honest when I had asked if the machine ever experienced any water ingestion.

Sorry for the long post, but I wanted to accurately describe where I was at and looking for suggestions on what I might do next. Thank you in advance for your time taken to read this.

Tim

[Technical Support]

Re: GTI with SBT engine & low compression ?

There is no question, you have some good damage on that low cylinder. If you are still under warranty, call in and we'll get you a new block and see what happened to this one.

[Bill O'Neal2]

Re: GTI with SBT engine & low compression ?

Did you fellas rebuild the carbureator when you replaced that first motor?

NEVER replace a motor without verifying that the carbs are clean and in good working condition.

Think about this senerio. What if the previous owner had a carb clogged up and that is why he got towed to shore flooding the motor with water. Then he got the water out ( after doing some bearing damage) and got it started up agian and sold it to you. After a few hours, the rusty bearings started coming apart and you ended up buying a SBT motor and installing it.
If the orginal carb problem was never solved, maybe it is running too lean now in that clyinder, causing it to slightly sieze and lose compression?

I doubt that is what happened, but for just those types of reasons, I always rebuild the carbs and check the fuel delivery system completely when doing an engine replacement. Especially if I do not know the history of the pwc.

[65Taurus]

Re: GTI with SBT engine & low compression ?

Thanks Bill for you input. I can’t tell you how many times you have helped me and you didn’t even know it. It seems like whenever I had a question, I would search the forums and would find the answer, and low and behold it was almost always you. My wife thought I was nuts one evening when I exclaimed loudly “Thank you Bill” after I found a post where you suggested using the purple wire to the gauges for a switched power source. I had been hunting and pecking till I was about to tear the rest of my hair out while looking for a switched 12V for an add-on hourmeter.


Back to the issue… Yes, while we had everything apart, I had the carb rebuilt with a Mikuni kit and new internal filter (even though the old filter looked pretty good). I am curious though, since the GTI is a single carb 720 engine, is it possible to lean out only one cylinder since the same carb feeds both cylinders? I will have the carb set-up re-checked, but I thought everything was good since the machine seemed to be running so well.

Thanks again for your time and feedback.

Tim

[Bill O'Neal2]

Re: GTI with SBT engine & low compression ?

No, if you have a carb issue, it would be with both clyinders. But it is not uncommon for one clyinder to sieze before the other.

You better use your warranty at SBT. Maybe a ring broke or something.

[Riixxii]

Re: GTI with SBT engine & low compression ?

They should both blow well over 150, how was your break in done?

[65Taurus]

Re: GTI with SBT engine & low compression ?

The break-in was per the SBT Tech article. Premix of 50:1 (plus the injection pump) with SBT break-in oil for the first tank. I followed the schedule closely, but it took me a long time to get the nerve to go WOT. Certainly not because of the speed, but paranoid it was not ready. I finally opened it up briefly after around 8 hours.

My friend kept telling me I was going too easy on it. Wonder if he was right.

As far as the 150 PSI. I cannot vouch for the accuracy of my gauge and I have not checked it with a second one, but it was the difference in the cylinders that got my attention.

Thanks, Tim

[Riixxii]

Re: GTI with SBT engine & low compression ?

When you break in a new engine it needs the idle to W.O.T. and varying throttle positions. You should not make wide open runs, steady idle, or constant speeds for any extended period of time. 8 hours before a W.OT. run is way, way too long. You need to run it wide open in the first 10-15 minutes. After the engine is completely up to operating temp.

[Technical Support]

Re: GTI with SBT engine & low compression ?

If you waited 8 hours to go WOT you simply did not follow our procedure. Nor do we recommend going WOT withing minutes of use - nor do the OEMs. Our break-in article provides a surefire way for proper care.

[Riixxii]

Re: GTI with SBT engine & low compression ?

Maybe using the "first tank" theory it took him over 8 hrs. to burn his 13+ gallons of fuel. Idle, off idle, midrange, and varying throttle with no wot runs.

[Technical Support]

Re: GTI with SBT engine & low compression ?

It doesn't take anywhere near 8 hours to burn through the first tank, more like 2-3 max. We do it every week, sometimes 2-3 times here for testing.

[65Taurus]

Re: GTI with SBT engine & low compression ?

OK, I truly respect and appreciate the feedback to my questions, and I definitely would like to determine where the problem lies, but I could use some clarification on where I "simply did not follow" your (SBT) procedure for break in.

I'll go through each step and please let me know where I may have gone wrong.

1. "During the first 10 gallons of gasoline, there should be NO full throttle operation. First idle the engine for about 10 minutes to ensure that the engine is properly warmed-up".

I did that... shouldn't be any problem there.


2. "Then begin varying the throttle position up and down, up to 20% throttle for a half hour. Then come back and let the engine cool off completely - it should be a good 30 min break".

I did that... good so far I hope.


3. After it cools down, you may now ride it up to 50% throttle for another half hour or so. Then come back again and let it completely cool down again".

Did it. Went to 50% (based on throttle position) for short periods several times within the half hour.


4. "Now you can go out and ride up to 90% throttle. Continue this for the remainder of the 10 gallons, never holding one throttle position for more than a few seconds. Do not run at a steady throttle position and do not go to 100% throttle".

I think I am OK with this step as well. I kept changing the throttle position almost continuously with a couple of squeezes to 80 or 90 percent throttle position (subjective) for a few seconds, and no wide open. My first tank was more like 12 gallons and went through it in a little over 2 hours.


5. "After your first tank of gas, replace your plugs with fresh ones. For the second tank, you may ride normally with one exception - do not hold it at 100% throttle for more than a few seconds".

Replaced plugs, rode "normally" but was still careful not to hold throttle in one position for very long. I interpreted the "do not hold it at 100% throttle for more than a few seconds" as an indication it might be best to wait longer before WOT. I did not understand it to mean that I needed to run at 100% for a few seconds.

In the previous post when I said I did not run at WOT until after about 8 hours, that means that I waited that long before I tried going wide open up the lake. Prior to that, I regularly would run up to 80 or 90% throttle for short periods but held off pressing the throttle that last little bit till I went through a few tanks of gas.

Tim

[Technical Support]

Re: GTI with SBT engine & low compression ?

5. "After your first tank of gas, replace your plugs with fresh ones. For the second tank, you may ride normally with one exception - do not hold it at 100% throttle for more than a few seconds".

Replaced plugs, rode "normally" but was still careful not to hold throttle in one position for very long. I interpreted the "do not hold it at 100% throttle for more than a few seconds" as an indication it might be best to wait longer before WOT. I did not understand it to mean that I needed to run at 100% for a few seconds.
Tim

Normally means normally - which includes wide open. You interpreted incorrectly, it simply means exactly what it says.

ALL THIS however is a moot point - you did not do anything, nor did I mean to imply that you did, to hurt your engine.

[Bill O'Neal2]

Re: GTI with SBT engine & low compression ?

Tim,
Let me ask this. How does your motor perform ? Other than the fact that you think you may have low compression in one clyinder, does it run good? Are you happy with the way it ran?
I have not read anything here that says the motor does not perform correctly.

Maybe you just got a bad guage?