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Squish 787 engine too big?

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fleroux99

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Hi everyone,
1997 GTX 787 carb. SeaDoo with 210hrs


SBT 1.5mm oversized top end rebuild kit already installed last summer (150 psi PTO/148 psi MAG at cold). It came with a 6 holes aftermarket cyl. gasket. The engine run very badly from 0 to 70% throttle position. Engine does a lot of vibration, have no torque and didn't burn correctly his 91oct fuel (plugs are very wet of fuel and both piston have black carbon buil-up). Today, i mesured the squish and i have 1.8-1.9mm (tolerance: 1.2mm to 1.6mm).
1- Do you think a 3 holes (0.3mm) OEM cyl. gasket will be enough to reduce my squish into tolerances?
2- I was tinking to deck lower surface of the cylinders for a good sealing surface and also to reduce a little bit the squish.
Is it a good idea?

3- Sometime, the 2 little oil lines from the oil pump to the rotary valve cover have some bubbles in it.
Is it normal?

What I have noticed:
- Run bad 0-70% throttle position (lot of vibration, no torque, bogging, run rich, etc.)
- Run very rich at 0-70% except at 70-100% throttle position, seadoo run very nice (nice brown color)
- Idle good in and out water
- Start very well except when you leave motor off more than 20min. after riding and you try to start engine at home
- Good top speed
- Medium/good acceleration if you hit all the way down throttle
- Bad acceleration when you gradually accelerate

What i did:
- New AGM battery
- Add a extra engine ground wire
- New rectifier
- New rave valve including rave body (OEM setting)
- Cleaning Water regulator valve (OEM setting)
- Run with/without seat to diagnostic a exhaust leak (doesn't help)
- Try to pull choke when running (doesn't help)
- Carb. rebuilt with OEM needle and seat (28psi in both carb.)
- ALL fuel line changed with automobile 1/4" fuel injection hose
- Factory carb. setting
- Carb. are synchronised
- Factory flame arestor with factory air box
- New fuel selector valve and new fuel filter with o-ring
- Fresh premium 91 octane fuel
- Cleaned pick-up fuel screen
- New fuel/oil check valve and fuel pressure relief valve
- Fuel tank hold 5 PSI during 10 min.
- Trigger: 250.3 Ohms
- 2 brand new spark plug caps #1: 4.74 KOhms, #2: 4.54 KOhms
- 2 brand new spark plug wires: 12.58 KOhms
- Between all 3 yellow wires stator: 0.7 Ohms
- At 5500rpm, battery charged at 14.2V
- Spark plug BR8EIX SOLID 0.022" gap
- SBT rotary valve rebuilt (0.012" gap) and properly timed.
- Solas impeller SD-SC-XO with Solas intake grate GTX-IG-B
- Engine run with XPS full synthetic 2 stroke oil
- oil pump properly timed
- oil pump bleeded 3 time this summer and new oil filter
- 2 new 1/8" oil lines from oil pump to rotary valve cover
- All fuses are OK and all grounds in CDI box are OK
- Etc.

Thank you!
Francis
 
When you checked the squash... did you check both sides of the piston? And, did you check it over the wrist pin? If you check it on the wrong edge... it will be large.


Generaly a large squash band gap won't show up as poor perfoemnce, since it doesn't effect the compression too badly. So your perfemnce issues are probably just from carbs that aren't working. But, if you are sure you measured it corectly... then yes... put in a thinner base gasket.


It is normal to see some air in the oil injection lines, as the engine can suck some oil out. But if they are old hoses... and you suspect any issues with them... then replace the hoses, and bleed the system.
 
Dr Honda said:
When you checked the squash... did you check both sides of the piston? And, did you check it over the wrist pin? If you check it on the wrong edge... it will be large.


Generaly a large squash band gap won't show up as poor perfoemnce, since it doesn't effect the compression too badly. So your perfemnce issues are probably just from carbs that aren't working. But, if you are sure you measured it corectly... then yes... put in a thinner base gasket.


It is normal to see some air in the oil injection lines, as the engine can suck some oil out. But if they are old hoses... and you suspect any issues with them... then replace the hoses, and bleed the system.
Click to expand...

Hi Dr Honda,
Yes, i checked parallel to wrist pins and both sides of the piston. I checked squish as per this: http://www.seadoosource.com/squishgap.html
I checked many time with 1/8" (0.120") lead solder and my average low reading is 1.82mm. But sometime i read 1.90mm. It why i put 1.80-1.90mm for a average.

I'm pretty sure for my carb. and i took all information on the net for a proper carb. rebuilt. Also, needle and seat don't leak and have a constant 28 psi pop-off pressure with WD-40. (MAIN-142.5, PILOT-70, 1.5 N&S, 80g black spring, 23 - 43 pop-off pressure (I have 28 psi), LOW- 1 turn, HIGH- (MAG-0 PTO-1/2) turn.)
Yesterday, i checked inside the carb. without flame arrestor on engine and my dad was running the engine. I seen no dipping fuel inside the carburator. Note: it was after a pm ride and engine was still warm. Engine bogging down at this moment and not rev up rapidly from 1500-4500 rpm out of water. And yes, if i do not touch throttle, idlee stay at 1500 rpm out of water when it start at warm after a pm ride. If i touch the throtlle, idlee go up to 2950 rpm out of water.
The more time passes and more I realize the compression can be low when engine is warn due to a too big squish gap.
1- Do you think i need to perform a compression test when engine is warm after a ride to validate this?
2- Do you think a too big squish gap will afect timing and maybe it's why my engine acting like this?

Thank you Dr Honda!
 
Last edited by a moderator:
Is the engine still under warranty?

If it is call SBT. Also if you open it up they will void your warranty.
 
mikidymac said:
Is the engine still under warranty?

If it is call SBT. Also if you open it up they will void your warranty.
Click to expand...

It's not a complete engine from SBT. My bottom engine has 210hrs and it's the original one. I only rebuilt top end last summer with SBT 1.5mm oversized piston and cyl exchange.
 
If that is the case then your squish is too big from just using the generic gasket they provide in their kit.
Even with that squish it should still run better than yours is as Dr. Honda said.
 
mikidymac said:
If that is the case then your squish is too big from just using the generic gasket they provide in their kit.
Even with that squish it should still run better than yours is as Dr. Honda said.
Click to expand...

You're right. But i can try it... :facepalm:
What do you think about timing? I didn't have a programmer or ignition timming tools.
 
No need to do anything with the timing. Make sure the marks on the stator are aligned if it has them, on some the holes are not even slotted for adjustment.
The only timing to worry about is the rotary valve timing. It has to be within a degree or two of spec or it will not run correctly. Some of the aftermarket ones are not stamped correctly and will not get you as close as an OEM one. Also if there is too much clearance between the rotary valve, cases and cover they will not run either.

Since you will have it apart for the base gasket make sure you do a leak down test after it is back together and check the rotary valve clearance too.
 
Mag cover never removed since 1997.

All my problems have appeared after the top end rebuilt.
I'm very very sure about my rotary rebuilt. This rebuilt was done after the top end rebuilt. Before the rebuilt, rotary parts was never touched from the day this seadoo was born. I check many time to confirm everything is right in specs. So it's mean (i think) that isn't a rotary valve issue.
- New rotary valve shaft from SBT
- New rotary valve from SBT
- New rotary valve cover from SBT (exchange)
- New rotary valve cover O-ring
- Timming OPENING BTDC 147°
- TImming CLOSING ATDC 64°
- Rotary valve/cover clearance 0.012" after rebuilt and 0.016" before rebuilt. NO PERFORMANCE CHANGE before and after rebuilt.
 
I didn't have the tools to do a leak down check... :confused:
If it a bottom engine leak, it should run lean and hesitate right?
 
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I've ran into the same issue after rebuild.

Also used the 6 hole gasket in the kit and got almost 2mm squish.

Dropped down to a 3 hole after doing measurements without any gasket and settled right in at 1.4x. So I would think 3 hole is what you would need by doing the math but you have to do a no gasket test to be 100% sure what you need.

After assembling and running it I have to say that there is only a slight increase in pickup. As in throttle response is more instant than it was but everything below 5000 is pretty bad which is the same as its always been before...

I've also noticed the little bubbles in the oil lines.
 
Just do you guys know, sbt only uses and sells a 5 hole gasket so on a rec ski you won't see much difference

Sent from my SGH-T889 using Tapatalk
 
Mekanix said:
I've ran into the same issue after rebuild.

Also used the 6 hole gasket in the kit and got almost 2mm squish.

Dropped down to a 3 hole after doing measurements without any gasket and settled right in at 1.4x. So I would think 3 hole is what you would need by doing the math but you have to do a no gasket test to be 100% sure what you need.

After assembling and running it I have to say that there is only a slight increase in pickup. As in throttle response is more instant than it was but everything below 5000 is pretty bad which is the same as its always been before...

I've also noticed the little bubbles in the oil lines.
Click to expand...

Thank you Stephen, it help me a lot. Did you try to retard or advanced ignition timing ?
I was thinking to try that after a good squish gap.
 
Do you think a bad counter shaft/gear balance can be my problem?

Today, i checked for a twisted crackshaft and all my reading are good. :toetap05:

Bad MPEM?

Bad ignition coil?

Bad or loose pick-up magnetic (trigger)?

Bad stator?

Bad ignition timing after top end rebuilt?

My problem sound like ignition problem below 5000 rpm... :facepalm:
 
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Not that this is the problem but ditch the iridium plugs. These skis LOVE a good ole NGK BR8ES plug, I wouldn't put anything but an NGK in my skis. Don't get fancy. The plug wires, are the solid wire and not spiral core? Are the spark plug boots resistor type that meet the correct specs?

Sent from my SGH-T889 using Tapatalk
 
racerxxx said:
Not that this is the problem but ditch the iridium plugs. These skis LOVE a good ole NGK BR8ES plug, I wouldn't put anything but an NGK in my skis. Don't get fancy. The plug wires, are the solid wire and not spiral core? Are the spark plug boots resistor type that meet the correct specs?

Sent from my SGH-T889 using Tapatalk
Click to expand...

- Ok. I will put a good cheap BR8ES next ride. (But now, i'm sure isn't my problem because i tried BR8ES and BR8EIX last summer with a better good result with BR8EIX.)
- Solide wire (I bought OEM solid spark plug wires last summer)
- Yes boots resistor are good. (I bought OEM boots resistor last summer)
 
Let's see,

On mine I've tried:

Replaced mpem
Replaced pickup coil
Replaced stator
Replaced Regulator/Rectifier
Replaced ignition coil
Replaced wires
Replaced caps
Replaced raves
Leakdown tested
Rebuild carbs
Replaced fuel selector
Replaced water seperator
Replaced fuel lines
And reinstalled top end with proper gasket for squish
Replaced rotary valve

And still wtf!! :facepalm: Actually none of this even made a difference!!


Does yours sound like this ?
[video=youtube;V6kjnpPvCXo]https://www.youtube.com/watch?v=V6kjnpPvCXo[/video]

4500+ is all good but below that is just not right.
 
Mekanix said:
Let's see,

On mine I've tried:

Replaced mpem
Replaced pickup coil
Replaced stator
Replaced Regulator/Rectifier
Replaced ignition coil
Replaced wires
Replaced caps
Replaced raves
Leakdown tested
Rebuild carbs
Replaced fuel selector
Replaced water seperator
Replaced fuel lines
And reinstalled top end with proper gasket for squish
Replaced rotary valve

And still wtf!! :facepalm: Actually none of this even made a difference!!


Does yours sound like this ?
[video=youtube;V6kjnpPvCXo]https://www.youtube.com/watch?v=V6kjnpPvCXo[/video]

4500+ is all good but below that is just not right.
Click to expand...

Sound like this YESSSSSS!!!! But mine is worst.
I need to put the throttle lever up to 60-70% before engine stop vibrations like that. When i pressed throttle lever all the way down like on this video, engine responds great and accelerates well .
What is your oversize piston? 0.5mm ?
Every people suggest me to raise pop-off pressure between 36 and 41 PSI. If you have a small oversize piston, maybe it's why your seadoo responds better than mine. I have the biggest piston oversize available for this engine: 1.5mm. My pop-off pressure is 27 PSI. If piston suck more air because the piston are bigger than original, the pop-off want to open more easily. So, i will try to change my springs to raise my pop-off pressure to the maximum to see how the engine react.
 
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That is exactly 38 psi each and with the low speed set at 1, 1/4.
I'm not sure the oversize is the issue in this case but anything is possible.

Are you sure the carbs are synchronized ?
 
Mekanix said:
That is exactly 38 psi each and with the low speed set at 1, 1/4.
I'm not sure the oversize is the issue in this case but anything is possible.

Are you sure the carbs are synchronized ?
Click to expand...

- YES, carbs. are synchronised.
- How look your plug after 10 min. idling in water?
- Do you trying to close your low speed screw ? Like: 1 turn, 3/4 turn and 1/2

Ok what i see:
- We have the same problem after top end rebuilt.
- I have pop-off pressure at 27-28 PSI with low speed screw set at 3/4 turn
- You have pop-off pressure at 38 PSI with low speed screw set at 1-1/4
- I need to put the throttle lever up to 60-70% before the engine stop vibrations and before he want to accelerate well.
- You need to put throttle lever up to 30-40% before the engine stop vibrations and before he want to accelerate well.
Hummmmm...
 
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