Super Jet 760 compression

[Ahmed AN9]

Finally, I assembled the engine after the 62T 760 cylinder porting is done by Newmiller. I used ADA non-girdled head with 38 domes. I started by checking the compression and I was shocked when I saw 120 psi on both cylinders. New O-rings installed on the head. What is possibly the cause?
I run the engine on garden hose and it started well just need a little adjustment to the carbs.

 

[Chase Musetti]

It got new piston rings and fresh hone?


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[Ahmed AN9]

It got new piston rings and fresh hone?


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Yes

 

[peterkrunich]

Try a different gauge maybe

thank you

petey krunich

 

[Chase Musetti]

I just saw your old thread. If you're still dealing with this a year later then you need to bring the motor to someone who knows what they're doing


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[Ahmed AN9]

I just saw your old thread. If you're still dealing with this a year later then you need to bring the motor to someone who knows what they're doing


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The things I did, anyone had the same issue probably will do. And if I send it to someone only if he has a golden hand will fix it.

Am concerning the head as one of my friends has the same head and he is having a lower compression on both cylinders.

What if the base gasket is became lower in thickness, is it going to affect? Visually the gasket look acceptable.

 

[Freakie]

At 120 psi in both pots.....u got the domes wrong or gaskets too thick.....but good news...if it starts. Ride it....no damage too be done.....just no power....evens that's relative so....go ride and give some feedback

 

[Ahmed AN9]

At 120 psi in both pots.....u got the domes wrong or gaskets too thick.....but good news...if it starts. Ride it....no damage too be done.....just no power....evens that's relative so....go ride and give some feedback

The domes are the right size as per ADA using pump gas. And if you are talking about the base gasket, it is OEM and I think it became thinner than new.

I would do it once and do it right if anything missed will be pain in the @zz.

 

[Big Kahuna]

Your rings have not seated yet. Not going to get accurate reading. What guage are you using. Gauges bought at an auto parts store are notorious for being inaccurate. They can be accurate front to back, but not read the correct reading. Are you using the long shank threads or the half shank thread (Half shank will read lower since you are adding CC's to the dome volume.......

 

[Big Kahuna]

Also, If this has been ported for Racing, Then your ports may be raised significantly causing a drop in compression.. All of these issues combined could show a low PSI number on your gauge. But with a Snap-On gauge you may blow 175 lbs.

 

[Ahmed AN9]

Your rings have not seated yet. Not going to get accurate reading. What guage are you using. Gauges bought at an auto parts store are notorious for being inaccurate. They can be accurate front to back, but not read the correct reading. Are you using the long shank threads or the half shank thread (Half shank will read lower since you are adding CC's to the dome volume.......

Thank you BK for your comment. The gauge I have is Innova with a long reach or shank thread.

Yes this cylinder was ported for racing but I saw the porting numbers and they seemed fine.

 

[Barbieri972]

Try a different gauge if both cylinders read the same I bet the gauge is just off

 

[Barbieri972]

Also give the rings a squirt of wd before testing

 

[Big Kahuna]

Thank you BK for your comment. The gauge I have is Innova with a long reach or shank thread.

Yes this cylinder was ported for racing but I saw the porting numbers and they seemed fine.

Race porting typically raises your exhaust port height. This will effectively lower your compression.

 

[Ahmed AN9]

Yesterday, I had a chance to test the ski to evaluate its condition even with the low compression readings I got. The ski started and idled perfectly wihout any difficulties and the low mixture is also great. Maybe after 10 mins I felt like the ski is going to die on idle which had me to give some throttle to avoid it to switch off and I saw some air bubbles going to the rear carb only. Is this going to affect?

I kept the ski running for almost 1 hour on various throttle positions. At the 1/4 throttle the motor vibrate harder than in the idle.

After all I checked the compression with cylinder is warm and I found it almost 125 psi. Cylinder walls and the domes and everything was perfect, crosshatchs visable, no scratches or scoring.

 

[MTRHEAD]

Your ported cylinder could be ported to the point you need 23cc domes. Unless you know the exhaust duration it was ported too all bets are off. A real 85mm superstock race motor used 23cc domes for hot setup and 26cc for mild. I recall 195psi +/-5 on snap on with 23's and speedwerx pipe. 38's on a real race ported cylinder would make even less than your seeing

 

[john zigler]

Call Chris @ Newmiller and ask him what CC domes to run....

You are low due to port heights. ADA specs are for stock cyls, and in our findings tend to be very conservative. Your location, and elevation will make a difference also. I am guessing your gauge is reading a little low as well.

 

[Big Kahuna]

if your seeing air bubbles in your fuel line (intake side), you are sucking air and loosing fuel pressure. as RPM's rise you will lose fuel, which will cause ............ your engine to run rough. This is assuming your running dual with seperate lines. Single would run rough also.

 

[Ahmed AN9]

if your seeing air bubbles in your fuel line (intake side), you are sucking air and loosing fuel pressure. as RPM's rise you will lose fuel, which will cause ............ your engine to run rough. This is assuming your running dual with seperate lines. Single would run rough also.

Yes am running dual and a separate line for each carb.
Do you mean that the carb is sucking air from the tank to carb line? or from the carb body/primer nipple? because as I said bubbles were observed only in one carb fuel line.

 

[Ahmed AN9]

Your ported cylinder could be ported to the point you need 23cc domes. Unless you know the exhaust duration it was ported too all bets are off. A real 85mm superstock race motor used 23cc domes for hot setup and 26cc for mild. I recall 195psi +/-5 on snap on with 23's and speedwerx pipe. 38's on a real race ported cylinder would make even less than your seeing

Is that applicable when using pump gas 95 octane?