Question about 760 comp.

[jvann13]

Hi everyone. I have been reading a great deal on the site. I am new to the stand up ski world. I have a superjet with an oem 760 that has some porting work. The current bore is 85 I think. I am looking at bumping up the compression a little. I pulled my domes out to see what I have and I have 38's. My compression is just under 160. The ADA website states the 38's should be at about 180. The engine is newly rebuilt and is running great. The gauge is a snap-on so it is right. I know a lot of factors go in the compression. Is the ADA site just a base line or is some thing up I should be concerned with?

 

[Flat Brapp]

Porting can affect comp numbers, but i wouldnt know without seeing it. Where do you ride at? Im in Jville and always looking for someone to ride with

 

[Matt_E]

The ADA numbers are for stock porting, sea level altitude.

 

[jvann13]

Thanks for the info guys. I think I will try to find some 35cc domes and give them a try.

Hay Flat. I ride at jax beach jet ski launch. I think it is 6th st north?? We also ride at the Beach Blvd. boat ramp. We try to ride after work once a week if we can get away (Work has not been cooperating with me lately). If we make a day of it we go to Huguenot or the boat ramp under the bridge right past the entrance of Huguenot.

We have not been riding as much as I want. The two guys I ride with are painting their skies and taking way to long. :261:

 

[Flat Brapp]

word me and my neighbor live on pottsburg creek and cruise around here but we also go out to jaxB. ill pm you next time we go out there.

 

[jvann13]

word me and my neighbor live on pottsburg creek and cruise around here but we also go out to jaxB. ill pm you next time we go out there.

Cool. I am planning on getting out as much as I can!!! I finally got my ski back together and running good. I plan on putting some hours on it!!!!

 

[nikad58]

Thanks for the info guys. I think I will try to find some 35cc domes and give them a try.

Hay Flat. I ride at jax beach jet ski launch. I think it is 6th st north?? We also ride at the Beach Blvd. boat ramp. We try to ride after work once a week if we can get away (Work has not been cooperating with me lately). If we make a day of it we go to Huguenot or the boat ramp under the bridge right past the entrance of Huguenot.

We have not been riding as much as I want. The two guys I ride with are painting their skies and taking way to long. :261:

Without knowing the porting specs, Becarefull going to 35cc domes.. I would say your going to have problems if you open it up on pump gas..

 

[jvann13]

Without knowing the porting specs, Becarefull going to 35cc domes.. I would say your going to have problems if you open it up on pump gas..

Thank for the reply. I know I will be on the edge of it being ok. I very rarely open it up for more than a few seconds. I have no idea of how it was ported. The guy I bought it from was a scum bag, so I can't trust what he said. What is an easy way to tell how it was ported?? Is there an easy way to tell???

 

[Mouthfulloflake]

post some pics of the cylinder with the head off, and measure a few things, and you can get a pretty decent idea of the portings intended purpose

 

[jvann13]

This is all I have for pics for now. This is what I have from when I tore it down after I bought it. What do I need to measure? I should be able to do that tonight and get better pics.


http://www.x-h2o.com/picture.php?albumid=35&pictureid=313

http://www.x-h2o.com/picture.php?albumid=35&pictureid=312

http://www.x-h2o.com/picture.php?albumid=35&pictureid=311

http://www.x-h2o.com/picture.php?albumid=35&pictureid=310

http://www.x-h2o.com/picture.php?albumid=35&pictureid=309

http://www.x-h2o.com/picture.php?albumid=35&pictureid=308

 

[Mouthfulloflake]

just from looking, the casting flash has been cleaned up in the transfers, and the exhaust looks slightly messed with, it almost looks unsymmetrical there, possible a home brew port job?

you can measure the distance from the very top of the EXH port to the head sealing surface, and get an idea if the port height was raised. That generally lowers the indicated compression, and yeilds power higher up in the power band.

 

[nikad58]

I was just saying that with 35cc domes in a 701 is running the edge with gas the way it is now and with summer fuel coming (yes there is a difference) not knowing what those gas companies are going to come up with (how much oxygen added).. it will be really, REALLY risky running 35cc domes on a 760..
In other words.. 35cc domes on a 760 if over the edge all ready.. IMO. But I run lakes and do open it up.. Its not as bad with a superjet that just braps around..

 

[jvann13]

just from looking, the casting flash has been cleaned up in the transfers, and the exhaust looks slightly messed with, it almost looks unsymmetrical there, possible a home brew port job?

you can measure the distance from the very top of the EXH port to the head sealing surface, and get an idea if the port height was raised. That generally lowers the indicated compression, and yeilds power higher up in the power band.

The guy I bought it from said it was ported for bottom end, but he lied about everything else so I would not be surprised if it was ported for top end. It does rip up top. On another note I have been having trouble getting the jetting clean on the bottom. I did not get a chance to pull the head off. I got luck/blew off house work and went jet sking!! :cheer2: I will get the measurements tomorrow. Thanks for the info!

 

[jvann13]

I was just saying that with 35cc domes in a 701 is running the edge with gas the way it is now and with summer fuel coming (yes there is a difference) not knowing what those gas companies are going to come up with (how much oxygen added).. it will be really, REALLY risky running 35cc domes on a 760..
In other words.. 35cc domes on a 760 if over the edge all ready.. IMO. But I run lakes and do open it up.. Its not as bad with a superjet that just braps around..

So is there more to what you can get away with pump gas than compression?? By what I read if you run 180 psi you might be ok but could not run wide open. I try do flat water free style and wave jumping. I am only on the throttle for burst.

Thanks for the feed back. I am learning more and more everyday. I sure don’t want to do anything to hurt my ski. What harm is caused by running too much psi/ too small of domes?

 

[jvann13]

just from looking, the casting flash has been cleaned up in the transfers, and the exhaust looks slightly messed with, it almost looks unsymmetrical there, possible a home brew port job?

you can measure the distance from the very top of the EXH port to the head sealing surface, and get an idea if the port height was raised. That generally lowers the indicated compression, and yeilds power higher up in the power band.

Sorry for the delay. I just measured and it is right around 35mm from the top of the EXH port to the top.

Worked on the jetting this weekend and the bottom is alot better. Still not perfect but better.

 

[keefer]

So is there more to what you can get away with pump gas than compression?? By what I read if you run 180 psi you might be ok but could not run wide open. I try do flat water free style and wave jumping. I am only on the throttle for burst.

Thanks for the feed back. I am learning more and more everyday. I sure don’t want to do anything to hurt my ski. What harm is caused by running too much psi/ too small of domes?

You run the risk of detonation especially at high RPMs, if you get deto then the engine will go boom sooner than later. Cranking psi is a general baseline of where you are with an engine. The Pipe, porting, timing, jetting combinations will make a difference when you start running higher compressions. You can always tune it to around 180 psi, any higher and you might have to start mixing some race gas to raise your octane to be safe.

 

[jvann13]

You run the risk of detonation especially at high RPMs, if you get deto then the engine will go boom sooner than later. Cranking psi is a general baseline of where you are with an engine. The Pipe, porting, timing, jetting combinations will make a difference when you start running higher compressions. You can always tune it to around 180 psi, any higher and you might have to start mixing some race gas to raise your octane to be safe.

Cool thanks for the info. So how can a common man like my self figure out if I can bump up the compression with out going boom?? Is it best to leave it alone till I can afford to let a builder look at it? Can I post the info on Xh2o and let someone guide me on what to look for?

 

[jvann13]

bump. Mouthfulloflake I did the measurement. Do you need better pics??

 

[Big Kahuna]

I ran 83mm with 35cc domes(81-83mm domes) at 190-195 lbs with no issues on pump gas with some WOT and no deto.
but I would not recommend this. There is an article in SPLASH from years back where Protec built them a SJ that had about 195 lbs compression and they ran pump gas for years with no issues.

 

[JonnyB]

Sorry for the delay. I just measured and it is right around 35mm from the top of the EXH port to the top.

Worked on the jetting this weekend and the bottom is alot better. Still not perfect but better.

what carbs dual OEM 44s?