Super Jet Lowering compression

[Little Trumps Super Jet]

hey guys, i could use some help.I just bought a new square. Its a 701 61x 1.75 over, the cases were also cut on both sides and new pistons less than a tank on them. It has a high compression west coast head milled with 26cc domes, its running 210 psi right now. My problem is i want to drop the compression so i can run 91 pump gas for my trip in a few days. The guy i purchased the ski from gave me the stock dual cooled head for it, i also ordered a milled stock head, i want to put it on to lower the compression but he told me the pistons might smack the stock or stock milled head. Will that happen? I really need to figure out how to lower compression so i can run pump gas.

Thanks for the help

 

[Dmac80sc]

I would think you need to put on a different head. Then you need to check the squish and then check the compression. I wouldn't run more then like 185 on high octane pump gas.

 

[btmboards]

I don't think the pistons should contact the head unless he for some reason decked the motor way too many times. Just check your clearance before you go to start it up though.
Dmac: here in Cali it is a pain to get anything better then a really low grade 91 especially if you ride allot.

 

[Dmac80sc]

Aha! Then do guys tend to run lower compressions?

 

[Little Trumps Super Jet]

How do i check the clearance and squish?? There both stock heads and one was milled so il try the stock head furst then the milled head when it gets here

 

[ajmps1]

thicker head gasket...??

 

[WB1994]

If your running lakes wide open throttle all day long then 180ish compresion but if your riding surf and just blippin the throttle like I do , 200 is safe. Ive run 195ish for many years in the surf with no problems.

 

[Little Trumps Super Jet]

Im running flat water free ridig, blipping mostly and occasionl speed run. How do i check clearance though????

 

[xXx Octane xXx]

Pull your spark plugs out and disconnect the battery.
Take a piece of soldering wire (at least 8" long), bend one end (between 1.5" and 1.75" from the end) 90 degrees.
Rotate the motor by hand so the cylinder you're measuring the squish clearance in is near BDC.
Insert the soldering wire into the cylinder through the spark plug hole and position it so the end of the wire is touching the cylinder wall at the very top of the bore (keep this away from all of the ports).
Rotate the motor by hand so the piston comes up and makes contact with the soldering wire.
Continue to rotate the motor by hand until the piston reaches TDC and starts moving back down.
The piston will compress the soldering wire to the squish clearance of your engine.
Remove the soldering wire and measure the compressed section of wire with a caliper.
Repeat in the second cylinder.

 

[Little Trumps Super Jet]

Ok that partly makes sense, so i stick the 90 in so its flat with piston and up against head and im holding wire straight up and down? What do i do after i measure it? How much should i have. Im kinda confused, any video to explain and show it maybe??

 

[swannysxr]

.045"- .050" is ideal

 

[xXx Octane xXx]

Ok that partly makes sense, so i stick the 90 in so its flat with piston and up against head and im holding wire straight up and down? What do i do after i measure it? How much should i have. Im kinda confused, any video to explain and show it maybe??

Yes, the short (1.5" to 1.75") section of wire should be horizontal, or flat across the top of the piston.

 

[ajmps1]

you will need a decent digital caliper

 

[BruceSki]

you should find out how that motor was built.

Is it a stroker? were the pistons changed to non-oem style yamaha pistons? was the cylinder decked?

in that case you will need to have domes re-cut to match that motor. Maybe buying a ski with a motor built like that wasn't the best idea if you are unsure about motor building and how to get around that. They may have crazy race porting done to that motor and the high compression makes up for all the top end only style porting.

 

[Little Trumps Super Jet]

Im not sure if it has domed pistons, i dont think so. Its a great motor with not too much done or not enough. It was decked cases cut on both sides, ported. Group K did some work on it. I just want to run pumo gas. So do i check the squish with the stock head im putting on? What saulder wire size do i use?

 

[wanderer1]

Bro, you aint giving or have enough info for an answer to this question. What if the cases were decked and then the domes were relieved for piston clearance? Then you slap a stock head on? Need to tear it down or find out who did the work and what they did.

 

[Schmidty721]

Im not sure if it has domed pistons, i dont think so. Its a great motor with not too much done or not enough. It was decked cases cut on both sides, ported. Group K did some work on it. I just want to run pumo gas. So do i check the squish with the stock head im putting on? What saulder wire size do i use?

.065 wire should work.
Also, Run the piece of solder in line with the piston wrist pin to eliminate any piston rock and you turn the engine over and compress the wire. Ie, either have it facing the front of the motor or the rear inline with the crankshaft.

If you pull the head off and it has flat top pistons in it, A stock head dome won't work. It is not shaped correctly for a flat top.
OEM style pistons have slight dome to them.

 

[Little Trumps Super Jet]

Ok thanks Schmidty. How do i hand crank the motor over? So this should tell me if i can put on the stock head on, and do i check this with the current head or after i put on stock?

 

[Little Trumps Super Jet]

Pull your spark plugs out and disconnect the battery.
Take a piece of soldering wire (at least 8" long), bend one end (between 1.5" and 1.75" from the end) 90 degrees.
Rotate the motor by hand so the cylinder you're measuring the squish clearance in is near BDC.
Insert the soldering wire into the cylinder through the spark plug hole and position it so the end of the wire is touching the cylinder wall at the very top of the bore (keep this away from all of the ports).
Rotate the motor by hand so the piston comes up and makes contact with the soldering wire.
Continue to rotate the motor by hand until the piston reaches TDC and starts moving back down.
The piston will compress the soldering wire to the squish clearance of your engine.
Remove the soldering wire and measure the compressed section of wire with a caliper.
Repeat in the second cylinder.

Do i put on my "new" head im going to run or the "old" head it currently has on it? I can check it if i kbow which one im supposed to check

 

[Schmidty721]

I would check it with the head that is on there first. It only takes a couple minutes to do.
You should be able to spin the engine over by rotating the coupler off PTO side of the engine. There is a plastic guard that covers the midshaft and engine couplers that you will have to remove.