Pump gas compression? (SL750)

[Polaris_Nut#1]

What should my compression be with a pump gas modification?

Sent mine out 3 yrs ago, never again [img]/graemlins/angryfire.gif[/img]

Put them on got 178, 180, 178
Talk about popping head gaskets and melt downs [img]/graemlins/angryfire.gif[/img]

I have to run two gaskets on each cylinder now 148, 150, 148 psi

If you guys from Forest Lake, MN read these Thanks [img]/graemlins/angryfire.gif[/img] [img]/graemlins/angryfire.gif[/img] [img]/graemlins/angryfire.gif[/img]

[Bryan Glynn]

Totally depends on the ski, what shop did it, and what octane it was setup for. Need more info.

[Polaris_Nut#1]

Thanks for the response, Sorry I left out the details

It's a 1995 SL750

Hot Seat Performance was suppose to set me up for 92 Octane

I was running Sunoco 94 Octane gas, cause the gas here in Mass. is oxygenated
So I figured 94 would be better than 92

[Bryan Glynn]

That's a good assumption, but you may have also needed to retune the carbs somewhat. It's not possible to tell what happened without good detailed pics of the pistons though.

[imported_Tim]

I agree with Bryan!! Need more info. You need to check your squish clearance and let us know what that is. I have a hard time believing that your motor has 175lbs or better compression with modified stock heads running 2 head gaskets. If you know how to CC your head's, that would tell you what is needed in the fuel area along with your timing curve. What did the old damaged piston look like when you took them out, hole in the piston?? 4 corner seizure? melted down on the exhaust side of the piston? From there we might be able to help :D

[Polaris_Nut#1]

Thanks, I'll take a couple pictures

Tim with the 2 head gasket on each cylinder the compression is 148 Mag, 150 center, and 148 PTO

[Polaris_Nut#1]

Almost forgot each piston started to burn a hole in the center :(

[imported_Tim]

Take a piece of solder and insert it into the head parrallel with the crankshaft and turn the engine over and let it smash the solder. Take a set of calipers and then measure the thinnest area and let us knoiw what that is, it will also tell you the width of the squish (measure the area that was flatend out) and tell us that. Burning a hole in the piston may not be from the head cutting in as much as your engine timing/spark plug and fuel choices. Since you said you were running 2 head gaskets are you sure that they were not leaking when the engine was at temp? Water in the cylinder during the combustion cycle will cause detonation and burn a hole very quickly in the piston. Improper jetting , fuel supply ect can all contribute to holed pistons. Be glad to help all we can. PS I cut heads here all the time for these engines and do not have issues with them, so unless Hot Seat made a mental mistake I think it may be cureable [img]graemlins/uts.gif[/img]

[ August 05, 2002, 05:43 PM: Message edited by: Tim ]

[Polaris_Nut#1]

Here is my entire setup

Since 1995, I have added ocean pro flame arrestors, primer kit, reed spacers, pro series reeds, changed the exhaust cooling filter from the 2.3mm one on 94-95 650/750's to the 1.0mm one on 780's, Also added R&D intake grate, Twist ride plate, 6* pump wedge, cone extention, 6 vien pump, 14/19 swirl impeller, aluminum waterbox, 28 inch sponsons

This setup ran great 55-56 according to the MFD 58 on glass :D

Well, It ran great till I sent the heads out :(

[imported_Tim]

Adding compression will require more FUEL. If you have not re-jetted your carbs than thats were I say you need to start to eliminate your burnt pistons. You have added more air(flame arrestors)more compression(heads) Reed spacers and reeds. All of these items would definetly need jetting to be changed or modified in order to make the engine live [img]graemlins/uts.gif[/img]

[Polaris_Nut#1]

They burnt the holes with only one head gasket on each cylinder

The guy at Hot Seat is the one who told me to doubble them up to lower the compression, he even insisted that polaris was sending 780's out with doubble gasket, I think he was full of it

I tried 105 mains and couldn't lean it out enough

I have 102.5 mains in there now and it still seems like it's getting too much fuel

I got over 5 hours on with the doubble gasket set up and no pistion damage yet

it's ok idling and down low, after half throttle it feels like I'm dragging an anchor, It just won't go :(

[imported_Tim]

What spark plug are you running? Check the squish and get back to us or call me @ 260-637-7953 [img]tongue.gif[/img]

[ August 05, 2002, 09:50 PM: Message edited by: Tim ]

[SeaLion]

I think you were over loading the engine from your impeller selection of 14/19 coupled with the tail cone spacer that loads it even more. Take out the spacer and go to a 13/18 or leave the spacer and go with a 12/17. You will get rid of the load and improve performance everywhere plus gain rpm. BTW, what is your max rpm with your current set up?

[John Sharp]

FWIW, my ols 780 came from the factory with 2 head gaskets per cylinder. I thought it was a mistake and finally found out it was not a mistake, but I never knew WHY it was done.

[imported_Tim]

The 780's were to only have 1 head gasket. The gasket was a sandwich of a couple of thinner gaskets in an attempt at a better seal. To stack the head gaskets is asking for a leak and detonation problems, How many of you guys that have overbored your engine made sure that the head gasket was also enlarged in the bore area so it does not protrude into the combustion chamber. I have seen and fixed this many times. Also if you have overbored cylinders, did you recut the head to the overbore dimensions? This is usually not necessary on 1st over but anything after that can cause you piston problems