550sx Reed Compression questions

[1970Camaro]

I purchased a 550sx reed ski and when I took it out, I noticed it had a hard time getting out of a 180 or a 360, almost as if it was lacking power or something. When you first fire it up, it pulled hard, but then it seemed to lag as it warmed up. Initially I was thinking it had a bad stator, exhaust leak or restricted waterbox. But when I hit the kill switch one time I noticed it "dieseling" a few times. I later did a compression test, and found that the front cylinder had 210 psi, and the rear had 220 psi. Im guessing this is not even close to low enough for California 91 octane pump gas and its getting preignition. So, I popped the head off and sure enough it had been shaved, with a copper head gasket, piston also had a 1.00 on the top, I'm assuming thats 1mm overbore. Cylinders and piston look ok, and bearings seem ok. I thought I read somewhere that stock compression for these skis is 192lbs? If I were to swap out the copper base gasket for a paper one, and a green paper head gasket, would that lower my compression enough to run pump gas? Or do I need to look for a 550sx head that is not shaved or not shaved so significantly?

 

[fastgtfairlane]

I'd try to keep the stock base gasket and change the head gasket. Changing the base gasket will change your port timing by raising them. The stock head and base gaskets are .010 thick, copper are .020 I believe you can get a paper gasket a .040 I would try that. But it really depends on how everything was set up. Why not just get some 110octane fuel and enjoy the extra power? Will.need it in the 550.

 

[WFO Speedracer]

Stock compression on a reed engine is 170, I don't think a compression increase that large can be had by just changing gaskets, the head would have had to nbeen milled as well

 

[E350]

Just some pesonal thoughts.

Ramos Oil in West Sacramento has 110 octane race gas at the outside pumps.

I mix one gallon of Race gas with four gallons of premium gas plus 16 oz of Klotz 2-stroke oil for my 550sx with 180 psi both cylinders.

You would likely run something like two gallons of Race gas to two gallons of premium. Experiment with your mix. Too much Race gas and you lose high rpm.

To accurately measure compression apparently you not only need to hold the throttle wide open, but you also need to disconnect the fuel line.

Personally, IMHO, that engine is studly. I wouldn't change a thing, someone took the time to set it up as a high compression engine. Keep it as it is and just run the correct fuel mixture for it.

 

[TheBuzzard]

If the base gasket is thicker than stock, port timing has been raised unless its been decked on the bottom. Somebody probably set the motor up the way it is for a reason. If you start changing things, it may run like crap. Lowering compression by changing the head or enlarging the combustion chambers will be the least invasive and easiest. If it were mine, I get in the head with a die grinder and remove material. The head is already junk so you got nothing to lose. If you can get a thicker than you got head gasket too.

 

[Big Kahuna]

I think he said Base Gasket when he meant Head Gasket. Nowhere did he mention Base gasket until the end.


May could find someone to remachine your dome and squish area to increase dome volume or just find another stock head.

 

[1970Camaro]

I've got alot to think about. I like the idea of getting in there with a die grinder, but I dont think I could be precise enough. You all have convinced me its wise to leave the base gasket alone at this point. I may try to get a stock uncut head or an Ada head with 24cc / 170psi domes, since changing to a thicker head gasket likely won't reduce compression enough. It is the least invasive and most easily reversed.

I also want to try some of that race gas before I swap out the head just out of curiosity. Long term I'd prefer not having to source race gas when I want to take this ski out.

Thanks for all the input so far, I appreciate it.

 

[fastgtfairlane]

I've got alot to think about. I like the idea of getting in there with a die grinder, but I dont think I could be precise enough. You all have convinced me its wise to leave the base gasket alone at this point. I may try to get a stock uncut head or an Ada head with 24cc / 170psi domes, since changing to a thicker head gasket likely won't reduce compression enough. It is the least invasive and most easily reversed.

I also want to try some of that race gas before I swap out the head just out of curiosity. Long term I'd prefer not having to source race gas when I want to take this ski out.

Thanks for all the input so far, I appreciate it.

I can totally understand the race gas part. I had a ssxi 750 and head the head milled for 93 octane. That engine finally gave out and I ended up getting a sx 750 cylinder which has lower port timing so the milled head raised thr compression enough to need 96 octane minimum so now I have to mix 110 and 93. It is a pita and I'm on the fence on ordering a Ada head to lower the compression enough for 93. I think had 22cc domes with my ported 550 I ran several years ago and I ran 100 octane. Mostly avgas. Definitely a pita getting that. But the extra low end torque was also nice haha

 

[WFO Speedracer]

Just an idea but I am sure there are some guys out there looking for the exact setup you have, someone might be willing to trade, worth a shot

 

[TheBuzzard]

I see you're in folsom and that's not far from me. Pretty sure I have one stock head left and just about everything else you could imagine

 

[1970Camaro]

I see you're in folsom and that's not far from me. Pretty sure I have one stock head left and just about everything else you could imagine

Yea I would like that. I will PM you.