Bill,

I am no mechanic, so I will tell you what I was told to do by Jack(Mr. LT80 himself.)

I run the quad up to operating temperature (5 minutes ideling), then I use a screw in tester and crank the engine over either by starter or recoil until the gauge stops climbing....usually not more than 3 or 4 pulls on the recoil or turns of the engine....I don't know the "reasoning" for testing compression with the throttle held open, but I that is what I was told to do. I figured if I do it the same way each time, at least I am making equal comparisons......[img]i/expressions/face-icon-small-confused.gif[/img]

Todd

 

Compression should be checked with the throttle wide open.This allows the air to flow freely and give an accurate reading. This is what I've been told anyways. [img]i/expressions/face-icon-small-smile.gif[/img]
Asfas: I feel the head gasket will drop compression. If it is leaking, you will know when you take the head off. It'll show a spit mark.Like you said, it's well worn. I feel this is where you are loosing compression.

The jets thing:: First off averyone wants to think like a banshee/250R with their LT80 jetting.LT's come fat as heck cuz it's a yard machine to Suzuki.They want all the fuel mix they can get and still have it run.ALSO the bigger machines come more "race ready" so to speak. The porting is much more that way compared to the stock LT80 porting.
I feel the reason that I and others can use a smaller jet is the way I/we port them increases efficency.Getting as much burnt fuel out and as much fresh mix in is the key.The better you can get this the less mix it needs.
It's sorta like the 1970's cars getting 10 mpg and todays bigger/heavyer cars trucks getting 20. We strive for that efficency. The car makers do it when the feds make em [img]i/expressions/face-icon-small-happy.gif[/img]
I'm terrible at explainiong some (most) things, hope that helped [img]i/expressions/face-icon-small-smile.gif[/img]

 

Engines are air pumps. Checking compression with the trottle wide open is like checking water pressure with the valve wide open. Closing the vavle could give you a lower reading. Alot of it depends on the motor as to how much different the readings can be. Lower readings are a result in worn parts (mainly the rings) or a leak somewhere (head gasket). Doing a port job will also lower the readings because of raising ports and them being open longer. This is delt with by milling the head, reshaping the head, or milling the top of the sleeve. (or all of the above depending on the motor) From my past exspreence, our LT needs 90lbs to start. It usually runs around 140 lbs (before I ported it)

 

It was 140 before and it's down now eh...hmm.
How much did you raise the exhaust port?
How much did you shave off the head?

 

That was with the head shaved .25 . I checked it hot right after the 3rd rebuild. Since then I have raised the ex port approx 3mm (thats where I put my mark anyways) I also raised the transfer ports and widen them all. (the exhaust quite a bit) Blended all the places that I could to smooth out any transions. (between jug and sleeve) (I did take the sleeve out of the jug and done alot of it) Funny the stuff you can do when waiting on parts! Does all this sound right? Am I way off on things? I am going to try and get a good compression reading when hot soon (maybe tonight) and see where its at now. I checked ring end gap when I put it all together last time and I put the feeler gauge up after .12 so I know its not going to last long. (the paper work that came with the piston and rings said it should have .04 for every inch of bore, thus it should have been somewhere around .08 right?) We have a couple of races that my sons want to try or I would have fixed it up right.

 

thanks for all the reply's. i think i'm actually learning something here.[img]i/expressions/face-icon-small-wink.gif[/img]