compression....how low is too low?

[EXtreme-]

sorry...no time to search the forums tonight. Was hoping you guys could open some discussion on stock jug compression numbers and when the rebuild time is a MUST, vs just freshening up the motor. How low can you go before causing damage? thanks, gotta roll :macho

[250Renvy]

I don't know the definitive answer is but I know I've run as low as 140 with no problems. Although I've found smoking increases. Without looking it up I believe stock compression to be 170ish

[MossboysRacing]

I had that happen to me to. I could only start it if being pulled by someone else. I checked the compression that night and it was like 120, which is too low. Check your compression, but it sounds like you may need to freshen up your top end.

[EXtreme-]

I have 3 R's that I checked the other night. One's running 190, one at 150 and one at 140. Of course, the 190 flat out rips. The others run ok for trail use, but I don't want reliability problems from neglect.

I achieved the numbers from kicking (our) brains out. Can you get a better read by "pulling" the quad in 2nd? or is that a bad thing?

thanks so far, keep it coming please.

[zedicus00]

kick it hard several times wide open throttle. thas the best way to do it.... 160 is stock the 140 one could definatly use a rebuild... run a thin gasket after the rebuild will get compression up a tad an not hurt reliability. itl stillb pumpgass safe too.

[86 Quad R]

Originally posted by zedicus00
kick it hard several times wide open throttle. thas the best way to do it.... 160 is stock the 140 one could definatly use a rebuild... run a thin gasket after the rebuild will get compression up a tad an not hurt reliability. itl stillb pumpgass safe too.


yup! what he said. works great. :cool:

[dober250R]

Can you run a CR head gasket if your head has been milled? Not trying to jack this thread.......

[Iliketogofast]

I get 220 PSI with a milled head and Cometic gasket (thick).

[86 Quad R]

Originally posted by Iliketogofast
I get 220 PSI with a milled head and Cometic gasket (thick).


and what head are you running?

[C-LEIGH RACING]

Any time you bump up the compression, you need to be sure the other parts of the engine are in good enough shape to handle the extra compression, parts like the piston, rings, top end bearing, crankshaft & main bearings. Any of those parts not up to par, the extra compression will some cause the part to fail & destory the engine.
Neil

[dober250R]

Neil,

What are your thoughts about using a CR gasket with a milled head?

[C-LEIGH RACING]

Realy depends on how that head is machined.
If its cut for high compression & your compression is low at this point, you have problems somewhere. If you install the thin gasket more than likely something is going to fail. Better find out what the problem is first, correct that & then install the CR gasket.

You know how much time you have on that engine sence it was rebilt last & how much was done to it while it was being rebuilt.

As it is, if you up the compression & the piston ends up busting you could take out the cases, at the least ruin the crank. Once you damage those tins on the crank, its junk after that. Be safe & check it out first.
Neil

[Iliketogofast]

Originally posted by 86 Quad R
and what head are you running?

It's the stock one.

[EXtreme-]

Neil,
in your opinion, can a stock jug with 140 pounds be a reliable trail machine or is there cause for consern?

[C-LEIGH RACING]

140s low but I have seen some running lower than that.

Run time on the engine is what you have to think about, how long sence it had been rebuilt.
Parts are worn & metal fatigued after a good long time of running.

Compression on a 2 stroke is the key to know what kind of shape the engine is in. After a rebuild, while the engine is fresh, the compression reading should be noted. After awhile of riding another compression reading should be taken & noted, then after that anytime the engine runs funny it should be checked again.
When you check the compression after that & a drop is found, its telling you something is wrong & its time to start finding out why.

So, far as reliable, look at how long the engine has ran & the compression & it will tell you maybe or maybe not, depends a great deal on the rider.

Its like a slipping clutch in a ol beater straight drive pickup truck, some drivers can run it like that seams forever, & others after it starts sliping about a day or so & its gone.
Neil