MY KFX 80 HAS 65 PSI COMPRESSION AFTER TOP END REBUILD VERY HARD STARTING

The compression is far too low to run. Depending on how 'oval' the cylinder barrel , it you either need to hone the barrel (they glaze) and put new piston rings in, or you rebore and add an oversize piston and oversize new rings

What should compression be on CR250?

Engines running on premium typically have compression pressures from 175 to 195 psi. Porting or valve timing has a big effect
Usually you want the compression over 160 psi.

Apr 27, 2016

CR250R compression? - Tech Help/Race Shop - Vital MX

more here

https://www.google.com/search?q=Honda+cr250+compression+specs

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With the decompressor active, nominal is 85 - 90 psi. At 62 psi, you have a top end problem. Try dumping a bit of oil in the cylinder and re-test. If the pressure rises, you have a ring seal problem. That sounds like an top end rebuild.

this is the forum for the kids toy

A compression test is the accurate way to assess the condition of your top end. Although the spec's will vary from motor to motor, anything below 120 psi usually indicates it's time to rebuild it. Some motors that's as high as 150 psi, so do some research and find the service limit spec for your bike. Good luck!

Hey Anon,
The biggie is the compression difference between ALL the cylinders,,, there should not be a 5-10% max. difference between the lowest and highest readings.. if more.. I would question the rebuild quality,,(My V6 is all within 5 psi after 13 years - (I rebuilt it!!) The longevity is based on how hard you run it, the quality of the oil and fuel you run and how clean you keep inside the cowl....
The engine could last for a long time.. or break tomorrow...no way to know??

To test the compression of the engine, use a good screw in type compression gauge. Hold the throttle wide open and rotate the engine through six or seven compression strokes.

The manual says, "If the final readings are 90 psi or more and if the final readings do not indicate more than a 10% variance between cylinders, compression is considered normal".

Do a "wet" compression test as well by squirting 1/2 ounce of oil into each cylinder and redo the test. If the compression comes up considerably, a top end rebuild is probably in order.

At 8.5:1 compression ratio, 120 psi max.

Good Luck
Steve

Hi there

• Low compression, and lots of blow-by forcing oil into the air filter via the the crankcase breather. Sounds like you need a top end rebuild. If you can get your hands on a compression tester, then check it. If it comes in under 120, then certainly time for a tear down and top end rebuild.

• If you put a quick squirt of oil down the spark plug hole and if the compression pressure goes up on a second compression test, then rings/piston/bore are worn. If it does not come back up, then it may be leaky valves, or a blown head gasket.

• do the compression test first. That will tell you where to start. Or, if you have the tools, tear the top end down and inspect. A motor re conditioner/machine shop will help to measure up and tell you if piston, and bore are ok. May just need new rings to freshen up.

Happy to answer any questions that you may have.
regards robotek

forget the shop
it is basically a wheel lawn motor engine

At 55 PSI it will be hard to start and at the end of its life. 80 to 90 psi is the top range.

Hi crazy

Your bike has an automatic de-compression lever that helps when starting. This mechanism will make it feel like the bike has low compression. If it is maladjusted, it may be causing the problem

check here for adjustment procedures to be sure that it is set up correctly.

Other than that, low compression pretty much means top end rebuild.

regards
robotek