I'm about to rebuilt my RGV 250 VJ21a top end but I don't know wich rings I should use,cylinders are stock size but I had too little compression less than 85 psi. should I use rings at the 25 measure will it fit?

It is possible but unless you have wild cams and the test was done at the top of mount everest I doubt it. In a stock motor 75 psi would be less than 6:1 compression ratio. Was the testing done at wide open throttle.

There is always a better alternative than the dealership as far as costs. They want to put a new motor in a 2003 model? That is ridiculous to me- cost-prohibitive. Find a reputable independent shop. Look for references and customer satisfaction. A used, low mileage replacement engine is a better alternative, probably less than half the cost of a new engine.
There may be an even better alternative, where you refurbish the top end of the engine-the cylinder head and valve train. This is dependent on that a wet-dry compression test supports rebuilding the top end. What I mean by this is a compression test of the cylinders reveals low compression in one or more cylinders, okay. Next, they would squirt some oil (about a tablespoon) in the low cylinders and check compression again. If compression doesn't change, it points to the valves and valve train (the valve guides which wear and could let oil into the cylinder). But if compression is higher in the wet test, then the piston rings are worn and losing compression, and oil gets past them-this would indicate the engine needs a complete rebuild or a replacement is called for. The reason for the wet test is that the oil will temporarily seal around the piston rings and improve compression if the rings are the problem.
If the test shows the valves are the problem (and it is very possible on an engine with only 120K-your bottom end could be fine), then a top end rebuild of the engine is a much cheaper route than any other choices. You may not know this, but often a little oil burning in the cylinders at start-up is nothing more than the valve stem rubber seals are deteriorated-a real cheap fix. The dealership probably won't tell you this. However, with the loss of power, well, that points to something more severe than valve stem seals. So I would recommend the wet compression test, just to check if it is the top end or the bottom end. If it's the top end, you could have the cylinder head reconditioned and motor along for a few more years, trouble free.
Similar to a compression test, a "leak down" test of the cylinders is actually a more accurate method of finding where compression is lost, and it's only a one step test. They hold compressed air in the cylinder and analyze where it escapes to pinpoint compression loss. Loss of compression translates into loss of power. Have either test done before you decide. Find a good mechanic and trust his advice. (Stay far away from dealers. Greed is their motivating factor.)

Hi. There is two ways to determine compression. A dry test and a wet test.
Check the compression dry and write it down. Squirt some oil into the cylinders and check it again. If the compression doesn't come up the problem is in the upper end, If the compression shoots up higher, the problem is in the bottom. ie:rings. (depending on the year make model
2 stroke 4stroke etc)Good Luck Vern

Sounds like a carb problem. My 310 stihl did the same thing. Rebuilt the carb and now it will cut the gates of the hinges off of H**L

The end gap is derived from the bore size. For the 2.91" (74mm) Geo 3 cyl bore, the top ring end gap is .004" divided by bore, so that is 2.91" / .004 = .012". The second ring and oil ring should be bore divided by .005", so that is 2.91" / .005 = .015". Don't file the springy part of the oil ring, as it is designed to compress and fit snuggly; just file the top and bottom ring that sandwich the springy part.

Cylinder bolts @ 25 ft/lbs (34Nm)
Cylinder head nuts @ 18ft/lbs (25Nm)
Check these nuts again after first warm up and again after first two rides to let gaskets bed in and seal. Cheers.

sounds like you are running to lean,whenever you alter your exhaust system you must rejet your carb. when you have jetted correctly your spark plug colour should be chocolate brown. white or gray is to lean black is to rich.Change your jet to a larger size,install new spark and take it on a short ride at full throttle stop and check plug colour. repeat this till plug colour is correct. hop this helps james

take the measurement of your block.

having a smaller ring will ensure block not being damaged but prone to a Blow-By. a too-tight ring can get your block damaged. so for you to know which size to get regardless of the stock size, tyr to get the block measurement first, then off you go wich ring to get

Not for sure ,,but I would think around 100 pounds or more,,,sounds like rings or reed valve

(Remove all spark plugs and plug wires). You will need a compression test guage (find one at Advance, Auto Zone, Harbor Freight) for less than 20 bucks. Crank the motor with the guage held tightkly against the spark plug hole and note max pressure. Then pour a spoon or 2 of motor oil down the cylinder (remove plug, pour in oil, and run compression test again. If reading rises noticeabley you have bad rings. If not, more than likely it is going to be a bad exhaust or intake valve.or, worse case, damaged piston. Good luck....gary