compression psi

Can someone please tell me the right compression psi on my xj750 1982?

 

The Information Overload Hour

XJ650 and XJ750 air-cooled engines:
Minimum: 128 psi
Standard: 156 psi
Maximum: 171 psi
Max. variance between lowest and highest: 14 psi

 

Oh Crap! Im so down on my compression i got the blue's. Amazing it's running.
I dont even want to tell you my numbers. But my Crabs are clean as hell and not Leaking. Boom!
New Petcock ended all my fuel woe's.
Now to get that top end squared away!

 

How in the world can this bike need a top end already? It's been cherished and only has 23000 miles on it.

 

Have you checked the compression with the throttle fully open Nyeema, are the valves adjusted properly?

 

Numbers you posted from an earlier thread:

"I checked compression: 9,96,92,125."

Hopefully it doesn't need a top end rebuild as that is not very many miles for these motors. Check out the procedure below and verify the gauge is accurate.
With numbers that low the bike would likely be difficult to start.

Also from The Information Overload Hour:

http://www.xjbikes.com/forums/threads/the-information-overload-hour.27544/

- COMPRESSION TESTING:

Yamaha recommends that you perform a compression test every 5,000 miles or so, and that you should record the readings, per cylinder, for future comparison and evaluation. The acceptable readings (specified at sea level) are as follows:

To do a compression test properly, you should first make sure all of your engine valves are properly adjusted to their recommended clearances, as valves that are too "tight" (not enough clearance) will allow the intake or exhaust valve to be open more than is necessary, or at the wrong time within the compression stroke cycle, thus bleeding off compression that would otherwise be developed.

Do not use thread adapters or the like on your pressure gauge, as the added volume of air space within the adapter will reduce the indicated pressure readings.

a) make sure the engine is cool/cold, 60F is the ideal temperature to take measurements at,

b) remove all spark plugs, and then stick the plugs back into their caps and make sure the plugs are grounded to the cylinder head (or even better, disconnect your TCI unit).

c) remove the airbox filter lid and the air filter.

d) make sure the battery is FULLY charged, and remains so throughout the course of these tests! It is actually recommended that for purposes of compression testing that the TCI be un-plugged and jumper cables to a large capacity battery (i.e. car battery) be used to make sure that the cranking speed remains pretty constant between each reading. Slow or sluggish cranking speeds will reduce the indicated compression pressure.

e) open the throttle FULLY and keep it open during testing.

f) crank the engine over until the needle stops advancing.

g) Let the starter cool down for a minute or so, then do the next cylinder, etc.).

h) If the readings are below spec, then shoot about a teaspoon amount of motor oil into each cylinder, crank the engine over a few revolutions with the starter (to spread the oil around), and then re-test each cylinder using the above procedure.

i) compare the two results and analyze.

j) keep all of your figures, and note the date and mileage from your odometer, so you can compare the next time you take readings (every 5,000 miles or so).

 

I just ran the procedure and got 108, 118, 108, 100 with oil dribbled in each cylinder. The valve shims are good, is this evidence for a top end rebuild or new rings/gasket? Mine's an 82 xj750 maxim, haven't started it up since I went through the carbs, wondering if it's worth it or of I go right into the engine.

 

To you both:

Stuck rings. This happens when engines sit without being prepared for long-term storage.

They'll free up as it runs. A dose of Marvel Mystery oil in the gas, and Seafoam in the oil will help free them. Another method is to make a 50/50 mix of ATF and Acetone; put a tablespon or two down each sparkplug hole and let it sit for a few days before running the engine again.

 

Thanks, I'll try it out, mine was closer to 60-70 before I dropped some oil in.

 

Thanks so much guys. Off to the garage now!

 

I did check with throttle open yes. 97 on all cyl. I feel like it's my pilot jets still clogged. I replaced everything but those when i rebuilt carbs. Back to the drawing board. Ima get new pilot jets. Thanks so much for the input. This is my first time even touching a motorcycle motor. I have learned alot.

 

Jets wouldn't affect compression test since the test is done with Wide Open Throttle. The test is to check the condition of the cylinder, piston, rings, and valves. So if you have wonky numbers, it's one of those 4. Have you tried putting a teaspoon of oil in each cylinder before doing the test? If it is the rings, that'd be how you verify it.

 

why so concerned about compression numbers? what's the problem?
are you going to race this bike?

 

Or a compression gauge that's faulty or in need of calibration. We've run across that many, many times.