Cylinder Compression Test

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thundermax

thundermax

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Checked compression with warm engine, wide open throttle, all came in at around 120 psi. Bike runs strong but this PSI seems low. Is it? Is a 98 with 10k miles. Is this telling me I need a valve/lifter adjustment? Thanks.
 
cushmanscooter said:
Checked compression with warm engine, wide open throttle, all came in at around 120 psi. Bike runs strong but this PSI seems low. Is it? Is a 98 with 10k miles. Is this telling me I need a valve/lifter adjustment? Thanks.
Click to expand...

Many gauges do not display correct values. Best way to determine actual engine health is by doing a leakdown test. Regardless, your compression numbers are all even which is a good thing. If the bike runs strong, doesn't smoke profusely and doesnt exhibit any signs of needing valve adjustments, just keep running it and enjoy!:biglaugh:
 
I read on RoadStarClinic.Com that a person should unplug the decompression solenoid and without doing so will result in low PSI readings. Would like to have this confirmed and where is it?

I do take comfort that all cylinders are within 1-3 lbs of each other.
Thanks
 
cushmanscooter said:
Checked compression with warm engine, wide open throttle, all came in at around 120 psi. Bike runs strong but this PSI seems low. Is it? Is a 98 with 10k miles. Is this telling me I need a valve/lifter adjustment? Thanks.
Click to expand...

Buy this bike new? 10k is early for valve adjusting. 120 is way below minimum. I'm thinking, another gauge to try too? Cranking long enough?
Steve
 
Everything's probably fine. Most important indicator is how even the cylinders are...

Most likely a gauge issue....
 
That seems low. The manual spec is around 140-150 IIRC.

Though 120 isn't low enough that it would likely cause noticeable problems. Might not be running as strong as it could be though.

Try it with a few drops of oil down each cylinder, and with a different tester gauge. Autozone rents them for free. I've seen two gagues on the same engine at the same time give results that were 25psi apart, so they can get damaged or inaccurate in time.
 
+1 if it ain't broken don't fix it, the old saying goes. Runs well, it's prolly ok. I like to ride with other Max's to judge how it's running. There's nothing like a full throttle 2nd gear roll on with a Vmax similar to your own. That's my favorite way to check my engine health.
Steve
 
Last edited:
Used a Harbor Freight $20 compression tester. It looks solid, ect. and has three fittings. This am rented Oreilley compression tester. Got it home, did not even have the correct fitting included!

Will retest soon, what I did not do with first testing was to take all plugs out, then test one cylinder at a time. Earlier, took out a plug, tested the cylinder. Of course I pulled the caps off all other plugs first so bike would not fire.

Report back soon.
 
You get same or slightly less psi with all plugs removed.

Guess I will just enjoy riding it, I really do.
 
Sorry if this is the wrong place to post but have been going through this site ,this is the only place I managed to post, I have just purchased a 98 vmax and having a problem . The front left header stays cold.i have stripped and cleaned the carbs . The front left one twice. I have rigged the front 2 coils and switched the plug caps to see if the coil is no good ,I have switched the plugs round that are new,but still front left down pipe is cold. The cabs are balanced as I have done them. Does this sound familiar to any one . If I put the choke on then the header does get hot.
 
When you cleaned the carbs did you remove the jet block under the float bowl? They are a couple tiny pilot jets in there that like to get plugged up.

Based on what you are saying about the choke, that would be my first guess.
 
Constant vacuum carb slides don't move much when motor is off and the throttle is open. Try removing the air box cover and propping open the slide then full throttle. All plugs should be removed also.

Good Luck
 
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