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Compression check

90FXRS

90FXRS

Junior Member
Contributor
When you suspect rings are needing attention, how much oil should be put in the cylinder when checking the compression? Any information on what a bad piston sounds like?
 
IMO, NO oil. Oil film will help old rings make a better seal and give you higher compression. A warm engine with the throttle held full open for several revolutions with the starter will do it. Should be less than 10% difference between cylinders.
 
Not to disagree with Breeze3at but first check dry; take a couple of readings to be sure of data. Second check with oil and if the compression improves, good sign that rings need attention. If bike is CV carbureted, the OP will have to hold the slide up since it is not cable operated. I just remove the carb from the rubber boot that connects it to the intake. Make sure the battery is fully charged and in good condition; a weak battery will not show true compression.

A loose piston will typically make noise on start up but the noise will diminish as the motor comes up to operating temps.
 
I just checked my wife's '93 Dyna and came up with 150lbs in each cylinder. The manual says you should have 90psi or more with the 10% variance, (as noted by Breeze3at) to be considered normal, . The best investment I ever made is a HD Android Camera Borescope. It connects to your smart phone or laptop and has an adjustable LED light. Takes photos, or video too! I swear it was only like $16 on Amazon. I got the Fantronics brand. You can insert it in the spark plug opening and inspect your pistons and cylinder walls. The photos are really clear. Curious to why you think the rings need attention? Does it smoke any? Do you hear noise? I agree with dolt about checking dry first. The wet check is done only to verify questionable ring seal. Use 1/2 oz per the HD Manual and it's 5-7 revolutions with motor at normal operating temp, dry or wet. I rock the bike off the jiffy stand if it's on it to make sure it saturates all of the ring surface. But I've never pulled the CV carb to do it. Lot of trouble and extra work. Just lock the throttle open and with the air cleaner off, hold the slide up with your finger. Do you have a manual?
 
Really no that much extra work to slip the carb out of the boot once the air filter is removed. Remove the head breather bolts and pop the carb out of the boot; it doesn't have to come all the way off, just enough to provide clear airway;)

I have never understood why the manual says 90psi; totally unacceptable and rings should have been replaced long before compression drops to 90psi. The manual also does not discuss adjusting for altitude. The manual is a valuable tool but sometimes misleading to the uninitiated.o_O

Those little cameras are very handy to have around.:)
 
I'm going to stand by my original recommendation of not pulling the carburetor to do a compression test. In my experience, any time you remove the breather bolts and break the seal on the spigot to the manifold you risk also breaking the seals on the manifold to the heads. Since Evo's are notorious for being a royal pain to get sealed one should not tempt fate. If any "uninitiated" rookie tackles that he could risk anything from broken breather bolts to a whole slew of vacuum leaks since even the breather bolts will have to be resealed. Ten minutes of extra work can sometimes turn into hours and extra money.
 
UPDATE....The owner says it has stopped making any odd noises so she wants to ride it and see what happens. Thanks for all of the input.

Breeze, I was asking about putting a pinch of oil into the cylinders to see if the readings vary. If the initial reading were off and changed after adding oil, this might indicate rings instead of valves.
 
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