Low Compression (Bad rings or valves?)

[bikedave99]

Well, I went for a ride on Saturday for a bit and came back and had bunch of oil on my engine. Not the typical seeping oil but splashes. It looked like it was coming from my airbox so I figured it was leaking from the carb joint which goes through the airbox. I was right, one joint was loose and had a gap. But, there was more than the usual amount of oil in box so I started getting worried. I got a compression tester and just checked my compression and three cylinders were between 130-150psi and the front left was..... 30psi!! Ohhh boy, so, maybe the rings are bad in that cylinder and it is pressurizing my crankcase and blowing more oil into my airbox.

Of course, before tearing my engine out the next best thing would be to check my valves and make sure I don't have a problem there.... I would have thought I would notice if one cylinder was that bad, but I did also just put on some cobra slip ons temporarily until I can get my Kerker 4-1 fitted so I was already feeling that power slump.

What do you guys think? Oh, and I tested it at warm and operating temperature with the same drastic difference... help!

 

[rebar]

Yes not enough valve/cam clearance can hold the valve open during compression.

 

[bikedave99]

Ok, after doing some more quick searching on here I just performed the test again and got 30psi on that cylinder. Then I dumped about a tablespoon or more of oil in and did it again and it climbed to 70psi. Not stellar, but since it went higher, I'm beginning to suspect rings more than valves now. No doubt valves could be adjusted as I have never done it, but since it went up with oil... what do ya'll think?

Thanks rebar for info.

 

[rebar]

Take a look at this thread.
http://www.vmaxforum.net/showthread.php?t=13232

I would check your valve clearance before doing anything major. Getting the valve covers off is a PITA though.

 

[KJShover]

Another cheap test is smell your oil. Blow by is a two way street. Chances are if your at 30psi and the rest of your cyls are at 130+ and the cause is your starting to have ring failure, you would get some fuel in the oil.

If it was valves, you'd just sent the unburned fuel out the pipe.

A leak down test couldn't hurt either.

 

[bikedave99]

Well crap, I sniffed my oil and there is a distinct gasoline essence...

 

[bikedave99]

As a final(ish) test I guess I will hook up some compressed air to the cylinder at TDC and see where the air is leaking out primarily. If it is coming out of the crank breather heavy or out the exhaust.

 

[Rusty McNeil]

130 sounds awfully low even for your 3 "good" cylinders..

 

[sdt354]

How many miles Dave?

 

[bikedave99]

130 sounds awfully low even for your 3 "good" cylinders..

Yeah, looking at the FSM it looks like 200ish is ideal.

How many miles Dave?

I am a little over 40k miles right now.

I was relatively ok with all the cylinders being a little low (or a lot...?) just because of its age and mileage. At least the other cylinders were consistently worn. It has had bad airbox to carb boots as long as I have had it which is just a couple years. I don't know how long before that it was run with unfiltered air entering the carbs. I always tried to get them seated as best as i could but unvariable one or two would unseat themselves. I know that unfiltered air and dust will accelerate ring wear.

 

[one2dmax]

mileage shouldn't be an issue unless it's been unfiltered as noted. Usually I don't worry about the number as long as they are all within 10% of each other. Are you holding the throttle open during the cranking test? Cylinders warmed up?

Sean

 

[bikedave99]

mileage shouldn't be an issue unless it's been unfiltered as noted. Usually I don't worry about the number as long as they are all within 10% of each other. Are you holding the throttle open during the cranking test? Cylinders warmed up?

Sean

Sean, I was kinda thinking like you, as long as they are all close then I wouldn't worry about it. Now, to answer your other questions, I tested compression at both room temperature and at operating temperature and had the same big difference between the one and the other three. I didn't have the throttle opened though... Hmmm. What is the theory behind that? Just making sure the cylinder is pulling in enough air to compress? Thanks for your thoughts

 

[06VMAXIMUS]

Holding the throttle open while checking compression allows the maximum intake of air possible. Make sure to ground out your plug wires while checking/cranking.

 

[bikedave99]

Holding the throttle open while checking compression allows the maximum intake of air possible. Make sure to ground out your plug wires while checking/cranking.

I see, that is kinda what I figured I guess. Well, I guess that could be the reason my other cylinders seemed low then. I'll redo it when I got a chance. I had heard of pinning the throttle while doing compression checks, but only on cars where you do it to put it in open circuit so it isn't injecting fuel. This makes sense tho, and I'll give it a shot. Thanks!

 

[one2dmax]

Don't even turn the key on. Remove the front seat and jump across the solenoid. Make sure it's in neutral.

Sean

 

[Bayoublaster]

For us just starting out ( I'm also checking my compression), how do you ground out the plug wires while cranking? Thanks.

 

[one2dmax]

You won't need to do that if you crank it over with the key off (jumping the starter relay like I noted).