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Strange increase in compression!

chuckycheese

chuckycheese

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Past Site Supporter
Charter Member
I've been plagued with a cold cylinder (#4) for quite awhile and have tried everything from a carb rebuild to valve adjusting, to synching, to coils, to signal generator...and on and on and on. I'm getting fuel and spark but it's not running. I figured I probably had a burned valve or a scorched piston, a blown head gasket or some other horrible problem but realized that I first needed to check the compression. I was amazed at what I found.

I compared my compression numbers with those from several years back and they were almost exactly the same...around 150 PSI....except for cold #4. I expected it to be low. Instead of being 150 PSI (as it was before), it was 170 PSI. Do any of you knowledgeable people have any idea what could be going on??? By the way, I know my numbers are right because I checked them twice.

There must be something significant about the increase but I have no idea what it could be. I really appreciate any help with this because I simply don't know what to do next.:cry: Thank you!!
 
Did you check all the cylinders at the same time? If not, maybe they all will all indicate 170 psi because the gauge is failing. The only other thing I can think of is the cam timming jumped, but again, all cylinders will show an increase. Carbon buildup could lead to an increase in compression but 20 psi sound pretty extreme.
 
Thanks!

Thanks!

Thanks for your thoughts!! I understand about about the increased PSI from carbon build up so that may be it. I checked them all at the same time (a couple of times) and the numbers seem to be exactly what I would expect...except for #4.
I'm just really frustrated because the carbs are clean and like new, inside and out, the bike only has 20K miles on it, it's getting a great spark and plenty of gas...but #4 just doesn't want to go. I'm figuring it must be something simple, at this point, but I just don't have any idea what it is.:(
 
chuckycheese said:
I've been plagued with a cold cylinder (#4) for quite awhile and have tried everything from a carb rebuild to valve adjusting, to synching, to coils, to signal generator...and on and on and on. I'm getting fuel and spark but it's not running. I figured I probably had a burned valve or a scorched piston, a blown head gasket or some other horrible problem but realized that I first needed to check the compression. I was amazed at what I found.

I compared my compression numbers with those from several years back and they were almost exactly the same...around 150 PSI....except for cold #4. I expected it to be low. Instead of being 150 PSI (as it was before), it was 170 PSI. Do any of you knowledgeable people have any idea what could be going on??? By the way, I know my numbers are right because I checked them twice.

There must be something significant about the increase but I have no idea what it could be. I really appreciate any help with this because I simply don't know what to do next.:cry: Thank you!!
Click to expand...
If this cylinder has been "cold" for quite awhile, though running, it may be carboned badly, which will raise the compression (this however is not a beneficial change!). If it was merely not firing and washing down the cylinder walls, the compression would likely decrease. If it were mine, faced with the information you just provided, I'd want to confirm good spark......you could spray some combustion chamber cleaner in through the air box or individual carb inlet and see if you get more normalized readings (per the instructions). This might lead you to check valve clearances and possibly carb operation for that one cylinder especially. I know a lot of this you felt you've done already so perhaps some rechecking of these basics is in order. Are you running with a common airbox and 1 air filter? Are you using anything other than normal spark plugs (platinum plugs for example are notorious for odd part throttle ignition misses - and the resulting problems). Are you getting any signs of oil burning in that particular cylinder? This can indicate valve seal or oil ring problems without necessarily affecting compression except that burning oil will increase carbon buildup in an affected cylinder.

Just my couple of thoughts on the subject.
 
If he was leaking oil into that cylinder wouldnt it also increase compression values?
 
TheCafeKid said:
If he was leaking oil into that cylinder wouldnt it also increase compression values?
Click to expand...

This is a possibility, but the engine would be smoking badly when idling. This could cause the pot to run colder, but not likely!

Has the engine ever had a helicoil fitted to that spark plug hole. If so, it may have been fitted deeper into the head than original. This would cause the compression tester rubber to go deeper into that chamber, producing a higher reading for that cylinder!! Just a thought.
 
Good stuff

Good stuff

These are great tips and I really appreciate all of you trying to help. The bike is completely stock and I've done everything I can imagine. It has been running crummy for quite a long time (dumb on my part) but I thought I just probably needed to rebuild the carbs....which I just did several weeks ago. I had a thought that I want to run by you to see what you think.

You need compression, spark and gas...right? I've got all three but here's my latest guess: Before I checked the compression, I warmed it up for about 10 minutes on idle. When I pulled the plugs to check the compression, I noticed that they were all somewhat black and sooty from idling so long....all of them EXCEPT #4. It looked lean (as it always does). If gas were getting to it, wouldn't it be black and wet all the time???

Sooooooo, the only thing I can think of is that even though gas is getting to the float bowl, somehow it's not being processed further and not making it to the combustion chamber. Does that make any sense?? I did a very thorough job of rebuilding them (new "o" rings, Berryman's, air, etc.) but maybe something's plugged up. I appreciate all of your help!!!!!!
 
chuckycheese said:
These are great tips and I really appreciate all of you trying to help. The bike is completely stock and I've done everything I can imagine. It has been running crummy for quite a long time (dumb on my part) but I thought I just probably needed to rebuild the carbs....which I just did several weeks ago. I had a thought that I want to run by you to see what you think.

You need compression, spark and gas...right? I've got all three but here's my latest guess: Before I checked the compression, I warmed it up for about 10 minutes on idle. When I pulled the plugs to check the compression, I noticed that they were all somewhat black and sooty from idling so long....all of them EXCEPT #4. It looked lean (as it always does). If gas were getting to it, wouldn't it be black and wet all the time???

Sooooooo, the only thing I can think of is that even though gas is getting to the float bowl, somehow it's not being processed further and not making it to the combustion chamber. Does that make any sense?? I did a very thorough job of rebuilding them (new "o" rings, Berryman's, air, etc.) but maybe something's plugged up. I appreciate all of your help!!!!!!
Click to expand...

Re-check the pilot circuit on #4 carb. Remove the pilot jet, the mixture and idle screws and ensure that the associated passages are clear before re-assembly. These passages diameters are very small and you sometimes need to poke some fine copper wire down them before blowing out with compressed air. You should be ready to go then!
 
High compression could be a valve not opening enough. What are the clearances like on #4?
 
Whooooa!

Whooooa!

Guy....I recently checked, adjusted, checked and double checked my valves so I know they're set just right (although I suppose a problem could still exist there).

49er....I'm kind of excited about your answer...maybe I'll finally get this thing figured out. When I cleaned the carbs, I was very careful to get the pilot jet clean and clear but as far as the tube..who knows??

Since the carb in question is #4, I should be able to poke a wire up there without taking off the carbs, right? I'm thinking with patience and a hand mirror, it might be 'doable'. I can't get to it for a few days but I'll be sure to leave a post when I figure out and any other thoughts are always appreciated. Thanks, again, to all my advisers!:-D
 
chuckycheese said:
Guy....I recently checked, adjusted, checked and double checked my valves so I know they're set just right (although I suppose a problem could still exist there).

49er....I'm kind of excited about your answer...maybe I'll finally get this thing figured out. When I cleaned the carbs, I was very careful to get the pilot jet clean and clear but as far as the tube..who knows??

Since the carb in question is #4, I should be able to poke a wire up there without taking off the carbs, right? I'm thinking with patience and a hand mirror, it might be 'doable'. I can't get to it for a few days but I'll be sure to leave a post when I figure out and any other thoughts are always appreciated. Thanks, again, to all my advisers!:-D
Click to expand...

Be very careful how you poke the copper wire into those passages. If you break a piece of wire off in a passage, you will have a difficult time getting it out. Might be safer to take the carbs off. It's your call though!!
 
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