Topic: feeler on engine rebuild... bad compression

[xsmooth69x]

so i want to know what im getting myself into if i decided to do a rebuild....(which i will be)

i have 2 cb550 engines so if all possible i want to take the best of both engines instead of replacing everything inside.

my problem is that both engines have bad compression

120-220-230-235

180-210-220-225

thats the compression i get from kicking them cold.

what i have in mind i just taking off the top part of the engine replacing the piston rings and head gasket and the bike should be good right?

is their anything else i should look at?

[Duke McDukiedook]

You sure you are really getting readings of +200psi out of a 70's Honda?

[xsmooth69x]

You sure you are really getting readings of +200psi out of a 70's Honda?

granted the engines weren't warm and i was just kicking the engine till the gauge didnt move anymore.

i took off the spark plug poured a little engine oil into the cylinder to loosen and lub things up (did this to all of them and kicked the engine over a couple of times with the spark plugs off the whole time) hooked up the compression tester with the little connector for motorcycle engines and kicked the bike until the needle didnt move and those are the readings

[tlbranth]

The oil will give you a false reading. I'd hook up a battery and crank the engines when testing. And hold the throttle open while cranking.

[xsmooth69x]

The oil will give you a false reading. I'd hook up a battery and crank the engines when testing. And hold the throttle open while cranking.

so i crank the motor get it running and warm. stop the motor pull out one of the spark plugs attach the compression tester then start the engine and have it open wide open?

[TwoTired]

Did you read the engine FAQ?

[xsmooth69x]

Did you read the engine FAQ?

ya the only engine FAQ was about camshafts and camchain

[TwoTired]

Did you read the engine FAQ?

ya the only engine FAQ was about camshafts and camchain

Well maybe you should read the rest of it.
http://forums.sohc4.net/index.php?topic=369.msg476032#msg476032

[fire113]

The oil will give you a false reading. I'd hook up a battery and crank the engines when testing. And hold the throttle open while cranking.

so i crank the motor get it running and warm. stop the motor pull out one of the spark plugs attach the compression tester then start the engine and have it open wide open?

...not only one plug! pull all four plugs and then insert Your compression tester and push the starter button with the throttle full open. Do this by all four cylinders when the engine is warm.

Good luck...Georg

[Duanob]

You should do a dry reading first. If you get a low number then pour a tiny bit of oil in each cyl and retest. You probably poured way too much hence the super high numbers. If you still get a low number on the cylinder, your rings are probably good enough, your problem is in the top end. A new gasket, head rebuild, and valve conditioning will fix it right up.

[TwoTired]

Unless a cylinder reads zero compression.  I always run a long unused engine for a bit to see if it will re-seat the bits obstructed by liquids turned to glue or surface rust interfering with amicable contact.  ...for a street engine.
If I were going to race it, I would take the whole thing apart to ensure the parts were tip top.

I returned several SOHC4s from long disuse.  Some had "bad" compression on one cylinder making my worry a bit, until it had some run time on it.  One example was my 77 CB550F (my current avatar). It had 8K miles on the bike and one cylinder was 50 Lbs lower than the others from sitting for 15 years.  After 100 miles of operation, they all read nearly the same. This engine feels like it has more power than all the rest of my cb550s.  Don't know why.   I'm thinking it was a Wednesday or Thursday production example.  It's just got a like more "snap" to it than the others, and not I'm going to take it apart just to find out why.

If you happily learn from mistakes, go ahead and take apart the motor.  It will cost you at least a gasket kit.  And, whatever else you break off, strip, or talk yourself into replacing "while your in there".   But, it might not be necessary to have the motor run well.  IME

[xsmooth69x]

Unless a cylinder reads zero compression.  I always run a long unused engine for a bit to see if it will re-seat the bits obstructed by liquids turned to glue or surface rust interfering with amicable contact.  ...for a street engine.
If I were going to race it, I would take the whole thing apart to ensure the parts were tip top.

I returned several SOHC4s from long disuse.  Some had "bad" compression on one cylinder making my worry a bit, until it had some run time on it.  One example was my 77 CB550F (my current avatar). It had 8K miles on the bike and one cylinder was 50 Lbs lower than the others from sitting for 15 years.  After 100 miles of operation, they all read nearly the same. This engine feels like it has more power than all the rest of my cb550s.  Don't know why.   I'm thinking it was a Wednesday or Thursday production example.  It's just got a like more "snap" to it than the others, and not I'm going to take it apart just to find out why.

If you happily learn from mistakes, go ahead and take apart the motor.  It will cost you at least a gasket kit.  And, whatever else you break off, strip, or talk yourself into replacing "while your in there".   But, it might not be necessary to have the motor run well.  IME

kk sounds good