Topic: best way to check compression ??

[bultaco59]

Hello out there in cb land,

I have just bought a 78 cb 750k its a bit of a basket case, I was wondering the
best way to ck compression the motor will be cold seeing there are no carbs on
motor. are there any tricks that I can try. any thoughts,any help would be great

thanks
Bob in burbank.

[grepper]

1st get or barrow a compression tester.  Harbor freight has them for about $20 or you can do the lone a tool at your local auto parts place.

Usually it is suggested to test with the throttle wide open, but since the carbs are off, this is a moot point.

Doing the test on a cold engine will give you lower numbers in any case.
Take out all the spark plugs (makes for easier kicking over).
Screw the tester into the spark plug hole, hand tight.
Kick the engine over several times till the needle on the tester stops moving up each kick. 
Record your number.
There should be a button on the tester to let out the pressure, push it and test again.
I would test each cylinder a few times and take the average of the readings for each individually.
Then, I'd put about a tablespoon of oil in each cylinder and test them all again.  These numbers should be a little higher

The oil in the cylinders will tell you if a lower compression number is due to rings or valves not sealing up.

If the numbers are in an acceptable range in both cases, great.   
If the non-oiled test is low, but with in an acceptable range oiled, you may be OK, as when the engine gets warmed up the sealing in the rings will improve.
If the non-oiled test is low and you see little improvement oiled, your valves might be leaking.  Leaking valves won’t improve when the motor warms up.
Also, if your average between the cylinders is more than 5% or 10%, that’s not good either.  They should all be very close to each other.  The engine may run, but fine tuning/syncing will be difficult at best.

As for what is an acceptable range, that all depends on bike.   Others that know your model may be able to narrow this down for you.  Below 100 lbs is defiantly not good.  I had a cb650 with over 30K that ran about 110 lbs.  Much less than ideal, but it ran fine, just not as strong as it should.

[bultaco59]

Well I checked the compression its not good, comp. as follows (dry)
65, 85, 60, 90 OUCH !! (wet) 70, 90, 70, 95, so instead of dicking around with this
motor, which has 48K on it, i found a matching frame and motor bike that the guy
willing to sell motor which has great comp. and 16k miles on it for 600.00, it would
have cost me between 400 - 500 to rebuild it.
thanks for the help.

[Rigid]

I have 3 good SOHC 750 motors for sale on indiana lafayette craigs for 350 each.  Where were you all winter!  Lol

[lucky]

Not so fast!
The engine had been sitting.
Without oil all over the rings the compression will be lower.
Once the engine was in service the compression would be much higher than
a basket case .

[TwoTired]

If you used a compression tester for an auto or diesel engine, the absolute numbers won't mean a thing due to the volume added by the tester itself.  See the the engine FAQ for an explanation.

Newbies would be better served by a leak down tester, which would also tell you just WHERE any leaks are.

Further, an engine that has been sitting won't have parts that are still comfortable with each other.   A bit of operation usually cures that.
Your numbers rose very little and equally across the bank, which tells me you rings are good.  The valves may have a bit rust or carbon on the faces, which may prevent full seal.  This would wear off with a few miles of operation and compression would come back up.

48K is not that much for a 750 motor, if it has had decent treatment.  They easily last twice that long with proper maintenance.

But, if you are looking for an excuse to replace, your questionable data may be just the one you need!

Cheers,

[steam-powered man]

yes indeed!  my latest project (cb350f, 27K) had been sitting for 10+ years.  changed the oil, full tune-up, then checked compression.
1 -- 115
2 -- 75
3 -- 75
4 -- 75

got her warmed up and drove round for about 5 minutes (tops) to check shifting, then checked compression:
1 -- 140
2 -- 140
3 -- 130
4 -- 125

this spring when this project's completed, i'll change the oil in 200/300 miles and check compression again.  i
expect a little improvement. 

[matt mattison]

If you used a compression tester for an auto or diesel engine, the absolute numbers won't mean a thing due to the volume added by the tester itself.  See the the engine FAQ for an explanation.

Newbies would be better served by a leak down tester, which would also tell you just WHERE any leaks are.

Further, an engine that has been sitting won't have parts that are still comfortable with each other.   A bit of operation usually cures that.
Your numbers rose very little and equally across the bank, which tells me you rings are good.  The valves may have a bit rust or carbon on the faces, which may prevent full seal.  This would wear off with a few miles of operation and compression would come back up.

48K is not that much for a 750 motor, if it has had decent treatment.  They easily last twice that long with proper maintenance.

But, if you are looking for an excuse to replace, your questionable data may be just the one you need!

Cheers,

 +1. Get it running, get a coupe hundred miles on it and re test. Don't forget to do the 3000 mile tune up first.