Topic: CB750 compression test: Normal (acceptable) difference between the 4 cylinders?

[PeWe]

I did a compression test today so I'll know before opening the engine. I'll deck cyl and head that will crank up the compression a little. Interesting to see how much more pressure it will make.

- Acceptable difference of pressure between cylinders? cold engine, last started about 3 weeks ago.

cyl 1: 180, 2:164, 3: 175, 4:175 PSI  (electric starter and full throttle)
cyl 2 must have a hardware difference (problem), right? gasket leak..?

tester used was this cheap model bought locally in an auto shop selling cheap car tools, parts
http://www.cb750supply.com/images/cb750supply/parts/480/CB-70222.jpg

Pistons are not much used, about 6000kms. 836cc + ported head and cam numbers like an RC295

EDIT: Funny, same kind of question written while I wrote this post
http://forums.sohc4.net/index.php/topic,154269.0.html
I'll test again after a warm up, start and let it idle 5 minutes or so.

[flybox1]

You need a warm engine so the cylinders expand, and are oiled to get good seal at the rings.
Run the bike to op temps and retest.

all cyls within 10% is acceptable

[70CB750]

Yes, 10%

[PeWe]

OK with 10% rule then
+/- 5 PSI  feels better for me due to the fresh pistons in old  rehoned bores . Based on 0 experience and knowledge about it.
I hope cyl 2 will increase when warm and after machining head + cyl block with new gasket.
(new valves planned to fly in as well, 5mm conversion with 34mm inlet, std ex.)

[flybox1]

OK with 10% rule then
+/- 5 PSI  feels better for me due to the fresh pistons in old  rehoned bores . Based on 0 experience and knowledge about it.
I hope cyl 2 will increase when warm and after machining head + cyl block with new gasket.
(new valves planned to fly in as well, 5mm conversion with 34mm inlet, std ex.)

With your dry readings you're actually close to 5%
I would guess that might improve with a 'hot' test....
IIRC, 170psi was HONDA spec for a stock engine. 

[seanbarney41]

OK with 10% rule then
+/- 5 PSI  feels better for me due to the fresh pistons in old  rehoned bores . Based on 0 experience and knowledge about it.
I hope cyl 2 will increase when warm and after machining head + cyl block with new gasket.
(new valves planned to fly in as well, 5mm conversion with 34mm inlet, std ex.)

With your dry readings you're actually close to 5%
I would guess that might improve with a 'hot' test....
IIRC, 170psi was HONDA spec for a stock engine. 

HONDA spec is mostly meaningless with the RC295 cam, as increased overlap is gonna bleed off cranking compression

[PeWe]

Update:
I did a new test today with warm engine. Idle in about 5 minutes. Final result about 10 minutes after.
Better result    Earlier a little bit too dry and cold after weeks of no start.
185, 175, 175, 190 PSI.
Difference due to a banana head? I'll see when the head and cyl block is decked at the tuner.

Head must be completely flat when tightened. Cam will be bent in the holders, and wear the bearings otherwise.

[CB750F2]

Before you take the head off I suggest that you do a leak down test to see if there is a difference in leakage. After removing the head measure the volume of each chamber. Also take note of the amount of carbon on top of the pistons - pistons 1 and 4 may have more than 2 and 3. Just my thoughts, Pat

[PeWe]

I'll replace all guides and valves anyway. I'll tell my local tuner to check the volumes of all chambers, adjust if needed. He ported the head 32 years ago.