Compression Test woes

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ThunderMinstrel85

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Hi.
1980 CB750 Custom
Just did compression test and was surprised by the numbers.
Cyl. 1 = 95 psi
Cyl. 2 = 100 psi
Cyl. 3 = 50 psi
Cyl. 4 = 70 psi
How can these be this low and bike starts and drives? Valve clearance must not have been done by previous owner and if I understand right, the clearance is basically nil..is that what I can expect when I check valve clearance?
Next step is to check valve clearance and replace shims as needed. Wondering if any gurus out there have been down this road and can save me any time/money. Got the bike cheap enough with 38,000 miles on her and was hoping to learn the engine and get her up to speed mechanically. Not seeing smoke out of exhaust except cyl. 2. First time compression testing a DOHC. Did I mess up? I thought 90 psi or below was considered a dead cylinder??
Bike starts right up and drives.
Any advice or wisdom appreciated.
 

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I briefly warmed the engine up (2 mins) and I did not crank the throttle wide open. I did take several readings each cylinder to be sure it wouldn't go higher.
 
Throttle needs to be wide open, also better if all plugs are out. No way two of those cylinders are firing, I would guess an inaccurate compression tester showing lower than actual compression. I consider lower than 120 to be bare minimum and indicates a fault. Setting valve clearances is a must...set to .005" not honda spec. then see if it is better, if not the damage has already been done from the tight valve clearances and will require at least rebuilding the head.
 
Spark plugs out, good charge on battery or use jump box, throttle wide open while cranking, crank until compression gauge stop climbing, warm engine will show somewhat higher numbers.
 
Redo on the Compression test: (engine cold, full throttle, no spark plugs)

Cyl. 1 = 100 psi
cyl. 2 = 100 psi
cyl. 3 = 115 psi
cyl. 4 = 100 psi

Also, checked valve clearances:

Cyl. 1 Intake .08 and .09mm
Exhaust .03 and .05mm
Cyl. 2 Intake .12 and .12mm
Exhaust .06 and .08mm
Cyl. 3 Intake .07 and .07mm
Exhaust .03 and .08mm
Cyl. 4 Intake .09 and.05mm
Exhaust .10 and .10mm

Cam Tool coming in the mail so I can manipulate the shims next. Any insights guys? Thanks so far!
 
Has this bike been sitting for a long time?
If it was sitting for a long time, i would spray some PB Blaster or WD40 inside cylinders while you waiting for the valve adjustment tool. Reason for this is if bike was sitting you may have some stuck rings inside piston grooves… point is to try everything before engine tear down, just keep in mind when you are done adjusting valves to spin engine with starter couple times to purge what ever rust inhibitor you sprayed inside cylinders.
 
Last edited:
Sounds good. I will do that now. Notbsure on the history of this bike but obviously neglected by previous owner somewhat. Can you explain in better detail how to spin engine with the starter when done with valves. Im just not familiar. Thanks
 
After you are done with your valve adjustment, leave spark plugs out and just press start button like you want to start the bike.
One reminder, not related to purging rust inhibitor from cylinders but very important during valve adjustment!
DO NOT rotate that engine while shim(s) is /are out of bucket!!!
 
It would be useful to know where you got that compression tester. You generally get exactly what you pay for with diagnostic instruments. The cheapo ones are worth their weight in poop.

I've got an old Craftsman one that reads within 5psi when I poke it with the air compressor, relative to the compressor's own gauges. Once bought a cheap Amazon special one -- it was low by about 30psi in the same test. Immediately returned it.
 
Last edited:
ThunderMinstrel85 said:
After the botched first compression test with a rental tester, I bought a $50 tester for the second test. Not sure on the brand, but will post it soon. I hear ya on the quality concept.
Click to expand...

Update:
My friend informed me that I have to adjust compression readings for elevation, which I did not know, so I feel a little better about my second readings. Actual readings would be:
Cyl. 1= 116 psi
Cyl. 2 = 116 psi
cyl. 3 = 133 psi
Cyl. 4 = 116 psi

Adjusted for 6200 feet above sea level.
Still need to pull shims and increase valve clearance all around.
Will be back on here soon with shim questions. As always, any advice is appreciated.
 
Shims:
I've pulled all but 4 stubborn shims to measure them.
Quick question: If my valve clearance on intake cylinder 1 is .08mm and I want it to be .12mm and the shim measures 273mm, do I replace current shim with a 269mm to get there? Want to be sure about this before I buy replacement shims. Thanks.
 
I hope you're not turning the engine over without shims in it. Or shoving the buckets down hard to remove the shims while any given cylinder is at TDC.
Basically, be careful. Those stubborn shims may get easier if you turn the engine over (WITH ALL THE SHIMS REINSTALLED) to a different spot, ideally when the particular cylinder of the given stubborn shim is at BDC.

You've got the math pretty much right, but in general the shims you can buy nowadays are only available in .05mm increments. Unless you fancy some hand lapping to get your shims to exactly the right dimensions. Generally you just get as close as you can to a clearance of 5 thousandths or that .12mm you mentioned. It's better to err slightly on the wide side of that number than the tight side IMO.
 
ThunderMinstrel85 said:
Hi.
1980 CB750 Custom
Just did compression test and was surprised by the numbers.
Cyl. 1 = 95 psi
Cyl. 2 = 100 psi
Cyl. 3 = 50 psi
Cyl. 4 = 70 psi
How can these be this low and bike starts and drives? Valve clearance must not have been done by previous owner and if I understand right, the clearance is basically nil..is that what I can expect when I check valve clearance?
Next step is to check valve clearance and replace shims as needed. Wondering if any gurus out there have been down this road and can save me any time/money. Got the bike cheap enough with 38,000 miles on her and was hoping to learn the engine and get her up to speed mechanically. Not seeing smoke out of exhaust except cyl. 2. First time compression testing a DOHC. Did I mess up? I thought 90 psi or below was considered a dead cylinder??
Bike starts right up and drives.
Any advice or wisdom appreciated.
Click to expand...
Did you have the throttle fully open when doing the compression test.?
 

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