Topic: How low can she go? (low psi - K1)

[Prospect]

So I picked up a 71 cb750 last fall.  It was a real high mileage machine that measured around 90 psi in each cylinder.  Not great by any means.  Cosmetically rough, odd exhaust, weak spark, spray can paint, 9 owners in the last 30 years, but the owner said she ran well.  I didn't really believe him but took it anyway because it was too good a price.   Yesterday I put in a new battery, gas and she started right up.  I couldn't believe it.  No leaks from the carbs and all the electrics work.  My question is, how low can the psi drop before she isn't going to start?  The good thing is that all cylinders are within 5 psi of each other. 

[GammaFlat]

You may try some of the "miracle" cures.  Sometimes it seems the rings can get stuck.  I'm told that BG44K (a BG product) works very well to free up stuck rings.  If you are heavily "carboned up", you may get a nice increase in your compression test. 

[Gordon]

I wouldn't be surprised if those numbers come back up after it has been run some.  Also, correct psi readings on an engine this small can be difficult to get when using a compression tester with a long hose designed for a car engine.  Really what you're looking for is that there is no significant difference between any of the readings, and since you say they're all within 5 psi then I think you're probably in good shape.   

[Grabcon]

You pose an interesting question. How low is to low for compression. If all four are within 10% on psi and there is no smoke during idle, acceleration or down steep inclines with the throttle closed don't worry about it. You will have a lose of power when you start nearing the 110psi range but if it runs good and you can sync the carbs run it til she smokes.

I have had bikes that had 100 PSI when they were supposed to have 180 and they ran fine. Just weak on the power. Noticeable at higher RPMs.

I have run Seafoam and Marvels Mystery oil and have gotten a few more PSI but nothing significant.

[Duke McDukiedook]

How low can it go? Only the bike knows.....

[wrenchmuch]

I don't know if this applies to the procedure you used or not but the throttle and choke must be open all the way or you will get very low readings . Also as stated before compression testers with long rubber hoses can lower the reading as well . Two Tired posted once that the gauge Honda used for compression testing had hard metal lines (not rubber which expands to varying degrees under pressure) . Comparing your tested values to the manual is only relevant if you have the Honda gauge the manual values were obtained with .

[Prospect]

I don't know if this applies to the procedure you used or not but the throttle and choke must be open all the way or you will get very low readings . Also as stated before compression testers with long rubber hoses can lower the reading as well . Two Tired posted once that the gauge Honda used for compression testing had hard metal lines (not rubber which expands to varying degrees under pressure) . Comparing your tested values to the manual is only relevant if you have the Honda gauge the manual values were obtained with .

I'm aware of the compression factor when using rubber gauges.  I have a metal hose which is short limiting the error. I didn't have the throttle fully open but it was close (the throttle cable is broken so I manually adjusted the air intake by hand)

Either way 90 psi is pretty low.  I'm not sure what I'm gonna do with this one but probably turn it into a touring machine with a full fairing and bags.  I scored a calafia fairing from a member along with bags.  (thanks again!)  I'm just glad it runs.  I thought this might have been a really good parts bike but it turned out to be a pleasant surprise.

[TwoTired]

You should take a look in the FAQ:
http://forums.sohc4.net/index.php?topic=369.msg476032#msg476032

Compression tests are only good for measuring comparisons between cylinders.  If one is grossly different, it indicates a problem.
Absolute numbers are pretty meaningless, unless you are pretty certain of absolute correct calibration of the tester and know that it adds very low less that 10% volume to the compression chamber.

If you want to know if the rings or valves are leaking then you need to do a leak down test, where you supply a known air pressure to the cylinder and see how much it holds.  If it's leaking, you can then hear it in the exhaust or valve ports (leaking valves), or in the crankcase vent/ leaking rings.  Any cylinder test is most valid with an engine that has routinely run recently, rather than sitting for months and/or years, as the parts all get more cozy fit tighter with frequent exercise (until it it worn out).

Cheers,