Need advice on a rescued 1985 CB 700 SC, low compression, idles amazing but...

[Larry Fine]

It is said that the variation is more important than the absolute pressure.

Even a cheap tester will work well if it will read one cylinder consistently.

[draser]

Did you use the compression tester on a different engine and worked correctly (you tested at WOT right)? Also, do you see an improvement as you keep on riding the bike, due to rings getting loose, valves sitting better, maybe??
Is it quiet up top when running, do the lifters work okay?
Can you borrow/invest in a borescope to look inside cylinders?

[Pete in PA]

For anyone buying a new compression tester the Schrader valve needs to be at the spark plug end of the hose. I can test my 30cc leaf blower and get 120 psi.

You can also test the reading if you have an air compressor with a guage and regulator.  A blow gun tip with rubber end can be pushed into the end of the tester.

[Allen F in NH]

UPDATE:
Hi Folks,

Ok, so I replaced the coils, and it didn't seem to make a difference.  I did do the compression test again, with and without added oil. It was about 120 (W/O throttle) without oil, and 170 with oil.  It runs beautifully of course and will rev right up in 1st but gets a bit funky at high RPM's.   2nd, 3rd and up it acts like it's starving for fuel as it rips above 4-5000 RPM.  I put replacement jets in it, and the rest of the fuel system is refreshed, but I now wonder if the replacement jets are not big enough or perhaps it's the modern fuels?  Anyway, thanks for the input so far.   And FWIW my compression tester is an old Craftsman.   

[mollusc]

That's what mine was like when I had a clogged air filter.

[Allen F in NH]

well, the air filter is new, but it's an easy to check... 

[Bob H]

2nd, 3rd and up it acts like it's starving for fuel as it rips above 4-5000 RPM.  I put replacement jets in it, and the rest of the fuel system is refreshed, but I now wonder if the replacement jets are not big enough or perhaps it's the modern fuels? 

It's not the fuel, or all vintage bikes would run like crap (they don't).

• When you did the jets, did you ultrasonic the individual carbs full disassembled?
• Blow compressed air through each passage and ensure clear?
• Smallest jets fully clear?

I have experienced what you describe when there is a motorcycle that was stored, small jets clogged, idle adjusted up so there is enough fuel to run. But at higher throttle not all jets are contributing and it won't achieve full RPM.

[mollusc]

Have you run a couple of cans of B12 Chemtool or Seafom through it?

Also, have you tried spraying starting fluid around the carbs and intake boots while it's idling to see if the revs increase, indicating an intake leak?

[Allen F in NH]

Yes, I sprayed starting fluid around the intake boot seals...no leaks.   I did not try running chemtool through it but it did sit for several weeks with a combo of ATF, PB Blaster etc in the cylinders.   It was locked up of course from sitting, but eventually did come loose with gentle persuasion.

[Allen F in NH]

AS for the carbs and ultrasonic cleaning, I did thoroughly clean every passage.  These are quite simple compared to other carbs I have rebuilt, but again, I used aftermarket jets.   The needles looked fine and original.  Maybe I will clean and reinstall the factory jets just to be sure.   

[Willie]

By any chance did you take a close look at the diaphragms? They might have cracks.

[mollusc]

Putting stuff into the cylinders isn't going to clean any gunk out of the carbs.
I know that you say you cleaned the carbs, but you probably know that they can often still have debris trapped in some of the tiny passageways.  Just because the jets are clear doesn't mean that all the passageways are fully open.

[Allen F in NH]

Diaphrams looked good.   Can't hurt to reclean the carbs.  Pretty sure I cleaned every passage, but who knows.  It's a pretty straight shot from the bowl through the jets.   Odd that it will rev happily in 1st all the way up but chokes in 2, 3, 4 etc. 

I am going to do  valves and rings and will circle back.   I have 2 other bikes to ride right now and there's no point in fussing with it until the crappy weather comes.  I intend to put the old factory jets back in as well.   

[Bob H]

I am going to do  valves and rings and will circle back. 

It only has 22k on it...

I bet you can (eventually) get her running great without pulling the head.
My late model NH 750 has 40,000 miles and all I have had to do is carbs and new spark plugs.
My goal is to not pull the head until I am well beyond 100k.

[Larry Fine]

Odd that it will rev happily in 1st all the way up but chokes in 2, 3, 4 etc.

That sounds like clogged pilot jets to me.