Topic: Compression check

[Dusthawk]

This is what my Rita, a '71 CB750 K1 Chopper, tested out at:

without Marvel Mystery oil:

1: 150 pounds
2: 110 pounds
3: 110 pounds
4: 150 pounds

With oil

2: 210 pounds
3: 185 pounds

I was so surprised with the 210 that I forgot to check the other two with oil.

I figure that the rings are toast but isn't 210 high? We checked it twice to make sure it wasn't a fluke. I wasn't as surprised by the 185 as it was still a reasonable measurement.

Ideas?

Jeff

[750goes]

210 is pretty high, so is 185.

Getting correct readings on the inside two cylinders is sometimes not easy if using a handheld compression tester
what sort of tester did you use,,, hand held or screw in style..
was the motor warm/cold ?
throttle open/closed ?
choke open/closed ?
they will have an effect on the measurements
so will badly adjusted valve clearances..

I would do the measurments again - or let us have the answers to the above questions for a better diagnosis.

whats the reason for the test anyway??
bike blowing smoke if so what colour??
leaking oil everywhere where from

keep asking questions you are sure to get a few more answers here from more members.
 

[scunny]

wouldn't the amount of oil put into the cylinder raise the compression. ?
ie.. more oil, higher reading.

[750goes]

Scunny,

The amount of oil put into the cylinder makes a difference in the compression readings, if the rings "are toast" then the oil will help seal this off and report back via the compression reading with higher PSI.

I would think that if excess oil was put down the cylinder, it will be blown out the exhaust after a few strokes or past the rings into the crankcase,, if so ignore the first test, and do it again straight after (enough oil in the cylinder now for a reasonable test result)

Measure the three results
1 - no oil
2 - lots of oil maybel
3 - less oil

I used a syringe without the needle to measure and squirt oil into the cylinders when I did my tests.

[Dusthawk]

Screw-in style gauge
Engine cold
Throttle wide open at the stops
Choke full off
Valves adjusted 'perfectly' Rechecked three times working on problems.
Plugs(D8EA) fouled black with dense powder deposits
Smoke(Greying White) out the pipes
Some oil or at least black stuff coating the insides of the straight pipes
a couple of squirts from a small oil can of Marvel Mystery Oil

The reason for the test was to get definitive conclusion for bad valves/guides. With the fouling plugs it looked like oil fouling to me so I thought how could the oil get there and the reasonable thing I came up with was oil getting by the valve seals and guides.




Jeff

[scunny]

my understanding of smoke issues is that if it's valve guides the motor will smoke straight away after it's been sitting for a while and if it's rings you get smoke under acceleration

[750goes]

Jeff,

Does the bike flow the same coloured smoke all the time or only after it is warmed up

One condition that can cause high compression is overly rich running which will cause over a period of time a build up of carbon on top of the pistons or wherever else (the exhaust port gets lots) it can lodge itself.

Low compression can also be a result of a "Stuck Ring" on an engine that may not have been run for some time. They can come good after some spirited running..

What is the history of the bike as you know it -

If you rule out the tune up scenario items,    then it sounds like a bad valve seat or guide...or possibly the rings.

[Jay B]

As I understand it, if oil raises the compression, it's bad rings, if it doesn't its bad guides.

[Dusthawk]

750goes, first, thanks for your insights,

The smoke flow all the time, but I get higher flow during accel.

Up until this last year the bike never sat around for long. The first owner who built it into the chop had it until '94 and used it as his commuter from Vallejo to SF(about 40 miles each way) in everything less than a downpour. The next two owners rode weekly to 'get away'. The last owner used it as a commuter from Mill Valley to SF(about 15 miles each way) six days a week for work and UCSF.I have had the pleasure to own it since October 2004 and ride as much as possible when its running.

By the way, what happens when I run resistor plugs with the caps? I heard the caps are already resistor. I was running two resistors(DR8-ES-L) and two regular(D8EA), now running all four non resistor(D8EA).

Hope this helps,

Peace,

Jeff

[TwoTired]

In my experience, cold compression readings are lower than when the engine is hot because piston, ring and cylinder tolerances are greatest when cold.
Added oil will still seal off worn rings, etc. during the compression check, recovering most, if not all, of the piston-ring-cylinder seal.  The differentials between wet and dry readings are usually less with a hot engine, provided ring wear is is the problem.

Aslo, the combustion chamber is about 20CCs,  And, a teaspoon of oil is about 5CCs.  Oil is not compressible. So, it can have a substantial impact on compression when used to excess.  There is also the volume of the tester to consider, and if it is storing oil or not.

All this information doesn't change the fact that along with your oil smoke, you're probably going to have to do a ring job.  The extent of which will be determined after physically measuring the dismantled components for wear factors.

By the way, what happens when I run resistor plugs with the caps?

Assuming a points ignition, misfires when the plug nears end of life.  And, a shortened plug lifespan with this ignition system. (interactive)

Cheers,

[Dusthawk]

Thanks everyone, yesterday I found a guy who has boatloads of parts and he is giving me a partially disassembled rebuildable for $150 delivered. He said he has tons of parts and wants to get rid of them all. Need something? Let me know and I'll see if he has is etc.

Anywho, I'll ride Rita with the current engine until I get the new engine done then its off to take a long ride somewhere.

Peace and Long Rides,

Jeff

[Boomologist]

Did a compression test on a VT700. I had the gage in the rear cyl and it took off running. Using just the starter I was getting 100psi but when it started it read 160.