Topic: compression test

[emilios]

Hi
I just bought a compression tester and i checked my cb500K1 four..
I put throttle to full open position and air lever too..i dont have a kickstart so i push starter button for a few turns..
On all four cylinders reading was 100 psi only...
What that means?
I have a problem?
Bike was cold.
Please help
Thanks

[TwoTired]

You don't have a compression problem.

See the engine FAQ for compression test number explanation.

[eurban]

TT replied while I was typing this; yes read his thorough explanation in the FAQ. . . . .

This type of question comes up fairly frequently here.  In all likelyhood, your test equipment is adding enough volume to your combustion chamber to significantly lower your compression readings.  Testers designed for cars (which have a much larger combustion chamber) can easily add 5cc to your tiny 14cc combustion chamber during the test and this effectively lowers a 500s compression ratio to around 6.5 to one.  Guess what the result is?  Lower than expected compression readings.  Your readings are all even. This is what you should take away from the test and it is good news.  Engines simply don't wear out evenly so the chances of getting an actual real compression reading in all four cylinders of 100 psi is very small. . . . In all likelyhood your engine is fine.  Why are you testing compression? . .   BTW your engine should be warmed up for a proper test . . .

[emilios]

Thanks guys
Well im testing my compression cause i have a strange problem.
I run bike for a 20 or 30 minutes and bike start to work "heavy" under 3000rpm..when i stop to traffic ligths and leave bike idle(1200rpm)
bike suddenly stop after 4 or 5 seconds..
My bike is standard pistons,carbs,air filter,exsaust etc...
I start from compression test and i also checked valve tappeds and points too..all are ok.
Carbs are balanced.
What could cause that problem?
Thanks

[emilios]

Help

[TwoTired]

When the engine stops, can you spin the crankshaft with the kick starter?
Can you describe the spark plug deposits when the engine stops.
Have you checked the carb bowls for debris that may block the jets?

[emilios]

  my kick starter mechanic is broken so i cant check that..
the other 2 things you said i didnt checked them..i will do it next days and i will tell you..
You think is carb problem as i think?
If crnkshaft dont spin when engine stops what it means??
Thanks for reply

[TwoTired]

I'm trying to determine if the pistons are seizing in the cylinders due to overheating, oil starvation, or both.

[gkaraolides]

Hi Emilios,

I had been having some strange problems similar to yours, with my engine behaving really strangely after say half an hour.  I replaced my fuel hoses with the proper 5.5mm internal diameter hose form David Silver Spares, and fitted the correct Honda clips, and the problems seemed to go away.

My 750K has been off the road while I was trying to get the alternator rotor off to get at the starter clutch, and fix a problem with the starter spinning and not turning the engine.  I hope to sort that out soon, and then go back on the road to verify that the 5.5mm hose and clips have really solved the problems.

Best regards,

George

[emilios]

Ok
I will put clip on fuel hose..
If problem is still there i will check plugs and i will try to spin crankshaft from the crankshaft nut behind points cover..
This will start showing me where the problem is..
Thanks guys...i will let u know..

[TwoTired]

If problem is still there i will check plugs and i will try to spin crankshaft from the crankshaft nut behind points cover..

Don't put very much torque on that nut.  It will just snap off.  It looks big, but it is not strong.

Does the electric starter turn the crank when the problem occurs?

[emilios]

Yes...electric starter turns crank when problem occurs..and bike start quickly.

[TwoTired]

Yes...electric starter turns crank when problem occurs..and bike start quickly.

OK, that eliminates the seizing possibility. (Hooray)

How about the tank vent?  Is that clear?  When the problem occurs is there an inrush of air while opening the gas cap?

How full is the gas tank?

[emilios]

i just put gas in tank..
I will go for a ride later...and i will check the things you said.
Thanks

[emilios]

I forgot to tell you that yesterday i was checking my ignition timing and i found it retard...so i set it again correct..
This could cause overheating...i hope the correct timing will help me..
we will see today

[emilios]

Ok
So i did the trip with full tank..but the problem was still there..i opened the tank cap and no inrush air out of tank..
threre is the foto of sparks when problem occurs.

[emilios]

I just checked all 4 plugs..and here what i found..
Number 1 and 4 are grey(good i think) number 2 and 3 black(not oily just black)
P.S. When i was restoring bike one coil wire was badly damaged so i order a new one from ebay for cb500four and i put it to number 2,3 cylinders..
Can this coil cause the problem?
I have a new one that i bought from david silver before a week.
?

[TwoTired]

How old are the condensers for the points?

[emilios]

the condensers and points are new. i put them before 2 weeks

[emilios]

Ok
I did some measure to my coils..
I removed plug caps..
Old geniune coil(1,4) from lead to lead reads 14.7 k ohm and primary connection reads 5 ohm

New coil from ebay(2,3) reads 17.75 k ohm and 5.3 ohm.

The new coil that i bought from David Silver(not on bike) reads 11.78 k ohm and 2.4 ohm.

In my experience i cant use David Silver coil..i cant even believe that the sell it for cb500four...strange.

Can my coil number 2,3 cause my problem with black,carbon plugs?
Is that my problem?

[TwoTired]

the condensers and points are new. i put them before 2 weeks

It is possible to buy defective parts.  Electrolytics in particular don't do well in storage.

Maybe yours are fine.  However, if you swap sides with condensers, and the problem follows the condenser, it's a good clue.

[emilios]

I will swap condeser..if problem still there i will swap coils...if problem is still on 2,3 cylinders it means that is carb problem right?

[TwoTired]

I will swap condeser..if problem still there i will swap coils...if problem is still on 2,3 cylinders it means that is carb problem right?

Seems like a reasonable approach to troubleshooting, to me.  Eliminate variables until the last one remains and points to itself as the problem.

The equation looks like this:
Air/fuel + Compression + Spark = engine running.

You already eliminated compression, right? 

I would add swapping the spark plugs to different cylinders (1 with 2 and 3 with 4), or replacing the sooty ones with new to see if the problem can be moved to different cylinders by ignition components.  Soot can reveal a spark related problem, but soot also implies improper mixture from the carbs.

Cheers,

[emilios]

I check bike today after swapping condesers..
Problem still on 2,3 cylinders..
Next to check is swapping coils..
I will let you know.

[emilios]

I swap coils and problem still there..
So i took carbs off bike and check again...
Then i notice that i had put some aftermarket idle jets that was shorter than the originals..
I order new (original) idle jets,i put carbs back and problem solved..
Now bike idles great...
Thats the end of the topic..
I just learn that idle jet length its important..