Topic: Cb500 Running on three cylinders

[Goldbug]

So I put in a used "good" motor. I've gone through my carbs three times now. Admittedly, the carb bowl in on the non-running cylinder was filled with rust and other junk and the engine was run with this crap in there briefly. I suppose it's possible that junk got into the engine and scored the cylinder.

The cylinder gets spark and plugs have been swapped to no avail. The bowl fills with fuel and the jets have been checked and rechecked but the carbs have not been totally torn down so I can't say if every passage is clean. It did appear that there was gasoline getting past the carb because when I pulled them it looked wet inside the rubber coupler.

When I put my hand over the carb inlets on the other cylinders I can feel the vacuum. I can't on the bad cylinder. My friend said this indicates low/no compression because he said the vacuum would exist even if it wasn't firing. His logic seems reasonable but I just wanted to see what you guys thought.

I have not done a compression check but I will ASAP.

[Gordon]

Remove the valve tappet caps and see what's going on when you rotate the engine. 

[Goldbug]

Ok, I will do that. Thanks Gordon.

[Gordon]

Definitely still do the compression test, though.  Checking the valve tappets is just a quick way to see if there's anything major wrong with the valve stems or rockers. 

[Goldbug]

Yes I will for sure. I need to get a compression tester though.

[Goldbug]

The valves seemed to actuate fine. But it has low compression and it's probably rings/cylinder because a wet test gave me a 30psi improvement.

Most of the cylinders were 105ish, bad one was 60 dry and 90 wet.

So much for a "good running motor." I guess it runs OK, just sounds like a VW bus and is slow. Glad I only paid $200 for it.

[Goldbug]

So I've been reading some other threads about low compression, is there anything I can do to try and identify and/or fix the problem if it's just from the motor sitting?

Adjust the valves?
Use some sort of cleaner?
Bang on it with a hammer?
Do donuts until it runs of 4 cylinders?

I did notice that when I was doing the first dry test my gauge jumped up to 90-ish but then all subsequent tests were lower, could it be an intermittent problem from deposits or crap in the cylinder?

I just want to make sure I've exhausted all options before tearing the motor down.

[Oddjob]

It's possible the bad cylinders rings are stuck in the piston grooves, try removing the plug for that cylinder and drop some redex or similar down the plug hole, leave overnight so it has a chance to soak in. MOST important don't install the spark plug until you've span the motor over a few times, if the cylinder is full of oil and on the compression stroke and you seal it with the spark plug it will bend the conrod. Make sure you give the oil a chance to escape first by leaving the plug out whilst spinning the motor over. Do another compression test after it's run for a while and see if it improves.

[Goldbug]

I used to drop oil lamp wicks down sparkplug holes to soak up residual oil, or treatments or whatever. Not sure if that's a new idea or an old trick.

[mrbreeze]

I don't know if you have any mechanic friends or not but I don't think a repair shop would charge you a whole lot of money to do a leakdown test on it. Its the best way to know whats goin' on before tearing it down.