Topic: Cylinders 2 and 3 not firing, gas in my oil 79 CB650

[bulldog]

Hi guys.  I've got a 79 CB650 I've been working on.  I rebuilt the carburetors and it was sure running great.  I was driving one day when the center two cylinders died, then came back, then died, back and forth a few times until they quit firing altogether.  When they were quitting, it seemed like it would run better at low throttle.

At first I thought electrical since 2 and 3 fire together, but I went all through the electrical system and everything is fine.  Then I noticed my oil was full of gas.

What does this mean?

Thanks a bunch for any info you can provide.

[ekpent]

The only way lots of fuel could be in the oil is if it is not draining out of the proper overflow tubes of the carb and back into the engine. Maybe float stuck,not turning off petcock if needed etc. Some fuel could get in there if a couple of cylinders were not firing at all.Are the overflow tubes clear ?

[bulldog]

I rode it for a while with the center two cylinders not firing.  A couple miles anyway, until it wouldn't even run.  It seemed to run progressively worse, it wasn't an instantaneous thing.  Cylinders 2 and 3 were either firing or not firing together, it was really odd, do they share an overflow tube or something?

This incident happened years ago and the bike has been stored ever since.  I'm finally getting the opportunity to work on it again this winter so I'm trying to plan out my repairs.  I've got the bike all disassembled for a cleaning so I can't try anything and see if it works.  I'm just looking for pointers as I make a list of things that need done.  Looks like I have to double-check my work on the carburetors.  Thanks!

[jason41987]

it just seems like such a coincidence that its 2 and 3 at the same time, my first thought would have been a bad coil as well, or a bad point or wire leading to that coil... what did you do to test them?

[BobbyR]

2&3 and 1&4 fire together. This is a waste spark system and the engine block is part of the circuit. One bad plug can cause the other not to fire. I would look from points to the coils. If you have a Dyna I know that is your problem. The Carib overflow is a separate problem and a coincidence.

[jason41987]

often overlooked too, is the spark plug caps, they actually have a little resistor inside that can go bad... a couple of mine did and the previous owner never thought to check them, then figured the bike simply wouldnt run and sold it to me... changed a few $2 caps, cleaned the carbs and it had no starting or running problems at all besides that.... for some reason, i think it was the 2 and 3 caps that failed

[bulldog]

I have two of everything in the ignition system from the spark plugs on back to the pulser generator.  I thought it was something electrical too because they're linked like that, so I bought replacement everything and tried them one by one to see if they fixed the problem.  I know everything there works, it's getting a good spark in both cylinders.

My dad suggested blown head gasket between 2 and 3?  Does that sound reasonable?  It's probably on the factory gasket.  I think it running for a long time on only two cylinders along with the blown head gasket may be enough to explain the gas in the oil without any carburetor problems.

Edit: One thing just occurred to me, what if the timing was off for just cylinders 2 and 3?  Is that possible?  I watched them both spark but perhaps it wasn't at the right time.  One thing I didn't check in too much detail was the points, I have no idea how that works to be honest.  I'll go do some reading on that now.

[iron_worker]

Yes it's possible. You set the timing for 1 and for and then you set the timing for 2 and 3. Check the timing.

A blown head gasket between 2 and 3 would cause a loss of compression and possibly cause it to miss altogether if it was really bad. A compression test would verify this.

Still seems to me like it could be an electrical problem since you say it would come and go. Generally mechanical problems don't just go away.

IW

[LesterPiglet]

I would do a compression test. How would fuel get in the oil? Past broken rings possibly. 2 and 3 are the inner cylinders, was there a partial seizure?

[curemode2002]

No Points on the 650's they are all electronic ignition from honda timing could be weird. If you look under the timing cover you will have two pickups and the rotating magnet. Make sure the spring on the rotor is free and moving, if it is frozen up your advance will be all messed up (especially on the higher RPM range), past that you can't move the pickups much individually so timing is pretty spot on.

+1 on compression testing too that will tell ya real quick if you have a issue with the head gasket or rings

From what I can remember on the 79's I believe the carbs have always been level on the 650's so an overflow seems to run in the airbox before the motor. But if she has been sitting clean those carbs.

[spanner boy]

Fuel gets into the oil because its not being burnt in the cylinder in circumstances such as misfire. It then gets pushed past the rings on the compression stroke. I'm with the electrical problem theory. I would start with a static timing check and then swap components with 1 and 4. If that doesn't show the problem start checking the wiring. Good luck.

[Bailgang]

All of the above is great advice but if you haven't already done so then the absolute first thing to do is change that oil and keep checking it for evidence of gas contamination until you figure out the problem. You may end up changing the oil a few times or more depending on how quickly you can figure out the problem but you can ruin the engine real quick if too much raw gas is allowed to contaminate the oil.