Topic: dead cylinder during the inaugural ride of my cb500

[phaedrus314]

Hi all, I finally got my 73 cb500k2 out on the road this weekend.  It is all stock and original, carbs recently cleaned with a Berrymans dip, air, and ultrasonic simple green bath with all new rubber parts and original hard parts, and a bench sync.  6,500 miles on the bike.

Anyhow, I took it out this weekend for the first ride of any length and after a bit noticed an occasional slight backfire and crummy idle.  I was out for about 30-40 mins and filled it up at a pump during this time.  I don't think it was acting up before the fill-up.

So long story short, I lost #1 cylinder, and when i drained the carb bowl i found some small chunks (possibly rust.)  The Plug is heavily sooted and the pipe on that cylinder has had some bluing from some point in its life.

After some research, I discovered the filter screen in the petcock has separated a little in the area of the post so I assume the rust is from the tank.  All fuel hose is new 5.5mm Honda stuff.

I already dropped the fuel bowl and pulled the jets and mixture screw and flushed carb cleaner through the passages with no change.  So, where am I now...should I cut my losses and rip off the carbs and go through them again, or is there a less drastic approach that has a reasonable chance of success?  Is there any solvent that will do anything to rust?  Is there a more likely culprit than the carbs?

[Dimitri13]

Since you found rust in your float bowl, there's likely rust not only in the jets, but the passages as well. Depending on how back the rank rust is, people will either suggest evaporust or a full tank liner kit.

Remove the carbs, blow out all the passages every which way with compressed air and carb cleaner. Clean/line the tank, and fix the petcock filter problem. In-line fuel filters may also help.

[phaedrus314]

Is it really a problem to have very small bits of rust in the tank?  If the screen is functioning properly won't that prevent pieces big enough to matter from getting into the carbs?  I'm not a big fan of tank liners.

I am not afraid to tear down the carbs, and will if need be, but I'd rather avoid it if possible...if not so be it.

BTW, thanks for the input.  -Chris

[w1sa]

Rust in the system is definitely a concern.......but,  a soot fouled plug might be more indicative of poor spark,  rich mixture, blocked air jet/passage or a combination.....how did the other plugs look?

[scottly]

It only takes a small piece of rust to hold the float needle open. Did you remove the needle and seat and flush it out? I'd do the simple "clear tube" fuel level test first to see if it is a float needle issue. Also, after your last cleaning, did you replace the carbon fouled plug?

[phaedrus314]

When I asked about rust in the system, I was referring to small/tiny bits (none bigger than a grain of sand or 2) in the tank which I suspect may have been stirred up when the tank was filled.  Wouldn't they be stopped by a properly functioning screen filter? 

Wouldn't a stuck float needle cause the bowl to overflow?  I didn't pull the float needle and seat when I rinsed it but did flush the other 2. 

I didn't replace the plug yet.  As this was a very early test run, I haven't gotten to dial it in yet at anything above idle or under load.  It was initially running fairly rich and sooting up all the plugs.  The other 3 are sooty also from initial start/tune but now have a more normal burn pattern developing.  The cold cylinder's plug was still very dark.

I didn't do a visual spark test but did clamp an inductive timing light onto the lead and it was strobing normally.  Will a timing light strobe on a non-firing plug?

[scottly]

I didn't replace the plug yet.

Replace the plug. I went through hell once, after fixing a flooding carburetor problem and not replacing plugs fouled before the repair. It acted the same. After days of agony, rebuilding the carb, swapping on another carb, rebuilding it, an older, wiser fellow told me "Your plugs are fouled". I told him I had cleaned them with a wire brush and carburetor cleaner. He said "Your plugs are fouled, replace them". I replaced the plugs, and the motor ran fine. The first time I rebuilt the carb, I had fixed the problem, but it was masked by the fouled plugs...

[cgswss]

As you are early in the process of putting it back on the road, just swap plugs with your "best" cylinder.  I'm assuming you have at least one plug that is a nice tan color.

[scottly]

As you are early in the process of putting it back on the road, just swap plugs with your "best" cylinder.  I'm assuming you have at least one plug that is a nice tan color.

If the plug is fouled, then the "best" cylinder won't fire anymore.