Topic: Suggestions? CB500 firing on cyl 1 & 2, no 3 & 4

[Jbird2200]

Hi all,

Long time reader here, but don't post often/ever.  I've learned a ton from this website and am currently in the process of rebuilding a 71' CB500k.  I've redone the wiring entirely on the bike, as the previous owner had spliced into just about every wire possible.  I was very careful in doing the wiring, and am certain I have power going to and coming from all needed sources, with one exception it seems.

 I fired the bike for the first time two days ago, and tried to set the idle today when I realized that only two cyl were running, the header off of 3 and 4 was cold to the touch.  I'm thinking it's an ignition issue, with my first thought being related to the points in some way, but I honestly know very little about these older ignition systems.  I'm simply looking for a guideline here as to where to start.  Any help is greatly appreciated...I really want to get this thing running right.  I've read the oldmanhonda how to on ignition timing, but I'm not sure how to start diagnosing it...would make much more sense to me if it were 1 and 4 or 2 and 3 not firing, as that would point to one single coil.

I've throughly cleaned and rebuilt the carbs, and currently getting fuel to the bike through a rigged system with a funnel and new fuel lines, as my tank is in the process of being redone.  I'm not sure what other info woul dbe helpful to know.  Thanks again to anyone willing to lend a hand.

[rbmgf7]

If it's 1-4 and 2-3, then it's possibly the ignition. Since it's 1-2 and 3-4, check to see if 3-4 are even sparking. Could be wire related or a bad resistor/boot. Next, are the 3-4 carbs getting fuel? Not sure on the plumbing for a 500. If not, check to see if the floats are stuck and not allowing fuel to fill the bowl.

[Jbird2200]

Thanks for the quick response. I'll be sure to double check the carbs, but having just rebuilt them, I've gone through them all a million times to make sure they were functioning properly before reinstalling.  I need to try to test the spark at the boot this week. 

So if I have spark at the boot, then it must be a fuel issue?

What if there is no spark at the boot, then where should I turn to next?

Thanks

[rbmgf7]

Thanks for the quick response. I'll be sure to double check the carbs, but having just rebuilt them, I've gone through them all a million times to make sure they were functioning properly before reinstalling.  I need to try to test the spark at the boot this week. 

So if I have spark at the boot, then it must be a fuel issue?

What if there is no spark at the boot, then where should I turn to next?

Thanks

Carbs are mischevious. I had an overflow problem which I disassembled, cleaned, and reassembled a set of carbs three times in a row due to dirt propping the float needle open.

If you have spark and the plugs are dry, then yes, it's fuel.

If no spark and wet, check to see if your boot is seated into the wire and is contacting the internal wire. May need to snip a little off the end of the wire to get a good contact. If there still isn't any spark, check the resistance in your boot. You'll need a multimeter and reference the proper resistance needed.

[dave500]

the twin outlet fuel tap might have a blockage?drop that filter bowl and have a gander,also if the tank is low and on reserve it might act up like that,check the fuel line isnt kinked to 3/4 carbs,i dont think its ignition to have that pair out of action,plug caps on the correct plugs though?

[scunny]

sure you haven't got 3 & 4 spark leads reversed ?

[the technological J]

make sure you bike is level or on the center stand.... make sure the bike isnt on the side stand

[grcamna2]

+1 on what the technological J just said...having it on the centerstand will get fuel into all 4 carbs better and keep the float bowls filled...as long as your temporary fuel tank has unrestricted flow and plenty of fuel in it.

[dave500]

ok so was it doing the same with tank installed?i just scanned over the thread and now see you have a temporary fuel set up,im guessing the temporary set up isnt feeding carbs 3/4?

[TwoTired]

If you are sure carbs 3&4 are getting fuel (there are two fuel lines for the 4 carbs), and you haven't swapped ignition leads on 3&4, then check that the valves move on those dead cylinders.  If the cam snaps, then two cylinders usually go dead.

[dave500]

youd be flat out snapping a 500/550 cam i think tt,,i think it would be making some other ghostly noises if the camshaft had snapped,although then again some will try and start anything and never surrender?till the battery runs out?

[Tugboat]

Make sure you have the floats installed correctly on #3&4 carbs.

[Jbird2200]

Thanks for all the thoughts guys. I work 10-12 hour days, so I wasn't able to look at it today, but I'm gonna get out there and try all that you've suggested, and I'll be sure to make 100% sure gas is getting to all 4 carbs, I'll check the spark also, as that will be an easy way to exclude a lot of the possibilities.

The bike is on the center stand though, and the plug wires and boots look like they're relatively newer ngk replacements, but once again I haven't checked em' to be 100% sure.

I'll report back my findings after I'm able to look at it tomorrow.

Thanks again

[:c:]

Another vote for carbs. It took me a while to realize the fuel line was kinked and fuel wasn't flowing. I suggest clear lines for the setup and you can change them later. after all, fuel and spark go boom and that's all there is to that jazz.

Pull plugs, crank, look for spark, return. If you have spark you don't have gas.

Alternatively, you can squirt some starter fluid in 3 and 4 and see if they catch (listen to the pipes). This is the more "exciting" way to find out. Use caution... YMMV...

[Jbird2200]

Well thanks once again guys for all the quick responses.  After taking a close look at my temporary fuel set up, it was easy to see that the fuel was freely flowing to 1 and 2 carb, but not to 3 and 4.  I now have all 4 running, albeit roughly (this bike sat for 4 or more years). 

After letting the bike warm up, it seems I may have a valve ticking somewhere in there, not too surprising..

I also have an issue it seems with how the exhaust is mounted up as I can feel the exhaust coming out of the head on the number 4 cylinder, and the header is at somewhat of an angle, doesn't seem to be flush....

So once I solve the header mounting issue, I can move on to the broken clutch rod, and see if I can't get the clutch to engage. 

Thanks again, you all made a simple solution eliminate a lot of guess work for me