1975 Honda cb500t twin running on one cylinder

I'm stumped. Let me preface this by saying Im not a mechanic, but I've followed the book, and worked on this bike multiple times over 4 years.

I had the bike apart to fix some leaks, replaced the piston rings, replaced all 4 valves (3 were bent), put it back together with all the timing marks lined up. After installing new points I checked compression and spark (good on both cylinders). I got it to fire but only the right cylinder, so I rechecked the timing, points, spark, compression....tried again, only ran on right side. I shot a lil ether into the left carb and it sputtered a bit but wouldn't run (in the past I've done this to kick the left side into action and it worked.

So I think I'm getting no fuel (keihin carbs, they are side specific or I'd swap them to check). I removed the drain screw and very little gas came out hinting that no fuel is getting into the carb, so I replaced the fuel lines and the fuel line T splitter, cleaned out the orifice into the carb, checked the float and float valve (is that what it's called?), all clean. Put it back in, it still seems like fuel is not going into the bowl. I even bypassed the other carb thinking maybe ALL the fuel was going to the right carb. Nada. Does the carb need vacuum to fill the bowl? What would cause the bowl not to fill? WIth the bowl off the float raises freely . The carbs aren't heavily covered in the book I have. I can track down my mechanic this week, but I've been pushing on this bike, and being stuck at this point is bothering me a ton.

I have cb450 which is essentially the same bike. Do you have the stock dual petcock? That may need to be disassembled and cleaned. Those Carbs should also be taken apart and cleaned. It's not that hard but I understand how it can be daunting. You should check out hondatwins.net. they have model specific forums and Honda twin gurus over there. Tons of info.

It sounds like you're more of a mechanic than you think. You've already got the problem diagnosed. Take the float bowl off the bad side carb and put the fuel line that goes to it in your mouth. Blow through the line while lifting and lowering the float gently. When the float is down you should be able to blow through the line and when raised you should not. It sounds like the fuel inlet or float needle valve on that carb is plugged. If that's the case, remove the float and the little needle valve and shoot some carb cleaner throught the fuel inlet. You may need to do this several times and even possibly use compressed air. That should take care of the problem for now. Those old carburetors were strictly gravity-fed and require no vacuum or anything to fill the float bowl. They are simple to rebuild if you just take your time and do one at a time. I sure would like to hear if you get it running right. Another thing, the ethanol in today's fuel is hell on carburetors. It plugs them up in a matter of days. If you can get non-ethanol gas, use it.

Originally Posted by Stadger

It sounds like you're more of a mechanic than you think. You've already got the problem diagnosed. Take the float bowl off the bad side carb and put the fuel line that goes to it in your mouth. Blow through the line while lifting and lowering the float gently. When the float is down you should be able to blow through the line and when raised you should not. It sounds like the fuel inlet or float needle valve on that carb is plugged. If that's the case, remove the float and the little needle valve and shoot some carb cleaner throught the fuel inlet. You may need to do this several times and even possibly use compressed air. That should take care of the problem for now. Those old carburetors were strictly gravity-fed and require no vacuum or anything to fill the float bowl. They are simple to rebuild if you just take your time and do one at a time. I sure would like to hear if you get it running right. Another thing, the ethanol in today's fuel is hell on carburetors. It plugs them up in a matter of days. If you can get non-ethanol gas, use it.

I'm still not a mechanic, lol, I've just worked on this bike way too many times. Actually my mechanic buddy tells me if I ever get to work on a simpler engine I'll be pretty good. This engine has even stumped him a few times, and he's built drag bikes for years. I'll try that trick you suggested, I have a length of new fuel line I can use.

I've cleaned the carbs several times, I removed the float needle valve assembly and used a small straight pick to make sure the inlet was clear and it is. I reinstalled the entire float assembly and the float needle valve rises freely during visual inspection off the bike.

In the float needle valve, there is a small spring loaded tip that the float itself hits, this is inside the larger valve. I might have to take a photo because its hard to describe. The valve itself is like 1/4" diameter 1" long, this small tip I'm talking about looks like a bic pen tip and is opposite the end that seals the larger valve. Is this an orifice that needs cleaned, or is it just to pad the float itself? I don't see where it goes all the way through. Any clue on that?

The other thing that bothers me is I have a single petcock (which doesn't dump fuel as fast as I think it should, so I'll clean that out). The fuel line exits that and goes to the rear of the engine and then I have it T'd to the carbs . It's clear fuel line and it's got a big air bubble in it (even with the gas cap off). What would cause that bubble to not be able to exit? There is a low spot but I can push it down so there is a constant downward flow available, and I remember from science class that as long as the other end is lower, it should flow freely to the bottom.

Sounds more like an ignition issue. Are you sure you have spark at both cylinders? Most twin parallel twins actually fire 180 degrees apart if I'm thinking correctly. Not both cyl at the same time, but some coils have a wasted spark too. Have you put a different ignition or coil on it?

If you have gas in the carb bowl and its pulling air then I'm thinking it's something else

The orifice that the "pen tip" seals is what you need to clear. If the pen tip is removed the gas (or blown air) should go from the fuel inlet through to the orifice easily. Right now I'd stay out of the petcock. Single Honda petcocks supply gas to 4 carbs with no trouble. If the petcock is in trouble, you'll notice it at high speeds long before it causes one cylinder not to fire.

Good point with the petcock but I had to replace mine because the screen on top was fucked. I actually only fired on one cylinder when I got my 450. I had to clean the points to get it to fire but it was still troublesome. So I replaced the coils. Solved my problem. She's been pretty reliable since.

Fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuckkkkkk yeeeeah!

I just got it running on both sides. It's loud as all hell so I'll wait til tomorrow to dial it in since the neighbors are probably all sleeping....

Turns out I was setting the float off the wrong point so it was burying the float needle valve as soon as I put the bowl back on. I used the trick Stadger suggested, blew into the fuel inlet and flipped teh carb upside down and back and it was always closed. Read the book more closely, reset the floats, and off she went. THe right side carb is overflowing, so I gotta fix that. but I should be able to test ride it in the next couple days weather permitting. Oh, and the fuel flowed to the carbs right away with the better float setting, so I might still clean the petcock, but it's not a problem.

Thermo: The bike also does this wierd thing where (each time I've wired it anyway) something in the ignition setup grounds out the bike, so if I hook my test light to the negative on the battery and put the tip on the block, it shows hot power. Like, maybe the points ground out or something. I noticed the end of my exhaust was arcing back to the bikewhen I had it running (hadn't bolted the outermost mount yet). Is this at all common, or do I need to hit up Hondatwins for some electrical info?

Cool you got it running!!! Feels awesome when something works when u put effort into it.
But as far as the hot on the test light. Is the ignition on when you use the light. If something like let's say your regulator is grounded to the motor (which is how my xs was when I got it. Everything was grounded to the bolts on the motor) then I could see how you would make that circuit. But I wouldn't consider that normal unless you were putting hot to the motor. I could be wrong as I'm also not a mechanic or an electrical guy and I have limited experience In both. Other than what I own. Nahmean??
Also the arching at your muff is not normal and sounds like you have a short possibly touching your frame somewhere. ? And you should check out hondatwins anyway just cuz those guys know shit about these little twins, And its a good forum.