Very confusing carburetor issue...

[Davisgriffin]

Hey guys! I've got a 1975 CB500T, it's been a long project getting her going and I'm so close! But... the right carburetor has been giving me trouble when I give it gas. At first I thought it could be the timing or valves so I put the left carb on the right side, but it worked perfectly, revved smooth up and down. When I give just the right carb gas with my hand it will rev up but sputters and groans at about 6000. If I try to keep it around 3000 it will just go completely quiet. I've taken the damn thing apart about 10 times now and I just have no clue what it could be.

Is it possible that that particular carb body is not seating well on that boot?

I took it for a hard ride around the block, it will ride fine, but I can hear the right side popping and being a shithead, and when I got back I noticed a sort of sweat on the carb next to the boot. Any ideas?

 

[focusinprogress]

if they are clean, they may just need synchronized....that's usually what causes one carb to pop and sputter if everything else is in order. If you're concerned one may not be seated properly or that the boot could be leaking, spray a bit of carb cleaner at the boot while it's running and see if it changes the idle dramatically. if it does, you have a leak.

 

[joevirus563]

Usually it's cause by dirty jets.

But! If they are not sync for sure it can cause problem.

Bench sync then at least and try it again.

Are you sure that fuel line is not kinked?

 

[joevirus563]

If the boot would leak then the rpm would be up high

 

[MotorbikeBruno]

Let's start at the beginning. You have pulled them apart to verify they are clean? Yes? Did you make sure the spring loaded tail on the float needle is springy and not stuck? Yes? Then have you verified float height, from the correct surface and with the floats JUST touching the spring loaded tail on the float needle? Yes? Double checked main jet size is same as other carb? (I'll skip the "yes" part from now on and assume you get what I'm doing...) What is the mixture screw at? Put it back to stock, and if testing without airbox, do half turn more rich to help with starting and testing while the airbox is off. Verify the slides don't bind while installed IN the carb All of that cool? as stated before, spray a little bit of carb cleaner around the boots on the engine side to see if they are sucking air. Probably not since you said it ran fine when you moved the OTHER carb to it's spot. More likely a simple setting or air cutoff failed (see below)

The thing I didn't see mentioned yet was the air cut-off valve. Sticks out the bottom side of the carb. If that's toast, it can let too much air into the system and cause some issues. Have you pulled that apart to see how the rubber looks etc? Sounds like a GL1000 cut-off valve will work in there but don't quote me on that...

LASTLY after you have checked all of that. Synchronize the butterflies the best you can. There's all sorts of methods (the "drillbit" method comes to mind) but if they are close, she should be able to run relatively well.

 

[MotorbikeBruno]

If the boot would leak then the rpm would be up high

Somewhat true It will show itself when the bike is at IDLE. Running lean at idle will cause your throttle to "hang" up and then slowly fall back down, sometimes never coming back down. The other way it shows itself is similar to his issue where at the high end of the rpm power dies off as too much air is being introduced and leaning the mixture causing failure.

 

[joevirus563]

Let's start at the beginning. You have pulled them apart to verify they are clean? Yes? Did you make sure the spring loaded tail on the float needle is springy and not stuck? Yes? Then have you verified float height, from the correct surface and with the floats JUST touching the spring loaded tail on the float needle? Yes? Double checked main jet size is same as other carb? (I'll skip the "yes" part from now on and assume you get what I'm doing...) What is the mixture screw at? Put it back to stock, and if testing without airbox, do half turn more rich to help with starting and testing while the airbox is off. Verify the slides don't bind while installed IN the carb All of that cool? as stated before, spray a little bit of carb cleaner around the boots on the engine side to see if they are sucking air. Probably not since you said it ran fine when you moved the OTHER carb to it's spot. More likely a simple setting or air cutoff failed (see below)

The thing I didn't see mentioned yet was the air cut-off valve. Sticks out the bottom side of the carb. If that's toast, it can let too much air into the system and cause some issues. Have you pulled that apart to see how the rubber looks etc? Sounds like a GL1000 cut-off valve will work in there but don't quote me on that...

LASTLY after you have checked all of that. Synchronize the butterflies the best you can. There's all sorts of methods (the "drillbit" method comes to mind) but if they are close, she should be able to run relatively well.

Cb500t are CV carb so no slide. Of course you need to check the throttle cable and the fuel line for pinch.

 

[Davisgriffin]

I'll try spraying carb cleaner around the boot, but I'm afraid thats not the issue. I think I need to pull apart the air cut off and make sure that is all perfect. I did buy brand new gl1000 gaskets for the air cut off, but maybe something is up. I'll also re adjust the right float height as well. Float heights affect performance greatly. Before I knew what I was doing I moved the actual float bowl up and down, and not the tang. If I've got it sitting vertical and the bowl just barely touching the needle valve at 20mm, would the bowl position have any effect? I'll keep on and report back. Thanks for the help guys.

 

[Davisgriffin]

Any ideas on why the right carb would have "sweat", like condensation, around the boot after a hard ride (only on the right)?

 

[hillsy]

Cb500t are CV carb so no slide. Of course you need to check the throttle cable and the fuel line for pinch.

All carbs have slides - although CV slides are air/vacuum activated as opposed to cable/manual.

Still, if they are worn or not moving freely they will cause problems.

 

[joevirus563]

Any ideas on why the right carb would have "sweat", like condensation, around the boot after a hard ride (only on the right)?

Heat vs cold fuel? Did you checked the plug?

 

[joevirus563]

All carbs have slides - although CV slides are air/vacuum activated as opposed to cable/manual.

Still, if they are worn or not moving freely they will cause problems.

Yeah that's what I meant. Cv carbs slide are not moved by the cable so usually they are not stuck. Since the op put them into pieces I tought he already test the carb for full working.

 

[hillsy]

Yeah that's what I meant. Cv carbs slide are not moved by the cable so usually they are not stuck. Since the op put them into pieces I tought he already test the carb for full working.

It's more important that CV carb slides have absolutely no resistance to free movement than manually operated slides.

I'd assume as the well that the OP has checked this, but ya never know.

 

[Davisgriffin]

Slides are in working order.

 

[MotorbikeBruno]

It's more important that CV carb slides have absolutely no resistance to free movement than manually operated slides.

I'd assume as the well that the OP has checked this, but ya never know.

Yep, ^^^ I knew they weren't cable operated, just tossing out more things that can cause his issues

As for the condensation...that's an odd one. Is it ALL around the boot, or is it in one location? Got a picture or can you draw with Microsoft Paint to give an idea? Only thing I can think of is a vacuum port isn't fully sealed (which could also cause some of your issues...) cracks in the boot itself or are those new? Is it fuel or actual condensation (water of course)?

And I went and re-read your first post and realized you haven't said a word about the spark plugs. That tells you all sorts of things. Popping can be too lean, or FAR too rich....pull those bad boys out and take a picture, that could help us help you.

 

[MotorbikeBruno]

I'll try spraying carb cleaner around the boot, but I'm afraid thats not the issue. I think I need to pull apart the air cut off and make sure that is all perfect. I did buy brand new gl1000 gaskets for the air cut off, but maybe something is up. I'll also re adjust the right float height as well. Float heights affect performance greatly. Before I knew what I was doing I moved the actual float bowl up and down, and not the tang. If I've got it sitting vertical and the bowl just barely touching the needle valve at 20mm, would the bowl position have any effect? I'll keep on and report back. Thanks for the help guys.

You don't measure carbs sitting vertical. You measure them at a 45 degree(ish) angle. Give that a shot And if you have them open...shake the floats to verify you don't have fuel in them as a pin hole is hard to find and will sink your float :'(

 

[Davisgriffin]

So I've got it mostly figured out, and yes I figured it out with the plugs. I think the float bowl was bent during cleaning and was affecting the level of gas even though I set the height at 20mm. I bent the actual bowl down and got a drastic improvement, although still not perfect. I went and bought a few new plugs and kept tweaking until I got it right. I had no idea the float level had such a drastic impact on performance until now. It's still not exactly perfect, but I'm getting there. Thanks for all the help..

 

[MotorbikeBruno]

So I've got it mostly figured out, and yes I figured it out with the plugs. I think the float bowl was bent during cleaning and was affecting the level of gas even though I set the height at 20mm. I bent the actual bowl down and got a drastic improvement, although still not perfect. I went and bought a few new plugs and kept tweaking until I got it right. I had no idea the float level had such a drastic impact on performance until now. It's still not exactly perfect, but I'm getting there. Thanks for all the help..

Killer! Glad to hear you got it somewhat sorted. I'd assume now that your synchronizing will be the culprit if the float is taken care of If you are crafty you can make a bowl fuel level gauge so that it shows you exactly what would be in the bowls. Kawasaki always had us doing that crap :/ Attached are a couple I found from a search so you know what I'm talking about. That way you can verify that the carb that was messed up is actually in-line with the carb that you know has good floats. There's a reason the book is very specific on float heights usually within 1mm of the spec...now you know how important it is! 8) 8) 8) Go ride the heck out of that bike now!

 

[Davisgriffin]

Does synchronization have an effect above idle speeds? It seems like once you open the throttle that minuscule difference in adjustment wouldn't matter. Am I wrong?

 

[focusinprogress]

Does synchronization have an effect above idle speeds? It seems like once you open the throttle that minuscule difference in adjustment wouldn't matter. Am I wrong?

yes. synchronization means that both carbs are drawing exactly the same amount of vacuum at all times. It's equally important throughout the revs.