[Gabedini]

[solved] Can't sustain RPMs above 3000?

Hey CX500 fam,

My 82 CX500C seems to not be getting enough gas to sustain RPMs above about 3000. I had taken the carbs off to fix an air leak issue (the metal part between the carb boot and the cylinder head was cracked, I got the bike that way), and now I have re-assembled them (they were very clean on the inside), put them back on, and have it running fine as long as I'm below 3000 RPMs.

Basically, when I give it gas I will go to whatever RPM I want for 3-4 seconds and run as normal, but then it will start to stutter and rev down, then will almost stall it will usually backfire a time or two or three until I let up on the throttle, give it some time, and then I can rev it back up again. My supervisor thought that it was the floats needed adjusting, but I read somewhere they aren't really adjustable because they're plastic... Another friend thought maybe a jet was blocked, but I don't think that's the case either because the carbs were very clean even before I took them apart.

I ran it with the choke on, that didn't make a difference, then I took the air filter out to let it suck air more easily and that made no difference either, so I don't think it's a mixture issue...
edit: I removed the fuel filter and ran it like that, made no difference there either...

My best guess, is that the accelerator pump diaphragm/rod piece is either bad or misaligned, but I'd like to hear your thoughts if there is anything else I should try before taking the carbs all the way back off.

Does that sound right? Has anyone else had trouble like this?

edit: also adding to the original post but you'll see it further down as well... turns out I'm a total noob (this is my first carbs I've ever taken off of a bike)... I had simply not lined up the vacuum cylinders with the little groove on the side (I did not realize it was necessary to do so, and my manual didn't tell me otherwise)...now it runs just like it did before I made the poor decision of removing the carbs in the first place (it wasn't necessary, my only issue was the crack in the piece that goes between the cylinder head and carb boot).

thanks - you guys/this forum are great,
gabe

 

[CXPHREAK]

Try opening the fuel filler to check for a blocked filler vent.

 

[Gabedini]

Try opening the fuel filler to check for a blocked filler vent.

er, wait a second... disregard my previous reply
what exactly do you mean by that? Is that one of the lines associated with the vacuum for the tank?

 

[JC]

Check out the pickups;

https://motofaction.org/motorcycles...timing-pickups-honda-cx500-gl500-cx650-gl650/

 

[CXPHREAK]

Floats couldn't be upside down or the needle not inserted to the float properly?

Did you use aftermarket float needles? Sometimes they're not the same exact length as OEM and don't give the correct fuel level.

Get yourself some 4 mm transparent fuel line, connect it to the float bowl drains. Run this up the side of the carb and open the drain screw. The fuel level will show in the clear tube. It should be on, to a mm or two below, the joint between carb and float bowl.

 

[Gabedini]

Floats couldn't be upside down or the needle not inserted to the float properly?

Did you use aftermarket float needles? Sometimes they're not the same exact length as OEM and don't give the correct fuel level.

I did not replace the float/float needles at all, so I believe they are OEM... upside down or misaligned is certainly possible, I was following the Clymer manual I got with the bike but I'm a noob...

Get yourself some 4 mm transparent fuel line, connect it to the float bowl drains. Run this up the side of the carb and open the drain screw. The fuel level will show in the clear tube. It should be on, to a mm or two below, the joint between carb and float bowl.

I'm assuming if it's too low, then that just means the floats are not allowing the bowl to fill all the way?

 

[Gabedini]

Check out the pickups;

Thanks for the link!... this implies an ignition issue, correct? (I'm pretty new to this whole fixing vehicles thing)...what needs replacing if those don't read correctly? The pickups themselves?

 

[OCR]

If the air vent hole in the gas cap is plugged, you will have a fuel starvation problem after it runs a bit as vacuum builds up in the tank.
Loosen the cap and see if it fixes the problem.

 

[Gabedini]

Check out the pickups;

https://motofaction.org/motorcycles...timing-pickups-honda-cx500-gl500-cx650-gl650/

Hey JC,

If I tested them and they appear to not being getting a high enough reading, is that an engine - out procedure since it connects up on the back of the engine? Or is it simpler than that?

thanks so much for the help

 

[JC]

If the coolant overflow tank is in place I don’t think you would be able to change them out with the engine in place and even then I’m not sure how easy it would be. I have only ever replaced them with the engine out of the frame.

 

[JC]

If you do replace it - and how far off are the numbers? -

Removing the cover:

https://motofaction.org/motorcycles...move-pulser-cover-ti-cx500-gl500-cx650-gl650/

Removing the pulse generators:

https://motofaction.org/motorcycles...lse-generator-atu-ti-cx500-gl500-cx650-gl650/

Setting up the timing after installing new:

https://motofaction.org/motorcycles...-timing-procedure-ti-cx500-gl500-cx650-gl650/


Edit: Carb/vacuum leak/blocked gas cap vent issues are much more common than the pulse generators having an issue on these bikes so as suggested below rule the easy stuff out first...

 

[nolimitz]

What values are you getting when measuring?

 

[nolimitz]

If it were me, I would be 100% certain my carbs were spotless clean/rebuilt before I pulled the engine. JMHO

 

[JC]

If it were me, I would be 100% certain my carbs were spotless clean/rebuilt before I pulled the engine. JMHO

^^ Definitely worth thinking about... and your carbs can never be TOO clean!

 

[Blindstitch]

I would remove complication and eliminate the gas tank first. Connect up a small test tank and make sure it isn't petcock/fuel valve related. Also did you reinstall orings between the head and carb.

 

[Sidecar Bob]

Good point. A clogged strainer inside the tank could cause this symptom. As could a cracked, loose or otherwise leaking vacuum line (runs from the left side of the right carb to the small barb closest to the petcock's main body). But as has already been suggested twice, open the fuel cap when it happens to see if it is just a blocked vent hole causing the problem.

BTW: I have replaced the pulsers in mine with the engine in place. It would be impossible with the coolant recovery tank in place and even with the tank removed (which you have to remove either the engine or the swingarm to do) I would not recommend for someone who is not familiar with assembling pulsers & ATU and setting the timing to attempt it.

 

[murrayf]

oull the air cleaner and check the air box screens are clear go for a ride with no air cleaner and see if it makes any difference

 

[Gabedini]

^^ Definitely worth thinking about... and your carbs can never be TOO clean!

I finally had some time (I've been swamped with midterms and work) to do some checking... I think the pulse generators are fine and I found (at least part of) the issue. The vacuum cylinder was not lined up correctly with the little slot that it's supposed to line up with, so it would not go down all the way. I will let you know when I get them back on there whether or not this completely solves the issue. (I'm currently waiting on a part in the mail, so hopefully that will be here Tuesday and I can put them back on Wednesday afternoon)

Thanks again for the help, ya'll are great

 

[Gabedini]

I think that the issue was with the carbs, my vacuum cylinder did not appear to be lined up right (the manual didn't warn me about that, I'm a noob lol)

 

[Gabedini]

I think the trouble was with my carbs.. I had tried that though and it didn't work, sadly... however apparently the PO had never opened the airbox because there was a whole bunch of bird seed in the air filter