Popping from one carb, bike only runs on full choke!

[AnXSofBikes]

I have a 1981 XS400, and the air filters, carb boots, fuel line, and vacuum line were all just replaced. When the bike is started, it will fire up just fine. When you try to give it gas without choke, the right carb starts popping horribly and backfiring. Other side has no pops. If the choke is left on, the bike is perfectly drivable with no issues.

Is this probably just a tuning issue in the one carb? Is there a specific jet or air circuit that could be causing this issue?

Any help is appreciated, thanks

 

[CaptChrome]

Welcome!

First, check for solid spark on the right side. You might want to put in a new set of plugs if the ones you are running are old.
Second, you should clean the carbs, the right one especially. There is probably some clogging in the pilot and needle jets. Make sure you blow through all of the internal passages with air or brake cleaner. After that, make sure you have a filter in the line before the carbs.

Let us know how it goes and if any of this helps.

 

[AnXSofBikes]

Thank you for the tips, I will try out the plugs tomorrow and see what happens. Carb bowls and jets were taken out and cleaned with carb cleaner at an earlier date. But it wouldn't hurt to check again if the plugs don't work.

Thanks!

 

[CaptChrome]

Carb bowls and jets were taken out and cleaned with carb cleaner at an earlier date. But it wouldn't hurt to check again if the plugs don't work.

Sounds like a plan. On the prior cleaning, if there is even a speck of grit in the tank or the petcock, it will find its way into your freshly cleaned carbs and absolutely wreck your day. Running 40+ year old tanks makes the chance for this even greater. See how it goes with the spark end of things, but chances are (and I might be willing to wager a candy bar on it) the carbs are the culprit.

 

[Itschrisb]

On the xs400 I just picked up my plug wires and caps were pretty corroded internally. The caps were barely hanging on once I started messing with them. Might be worth checking those out and cutting back to some fresh copper to see if anything changes.

Also have you checked to make sure the carbs are synced and that the mixture screws are set correctly?

 

[AnXSofBikes]

Alright, pulled both plugs and one was wet, other was dry and looked good. Replaced both and didn't see much improvement.

On the xs400 I just picked up my plug wires and caps were pretty corroded internally. The caps were barely hanging on once I started messing with them. Might be worth checking those out and cutting back to some fresh copper to see if anything changes.

Also have you checked to make sure the carbs are synced and that the mixture screws are set correctly?

Thanks for the tip! I checked the wires and caps and they all looked good as new. Just minor cracking on the rubber, no corroded metal.

I think I will just bite the bullet and fully disassemble the carbs for a recheck/cleaning. Girlfriend's family gave me a HF gift card for Easter so I might just get an ultrasonic cleaner!

 

[tstidham]

When I cleaned my carbs, it took 3 attempts to get everything thing clean. In the end, I took a single strand of some 12 gauge multistrand copper wire and use it like a ream. It pushed out quite a bit of gunk from the pilot jets. I used it in every passage way as best I could as well.

 

[Itschrisb]

Alright, pulled both plugs and one was wet, other was dry and looked good. Replaced both and didn't see much improvement.

Thanks for the tip! I checked the wires and caps and they all looked good as new. Just minor cracking on the rubber, no corroded metal.

I think I will just bite the bullet and fully disassemble the carbs for a recheck/cleaning. Girlfriend's family gave me a HF gift card for Easter so I might just get an ultrasonic cleaner!

I’d still suggest pulling the caps off the wires and the wires off the coil and looking at the copper inside the wires. Again visually mine looked fine but once I took them off the wires the copper was corroded and broken down.

Are you idle mixture screws exposed? If they aren’t I suggest drilling out the brass covers, replacing them with new ones and adjusting them properly. They’re set on the lean side from the factory.

The carb boots you replaced, are you referring to the manifolds that bolt to the engine? If so double check the bolts are snug. I had to retighten mine after a few runs.

You didn’t mention if you synced the carbs after rebuilding them.

 

[The Wang]

I'm also having carb issues, I just posted a thread but reading this one opened my eyes

 

[kebabs]

I have a 1981 XS400, and the air filters, carb boots, fuel line, and vacuum line were all just replaced. When the bike is started, it will fire up just fine. When you try to give it gas without choke, the right carb starts popping horribly and backfiring. Other side has no pops. If the choke is left on, the bike is perfectly drivable with no issues.

Is this probably just a tuning issue in the one carb? Is there a specific jet or air circuit that could be causing this issue?

Any help is appreciated, thanks

did you ever get your bike running good

 

[gpounce32768]

I was kind-of thinking of diaphragm issues eg missing or broken spring, or maybe the needle positioned way too high.

 

[tstidham]

I can say, after replacing the shaft seals on my 78, it's running better than ever. I always had some slight idle misfires and slightly changing idle speeds since I got her running 3 years ago. And tons of exhaust popping when downshifting or deceleration and that is now all gone. If you can get it to idle, spray some carb cleaner right on the throttle shaft washers and see if the idle changes. The ones I took out weren't broken or cracked but were clearly sucking air. To be expected of 44 year old seals I guess though.

 

[kebabs]

I can say, after replacing the shaft seals on my 78, it's running better than ever. I always had some slight idle misfires and slightly changing idle speeds since I got her running 3 years ago. And tons of exhaust popping when downshifting or deceleration and that is now all gone. If you can get it to idle, spray some carb cleaner right on the throttle shaft washers and see if the idle changes. The ones I took out weren't broken or cracked but were clearly sucking air. To be expected of 44 year old seals I guess though.

Well thats good you got it running sweet those bloody carbs

 

[tstidham]

Yea man, just took it out for a quick run and man does it sing now! I have to repack the mufflers again, though I'm thinking it went through the packing every year due to the lean pops just burning and percussing the fibers all this time. I may even spring for decent stuff this time.

 

[kebabs]

Yea man, just took it out for a quick run and man does it sing now! I have to repack the mufflers again, though I'm thinking it went through the packing every year due to the lean pops just burning and percussing the fibers all this time. I may even spring for decent stuff this time.

brilliant. it's a good feeling when running sweet .