I have a problem with my America 800 it's running lean on the right cylinder and when I adjust the mixture screw it makes no difference. The mixture was checked using an AFR meter. The rubbers between the carbs and manifolds have been replaced today still running lean on right cylinder.

The bike sat for 4 years and when I brought it out to get it going again I found the carburetors full of gunk and it would not run so the carburetors were pulled down and cleaned, and while they were apart I put bigger main jets in and blocked the air recycle system.

Someone has told me that the coil is faulty on the side that is running lean but that doesn't make sense. I have check there are no air leaks.

Could it be the idle mixture circuit ?

  • 2
    Couple of questions. If you pull out the mixture screws do you have a spring, a washer and O-Ring that have matching order on both left and right? Where were you measuring at with the AFR meter? Is it a homemade AFR meter? How did you block the air recycle system? Welcome to the site. Cheers. Commented Apr 28, 2016 at 7:52
  • Yes the mixture screws are complete. No it's a proper AFR meter and was measuring at the end of exhaust pipes. I bought a kit to block off air injection. Thank you for your response and the warm welcome to this site Commented Apr 28, 2016 at 10:15
  • How many miles on the Triumph, its it lean only at idle?
    – Moab
    Commented Apr 28, 2016 at 13:08
  • Do you have one these to test/check temps of exhaust pipes and what not. It's a good way to run your bike at half throttle, pull over and check. Lean at mid will increase your temp noticeably. goo.gl/ZzE2uU Commented Apr 28, 2016 at 19:52
  • No I don't have a temp thing to check I haven't ridden it except around the block but at idle the right side is getting hotter than the left. At mid throttle the left side is running a little leaner than the right. The problem is only happening at idle. Would the carbi balance or the throttle sensor be causing the problem Commented Apr 29, 2016 at 2:47

1 Answer 1


You might have a tear in your CV carburetor diaphragm

At the tops of your carburetors there is a cover with 4 screws that gives access to your CV diaphragm and slide.

The CV (constant velocity) carbs adjust for barometric pressure automatically. The difference in pressure from your intake tract and the atmosphere raises and lowers you slide that has the jet needle in it which meters fuel from you main jet.

An unmetered air issue can cause all of the symptoms you are describing and if you have inspected and tested all of the other areas of the carb between the throttle butterfly and the head perhaps this is the area that is giving you issues.

A tear in the diaphragm would cause what you are illiterating.

here is an image of a slide, jet needle and diaphragm

enter image description here

Inspect the Diaphragm

Pull on it a bit, put your finger behind an area and stretch it across you other finger and look for holes and cracks. Also, ensure that when you put it in the carb you put the ring around the outer edge back in it's groove properly in the top of the carb body. It's very easy to pinch it and damage in when you put the tops back on it. You will want to press the cover on and feel that the diaphragm is proper inside the carb body groove before you screw it down.

  • Thanks for that I am now giving up on afr's and tuning buy the plugs and they both show a rich mixture and I am now experiencing more trouble with the right cylinder it is not firing when started from cold and once it warms up it fires and starts to run ok any ideas why this would be happening Commented May 5, 2016 at 1:24
  • It would be very nice to get the temp of the right and left with markers 30 seconds with on and off choke. It's weird, so a misfire could be tested by swapping the coils perhaps. You want to rule that out. Commented May 5, 2016 at 1:32
  • running hotter is a symptom of lean but the plugs are Tblack......conflict mentally with that. Temps help so much. gearbest.com/temperature-instruments/… Commented May 5, 2016 at 1:34
  • @DaveAnderson Have you done an air leak search on your bike with some carb cleaner to ensure you don't have any unmetered air going into your system? Commented May 5, 2016 at 17:09
  • I have done an air leak test but I am doing another one today as yesterday the bike would not even start so when I get it to run I will be going back over everything again. I have checked the spark and its strong but when it would start I pulled out the plugs and they whet wet so the mixture must be way out is the only reason for the no start. Will let you no once I have checked for air leaks again Commented May 5, 2016 at 23:58

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