bs38 float adjustment, running rich on left side

[nooch]

1978 xs650 pamco, PMA, cam chain good, timing good, valves good. .30 pilots, 145mains, slide needle 2nd notch... carbs were synced, if not very close, both plugs looked the same (good). I had a flat spot off idle (lean) and slight popping when cruising. floats were set too low (27mm) so I adjusted them to 25mm, this cured the flat spot and it pulls much better and harder, but now my left side is running rich at idle and backfires every 5-10sec or so, it actually popped the left carb out of the holder at idle.

So, my question is, can I lean the left float slightly to try and lean that cylinder, or should I be looking more towards my carb sync (could it have been knocked out of sync when the carbs popped off?) I know the sync screw on my bike is VERY touchy.

any help would be great, thanks!!

 

[5twins]

The float spec for the '78-'79 carbs is 24mm ± 1mm. Although 25mm is in that range, it's at the lean end of it. 1mm may not seem like much but it can make a difference on float settings. I experimented with the full range of settings on my '78 (23mm, 24mm, 25mm) and it liked the 24mm best. Also, there is a 32.5 pilot available now and sometimes it's just what you need for the '78-'79 carbs. Where are your mix screws set?

 

[grizld1]

Does your ignition have electric advance? If it's the earlier version that retains the ATU, you might want to check the advance interval.

 

[5twins]

I guess I should add that while you say you're running rich, backfiring is a sign of running lean, or the timing being off. But, with a Pamco, both cylinders are timed the same off one mark or point, so you would think both would act up. How's the carb sync?

 

[nooch]

My mix screws are set at 13/4 turns out 1/2 less than stock. I have a set of the 32.5 jets but correct me if I am wrong I am still learning but if I am already rich on one side with my mix screws where they are would I need bigger primaries? Or should I just try adjust my floats first? And is it bad to have one float at 24 and the other at 25?

 

[nooch]

5twins that is what I was thinking, my sync being off, it was fine but I think when my left carb popped out it might have thrown off my sync.?? The left plug is rich and that's the cylinder that's backfiring, the right plug looks pretty good to me... I don't have electric advance

 

[5twins]

I'm not going to say it's bad but it is wrong. Both carbs should be set the same. If you have to use differing settings like that, you're compensating for some other problem. When you set the floats, did you measure each float "bulb" individually? Many times the floats are twisted on their mount, one "bulb" sitting higher or lower than the other. If you don't check both sides, you won't catch that and what you think is a proper setting may not be.

With your mods, jetting, and mix screw settings, you shouldn't be running rich (well, maybe your mains are a bit large). Your mix screw setting in particular is rather lean for that carb set. Even with mods and larger pilots, this carb set usually doesn't like settings below 2 turns. That could cause a lot of popping at idle.

Another possibility is a bad float needle and/or seat. A slow leak might be compensated for while running down the road but at idle, the bowl could be slowly over-filling. That would make it run rich and eventually start dragging the idle speed down on that side. A bad enough leak would eventually start dripping gas out of that carb.

So, you say you're rich on the left side. How do you know? Is the plug black or is that side fouling the plug?

 

[grizld1]

Here's a tip for bench synching your carbs. Instead of looking for light passing the throttle plates, look at the alignment of the throttle plates with the pilot nozzles (little holes in the carb spigots).

 

[nooch]

alright I was thinking the same thing, I didn't think it would be right to have different floats settings. they were the same before and both plugs were burning the same, which is why I'm leaning toward my sync being off after my carbs popped out...I have measured/tweaked my floats both sides are even. my floats do not leak and I don't think the needle and seat are bad because I have left my petcocks open for a day and no leakage... I had my mix screws set at 2 turns out which seemed to be ok but I turned them in trying to get my left cylinder leaner. I will go back out to 2 1/4 turns out and re-sync the carbs. My plug on the left side was black and slightly wet when I pulled it... what would you say about that? let me know if you think I am heading in the right direction...

 

[5twins]

Correct mix screw setting is determined by where that cylinder runs (idles) the best, smoothest and at the highest speed. You turn the screw one way (in or out, doesn't matter) until that cylinder starts slowing and stumbling, then back the other way until the same happens. Best setting is somewhere in between those two points. There is a range, maybe a half turn wide or so, where the cylinder will run the best. We call that the "sweet spot". Some advocate setting the mix screw right in the middle of it but I find going as far to the rich side as possible can help with excess popping on decel if you have that problem.

Something does seem amiss with that left carb, unless the plug reading you're seeing is being caused by an engine problem and is oil, not gas. You might want to inspect the choke plunger to insure its rubber tip is intact and sealing. If it's bad, that could be causing the rich running on that side because the choke isn't shutting off completely. You don't need to take the whole choke assembly off the carb body, just remove the plunger from the top of it.

There's also a chance you've got a bad plug, it's rare but it does happen. I would swap them side to side to see if it burns clean in the other cylinder and the "good" plug turns black. With a Pamco, the only other ignition related problem that would cause bad spark in one cylinder only would be the plug wire and cap on that side. Check that the connections are good. You could also measure the cap's resistance. When they go bad, it climbs, eventually choking off the spark.

 

[nooch]

so you don't think that the carbs popping out of the boot could knock the sync off then, right? I will do the dead cylinder method again when I get home tonight, but would you recommend double checking my sync before I start with the dead cylinder method? or not? and I think I remember reading how to sync the carbs using the dead cylinder method in the carb guide, do you recommend this?? ill check that plunger tonight before anything also. I appreciate all the help I will let you know how I made out tomorrow if I can get some time in the garage tonight! haha thanks again!!

 

[5twins]

The carbs popping out shouldn't have disturbed the sync but I guess it's possible. Sure, give the sync a quick check first. I rarely use the dead cylinder method because I have a manometer, also I can usually hear what my mix screw adjustments are doing without having to kill a cylinder.

 

[nooch]

well I got it running better not perfect but pretty decent. I synced the carbs using the dead cylinder method, I know its not really preferred but it worked ok. then I played with the mix screws and I have my left cylinder at 2 1/4 turns and the right at 2 1/2 turns. that's where it seemed to like it the most. checked my plugs after a little ride and they were fairly even the right side looked a tad bit hotter mind you I am looking very closely from a foot or 2 back they looked identical, I guess I am just being picky. I rode to work and back yesterday which is about 65kms... got home checked plugs and the right side was running leaner again, but the left wasn't as bad as it was before. it was a double double coffee kinda colour haha which I think is ok, not too lean or rich. I adjusted my sync screw last night 1/4 turn CC so I will check plugs again after work to see if it made a difference. it seems to be running pretty decent so far. I swapped the plugs and it acted the same so I think my plugs are ok. if its still rich on the left side tonight I will further inspect the plunger. thanks again for your help, I will keep you posted.