One Cylinder Just a Touch Too Rich

[fenbrancis]

I've posted a few times about my basket case of an 82 heritage special. Stock carbs, stock exhaust, stock intake, gonzo ignition.

I finally got this thing purring after a COMPLETE carb rebuild. Whilst battling carb issues my left cylinder has always run rich. And by rich I mean the plug is black. Once I got my carb cleaned thoroughly and rebuilt the bike runs amazing except the left cylinder still is rich. So rich my axle is covered in soot.

This motor has 13k original miles and runs smooth in all ranges, just when I really rev it out on acceleration I see some black smoke come out of the left pipe.

Troubleshooting done:
Fresh plugs (left one immediately black)
Leak down test (held compression within range of "Good")
Leaned mixture screw (I run my mixture screws at 3 turns per the 'carbguide')
Ran some seafoam through to clean
Complete carb rebuild (every gasket, seal, jet, screw, float)
Cam chain adjusted
Timing set
Valves adjusted
Carbs in sync

Am I missing something? Am I being too picky?

 

[jpdevol]

Spitballing possibilities:
Plug cap with too much resistance (test Ohms, switch sides, replace)
Needle jet - if not removed - still has gunk between carb body.
main jet non-mikuni and not as marked
jet needle non-mikuni and not the same or the spacer ring not seated in slide (or some other assembly issue)
restriction in left exhaust (check for equal flow R & L)

 

[fenbrancis]

Thank you kindly! Will check all of those!

 

[Jim]

The choke plungers have a rubber seal molded into 'em. Possible the rubber's hardened and/or cracked on the left carb... giving a constant gas dribble on that side.

 

[fenbrancis]

May I ask:

If it were a bad head gasket or worn rings, would it smoke all the time not just on hard acceleration? Wanting to do my best to rule this out if I can.

Because when I say smoke I mean on acceleration it gives a little puff of black but nothing that is alarming.

 

[Jim]

The choke plungers have a rubber seal molded into 'em. Possible the rubber's hardened and/or cracked on the left carb... giving a constant gas dribble on that side.

Also possible the spring has broke on that plunger... not holding it closed.

 

[jpdevol]

May I ask:

If it were a bad head gasket or worn rings, would it smoke all the time not just on hard acceleration? Wanting to do my best to rule this out if I can.

Because when I say smoke I mean on acceleration it gives a little puff of black but nothing that is alarming.

Those issues - as well as burnt valve - would have shown on a leakdown test. The puff of smoke - esp. gray or black - sounds more like fuel (rich)

 

[jpdevol]

The choke thingies Jim mentions are definite possibilities and , on BS34's, also effect the diaphragm chamber so the slide will also act funky

 

[Jim]

so the slide will also act funky

Yup, lift the slide and cover the oval hole at the carb bellmouth (inlet). If the slide drops rapidly after you release it, it's a clue the choke's not sealing.
Try that and let us know?

 

[fenbrancis]

Thank you all!

I’ll give these a try and report back. I did not change the needle jet so I’ll look for some wear on that one.

Also, I did see a vacuum slide with the oval hole covered should take about 15-20 seconds to fully close. Sound about right?

 

[Jim]

Also, I did see a vacuum slide with the oval hole covered should take about 15-20 seconds to fully close. Sound about right?

Yup, anything over about 8-10 sec is usable actually. If the choke plunger's leaking, it'll drop like a rock. If it does drop that fast, find a third hand and have 'em apply some force to the choke plunger and see if that slows the slide down... assuming it drops fast of course.

 

[fenbrancis]

Yup, anything over about 8-10 sec is usable actually. If the choke plunger's leaking, it'll drop like a rock. If it does drop that fast, find a third hand and have 'em apply some force to the choke plunger and see if that slows the slide down... assuming it drops fast of course.

Nice! I will give this a try

 

[jpdevol]

did not change the needle jet so I’ll look for some wear on that one.

I don't think they wear on the CV carbs, but they have lateral holes in them to emulsify the fuel air mix and gum/gunk does get in between them and the tube in the carb body - making it rich. Remove main jet and washer, reinstall jet without washer and tap from float chamber side. It will exit into the venturi.

 

[fenbrancis]

I don't think they wear on the CV carbs, but they have lateral holes in them to emulsify the fuel air mix and gum/gunk does get in between them and the tube in the carb body - making it rich. Remove main jet and washer, reinstall jet without washer and tap from float chamber side. It will exit into the venturi.

Is it the needle that causes the issues or the jet the needle slides into?

I didn’t replace the needle but did replace the jet the needle slides into.

 

[jpdevol]

Either can cause an issue. I mention the jet - emulsion tube in Britain - as many folks don't remove them for cleaning. If you did, then likely not a source of the problem

 

[fenbrancis]

Either can cause an issue. I mention the jet - emulsion tube in Britain - as many folks don't remove them for cleaning. If you did, then likely not a source of the problem

Ah okay. Yeah I’ll definitely pop it out when I check the needle and make sure all those holes are clean. Will report back!

 

[JJames]

Double check your float level as too high could cause the symptoms you describe. Also problems with rings or valves likely cause bluish white smoke where black smoke is usually gas.

 

[fenbrancis]

Will also give float height a check.

And nice, yeah as mentioned it’s pretty sooty not really a white blue. Thanks for that tip!

 

[fenbrancis]

Alright wanted to give an update:

Float heights are good.

My slides were falling a little quicker than I wanted with the air inlet shut off so I swapped the diaphragm with a set I purchased that I thought were good. I also took the choke plungers out and they seem to be good. With this set up the diaphragms slide smooth and take 15-20 seconds to fall when inlet is plugged.

Now the bike has no bottom end power. Idles fine but wants to die on take off. When I do get going it has not really snappy bottom end, almost like it’s staving for fuel or it’s not getting spark. (But it’s got to be getting spark as it idles perfect)

Going to throw the original diaphragms back in today as that’s the only thing that was changed.

(Just to ensure no vacuum leak from putting the air box and carbs back on incorrectly, I sprayed carb cleaner all around the intake boots and no change in idle.)

 

[jpdevol]

Well that's just odd......are the changed slides used or new aftermarket (non-Mikuni)? Maybe post pics when ya get them out.

The other thing(s) that can cause strange nagging problems with carb adjustment is some of the cheap aftermarket parts; so maybe list the source and sizes of any replacement parts - floats, jets, etc.

Carbs are very precise things, it doesn't take much difference vs stock to mess them up