‘75 carb issue

[willis]

A recent issue with my 75 carb popped up yesterday. These carbs were gone through professionally a few years ago and not a lot of miles on it since. But the right carb has never performed right. At idle it misfires until RPM picks up a little and then it ran well. I’ve been through the jets and replaced with real Mikuni units and replaced the seals on the emulsion tubes. Checked for air leaks and confirmed float heights. Never could figure it out. Yesterday it started running like shit. Hard to start, popping and backfiring. Just running like crap in general, but only the right side. I changed the plug on the right cylinder at my buddy’s garage and it was definitely fouled. On the way home it just got worse. To make matters worse the battery died too, which is only a few weeks old. Obviously the battery issue is another issue on its own. But today I tore down the carb to have a closer look. The carb is still very clean. No crap inside. Diaphragm is still good, as it was replaced when the carbs were redone. Jets are clean and passages are clear. The throttle shaft seals are still pliable, but I ordered some Suzuki replacements today just to be sure. I also ordered some new JBM intake boots, as there is a small cut in the bottom of the current one, but does not appear to be cracked through. A few other things caught my eye and am wondering about input from those of you far more knowledgeable about carbs.


First, I noticed some wear marks from the butterfly in the carb throat. Not enough to catch a fingernail, but questionable at least.


Also, I noticed under the choke cover a small nick in the mating surface. I don’t think this would affect the idle with its location, but I’m could be very wrong.


A few other pictures just for reference.

 

[JOHNDADDY]

I'd go after the bat and charging system first and make sure all is well.
low voltage can give all kinds of issues.
Best of luck with it.

 

[willis]

I recalled Mailman's similar issues with his '77. I just re-read a lot of his thread documenting a lot of the same issues. It can be found HERE. Looks like some of this wear is normal inside the carb body. Hopefully seals and manifolds will be here this weekend. In the meantime the battery is back on the charger. I do have one of Jim's rotors on deck in the parts stash if needed. This bike is in physically great shape, but I have never been inside of it. I also plan to run a compression test this weekend. Just so I know. I do still wonder about the nick in the choke cover though. Any thoughts from you all on this?

 

[JOHNDADDY]

Far from the heart, don't think it's a problem.

 

[Niels B]

I’m almost 100% sure the nick in the choke housing is not an issue - I would however fill it if you can. Do you know why it was rebuild with a 130 main jet ? If all else is stock I would have started out with a 127.5 as original required. Assume you run 130 in the other carb too ?
I think the carb holder crack could cause the issue - even that it seems to have no cracks all way through. When it’s that brittle it may cause false air to come in at the carb during vibrations and load. Get a new pair

 

[willis]

jetting was increased when I changed pipes and mufflers. Much more free flowing exhaust.