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Ok so I had a topic going in General Maintenance but I figure this is getting kind of involved so I moved it here.

I've been having a hard time getting my bike running right. It's a 1980 Yamaha XS400. It seems to be running lean and only idles on 1 cylinder until you hit the throttle and then the right bank fires up. Yesterday I pulled the carbs completely apart and cleaned them thoroughly. When I got everything back together, it seemed to idle better but still the same issue. You can definitely tell that it is only idling on the left bank just by the feel of the exhaust pulse and the sound. Once you hit the gas it sounds totally different.

Anyways, while I was trying to get the carbs adjusted, I happen to push on the throttle shaft of the right carb(the one that doesn't fire at idle) and it immediatly sprung to life. I looked closer and you could see there was some varnish around the outside of the shaft which I'm guessing can't be good. I suppose it's causing a huge vacuum leak. My question is, on this particular model of carb(Mikuni BS34) is there a way to fix this? I looked in the manual and it shows a seal on each carb on the throttle shaft on the outside of the carb. Can you order these? Thanks in advance and here's a picture of what I'm talking about.



Oh and here's a picture of the bike in case you were wondering.

I don't have an aswer or solution for you but your problem resembles mine a whole lot. Check "1982 maxim 400 running on 1 and a half cylinder" and I will keep checking this thread. If you come up with a solution please let me know! I will do the same.

Yeah I actually did read your post before, hoping for an answer. I guess we'll both keep our eyes peeled.

No man, I don't think you can get them anymore. Once the throttle valve stems are shot, they're shot. Time for new carbs.

On the bright side, If you get new carbs, you can get slide-type carbs instead of CV carbs ... the give a better response. (Usually for offroad use)

Or you can find a set of used carbs...

Yeah I've thought about some used carbs but I'm afraid of running the same risk as the carbs I have now. Any suggestions on some slide carbs?

I believe the number of the carbs you posted indicates that they have 34mm throttle bodies. Just find similar offroad carbs that have the same dimensions and you should be OK. Slide carbs aren't as complicated as CV carbs so they cost alot less(`$100 per) . But you will need to run a dual throttle cable (One to each carb) then you just balance the carbs by adjusting the throttle cable.

You could also run a larger single carb to both cylinders. This means you never have to synch, but then you's have to fabricate an intake.

You shouldn't run the same risk w/ another set of used carbs. that problem is pretty rare in general (Though it may be a problem with these specific carbs)

Maybe Mikuni or Keihin make a dual bank of carbs that you can buy new. Or maybe carbs from a GS500 or EX500 will fit. I can't tell you for sure. Time to do some research!

Ya know, I've beeen thinking about a single carb for a while now. I'm just not sure what size I should look for and from what. I would love to run a single on that bad boy. I can tune a single carb!!

Also I checked the carbs again last night, and i'm not real sure if it's that seal or not. It wasn't behaving the same way yesterday. It still idled like poo but didn't make any difference when I sprayed WD-40 on the seal area. But everytime I hit the throttle or open it up real slow it finally "catches", and the right bank springs to life. I really am thinking air leak for sure now but I can't find one. With the carbs being a little out of synch it should still run on both banks right? I've got them very close with feeler gauges but haven't hooked up the manometer yet since I can't get it to idle stabily. I'm ready to cry.

By the way, I'm in Colorado Springs if anybody wants to take a peak at it.

I had a prob on a bike once, and a certain cylinder would go out every now and then - or it wouldn't fire until hiogher revs. The problem turned out to be a bad plug. Try replacing your right one (Or just swapping out left and right) and see what happens.

Sweet, I'll try that. I'm also wondering if maybe I have the wires to the ignition coils flipped. Would it run bad if the coils were connected to the wrong cylinders on the XS400? Like say left coil is on right bank and vice versa?

I would think that the bike wouldn't run at all if the coils were reversed - Maybe just a backfire or 2 here and there. but anything is possible! I once ran a 1975 bmw (6-cyl car) for about a week with the Distributor turned 180 degrees... Don't ask me HOW it happened , but it did ...