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Hey all, I'm Jeff from Colorado Springs and I'm finally finishing up a project bike. It's a 1980 Yamaha XS 400. I got the bike for $50 a couple years ago, a friend and I chopped it up a little. We stretched it forward slightly and raked it a little and dropped it alot! I've finally got it all back together and the carbs are clean but I can't for the life of me get it running right. It fires up right away and runs great with a good amount of power for a 400cc but the throttle response is very "flaky". I can get it to idle real good and then once I ride it for a few minutes, then let off the throttle, the idle is super high. I have to put it in 1st and let the clutch out slow until it drops down and then it finally settles down again after a minute. Is there anybody in the Colorado Springs area that wants to help out?? I would be greatly appreciated.

Thanks in advance.

It's a carburetion problem. It's indicating a lean mix. Either your pilot jets are clogged, you ahve a pinched diaphragm or vacuum leak, (Or various sundry other causes)

Would love to see some pics tho!

Thanks for the reply. I was pretty sure it was indicating a lean mix but wasn't totally sure since it runs ok "sometimes" but after riding it throws the idle way up. There are some different things that I've done to the bike to cause this I know. I am running HD bullet mufflers "opened" up and pod air filters. I did talk to a guy who owns a motorcycle shop in Fort Collins a year ago about rejetting the carb and told him exactly what I had done and he told me at our altitude, I shouldn't have to rejet the carb at all. I thought that sounded a little crazy. Is this true? I know that I have no vacuum leaks, the pilot jets are clean(I cleaned the carbs during my build process) and I can't say I'm sure about the pinched diaphragm. I think at first start, I did have a stuck float because once I kicked on the gas and the float bowls filled up, I had gas running out of the back of my carb. But after a few taps and shakes it was free. Should I try and rejet the carb?

The problem is, I know more about car carbs than bike carbs. I'm trying to learn but dealing with tuning 2 carbs simultaneously is a pain.

Thanks for any other help.


EDIT: I'll try to get some pics up soon but I'm trying to get my digital camera back from the ex.

Hiya Stratus - The bike is almost 30 years old - it's possible that in its 30 year history someone else has messed with the carbs (Maybe even changing jets)

The best way to know is to compare the jets to spec. You could also enslist the help of a shop with an exhaust gas analyzer.

There's really not much difference between bike and car carbs. To balance them, there are a number of tools out there (Like Mercury sticks.) It's easier to balance carbs on a twin - I do my bmw without using any tool at all - I simply unplug one spark plug wire and get the bike idling roughly to spec, then I do the opposite cylinder the same way. When the bike idles the same (on both cylinders, independantly) they are synched.

That's a good way to balance! I've tried synching them with a manometer and they are very close but I think it's still something with jets. I guess I may have to enlist the help of a shop around here and see what I can do. Thanks again for the reply.

Wow, so I started tinkering with the carbs again and just can't get this figured out. The bad part is, it's not a power issue at all but it's more of an idle(or fast idle) issue. The thing just revs like crazy after riding for a minute. If there IS anybody in the Springs that could help, trust me, it would be appreciated. I'll buy beer!!!



EDIT: Just had a thought; when I cleaned the carbs, I cleaned the slides real good. Do these need any kind of lube or anything? Would this be a case if the slide was sticking open after riding causing more fuel flow to the engine and running the idle up? Like I said after you ride and say come to a stop, the idle is super high(even after taking off hard in first and shifting to second, the idle is stuck high), and then settles down after about 20 seconds. Whatta ya guys think?

Something is causing the bike to be lean - you have an air leak maybe. An easy way to check for this is to spray carb cleaner or wd 40 around the rubber bits on the intake and vac lines and see if it affects the idle in any way.

The other possiblity is that you pinched a diphragm or have a hole in one. It is also possible that your jets (OR passages!!!) are still clogged with a bit of crud. Your valves may need adjusting or your carbs may need to be synched (And it's pointless to synch carbs if the valves are out of adjustment)

I know it's tough to figure out these problems, but you have to bite the bullet and double or triple check everything and maybe find out that you missed something or didn't do it as well as you thought. Either that or take it to a mech

I know you can do it !!!!

Hey thanks again for the reply. I did pull the carbs yesterday and noticed that one of my needle jets was stuck. As in when the needle is put into the slide there is a small "washer" or plate that goes over the top of it with 2 screws, under that there is a small spring that lets the needle move a slight amount up and down. Well, I had that in wrong. There is a small plastic peice on the needle under the c-clip that has a "nipple" on it that fits into a hole in the slide. I didn't have that in right. I also pulled the carbs all apart again and am going to check everything else to make sure it is all good. Also the valves are adjusted. I did that about a week ago. Thanks again for all the help.

By the way, you asked for a pic, so here ya go. There's more pictures here: http://totalmotorcycle.com/BBS/viewtopic.php?t=12648.

Dude, that's Fkn Sweet. Seriously - looks awesome.

Thanks man, I'm very happy you like it.