First problem with new bike...carb related of course.

Put about 60 miles on it the other day, ran decent, sat yesterday, took it out today and within a couple miles it was spittin' and sputtering unless I was at full throttle. Met up with a buddy to ride a few miles from my house, when I stopped gas was pouring out of the left carb.

Bike is a 79'. Motor is stock and as far as I know the carbs are stock settings but I don't know. I am guessing at the very least they need a good cleaning and set back to stock settings to start. Does this issue sound like more than just needing cleaned? I don't know anything about carbs so this is a learning experience and I have started reading the carb articles in the tech section.

Also, before I tear into these things, are there any special tools that I need to grab while I'm in town tomorrow or anything that will make things easier to work on them?

And last question for now, petcocks, the ones I have don't match. I don't know if it's an issue but it bugs me...maybe a little OCD...so I want to replace them. Other than making sure they fit the tank, is there anything particular I need?

Thanks...sorry for the newb questions.

Chad

gas coming out of the carbs can be a stuck float which will screw you, a badly seating float needle, or a malfunctioning vacuum petcock.

maybe others, but those are the most obvious failures.

Thanks...I'll certainly read up on those issues and how to sort them out and see what happens. I guess it's a good thing I want to replace the petcocks anyways.

Another dumb question...how do I tell for sure what carbs are on my bike? Like I said, pretty sure they are whatever is stock for the year but just to be sure. I know they are Mikuni...that's all I know.

Go to the tech section. Follow the link to the Carb Guide. Print it off, read all of it, then read it again.

The carbs from any year 650 can be swapped onto any other year. For that reason, it's possible that the carbs have been changed. Post a pic of them and compare the jetting in them to the chart in the carb guide.

They are off the bike and torn down. Sprayed them out well with carb cleaner and they seem pretty clean to begin with. Tiny bit of stuff floating in the bottom of one of the bowls, one carb had an inline filter, the other didn't. No idea why, I picked up two new ones today so both will get filters. I am replacing the fuel line as well just to be safe. Here are some pics as requested...these are pre-cleaned so they are still a bit dirty...

That filter looks like it has a lot of crap in it.

A single piece of rust flake can jam up your float needle, causing your carbs to pour gas out the air filter.

Fuel filters are cheap insurance.

look like they could be '79 BS38's. Take a pic of the inside of the slide (where the needle goes) and post.

what's up with the screw in the throttle?

Also, you'll want to pull those screws on the sides out (VERY CAREFULLY), pull the springs, then under the springs will be a tiny little washer, and under that a tiny little o-ring. get all that stuff out and blow those out with cleaner/compressed air as well.

And if you don't have the washer/o-ring, get them.

Yeah...that's the filter that was on it when I got the bike. I have new ones now that I will install on both sides along with new fuel lines. The pilot jets in the carbs I just found are 27.5, not sure how to check the main jet though.

Main jet is under the drain plug on the bottom of the float bowl. Those appear to be '78-'79 carbs so stock main would be a #135. 27.5 is the stock pilot size. Your mix screws will have an o-ring on the screw above the threads, not a washer and o-ring in the bottom of the hole, that's a BS34 thing.

Pay attention to the carb top screws. There were longer ones used on the 4 inner holes through the butterfly shaped bracket and shorter ones on the 4 outside holes. Yamaha actually cheaped out here because there should be one more longer screw in the left front outer hole where the throttle cable bracket mounts. Swap a longer one into that hole if you can. The original short screw only catches by a couple threads and is easily stripped.

Lots could be wrong, you need to make sure you replace those filters and blow everything out real good. One little piece of crud can change your carbs completely.

What they are doing however has a limited number of causes, and it is either the vacuum petcocks or the float operation.

did your floats have gas in them when you took the carbs apart?

Floats where good, petcocks are not vacuum. I cleaned the ever living heck out of it and blew it out really good. Hoping it was just a little dirt. Looked in the gas tank...not the cleanest place for sure, not terrible but not great. I'll have to give it some attention too very soon. The left side carb failure makes sense though since that is the side that for whatever reason did not have a filter. Now both sides do. I am getting ready to hook up the gas lines and fire it up to see what happens...fingers crossed.

All hook up and she fired right up. Idles good, revs good, no gas leaking from the carbs though I think one of the petcocks is on the way out and leaking just a bit. Might rebuild, might replace...who knows. Anyways, seems to be better for now, but I can't really say for sure until I can get out and ride a little. I installed new forward controls today and until I get a new rear brake setup and the brake pivot welded on the bike is grounded. I hope to get all that taken care of in the next couple of days.