idle issues

[smiles79]

So I have a 1975 XS650 with a pair of Mikuni round slide carbs. It was at the point where it would idle but die as soon as I give it gas (unless the choke was on). When this happens I can hear a noise come from the carbs. I now have it to where it will maybe cough a little with the choke off then rev up properly, but then sometimes it will stick at a high idle, usually around 3000 rpm (from what I've gathered, this means it's probably running lean?). I tried adjusting the air screw (mixture screw?) And with the brake side all the way in, the bike was running the same same as it was with the screw 1.5 turns out.

As of right now the air screws are 1.5 turns out and the idle adjustment screws are 4 turns out. Also, I can see a flame come out of the exhaust pipe when the engine has almost completely stopped after shutting it off, which would lead me to believe it's running rich (contradictory to what I was thinking above). Any ideas as to how to fix this?

Thank you all in advance, I hope you can help me.

 

[retiredgentleman]

Failure to respond to throttle, and the need to have the choke on, means the pilot circuit is not supplying enough fuel/air mixture.

Usually it means the pilot jet is partially blocked with dirt/debris. Also could mean the pilot passageways are blocked..................look for the 3 tiny holes in the carb throat..............1 single hole there as well.

First step is to remove the pilot jets, and clear them out if they are plugged up.Take note of the size that is stamped on them.

 

[smiles79]

If the pilot jets and passages are clear, should I try the next size up?

 

[smiles79]

Ok so the passages and jets (165 mains and 25 pilots) are clear, still having issues. The intake boots have some small cracks, gonna check them out (I did the carb cleaner trick but it didn't affect anything). Any other ideas?

Thanks

 

[smiles79]

Ok, maybe the pilots are 52? I guess I'm not sure which haha.

 

[smiles79]

It seems as though I'm starting to just have a conversation with myself, but I put a pair of #35 pilot jets I had laying around (I found a chart that says 35 is stock) in the carbs and it hasn't changed a thing. Any and all help is greatly appreciated!

 

[angus67]

how old are your carb boots? sounds like serious vacuum leak. Point a unlit propane bottle around the carb boots, see if the idle changes. if it does, new boots are in the cards for you. What pipes you running? you may need more pilot than stock.
But varying idle, and high hanging idle are vacuum leaks.

 

[smiles79]

The boots have some small cracks, but don't seem to go all thE way through. I sprayed carb cleaner around them and engine rpm didn't change.

As far as pipes, the guy I bought the bike from took the baffles out and put in a washer with like 6 holes in it on each side.

I should mention that before I put the #35 pilots in, there were #25s and doing this change doesn't seem to have affected anything.

 

[Fndirtygreaser]

What needle and jet needles do you have ? Pilots should work. And mains are a bit on lean. 180-190 is what people usually use. What kind of filters are you using ?? And are they vm34's????

 

[smiles79]

I'm not sure about needle jets and jet needles. They are vm34s with aftermarket (I assume) cone shaped filters, haven't looked for brand.

 

[smiles79]

Air filters, that is.

 

[smiles79]

I was told to try a couple sizes up on the pilots and mains since I have pod filters and less restrictive exhaust, so I'm going to start with #40 on the pilots. Is this a common size for this bike?

 

[5twins]

You can "smiles" 5000 times if you like. It won't help. The required jetting for VM34s when used on a Yamaha 650 is well documented (and #40 pilots aren't in the mix). In fact, there was a whole in depth write-up done on it a few years back. Basically, all your jetting is wrong, so are your air filters, and well, so is your whole way of going about trying to solve your carb problems. You're trying to sort high performance aftermarket carbs and you have little to no carb knowledge. What's wrong with this picture???

The Mikuni round slides have so many jetting choices available that you could spend your whole life dialing a set in on a new bike that never used them before. Fortunately, they have been used and dialed in on a 650. There are baseline jetting specs established. Find them and use them, or spend the rest of your life fucking around with your carbs, lol.