Noob vs. Mikuni VM carbs: Questions

[ippytattoo]

From what I can see in the video what you are opening when the idle raises is actually the enrichers (choke) so it would make sense that the idle would go up. Try opening them when you try to start it rather than using starting fluid. Also if your bike has the stock TCI ignition your timing is non-adjustable so you wouldn't need to adjust your timing.

 

[5twins]

Do as Griz suggested and pull one of the carbs apart. Write down all the jetting including the needle and needle jet. You won't get anywhere trying to adjust the carbs if the jetting hasn't been changed to a 650 "baseline". Part of the 650 set-up is removing the air jet and running without one. If you find an air jet still installed, it's a pretty safe bet that your carbs aren't 650 baselined .....

 

[vdubplate]

I checked and I done have an air jet. I pulled the carb apart and found that I have a 175 in there which is high considering I live in Denver. G&L choppers said that they jetted the carbs which is bs because it seems obvious that these were prejetted like Griz suspected. I have a sport quad thats jetted w a 118. So you saying if I set the baseline to xs650 standard on these carbs it shoul work the same as stock?

 

[5twins]

Mikuni VMs are custom high performance carbs. They're going to take some tinkering to get right. Just checking the main jet size isn't going to cut it here, YOU NEED TO CHECK ALL THE JET SIZES. The standard VMs are sold with jetting for a 2 stroke dirt bike. ALL of that needs changing to work on a 650. You need different mains, pilots, needles, and needle jets than what comes in the carbs. The 175 main really isn't bad, it fits the 650 baseline which calls for mains in the 170-190 range for your altitude. Read the VM guide over at the 650Garage that Griz referred you to. It gives the baselines and all the other pertinent tuning info.

 

[vdubplate]

Man, I'm intimidated lol! I read the writeup on trying to home in on the correct setup and it looks like a ton of work. I can barely get my bike to hold an idle let alone home in on the perfect jet setup for these. I'm suprised that the 175 is correct because my brand new plugs that I got yesterday are fouled with carbon deposites and my other bikes are running a 118? Are jets different depending on the bike? I guess I'll order all those jets today and see how far I get. If worse comes to worse Ill throw stock carbs on.

 

[5twins]

Stock carbs may be easier for you to deal with. It helps to have a little more than basic carb knowledge when dealing with aftermarket high performance carbs like the VMs.

If you're mostly just idling the bike in your driveway, the mains probably aren't responsible for your black plugs. They have their greatest influence at higher speeds, RPMs, and at larger throttle openings. Black plugs at idle point towards the pilot jet size or air screw setting. Needle clip position can also have a big effect here.

What you have to understand about these or any carbs is that the 3 circuits in them overlap. Changing one influences the one next to it and may require a change there as well. Increase the pilot jet size and you also richen the lower part of the midrange. Increase the main and you also richen the upper part of the midrange. Lean the needle setting and you also lean both the lower part of the main and upper part of the idle circuits.

 

[vdubplate]

Maybe I'll get the jets for the vm34s and a bunch of spark plugs and see how far I get. If worse comes to worse I'll use the other set of carbs I started rebuilding. So a question that I have about the VMs, If I screw in or unscrew the idle adjust the slide is supposed to raise and lower isn't it? Mine doesn't move a millimeter at all if I mess with that adjustment.

 

[5twins]

There are 2 adjustment screws on your VMs, an idle speed adjustment screw and an air screw. The idle speed adjustment screw raises the slide as you screw it in which increases the idle speed. It is about in the middle of the side of the carb body and at 90° to it. The air screw is more towards the air filter and angles in from above. It controls the amount of air fed to the pilot circuit so turning it out (more air) leans the mix.

 

[vdubplate]

Yea, I've done a lot fo research on those screws. So I was under the impression that when you screw the mixture screw in tight that leans and out increases the fuel is that correct? I also have to check into my idle adjust and throttle. I read up on the drill bit technique to measure carb slide height. It seems as if the idle adjustment screws are not working because the slide doesn't react to the screwing in or out of that knob.

 

[5twins]

On a true fuel regulating mixture screw, yes, screwing it in reduces fuel flow (makes it leaner) and out adds fuel (makes it richer). You don't have fuel regulating mix screws on your VMs, you have AIR regulating screws. They work just the opposite of fuel regulating screws - out adds air and leans the mix, in cuts air and richens it.

I don't know what's up with your idle speed screws. They're a simple mechanical set-up on the VMs. The end of the screw has a rounded taper on it and that pushes into an angled "ramp" cut into the side of the slide. Maybe you just haven't screwed them in far enough yet for that to happen.

 

[vdubplate]

Yea, your right I went out and messed with the idle screw which will only higher or lower my slide by about a millimeter or two.

 

[5twins]

That may be normal and all you get. I can't say for sure because I don't run the VMs on my 650. Another possibility is that your throttle cables are adjusted too tight. That would have the cables holding the slides up a bit already and taking away from the adjustment range of the screws. Check to see that your cables have some freeplay at the tops of the carbs when the twist grip is fully closed. If they don't, add some by screwing the carb top cable adjusters in a bit. To get the VMs to run "in sync", you need to match the amount of freeplay in the cables to one another. Then the slides will lift together. One will begin lifting before the other if the play in the cables is not matched.

 

[vdubplate]

I went out to the garage set the slides to they are all the way down, changed my spark plugs and usually I start with 3 to 4 half turns out on the mixture screw. This time I started on 1.5 turns and resorted to 1 half turn only and the bike is running almost good. I also set the gap on the plugs to .28 instead of .30. Its still rough and I need to check the plugs again to see how fouled they are but at least the bike is moving.

 

[5twins]

Nobody speaks of their mix screw settings in so many half turns out, you say it in full turns and fractions of that. I don't have a clue what setting you're at by your description. I'm pretty sure the spec for the VMs is around 1.5 turns out to start. Mikuni likes you to end up between 1 and 3 turns out after final adjustment. If you are outside that range, it indicates a pilot jet change is needed.

 

[vdubplate]

I know they don't but these carbs are so finicky that half turns can make a big difference.

 

[5twins]

Well yes, of course they can, but when you tell people what your mix screws are set at, you don't tell them "6 half turns", you say "3 turns out", or at least you do if you want anybody to understand what you're talking about.

 

[vdubplate]

Noted
I was able to ride my bike a few miles tonight. Very satisfying I must say!

 

[XS1961]

Part of the 650 set-up is removing the air jet and running without one. If you find an air jet still installed, it's a pretty safe bet that your carbs aren't 650 baselined .....

?
Ive read this before, but Ive never had to remove the air jet from any of my 650 installed VM's & theyve all run perfect,and still do ...Is this something to do with weather/temp differences over in the States?

 

[5twins]

Maybe it's weather related or maybe it has something to do with differing gas formulations between here and where you are. But I think the main reason was that the chosen needle and needle jet (6F9 and P series needle jets) run too rich with an air jet in place. Do you run that needle and series of needle jet or something else? There are so many tuning parts available for these carbs that it probably is possible to find a needle and needle jet that will work with the air jets in place.

 

[grizld1]

It has more to do with the demand characteristics of the engine. Some high-revving machines lean out at WOT when they hit high rpm's, and that's what the main air correction jet addresses; RS-series carbs (made for high-revving sport bikes) come with plugs installed in the MAJ holes that amount to undrilled AJ's, for maximum correction.

That's not much of an issue with our old twins. A full-race motor under race conditions might benefit from a bit of restriction. This is something I've rethought over the years, but in street use, the difference between the #2 AJ installed by Mikuni and an open hole would be pretty negligible.