Single carb conversion thread...

[JasperLP]

TeeCat,
You need something to read. I don't know the way to put these in here, but go to these sites and download these. Print them out if you can, if you're like me you'll be reading them over and over again. My 61 year old brain is slow to comprehend, maybe I shouldn't have inhaled so much back in the seventies.

www.amckayltd.com/VM34-36.pdf

www.mikuni.com/pdf/vmmanual.pdf

These will help you a bunch, if it was easy, children could do it.

Stay after it, it's just a carburetor...

 

[TeeCat]

Thanks, Jasper...

I had seen the second document and have bee reading it over and over again since before I even got the carb, but have never seen the first one.

I suppose I'll have to open the carb again and see if I can determine whether anything is wrong with the float setting. That would seem to be the most obvious cause of flooding, but something is definitely not right.

This combination has worked for a lot of other people, according to joe-wiseguy, so I know it has to work for me at some point.

TC

 

[joe-wiseguy]

when i first read through this thread i thought of weak compression right off. if you have had issues with other carbs and them not running well and with the info you posted here it sounds like weak comp as the engine once warmed up dies. a motor does not increase in compression once at operating temp teecat...... once it warms up to operation the cylinders etc expand. yes you have more oil on the cylinder walls but if the motor has low comp at cold it will not improve once tested when fully warmed up. now i rode my bike today.... 80 xs650 special. two into one intake tracker model just as yours.... 36mm mikuni VM roundslide carb... 25 pilot with 200 main. temp about 68 degress and fairly high humidity. it ran rich on choke but once warmed up ran like a cheetah. all ranges. if i change anything on this bike it will be to install a smaller pilot jet. no fuel leaking out of the carb. compression 147 left and 145 right. 14000 miles on the speedo. elevation 200 feet above sea level. guess i should have checked the air-fuel mix screw setting but it will be different at your location unless you are in the same area as me anyway.

now your bike. from what i have read it doesnt sound like it is running too rich to me. if it starts right up with choke on but will not start with choke off when cold it is not too rich. if it was tooooo rich you could probably start it with the choke off. you said the bike had good comp last year. check it again. cold numbers and warm numbers. if it is the case then no carb on the planet can fix weak comp.

next your carb. hopefully you didnt buy one of the few chinese knockoffs that are being sold on ebay. as i have had one customer of mine school me on just that. the chinese miks are impossible to calibrate.

take your carb and remove the bowl and inspect the chamber for shavings etc as previously posted. yes they can come messed up. next check the needle setting. the needle setting should have come factory at the middle setting on the circlip. if the circlip is in a lower notch setting then it will run rich as the needle is raised out of the jet and the jet needles are tapered. if the clip is lower in the grooves with the circlip then it will run leaner.

set the idle screw with the spring in to raise the idle..... when it is cold run it in until the bike will keep running without rider useing the throttle to keep it going. once warmed up you can adjust it back to where you want it. next keep the bike running and as it warms up start raising the choke lever to shut it off. you may have to operate the throttle if not warmed up completely while you adjust the air-fuel screw to get the bike to run at a peak..... running this screw in will lean it out, while running the screw out will richin it. rule of thumb is that if you run the screw out to 3 or more then you should compensate with a richer or higher number pilot jet. now if you turn it in and it runs better then go a lower pilot.

example...... say you are running a 22.5 pilot. it runs but is popping some when you release the throttle or it doesnt want to idle down when the throttle is realeassed..... both are signs of lean fuel mix. so you start running the air-fuel screw out and get to say 3.5 turns and it is running better.... well you have now taken it through the next pilot size with the screw.... so at this setting you are running the equal to a 25 pilot..... so what do you do.?? you install a 27.5 which is the next jet up from your current settings and turn in the air-fuel screw back to @ one turn out and see how it runs. if you have to take it out to 3.5 again to get better response then you repeat the steps above. you should be able to get it close and perhaps have to screw back in the air fuel and find good power and running in all ranges.

you can also move the needle in the jet up or down to do final tuning to achieve greater perf.

that being said. all across the usa, canada, australia and uk.... guys are using this same setup with 22.5 or 25. or 27.5 pilots, with either a 200 or 210 main with raising or lowering the needle to acheive good throttle response.....

if you were running two carbs with a good motor you could possibly run even leaner pilot and main as has bee suggested here, and that may be the case for your local so dont put it out of your mind here..... however the carb is setting a little ways away from the cylinder runners and this creates surface area tension that stock motors dont experience. therefor the case has been that a richer fuel mix has worked best for this intake application. just like in a car the further away form the intake valves your fuel air mix takes place the richer you need it to be..

finally i would suggest that you find someone locally that is hip to carb workings and use thier experience to assist you in this setup. surely there is a local shop or someone they know that either races or works with carbureation.... that can guide you locally in real tiime and put hands on your bike with you to help you through this.......

carburetion is as simple or as tough as you want to make it. there are still possibilities of other variables here with your bike that you need to look at first before you get discouraged. get the mikuni tuning manual that you can download and find the VM section and read it over and over. check your compression again cause i dont care what it had last year and neither does your bike. check the float levels, check the needle settings. once you get this dialed in you wont want to go back to dual carbs again. i have not yet had a customer that once got his bike dialed in with the 2 into 1 take it off and go back to dual carbs....

check also on the intake that the bung with plug is tight that is there for possible use of vac petcock. a vac leak could cause you some heartache here too. it should have been set with seal as i installed them after chroming. look for any possible leaks elsewhere.

you mentioned coils but if a coil is bad they do sometimes stop working after they heat up. so check that you still get spark at the plugs if the motor stops when warm just to make sure.

 

[5twins]

The VM carb uses an air screw to fine tune the idle circuit. Turning it out adds more air and leans the mix. If you have to run it out past 3 turns, you need a smaller pilot not a bigger one.

 

[joe-wiseguy]

did i say that ass backwards in my novel i wrote? i better go look at it again...

 

[TeeCat]

joe, thanks for your detailed post. It's a lot of info so I'm going to read it over and try to approach it logically, but I still think I need to try to start the bike with some other air screw setttings. (griz)... I have only been at 1 1/4 and down to 1, but that was before I read that they work backwards from the CVs. I'll start with half choke too, and if I have the same result as yesterday, I'll have to pull the carb off and go in again.

Btw... is it normal to have any fuel in collecting in the inlet? What does actual flooding "look like"? Know what I'm asking? Just trying to determine if it's really flooding, or if this could happen because of my fumbling around trying to find a functional starting point, etc.

set the idle screw with the spring in to raise the idle..... when it is cold run it in until the bike will keep running without rider useing the throttle to keep it going. once warmed up you can adjust it back to where you want it. next keep the bike running and as it warms up start raising the choke lever to shut it off. you may have to operate the throttle if not warmed up completely while you adjust the air-fuel screw to get the bike to run at a peak..... running this screw in will lean it out, while running the screw out will richin it. rule of thumb is that if you run the screw out to 3 or more then you should compensate with a richer or higher number pilot jet. now if you turn it in and it runs better then go a lower pilot.

Might try this today before I go yanking the carb off. But the air screw operates the opposite way from what you're saying, from what I have read, right? Maybe I should start with 3 turns out and run it in?

Off to cut grass, read some more, etc.

TC

 

[TeeCat]

SOME SUCCESS!!!! (With video for your diagnostic pleasure.)

Guys, here's what I did:

I went to get a handful of stock plugs for dialing in. While I was there, I was going to get a compression tester, but they didn't have the rather inexpensive one that I thought would do the job. So that will have to wait. Maybe you can tell me what fittings it will need.

Anyway...

I went out there and set the air screw to three (3) full turns out, with about half or so choke... maybe a little more to start... possibly full. Can't really recall. About 5/32 under the slide. The bike started on the first or second hot kick, even after I had left the choke on all night. Oops...

As soon as it started, the idle speed increased, but got sloppy, and "rich" sounding, so I started to ease it off the choke, but did as joe said and came up gradually on the idle screw. It did respond to that and I was able to keep the bike running with it as I gradually came off the choke. In a minute or so, it was off the choke and idling (slopily) with the idle screw. I used that to maintain idle til it got a bit warmer.

The bike takes throttle at low speed, but idles like a Harley and would smoke a little AFTER I'd crack the throttle, which it normally does not do... or does it very little.

So, I came in to about 2 1/2 turns on the air screw. It seemed to respond to that a bit, but that's about where I left it until I get further guidance from you based on this report and the video.

NOTE: RIGHT after the bike started the first time, there was a little dribble of fuel in the inlet. But I wiped it off, and now there is none, even after several warm re-starts. So it looks less and less like it's flooding, and more and more like its unhappiness with the initial air screw setting, and like a tuning issue. So, I'm encouraged now!

In trying to take a video for you, I had occasion to shut the bike off and restart it three times because I had to run in and out of the house for the camera (I kept messing up the video). The bike restarted first kick, every time, with basically no throttle and no dribbling. As I just put the bike away (rain coming) the inlet is dry.

I don't think the bike ever really came up to temp, because I have not ridden it yet, so any assessment on my part is really not fair, but as it warmed up I had to continue to play with the idle screw, and had to add a little when the idle dropped to around 1000 rpm. But even at that, a 3/16 drill bit fits loosely under the slide. In my (in)experience, that does not really seem like an unreasonable amount of idle screw setting to me; I think that's a pretty small aperture.

It takes throttle well and returns right away, but will bog/hiss under a hard crack, then recover. You'll hear it.

So, I'm encouraged, but a bit stumped as to what's next, so I'm anxious to hear what you have to say at this point. Maybe this will work out! Better day today.

Again: 388.52 feet above sea level, 25 pilot, 200 main, about 2.5 turns out on air screw (I didn't want to go too far in without more input), and maybe about a 3/16 bit loosely under the slide for a lukewarm 1000/1200 rpm idle with no choke.

And weather/time permitting this week, I have to get up the nerve to do my tappets... need to get serious about that if I want a good tune. But I'm a bit reassured now that this was not a huge mistake. Maybe all I need is some patience and a few good heads on this project. I have the second part here.. the first part is the issue for me.

TC

Video:

 

[5twins]

Try the 22.5 pilot. You could also lean the needle a step. That might clear up the bogging when you whack the throttle open and are transitioning from the idle to midrange circuit. Just do one thing at a time. Changing the needle may make things good with the 25 pilot. The 22.5 pilot may make things good with the needle where it presently is. Do both at the same time and it may be too much, making things too lean.

There's a few other things you should find out from Joe - what needle everyone uses and whether or not they remove the air jet. When you run a pair of VMs on the 650, you remove the air jet (run without one) and change to a 6F9 needle. Not sure if that applies to a single carb set-up, though.

 

[TeeCat]

twins, thank you. joe didn't mention those other things, so I'm assuming that we're talking about just these variables here. He'll hopefully be following this.

I only have a 27.5 pilot in addition to the 25. So I'll have to order a 22.5. Meanwhile, maybe try leaning the needle? Now, is that raising the clip toward the top slot, or dropping it to the bottom? So, you think I'm rich then, right?

And I understand in theory how the mix screw works, but am still really confused about when to touch it in relation to other changes I make.

Thank you, sir. I may have time to do the needle this coming week. And I'll order a pilot.

 

[JasperLP]

TeeCat,
I really like the way that thing is sounding! When you put the air filter on, you may have to adjust some more. I'm with 5twins when it comes to changing one thing at a time. Otherwise you don't know which worked. I kept a log when tuning mine, and still do.
Here's some more reading material that might come in handy when you get out there doing some serious testing. It's an old article, but not as old as your bike. That is a good looking '73. I put 35,000 miles on one of them back in the seventies.

www.strappe.com/plugs.html

Be a happy tuner!

 

[5twins]

Raise the clip to lower the needle. Needles usually come set in the middle #3 slot from the factory. This gives you 2 steps up or down to fine tune the midrange with. The slots are numbered 1-5 from the top down. #1 would set the needle as low (lean) as possible in the slide, #5 as high (rich) as possible .....

Yes, it sounds like you're too rich. The fact that turning the air screw out (leaner) makes it run better supports this along with the smoke puffing. If the smoke is black, that's a definite rich indicator. White smoke can indicate a rich setting as well. It may be the rich mix washing the oil out of the cylinders, giving you a 2 stroke style oil and gas mix. That's why it smokes.

 

[tonyc]

Good job TC you are on the right track. It want be long. I really like the idea of single carb. I am toying with the idea of an updraft Zenith for my next engine. I will have to see if there is enough room. Keep at it. The other TC. PS check out a photo of my bike in the photo section under More progress

 

[TeeCat]

Jasper, twins, tony, joe, griz, all... I appreciate your help so much. I get so damned mad out in my yard/little shop there sometimes that the squirrels pause from their acorns and look at each other... I know they're having fun at my expense. I'm convinced of it. Ah... I love the lil' snotters anyway. But I really do appreciate all this kind expertise even in the face of my blind fury!

(Tony... your bike is REALLY coming along nicely! MAN, I'd love to have your skill. Move to my neighborhood at your earliest, please and thank you. )

twins... you're making huge sense in your explanation. I get what you're saying, thank you. So, this week, I'll go out there and pop the cable/cap off the carb, pull the slide, get the needle out, and raise the clip one notch to lower the needle. Think I can do that without pulling the carb. Might also try to find a 22.5 pilot just to have it. Might be a few days (rain/time) but I'll keep you posted. This is making more sense now, but still fuzzled about when to touch the mix screw in relation to other changes like needle position, jetting.

Jasper, thank you. I'll read this! So, you like the way it sounds? I do too (in my gut), but I thought that asymmetrical idles was the exclusive domain of overcammed non-360-degree 4 strokes. Does that idle sound like it's approaching normal? She does have a wicked iff-idle bark, though, iimssms.

I am encouraged now!

 

[grizld1]

TC, don't assume that the smoking was due to a fat idle mixture. You may have bled fuel into your crank case. If the oil is fuel contaminated, change it right away.

I gave you a procedure for setting the air screw in your other thread. On VM's, don't attempt to control the pilot mixture by dropping the needle. Once you have the pilot circuit set up well enough to run (and you do), select and set main circuit components (main jet, needle jet and needle). Then correct pilot elements (screw position and pilot jet) in response.

When testing transitions, roll the throttle on fast, but don't snap it.

 

[TeeCat]

TC, don't assume that the smoking was due to a fat idle mixture. You may have bled fuel into your crank case. If the oil is fuel contaminated, change it right away.

I gave you a procedure for setting the air screw in your other thread. On VM's, don't attempt to control the pilot mixture by dropping the needle. Once you have the pilot circuit set up well enough to run (and you do), select and set main circuit components (main jet, needle jet and needle). Then correct pilot elements (screw position and pilot jet) in response.

When testing transitions, roll the throttle on fast, but don't snap it.

From the other thread:

Now with the motor warm, back the idle down to the lowest rpm it'll pull and tweak the air screw 1/4 turn one way, pause to let the motor respond, and turn it 1/4 turn the other way, looking for the position that yields higher rpm. Back down the idle again and repeat the procedure until you find the air screw setting that yields highest steady idle, and reset your throttle stop for 1200 rpm. If you find the idle spiking after a brisk ride to full warm-up, richen slightly--1/8 turn or so. Let your motor tell you where the "sweet spot" is. As to whether you're rich or lean, let the motor tell you that when you try to take a ride. If the motor falls on its face in off-idle transition, step up the pilot.

griz, your instructions in the other thread are received and noted. One question... when you refer to a "steady idle", is some "camminess" okay? My idle now sounds "cammy".

Now, considering all that has been said, here is what I think is on the agenda for my earliest opportunity, weather/time permitting:

1. Check oil for possible fuel contamination and change in that event;

2. Assuming that the bike starts and comes off the choke in current configuration, perform your air screw procedure as described in the other thread;

3. Once that hopefully produces a stable idle, try to put load on the bike and evaluate.

That would seem to be as far as I can go just yet, right? And if I can get as far as trying to take a ride, should I put the air cleaner assembly on?

TC

 

[5twins]

Follow Griz's advice. He has more 1st hand VM experience and knowledge than I do. The advice I was giving was just based on general carb knowledge. I forgot about that generic needle jet you probably have. It's possible you're both rich and lean. Too rich in the midrange with that needle jet installed (and the air jet) and too lean in the pilot circuit. That's why an established base line is so important when dealing with an aftermarket carb. There are so many jets and needles available for the VMs, you could spend the rest of your life trying them all if you didn't have a starting point.

Something else you could try for free is removing the air jet. If it helps, fine, if not, stick it back in. That will lean your midrange and main some (it feeds the needle jet). With that rich stock needle jet, it could help.

 

[grizld1]

TC, why are you trying to set the idle mixture with the air filter off? The filter influences the air/fuel mixture, so it needs to be in place when you tune. The idle may sound "cammy" for a couple of reasons. The main reason for using one carburetor per cylinder is to synchronize intake vacuum so that uneven performance between cylinders is somewhat compensated for; that was quite an issue sometimes on single-carb British twins. The uneven idle may also be due to asymmetries in your intake tracts. One thing to add to your list of chores: finish your jet list, including needle jet and needle!

 

[TeeCat]

twins, received and noted. It helps to know where you were coming from in your reasoning.

griz, I left the air cleaner off for a couple of reasons. It only "friction fits" presently; I still have to drill a little hole on the 60-degree elbow for the mounting screw.

Also, because I thought the carb was flooding (I think it's safe to now conclude that it's not), I was wanting to see what was going on in the inlet in terms of evidence of raw fuel. And because of the physical location of the carb, I cannot access the air screw with the air filter on. I have to use a tiny precision jeweler's screwdriver as it is.

It's a little tricky, but I suppose I'd better try to have the filter on, at least when I'm listening for the motor's reactions to what I do. I'll let the motor come to temp with the filter on when I'm working on the idle.

As to jet list, I have a 27.5 pilot ready in case I need it, and a 210 main, but as to the needle jet and needle, I'd better defer to your judgment on what you think I'll need to order.

Rain tomorrow, but maybe I can at least try to get the idle the bulk of the way sorted after work these coming weekdays.

MAN... this is a LOT to think about! But... I'm hopeful that this will be worth it. Just really glad I don't seem to have a flooding problem... just some serious tuning homework!

TC

 

[TeeCat]

Oh, and griz...

On VM's, don't attempt to control the pilot mixture by dropping the needle. Once you have the pilot circuit set up well enough to run (and you do), select and set main circuit components (main jet, needle jet and needle).

This is starting to make a lot more practical sense to me. So, in practice, if the bike starts and comes off the choke well net session and settles into a stable idle, from what you're saying here, I suppose I could actually try to place it under a cautious load? Little worried about that stumble in the video making itself evident in traffic... but maybe that was largely from my cracking the throttle as much as needing to select/set main circuit bits.

By the way, about a third of the way down this page, you'll see the specs for this carb as it shipped.

TC

 

[grizld1]

That carb will never tune right on an XS650 with a Q5 needle jet in place. That's the heart of the carburetor, and if you could get running with that you'd be lucky to get 30 mpg. That's why you're smoking--as soon as the slide lifts you're getting a massive fuel dump through the NJ. One more time--use a 159-P6 or maybe, in your single-carb application, 159-P8 needle jet and a 6F9 needle; the one in that carb (a combination I've never seen before) is entirely unsuitable. And get that main air correction jet out of your intake bell if you haven't already done it. I'm done on this thread.