Mikuni VM34's Tuning??!! Help!!

Justjuan78

XS650 Member
Messages
26
Reaction score
1
Points
1
Location
Miami
Mikuni VM34 Tuning Question??!!! Help!!!
Hello everyone... Hope someone can help.... I have a Hugh's PMA, Pamco, 1.5" free flowing exhaust and brand new Mikuni VM34's with UNI filters which I bought from 650central... My questions are the following... Bike is starting and remaining on at a quarter-half turn out any leaner and the bike shuts off if chokes aren't on... When I try to give it a quick throttle the bike makes a hissing type sound and doesn't really respond... If I increase throttle slowly it will respond okay but the bike seems to have no power when I run it up and down the street...

I have timed the bike on fire and full advance, checked for air leaks... Cleaned the spark plugs, tried to adjust the air and slide screws but it's just not responding well. I am no carb wiz... Can anyone help??!!

One more thing... A few times I've noticed that the carbs have actually gotten cold to a point that there is condensation build up on them... I live in Miami.... Is this normal?!!?
 
Did you sync. the carbs? The bike will not run right with out of sync. carbs. Check the cable to make shure the slides are opening at the same time. If you got some set up information with the carbs read it. Are the idle screws set right ? Do the Basics MMM will get you going for shure.
 
Remember that with these carbs you cant just snap open the throttle and expect it not to bog down. These aren't CV carbs, ya gotta roll the throttle. You needa do some research on these babies, even 650central carbs cant be expected to work outta the box every time.
I bought VM34's with stock jetting from a mikuni supplier, so I feel your pain as far as getting them set up properly. But seriously, they make a HUGE difference compared to the Bull Shit series carbs (c'mon, no one ever thought BS carbs were aptly named while dealing with rotted out diaphragms?) So stick with it.
 
Thank you all for the advice! Read the mikuni VM manual... Great info!! I have not synced my carbs yet but from what I read it sounds like the air/fuel mix is too lean... Was hoping to take the bike out this weekend but i guess I will just wait and contact 650central to see what he suggests on Monday
 
After reading the Mikuni manual, don't forget to give this guide a go:

www.amckayltd.com/VM34-36.pdf

As I recall, MMM usually ships VM34s (for bikes with pods/opened exhaust) with 25 pilots and 180 mains. A couple people here have had to drop the needle a notch to get that working for them, which suggests the jetting may be a touch rich. If you're lean, double-check the jets to make sure of what you have.
 
Maybe start with telling us what jetting you're running. Need everything - needle, slide, needle jet, pilot, main, air jet...

I'm running VM34's and love them. I can 'blip' the throttle without any real problem. I'll dig up my jetting. Was running perhaps a tad rich on my stock motor, but I slapped them onto my new 750cc motor with a ported head and enlarged intake valves and it's running like a top with no re-jetting.

Air screws are about 1 1/8 turn out.
 
Sorry to be slow responding.

First off, 1/2 turn isn't a bad setting, though you don't want to go much inside it. Run the bike a bit, and if you don't get lean signals like severely elevated idle after warmup, exhaust popping under engine braking, etc., be happy.

If 1/2 turn gives you lean signals during a longish ride, check float level. Even if level was carefully inspected before shipping, the carbs have gotten tossed around in transit without fuel in the bowls to damp the movement of the floats. If the level (as measured at the point on the float arms where they contact the horizontal pins on the floats) is at or near the minimum spec (max fuel level), try setting the needle clip at #4 (1 up from the lowest groove) and reset mixture screws. There are heavy interactions between needle position and pilot jet, and that step will fatten the mixture. If a full step is a bit much, get a pack of assorted washers at Radio Shack, return the clip to #3 (middle notch), and reinstall it with a washer under it--the one that fits (you'll see which it is) is .020" thick. That will give you a half-step change.
 
Ok - my setup:

Slides 2.5
Main Jets 190
Pilot Jets 25
Needles 6F9
Needle Jets P6
Needle clip position 3rd from top

I'm in Toronto so about 250 ft above sea level.

I ran this on a stock motor with Mikuni carb boots (with vacuum barbs blocked off with the caps from MikesXS), 6 inch UNI foam filters, MikesXS 1.5 inch headers and Birturo Megaton silencers which are pretty much free-flowing.
 
Read All the info... There is Alot! I will have to re-read a few times and then some... Stupid ass work is getting in the way of my motorcycle time :D
Going to call Michael at 650central today and find out all the specs on jets and needles so I can post...

Thank you all for the help so far
 
Instead of bugging Michael and wasting his time, open your carbs, check your float level, and make your own jet list while you're in there. It's the right thing to do.
 
Do you think he remembers what he put in every carb set he sells, lol. You wanted custom, high performance carbs, you got them. The ball's in your court now.
 
Actually He did.. I think I got the carbs pretty much tuned... It's been raining and hot as hell here in Miami so there has been hell of a lot of humidity which I believe has been one of the problems... What he sent me seems to be right on the money and yesterday after a some tuning with a buddy I took it for a ride and it ran hard with power and speed all the way through on the throttle and no lag or hiccups.

Thank You All for the Help and info!!
 
Hate to steal the show but I have a related question I have mikuni carbs on my xj650 from Murray’s carbs should I leave my vacuum line on 3 take it off or t off it and put one on 1and three I’m talking bout the one that goes to my petcock
 
You need engine vacuum from a cylinder's intake to operate the stock petcock as designed, doesn't matter which one. VM34's are typically not fitted with a port for vacuum, unless your kit supplier made a provision for one. I think there may also be non-vacuum operated petcocks that could replace your stock one.

Note: this forum is geared primarily to Yamaha 650 twin cylinder machines 1970-1985 and their modifications.
 
The choke enrichens the mixture, so if it runs with choke on but dies with choke off then obviously it's running to lean. Your plug colour will tell you that. Screwing the mixture screw in leans the mixture, out enrichens it. Screw the mixture screw out until it runs OK with the choke off, After warming up. From memory I think the bench setting is 1.5 turns out, over two and you need a bigger pilot jet.

I run VMs on my 74. It has 42 mm pipes, pods, longer manifolds (which make no difference) and pretty open DIY mufflers. Can't remember the jetting or needle position, but I did have to change them.

To start try the idle screw 1.5 out and needle clip in the centre groove to start, then read your plugs.

READING THE PLUG COLOUR IS KEY.

You also need to sync the carbs to get it revving cleanly. Either buy a set of vacuum gauges or make yourself a manometer to sync. A couple of coke bottles will suffice rather than going to the extreme I did. You need something with a reasonable volume of water, stay away from a looped tube, it'll only confuse and annoy you if you have no experience. Here's a video showing how to sync using the manometer shown.


With VMs, when synced and jetted correctly, you CAN snap the throttle open for a clean response, they will work better than BS carbs, that's why people run them.

Stock BS34 manifolds will fit VM34s, they should have spigots fitted for vacuum petcocks, if not you can fit them, or swap to manual petcocks.
 

Attachments

  • 9.jpg
    9.jpg
    288.9 KB · Views: 134
  • 6.jpg
    6.jpg
    287.2 KB · Views: 146
  • manometer2.jpg
    manometer2.jpg
    205.6 KB · Views: 148
  • 6.jpg
    6.jpg
    203.4 KB · Views: 140
  • 277997242_3239169633009727_2126554404949701837_n (1).jpg
    277997242_3239169633009727_2126554404949701837_n (1).jpg
    213.7 KB · Views: 125
Last edited:
The choke enrichens the mixture, so if it runs with choke on but dies with choke off then obviously it's running to lean. Your plug colour will tell you that. Screwing the mixture screw in leans the mixture, out enrichens it. Screw the mixture screw out until it runs OK with the choke off, After warming up. From memory I think the bench setting is 1.5 turns out, over two and you need a bigger pilot jet.

I run VMs on my 74. It has 42 mm pipes, pods, longer manifolds (which make no difference) and pretty open DIY mufflers. Can't remember the jetting or needle position, but I did have to change them.

To start try the idle screw 1.5 out and needle clip in the centre groove to start, then read your plugs.

READING THE PLUG COLOUR IS KEY.

You also need to sync the carbs to get it revving cleanly. Either buy a set of vacuum gauges or make yourself a manometer to sync. A couple of coke bottles will suffice rather than going to the extreme I did. You need something with a reasonable volume of water, stay away from a looped tube, it'll only confuse and annoy you if you have no experience. Here's a video showing how to sync using the manometer shown.


With VMs, when synced and jetted correctly, you CAN snap the throttle open for a clean response, they will work better than BS carbs, that's why people run them.

Stock BS34 manifolds will fit VM34s, they should have spigots fitted for vacuum petcocks, if not you can fit them, or swap to manual petcocks.
Hey Tog
Thanks for the time you put into vids and pics, I will be tackling this task by early spring with TM36 flat slides and will employ your gauge method and hopefully be tuned for the warmer months
 
Back
Top