Makuni VM34 Carbs = Backfire / Throttle Hang

MadMax1996

MadMax1996

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Yesterday I had swapped my failing BS38 carbs for brand new pre-jetted Makuni VM34 roundslide carbs. The fuel is now running through 2 fuel lines - each going tank to carb separately with shutoff valves and fuel filters in between. It seems to be running well enough to head down the road although I'm quite certain it could be tuned a bit better. It now has a tendency to backfire while either idling or rolling off the throttle. I'm hoping a good tune up with the correct pilot and main jets will fix the issue. Another new issue that the new carbs seems to be responsible for is an unpredictable throttle hang. I wonder if that has to do with my throttle cable not being long enough or if the slides are getting struck before eventually sliding back down.
 
Never saw a sticking slide in a new Mikuni carburetor; I'd guess the problem is either a sticking junction tube in the throttle assembly, lean pilot mixture, or, if your ignition uses a mechanical advance governor, a sticking ignition rod. Carbs bought prejetted only give you a baseline; final tuning is up to the buyer.
 
grizld1 said:
Never saw a sticking slide in a new Mikuni carburetor; I'd guess the problem is either a sticking junction tube in the throttle assembly, lean pilot mixture, or, if your ignition uses a mechanical advance governor, a sticking ignition rod. Carbs bought prejetted only give you a baseline; final tuning is up to the buyer.
Click to expand...
I realized I may have not had the fuel mixture screw at baseline. However, as I begin to turn the fuel mixture screw ahead 1/2 turns with 10 second intervals to check for improvement there has been no real improvement. It doesn't stumble any measurable amount more at any point up to 6 full turns. I'm going to change out the 220 pilot jet for a 180 to see if that gives me a response in the fuel mixture screw. I'll let you guys know if this does the trick.

Max
 
Hope you understand that the pilot mix on VM carbs gets leaner as the screw is turned out. The screw should never be turned out past 3 turns. There is no 220 or 180 pilot jet; those are main jet sizes. 220 mains are extremely fat. No more response from me until you post a complete jet list: slide, pilot jet, needle jet, and needle (already got the main).
 
I recently had those symptoms on a CV 34 carb
One side only ( left )
That seems to have been a dirty Pilot jet or idle circuit.
https://groups.google.com/forum/#!topic/rec.motorcycles.tech/EfiA6NtWrWk
"
There are two kinds of backfires related to dirty carbs. One is the
loud *bang!* associated with high fuel levels caused by dirty float
valves.

The other kind of backfire is a wimpy *piffle-piffle-snap!* on
deceleration, That's caused by dirty idle jets and passages."

But for heavens sake be careful...
Setting up carburetors is difficult. To lean --and it can go terrible wrong.
Not sure if this will help but perhaps
I don't have that carburetor but there appears to be setting up instructions
http://www.mikuni.com/pdf/vmmanual.pdf

Can it be to small pilot jets ??
 
I’m going to follow this thread with great interest. I have my ‘76 running half decent now with my PWK’s (or as good as I should expect), but I’m very interested in the VM34 round slide carbs. Disappointing to hear you may have slide hang up though. That’s one of the reasons I was contemplating going to the Mikuni’s. The PWK’s do have random slide hang up, not only at idle.

Good luck with the tune and keep us posted. Thx
 
bosco659 said:
I’m going to follow this thread with great interest. I have my ‘76 running half decent now with my PWK’s (or as good as I should expect), but I’m very interested in the VM34 round slide carbs. Disappointing to hear you may have slide hang up though. That’s one of the reasons I was contemplating going to the Mikuni’s. The PWK’s do have random slide hang up, not only at idle.

Good luck with the tune and keep us posted. Thx
Click to expand...
I'm pretty sure my throttle hang may have something to do with a slightly too short throttle cable. If it were a quarter to a half inch longer on the two split throttle cables it might clear up the throttle hang. Not to mention I'm still figuring out what pilot and main jets to use. The kit came with an assortment of different ones to try out. With all that said I already feel a considerable amount of throttle response increase since I put the VM34's on. Once they're tuned properly with a new, longer throttle cable they will be a huge improvement in my opinion over the BS38's that were on previously
 
grizld1 said:
Hope you understand that the pilot mix on VM carbs gets leaner as the screw is turned out. The screw should never be turned out past 3 turns. There is no 220 or 180 pilot jet; those are main jet sizes. 220 mains are extremely fat. No more response from me until you post a complete jet list: slide, pilot jet, needle jet, and needle (already got the main).
Click to expand...
You're right I had it backwards may have played into why I'm having difficulty tuning them :laugh:
 
Rule of thumb: when mix screws are located forward of the slide towers, turn out for richer, in for leaner. When mix screws are to the rear of the slide towers, out=leaner, in=richer.

In the Tech section there's a link to the VM34 and 36 carb guide that I wrote years ago for XS650 Garage USA. You might find it helpful.
 
Grizld1 knows his stuff! Would be worth spending some time reading his write-up.

With your symptoms...are the carbs synced and do the slides move at the same time? Also, ensure you don't have air leaks at the intake.

As to jetting, for a frame of reference, here's my set-up (Sea level):
175 Main Jet
20 Pilot Jet
P6 Needle Jet
6F9 Jet Needle
2.0 Throttle Valve
2.0 Air Jet (some people remove altogether--but I haven't found the need to)
3.3 Needle valve
1-1.5 turns out (air mix)
2nd from bottom clip on needle
 
Pay attention, Max--there are things that you can't derive from component-and-range charts. Component changes have very strong secondary effects in these carbies. The 2.0 slide used by some tuners is richer than the 2.5 slide used by others, and must be used with a leaner pilot jet (anywhere from 15 to 20) to compensate. Most tuners use the generic 2.5 slide, with a 22.5 or 25 pilot jet. Think the needle position only affects the midrange? Think again! Moving the needle even a single step can force a compensating pilot jet change. As Jon the bike freak says, get your cables straightened out before you try to tune.
 
Seems that I have cleared up the issue. Had to fabricate a bracket to hold the carb from bending down. The vibration of riding on country roads that are either nothing but dirt or nothing but pot holes would bend the carb down due to the rubber seat the carb slides into which slides over the intake. This bending would in turn pull on the throttle cable attached to the slides in the carbs causing the throttle to hang. Having the carb held up in position solved this issue. Now for the backfiring issue I got lucky and didn't need to mess around much with the current jets. I tried going both directions with the problem only getting worse. So a few adjustments to the fuel mixture screw (1.5 turns out) cleared up the backfiring. I must have been to rich. Or too lean. In reality God had my back on this one and got her where she doesnt have a hiccup all through the throttle and rev range. My main jet is 220, pilot is a 20 and the fuel mixture screw is set at 1.5 turns out after adjusting the idle screw to where 1/8 turn more out itll die. Thank you for all the help guys.
 
That’s great news MadMax. Can you also post up all the other details of your carb set up. Eg. needle number, clip position, Jet numbers etc. Thx
 
MadMax1996 said:
Seems that I have cleared up the issue. Had to fabricate a bracket to hold the carb from bending down. The vibration of riding on country roads that are either nothing but dirt or nothing but pot holes would bend the carb down due to the rubber seat the carb slides into which slides over the intake. This bending would in turn pull on the throttle cable attached to the slides in the carbs causing the throttle to hang. Having the carb held up in position solved this issue. Now for the backfiring issue I got lucky and didn't need to mess around much with the current jets. I tried going both directions with the problem only getting worse. So a few adjustments to the fuel mixture screw (1.5 turns out) cleared up the backfiring. I must have been to rich. Or too lean. In reality God had my back on this one and got her where she doesnt have a hiccup all through the throttle and rev range. My main jet is 220, pilot is a 20 and the fuel mixture screw is set at 1.5 turns out after adjusting the idle screw to where 1/8 turn more out itll die. Thank you for all the help guys.
Click to expand...
I know of one that ran to lean and damaged his engine so if this was me I would keep close attention to how the spark plugs looks. In the beginning .So I would be certain not running to lean. Probably you know this ..but I put it here anyway.
As I recalled it he had a Californian model setup that he tried to get more fuel to and : Fumbled it up .With severe damage to pistons and cylinders.
Even though he was experienced and talked to other that also should know. It has a reputation of being tricky to get right .That was a Mikuni bs 34 if i remember right
 
220 main, 20 pilot, and second clip position the rest I'm not entirely sure. It was a pre-jetted carb kit from niche motorcycle on eBay. The kit came with an assortment of pilot and main jets to use depending on the climate the carbs are used in. As for running too lean I'll be keeping a close eye on the spark plugs thanks for the tip!
 
Re. aluminum carb mounts, that "tube" is just a chunk of radiator hose. It should be trimmed to set the carburetor right up against the metal mount so that there's no room for sag. That might cure your cable problem too. 220 mains are going to run very rich unless lean needle jets have been installed.
 
grizld1 said:
Re. aluminum carb mounts, that "tube" is just a chunk of radiator hose. It should be trimmed to set the carburetor right up against the metal mount so that there's no room for sag. That might cure your cable problem too. 220 mains are going to run very rich unless lean needle jets have been installed.
Click to expand...
K thx, will have to source aluminum mounts that are the correct OD for my PWK’s.
 
Thx grizld1. I have read quite a bit and realize that these carbs are far from ideal. I have them working relatively well right now, but am seriously thinking about a transition to the Mikuni VM34’s. That may be in the budget for later this season, so right now I’d like to keep the bike going (if I can) with the PWK’s. I know it may be throwing good money after bad, but if I can keep the additional cost low enough, it should be too painful. Found someone in the Classified section that may have a set of the OEM metal carb support brackets for sale. Thx for your help.
 

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