mikuni vm34 on 750

baz · Jul 8, 2013

i have just fitted a 34mm mikuni to my 750 commando the engine is completely standard and the carb came from an 850 so is jetted 6dh3 needle 2.5 slide 35 pilot 230 main p.0 needle jet the bike runs pretty good it will rev out and tickover midrange is very good apart from at an 1/8 throttle opening it runs lumpy and pulls back when i did a plug chop at this throttle position the plugs are a perfect light tan i have tried adjusting the pilot screw in and out but it makes no difference to this flat spot it seems the slide cutaway may be wrong does anyone know what slide cutaway is recomended for the 750?, cheers baz

RoadScholar · Jul 8, 2013

I ship the 34 VMs for 750s with a #40 pilot, a 2.5 slide cutaway, a 230 main, a 6DH3 needle and a 159 O-8 needle jet; your flat spot could be coming from a number of foibles, but the #35 vs the #40 pilot jet would be a very inexpensive test that could bear fruit quickly. I am happy to help you if you want to call the shop, but be prepared to discuss the last time/mileage that you torqued your head, set your valves and checked your timing and compression; other forum members will, probably, need the same information to offer a well founded suggestion(s).

Bill.

christulin · Jul 8, 2013

Sounds pretty standard although some people run bigger main jets (wouldn't have much effect at 1/8 throttle). What is the needle position? Might be worth trying different needle heights. I think most people start with second notch from the bottom.

concours · Jul 8, 2013

baz said:
i have just fitted a 34mm mikuni to my 750 commando the engine is completely standard and the carb came from an 850 so is jetted 6dh3 needle 2.5 slide 35 pilot 230 main p.0 needle jet the bike runs pretty good it will rev out and tickover midrange is very good apart from at an 1/8 throttle opening it runs lumpy and pulls back when i did a plug chop at this throttle position the plugs are a perfect light tan i have tried adjusting the pilot screw in and out but it makes no difference to this flat spot it seems the slide cutaway may be wrong does anyone know what slide cutaway is recomended for the 750?, cheers baz

What air filter?

christulin · Jul 9, 2013

Yeah, I overlooked the obvious variables. The air filter can make a difference. So can the exhaust and the cylinder head. The airflow through a carb depends on atmospheric conditions (I live at 6000 ft), the amount of restriction of the air filter, flow through the head and the exhaust, valve size and clearance, etc.

pete.v · Jul 9, 2013

These carbs are very sensitive to needle to needle jet relations. There are numerous sizes of both and can be a real "hit and miss" endeavor.

It is common to get two numbers below and two numbers above of both. Now if you do the math you will see that the variables seem infinite particularly when you add the needle grooves into the equation.

I agree that the needle grooves is the place to start. If you get close but not quite there, you should be able to tell what to get and the direction to take.

You will get proficient at changing the needle clip after a few dozen times or so. Try to do it where the floor is open so when things drop and bounce away (and they surely will), you will have a good chance to recovering them.

baz · Jul 9, 2013

RoadScholar said:
I ship the 34 VMs for 750s with a #40 pilot, a 2.5 slide cutaway, a 230 main, a 6DH3 needle and a 159 O-8 needle jet; your flat spot could be coming from a number of foibles, but the #35 vs the #40 pilot jet would be a very inexpensive test that could bear fruit quickly. I am happy to help you if you want to call the shop, but be prepared to discuss the last time/mileage that you torqued your head, set your valves and checked your timing and compression; other forum members will, probably, need the same information to offer a well founded suggestion(s).

Bill.

engine has done around 800 miles since rebore and valves lapped in,last torqued the head about 300 hundred miles ago and checked valve clearences compression is very good althogh i have not checked it it runs lucas rita ignition the bike only does this with the 34mm mikuni i was running it on a 32mm CV mikuni but it would cut out at high revs with this one fitted

baz · Jul 9, 2013

christulin said:
Sounds pretty standard although some people run bigger main jets (wouldn't have much effect at 1/8 throttle). What is the needle position? Might be worth trying different needle heights. I think most people start with second notch from the bottom.

the needle was in the highest (richest) position i have tried moving it down 2 psitions this made no difference

baz · Jul 9, 2013

concours said:
baz said:

i have just fitted a 34mm mikuni to my 750 commando the engine is completely standard and the carb came from an 850 so is jetted 6dh3 needle 2.5 slide 35 pilot 230 main p.0 needle jet the bike runs pretty good it will rev out and tickover midrange is very good apart from at an 1/8 throttle opening it runs lumpy and pulls back when i did a plug chop at this throttle position the plugs are a perfect light tan i have tried adjusting the pilot screw in and out but it makes no difference to this flat spot it seems the slide cutaway may be wrong does anyone know what slide cutaway is recomended for the 750?, cheers baz

Click to expand...

What air filter?

it is running with a K&N pankake filter i tried removing it and it made no difference so i tried taping the filter up 1/2 the 3/4 of its surface it made no difference

baz · Jul 9, 2013

christulin said:
Yeah, I overlooked the obvious variables. The air filter can make a difference. So can the exhaust and the cylinder head. The airflow through a carb depends on atmospheric conditions (I live at 6000 ft), the amount of restriction of the air filter, flow through the head and the exhaust, valve size and clearance, etc.

it has a K&N filter no balance pipe on the exhaust and pea shooter silencers

baz · Jul 9, 2013

i should mention i have taped the throttle positions and marked them on my twistgrip and i know for certain that it only does it at an 1/8 throttle

RoadScholar · Jul 9, 2013

At 1/8 throttle the car is transitioning from the pilot circuit the needle. Try a #40 pilot jet with the needle clip in the center position.

Bill.

ILLF8ED · Jul 9, 2013

Ken Armann gave me the jetting that is perfect for VM34 on a 750, but I gave the slip away when I sold the setup. You can call Ken at 408 626-0061.

baz · Jul 10, 2013

i will get a 40 pilot jet and play with the needle settings as most recomend a 2.5 cutaway slide i wont change that just yet,the other thing i will check when i get time is for air leaks on the inlet manifold where it bolts to the head cheers baz

baz · Jul 20, 2013

ok i fitted a 40 pilot jet this made no difference so i tried moving the needle in fact i tried the clip in every position still no difference ,the only thing i have noticed is my carb dosent appear to have an air jet fitted or provision for one ,is an air jet needed on this carb? cheers baz

concours · Jul 20, 2013

baz said:
concours said:

What air filter?

Click to expand...

it is running with a K&N pankake filter i tried removing it and it made no difference so i tried taping the filter up 1/2 the 3/4 of its surface it made no difference

GOOD SCIENCE lad!

concours · Jul 20, 2013

baz said:
ok i fitted a 40 pilot jet this made no difference so i tried moving the needle in fact i tried the clip in every position still no difference ,the only thing i have noticed is my carb dosent appear to have an air jet fitted or provision for one ,is an air jet needed on this carb? cheers baz

You mean in the intake horn? Just a drilling on the sled VM's. I have had a partially plugged air jet passage (it was a hay seed sucked in while field riding, sleds have no air filter) that was a bitch to diagnose. I have seen them (removable air jets) on old VM's.

baz · Aug 12, 2013

finally got somewhere with this carburation flat spot,it seems the previous owner of this setup had modified the cap of the carburetor instead of the cable adjuster screwing into the cap it went through into a threaded disc this had come loose and was alowing air into the top of carb, such a simple thing that i overlooked!! the flat spot has gone so it was on to some WOT i ended up with a 280 main jet needle in the middle groove the bike will go from 0 to 100 mph without a hicup i am very pleased with it ..........baz

Hobot · Aug 12, 2013

Ugh you've added another number to list of things to consider discovering on VM's. Now go look up VM "Mikuni UFO carb kit" to add a bit more zest yet.

rpatton · Aug 13, 2013

You might want to check the float level just as a matter of course, especially on a carb that someone else has been 'tuning'. The tang level is usually a good measure, but not always.

mikuni vm34 on 750

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