Mikuni vm38 on 74" shovel

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clunker1
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Mikuni vm38 on 74" shovel

#1

Post by clunker1 »

I recently bought a Mikuni VM38-9 to replace my butterfly Keihn carb. I've looked on the knowledge data base but all comments refer to panheads. I have found references online that indicate tuning of Mikuni is different on shovels. I'm trying to find out if anyone has used a Mikuni VM38 on a 74" shovel and what jets and needle circlip position was used.
I'd like to get a close starting point to start test riding. The carb is tucked under the tank so changing needle position is even more difficult. I do plan to get rid of drag pipes and replace with stock 2 into 1 header pipes to improve tunability.

I will really appreciate any comments.
Dennis
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Re: Mikuni vm38 on 74" shovel

#2

Post by kitabel »

If you do it now, you'll have to do it again with the mufflers.
What jets are in it now?
clunker1
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Re: Mikuni vm38 on 74" shovel

#3

Post by clunker1 »

Thanks for the adviceabout muffler. I'm going to test ride after installing new exhaust system.
Currently, 330 main, 30 pilot, 6dp1 jet needle, needlejet 166 q2, air jet 2.0, needle circling 8n position 3 from top.
Thanks
clunker1
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Re: Mikuni vm38 on 74" shovel

#4

Post by clunker1 »

Meant to type needle circlip in position 3 from top.
kitabel
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Re: Mikuni vm38 on 74" shovel

#5

Post by kitabel »

Slide is 2.5? Marked on the bottom.
Looks good enough to try, but better .5 air jet than 2.0.
If the plugs are dark, lean out the needle jet to Q0 or P8.

Despite what you read, there is no direct equivalence between circuits: 1 notch on the needle is not equal to X change in slide, NJ, etc. They all overlap, but do not duplicate functions.
clunker1
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Re: Mikuni vm38 on 74" shovel

#6

Post by clunker1 »

Slide is 2.5 as you guessed. I've heard some folks will change needle position to richen or lean.
Looking at the fuel flow curves, it seems it riches or leans more at mid throttle than at low or high throttle. The middle slot for the needle seems more linear fuel flow with throttle opening. What situations would warrant a change in needle height rather than a change in needle jet?
Thanks again for your help.
Dennis
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Re: Mikuni vm38 on 74" shovel

#7

Post by Edaddy »

If you are going to mess with Mikuni - do yourself and buy http://victorylibrary.com/MIK-HD.htm online. It's legit, when I bought it was $10.

Has the jets and other settings for 74 inch Pan and Shovel. Mine is a pan with Shovel valves so I kept to the recommendations for that. Got mine dialed in perfectly. One two kick start - right power throughout the range - with Drag pipes and revursion cones and lollipops. All these are addressed in the little book.

Added a power jet so my main is smaller - the calculation for that is in there.
As you can see I HIGHLY recommend it for Mikuni.

I am switching to an Su HIF44 - the rollon with the Mikuni w/out an accelerator pump is the only thing I cant change and I hate it coughing or stalling if I roll on too quick.
kitabel
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Re: Mikuni vm38 on 74" shovel

#8

Post by kitabel »

Move the clip down 1 notch if the response above 1/2 throttle is flat, reverse if it smokes black or hurts mileage.
If it's better with even lower clip, add 2 to the NJ size and put the needle back.
clunker1
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Re: Mikuni vm38 on 74" shovel

#9

Post by clunker1 »

If it is lean or rich at mid throttle, would not be acceptable to go up or down one size on the needle jet? The reason I ask is that I can do this without removing the carburetor.
Thanks again all for the valuable input.
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Re: Mikuni vm38 on 74" shovel

#10

Post by melvinjo »

If you have stock tanks, remove the right one, and leave the left side tank on (Plug or pinch off the small lower and upper cross over line) That way you can still run the engine or ride it, but have easy access to the carb adjustment I do it all the time, especially after any top end work where I might have to readjust carb settings
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Re: Mikuni vm38 on 74" shovel

#11

Post by melvinjo »

Sorry: meant to say leave the top cross over vent line open, but close off the botton fuel cross over line. Just dont fill the left tank up too much to avoid gas pouring out the top cross over vent line
clunker1
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Re: Mikuni vm38 on 74" shovel

#12

Post by clunker1 »

Never crossed my mind to leave the left side tank 9ff for access to the carb.
Thanks all for the responses.
clunker1
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Re: Mikuni vm38 on 74" shovel

#13

Post by clunker1 »

My bad. Meant to say leaving off the right side tank during carb tuning is a good idea.
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