Mikuni VM34 tuning

[LightningTeg]

Predator 420cc + Mikuni VM34 tuning

So I’ve gone down 2 sizes in pilot jet and got idle close with 4 turns out on the air screw. Looks like another size down will get that wrapped up. Sitting at mid 11s WOT. I’d like to see 12.5 so maybe one side further down on the main jet.

Now the needle jet. it’s running way rich 1/4-3/4 throttle, like barley runs 9-9.5 afr lol. I’m not sure if I need to order just the needle or needle jet also or if there is a way to combine them? I still need to pull it and determine what it actually is now but I have an idea after researching.

ProTip: if you need an O2 sensor weld bung, go to auto store and find “spark plug defoulers”. You can cut the end off and they work perfectly

 

[vpd66]

What O2 sensor gauge are you using?? I can see its an Innovate but what model is it?

---------- Post added at 12:04 PM ---------- Previous post was at 12:04 PM ----------

Also raise the clip on the needle and see how much it leans it out.

 

[Kartorbust]

What O2 sensor gauge are you using?? I can see its an Innovate but what model is it?

---------- Post added at 12:04 PM ---------- Previous post was at 12:04 PM ----------

Also raise the clip on the needle and see how much it leans it out.

Zoom in on the picture if you can, it says LM1.

 

[Brianator]

That thing is pretty cool, I'd love to get my hands on one.

I wouldn't go changing the needle unless required, it probably has plenty enough adjustment as ypd66 mentioned.

 

[BigWes]

That's awesome

 

[LightningTeg]

What O2 sensor gauge are you using?? I can see its an Innovate but what model is it?

---------- Post added at 12:04 PM ---------- Previous post was at 12:04 PM ----------

Also raise the clip on the needle and see how much it leans it out.

It’s an old LM1 wideband.

Clip is at highest position already. It definitely needs a new net or needle I just don’t think I understand how to pair them or if I can / should just order one or the other.

 

[Brianator]

There's a good chance you'll only need a leaner needle in that case (it's all in the taper) unless you need to go extreme. I know there's a guide online that breaks down what the characters on the needle mean and how to choose a better one for your application, I probably came across it when researching my 2 strokes TM34ss carb a couple years go.

 

[LightningTeg]

The transition to part throttle is ok, albeit still rich. Just really in the meat of part throttle it hates life.

I’m glad I have the wideband I couldn’t be motivated to be pulling the plugs after every test lol. I’m spoiled with my EFI on my “race” Car so easy to tune.

 

[LightningTeg]

So I pulled the needle jet, its a 159-Q2.

Im gonna go down a whole letter size and see what happens since these jets are about 3 times more expensive then the others.

So plan is a 180 Main, 159-P0 Needle Jet, and a 25 Pilot. I should add this motor is a 420cc

I highly recommend using JetsRUs. They have been shipping same day on all my orders and I have been getting them in < 2 days!

 

[bob58o]

Out of curiosity, what kind of timing are you running. I have no experience with O2 sensors, but if timing is not optimized will less than complete combustion lead to rich readings on the sensor?

Throw some timing at her and see how she likes it.

 

[LightningTeg]

Out of curiosity, what kind of timing are you running. I have no experience with O2 sensors, but if timing is not optimized will less than complete combustion lead to rich readings on the sensor?

Throw some timing at her and see how she likes it.

Whatever factory timing is? I didn't even know you could change it lol!

Ignition timing does not affect air / fuel ratio. That is fixed after the intake valve closes. I always tuned AFR first on my car then moved to ignition timing for maximum torque. Never noticed it change then and that was 600whp

 

[LightningTeg]

On a side note, I just removed the governor on this thing (internals and all). Do I need valve springs yet or should I wait for cam/rod etc?

 

[bob58o]

Changing ignition timing on these is as simple as rotating the flywheel on the crankshaft. They sell offset woodruff keys for this purpose - or other people run without a key after lapping the flywheel to the crank.

When I modified my 420, I did valve springs, carb, exhaust, ignition timing, milled head, port work, .... and didn’t upgrade rod, flywheel, camshaft,..

There was a post from the guy at VegasCarts on buggies gone wild where he convinced me that the upgrades I did didn’t need a billet rod. He said things got real dicey the moment up upgraded the cam.


“With regards to bang for buck, a 420 clone with the following mods will make 20-22hp and doesnt need a billet rod:

-Timing Key
-Ported & Milled Head
-28-32mm ATV/Motocycle Carb
-Header & Free Flowing Exhaust
-HD ValveSprings

Can be done for $400 including the engine.

-Geoff

-Geoff Warner
702-335-4002”

 

[Kartorbust]

Though funny enough, the 460cc big block he sells, comes with a billet rod in the kit, because it has a better oil slinger.