Hobbit Tuning

Jack Hernandez /

Set up:

DR 70 kit / puch Aluminum head (super sport.

Proma Exhaust

MLM intake

VM 20 carb

HPI mini rotor ignition

TJT variator w/ 1.5mm shim

Yellow rear contra spring (ground nubs)

Stuffy crank

Pull start

Green clutch springs


Starts really easy

Idles fine

After a rev it wants to die when rpms are falling.

Main jet is currently at 195, however it did the same thing with 180, 182.5, 185,187.5,190

No obvious air leaks

Doesn't seem to be variating all the way

Carb leaks out of vent tube when gas is left on and bike is off.

Please Advise

Re: Hobbit Tuning

Brandon Weiss (Detective brandon to you) /

If you're leaking fuel from the vents, that means your float needle is stuck/not sealing. Check your float level and ensure your needle moves freely and theres no crud in the seat, and that your float actually floats.

That could be the root of all your issues, being too rich/flooding because of the float needle. That would explain the easy starts when cold due to the extra richness...and could potentially be causing power loss and lack of variation etc..

I dont know baseline jetting for these setups/vm20, so somebody would need to chime in there whether or not you're too lean/rich as far as jetting.

After timing/jetting, work on tuning the variator/pulley. If your jetting and timing is fine, and you're not variating, maybe your contra is too stiff/weights too light?

Re: Hobbit Tuning

i would go down to a 175 or so main

def a carb issue to start.

go up in weights on the variator and with your yellow spring in the rear, it's not stuck is it?

im running a polini front variator @18grams and yellow spring. malossi 70 h20 vm20 and a 172.5 main, 4pedal reedz

it's a pain in the ass but stay at it... when it's all together and running, it's a blast and worth it!

Re: Hobbit Tuning

DrGeorgeTompson (Alan) /

RPM dipping way low after a throttle blip is indicative of rich idle circuit. If it's leaking, that also doesnt help lol.

Re: Hobbit Tuning

Jack Hernandez /

Things started to go sideways since i added the rear contra. But at the time i didnt have the HPI on, or the shim in the front variator. I will pull apart the Carb again and try to clean the needle seat. Hoping its not bad casting.

Re: Hobbit Tuning

Jack Hernandez /

Update: installed new needle valve and petcock. Leaking from vent hose has stopped. I believe it was from the bowl overflow vent because the petcock was not turning off. Bike idles great, but Still dies after a big rev. Put a 175 jet in and same thing.

Re: Hobbit Tuning

Mine stopped idling. Pulled the head after checking everything else. (cleaned the carb 10x, swapped out the carb, sealed the intake, replaced the crank seals) Head gasket had burned through. Cleaned, reassembled and idle was back. Lapped and re-seated decomp valve too JIC. (edited)


Re: Hobbit Tuning

Your decomp channel is leaking into the combustion chamber... you’re just gonna keep blowing out head gaskets. Fill the channel or get a new head with no decomp.

Re: Hobbit Tuning

Was thinking the same. Not much gasket left with the cut head. JB weld OK for that?

Re: Hobbit Tuning

Get a pack of alumaweld rods and torch it.


Re: Hobbit Tuning

Jb will get the job done.

Re: Hobbit Tuning

decomp sounds solid to check. get new gaskets and a plug after you do

Re: Hobbit Tuning

Jack Hernandez /

This post took a turn. Please refer back to the first October 3rd post.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account