Vm20 idle circuit

I wanted to get some feedback regarding vm20 idle circuit tuning.

I have an e50 vm20 (190 main jet with 22.5 idle jet) with malossi reed and estoril.

The bike has always been a pain to start. I added the vm20 about 4 months ago.

I put the carb on the bike as shipped from Treats. With the choke on, I would kick the bike over several times (~10) with no avail. After a few second break, I would go back to kicking the bike over (~10 times) and give the bike full throttle and it would start. The bike would want to die unless I was giving it some throttle until the temperature reaches 225 degrees, then it runs fine.

I recently moved to Phoenix with garage temps around 100 degrees. I have been looking into the following guides and talking to some gurus and I am still lost -




The bike idle temp is around 260. From the above information, it seems that the mixture screw is to be 1.5 to 2 turns from being bottomed out to start the tuning process. What position do you put the idle screw when you start the tuning process and when do you change this? Choke or no choke?

Trying to understand the Mikuni manual, I found the bike wants to "stumble" when the mixture screw is 1 turn from bottomed out but never could get the bike to "stumble" when backing the screw out. At 3 turns out, the bike will reach an elevated RPM that does not change with continued backing out. I feel as if a minor adjustment to the idle screw can throw all the mixture screw adjustments off.

Sorry for the confusion. I am a newbie.

Re: Vm20 idle circuit

So you haven't actually changed the jets at all? They don't ship them tuned to your build... do you have idle and main jets in different sizes you can start tuning with?

Re: Vm20 idle circuit

I have been upjetting the main jet but I do not have any additional idle jets.

Re: Vm20 idle circuit

sounds like your pilot is too large. treats has a set.

Re: Vm20 idle circuit

Okay I can get some smaller jets.

How about the idle screw and choke?

Re: Vm20 idle circuit

Probably Fred /

Know what jets and parts your carb has and what they do


Get your bike to start and idle correctly first

Scroll down to see how to mark throttle on this tuning guide:


Re: Vm20 idle circuit

So down jet the idle and start there?

Re: Vm20 idle circuit

Jack Rutherford /

try a 17.5 idle jet - buy a range of them - maybe a 15 as well

Re: Vm20 idle circuit

Thank you for the advice. I will give the 17.5 a shot

Re: Vm20 idle circuit

The 17.5 did the trick. The moped is much easier to start now.

Now, I am having back firing with WOD. I have tried down jetting with no change in the back firing. The head temps are running at 324 degrees. The timing is set at 17 degrees and the points at set at 0.4 mm.

Is this more a carb problem or ignition problem?

The moped has a Estoril and I can see fluid coming out of the pipe when I am WOD.

Re: Vm20 idle circuit

What is WOD ?

Re: Vm20 idle circuit

Sorry -> WOT

Re: Vm20 idle circuit


Re: Vm20 idle circuit

do you mean backfire (intake tract combustion) or afterfire (exhaust tract combustion)?

the terms are often lumped together.

Re: Vm20 idle circuit

Check to see if your points are arcing

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