Mikuni 20mm Carb setup help

Hi there,

I rebuilt my Honda Hobbit with a 70 kit and a brand new Mikuni 20mm carb. The moped will only run with the choke on. Once I turn the choke off it dies. I messed with the idle screw and the mixture screw a bunch and could not get it to idle w/out the choke on. I messed with those 2 screws so much that Im not sure where my baseline was anymore. My question is...where do you set these 2 screws to start? And what is the process for getting it to idle normally w/out the choke? Im a noob when it comes to carb tuning so any advice would be great.

Ricky* Moved to this forum by grahammotzing(edited)

Re: Mikuni 20mm Carb setup help

I'd post more info

Pipe?

Intake?

Jets?

The vm20 is not easy to tune at all ......

Re: Mikuni 20mm Carb setup help

When I had a vm20 I ended up buying a whole Shit ton of jets.

Re: Mikuni 20mm Carb setup help

Yeah ok sorry here is what I gots going on:

70cc kit from treatland:

malossi cast iron 70cc cylinder

Pipe:

Proma

Intake:

MLM honda hobbit VM20 intake - https://www.treatland.tv/productdetails_popup.asp?productcode=motion%2Dleft%2Dhobbit%2Dintake%2D826

Jets:

What it comes with - treats website says "usually includes a 180 main and 22.5 idle"

Re: Mikuni 20mm Carb setup help

Dude, CARBS ARE NOT SETUP FROM THE FACTORY! Lol You need to tune your bike. buy a set of jets in the right range and get to work.

Re: Mikuni 20mm Carb setup help

yeah ok i see what you saying. how do i know "the right range" for jets?

Re: Mikuni 20mm Carb setup help

Sorry that wasn't meant to sound mean, I just don't understand how often people expect a universal part to be plug and play. It could be put on any bike!

Jetting is an art, you screw up and your engine seizes. So don't run it anymore until you get an idea of where you need to be jetting wise.

Running rich= four stroking, meaning it doesn't fire every revolution. No risk of seizing here as the engine is getting plenty of fuel/oil mix..

Running lean= bogging down.

You want to get the bike rideable rich, then slowly jet down until the four stroking stops and temps equalize...best you get a head temp sensor too, no-no zone starts at 400° be cautious....

There are thousands of threads on jetting here, prolly in the wiki too. Good luck. Someone will chime in with a range to start with(edited)

Re: Mikuni 20mm Carb setup help

aaron,

Thanks for the info. so the carb comes with a 180 main jet and from what i can tell looking at the comments under the page for the carb on treatland:

https://www.treatland.tv/mikuni-20mm-carburetor-p/mikuni-20mm-vm.htm

it looks like most people find luck around 140/150. So i guess running it today a bit should not cause engine problems because 180 is large and would be running rich right? just too rich i guess so i should jet down till it will idle?

Re: Mikuni 20mm Carb setup help

Idk if this is useful but here is a video of it running with the choke on:

Re: Mikuni 20mm Carb setup help

pretty quick vdo but lower yr idle. how she run?

Re: Mikuni 20mm Carb setup help

Yeah if it's rich ur not gonna seize it. I still wouldn't tho. But whatever not my bike.lol

Idle is way high. Slow that downnnn. But yeah get some jets n you'll be golden. Seriously tho, get a temp sensor.

I used this one. Has a tach too.

http://www.amazon.com/dp/B078YRRR5R/

Re: Mikuni 20mm Carb setup help

this video explains the tuning procedure

Re: Mikuni 20mm Carb setup help

you probably have an air leak. a 22.5 pilot jet is pretty large, that's a high idle, and you just rebuilt your engine. i would pressure/vacuum test.

Re: Mikuni 20mm Carb setup help

I diddnt think of that, but that sounds right.

Easy way to test is to spray carb cleaner on all the mating surfaces while running...crank seals, Reed block, cyl base gasket....of it dies you have a leak.

Re: Mikuni 20mm Carb setup help

hey Aaron does that tach/temp gauge read in celcius or farenheit. it kick ass that it has both tach and temp and you can replace the battery. iam gonna get one thanks

Re: Mikuni 20mm Carb setup help

Sounds like an air leak, I run a 170 and a 17.5 idle on my vm20 and she rips. This is on both the e50 and za50 with the treats supreme kit. I just run a non side bleed pipe is the only difference. Jets are your friend, and its actually kinda fun playing around with them. I really want that adjustable jet from mikuni!

Re: Mikuni 20mm Carb setup help

Dirty30 Dillon /

A) Requiring choke to run indicates a lean state, which with a 180 you should be anything but. You have a leak somewhere that is pulling in air.

B) VM's are easy to get running fair, but less crisp until you find the sweet spot. Most bikes I slap a VM20 on can be tuned enough for a life of riding within an hour. This is versus the 2 hours I would spend dialing in a PHBG, but I would typically have a VERY tight tune after that.

C) Don't buy the Dial-a-Jet from Sudco. For the same price you can have a full range of hard jets and tune multiple bikes.

Re: Mikuni 20mm Carb setup help

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> A) Requiring choke to run indicates a lean state, which with a 180 you

> should be anything but. You have a leak somewhere that is pulling in

> air.

>

> B) VM's are easy to get running fair, but less crisp until you find the

> sweet spot. Most bikes I slap a VM20 on can be tuned enough for a life

> of riding within an hour. This is versus the 2 hours I would spend

> dialing in a PHBG, but I would typically have a VERY tight tune after

> that.

>

> C) Don't buy the Dial-a-Jet from Sudco. For the same price you can have

> a full range of hard jets and tune multiple bikes.

What do you think the con of the dial jet is?

Re: Mikuni 20mm Carb setup help

Dirty30 Dillon /

> Will I am Wrote:

> -------------------------------------------------------

> What do you think the con of the dial jet is?

The biggest downsize is you have no frame of reference for what size jet you are running. The dial-a-jet is not indexed, so you have no way of knowing what orifice size you are actually getting.

That, and all it takes is someone or something rotating that screw on you, or vibrations turning it and you could fuck yourself. even if you don't seize, you have to go through the whole process of turning and tuning again. I would much rather have a hard jet that I can properly clean and replace without re-tuning every time.

Re: Mikuni 20mm Carb setup help

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> > Will I am Wrote:

>

> > -------------------------------------------------------

>

> > What do you think the con of the dial jet is?

>

> The biggest downsize is you have no frame of reference for what size jet

> you are running. The dial-a-jet is not indexed, so you have no way of

> knowing what orifice size you are actually getting.

>

> That, and all it takes is someone or something rotating that screw on

> you, or vibrations turning it and you could fuck yourself. even if you

> don't seize, you have to go through the whole process of turning and

> tuning again. I would much rather have a hard jet that I can properly

> clean and replace without re-tuning every time.

Makes sense, I do like the pleasure of knowing what im running, is what im running, when im running it...lol Thanks for the insight

Re: Mikuni 20mm Carb setup help

Hi all and thanks for the responses,

Ill order a set of jets today so that I can experiment. And ill order a temp gauge.

As far as an air leak... I cant imagine where the leak would be i build the engine last week and everything is to torque specs and all the seals and gaskets are new. i guess the only real way to check for air leaks w/in the engine case is to use a compression tool?

Id like to verify im running a 180 because like Dirty30 said, running with choke on is a symptom of running lean right? so with a 180 i should be far from lean. Is there a way to check and be sure im running a 180? is it written on the jet?

Does the wrong size jet commonly cause the bike to run only when the choke is on? or is that uncommon?

when you google "wont run without choke" everyone says its a blocked pilot jet. since my carb is new that cant be but maybe i need to adjust the pilot jet? is that an option? like i said im new to the art of carbs and jets.

Thanks for all the help.

Re: Mikuni 20mm Carb setup help

Buy a can of carb cleaner and spray all seals, carb, cylinder, intake....when it starts to die out you found your leak.

Re: Mikuni 20mm Carb setup help

> ricky miles Wrote:

> -------------------------------------------------------

> when you google "wont run without choke" everyone says its a blocked

> pilot jet. since my carb is new that cant be

You've checked the idle passage to validate your statement , right ?

New isn't always better or even correct .

Re: Mikuni 20mm Carb setup help

> ricky miles Wrote:

> -------------------------------------------------------

> Id like to verify im running a 180 because like Dirty30 said, running

> with choke on is a symptom of running lean right? so with a 180 i should

> be far from lean. Is there a way to check and be sure im running a 180?

> is it written on the jet?

it is written on the jet.

however, understand that the main jet is only used to limit peak fuel flow. it does not come into play at idle. the pilot jet and the air screw are your factors at idle.

Re: Mikuni 20mm Carb setup help

> Will I am Wrote:

> -------------------------------------------------------

> Buy a can of carb cleaner and spray all seals, carb, cylinder,

> intake....when it starts to die out you found your leak.

NEVER buy, let alone use , carb cleaner .

It eats rubber and rubber type materials . But , go ahead and shorten the life of your seals and such . The vendors love that and you really need the practice of tearing your motor apart .

If you want to clean a carb use brake cleaner . It cleans better , quicker , leaves no residue and doesn't destroy rubber .

If you need to find a cylinder leak , try a propane torch , opened on low and unlit .

If there's a leak , the motor will notify you , immediately , without any damage .

Re: Mikuni 20mm Carb setup help

> P D Wrote:

> -------------------------------------------------------

> > Will I am Wrote:

>

> > -------------------------------------------------------

>

> > Buy a can of carb cleaner and spray all seals, carb, cylinder,

>

> > intake....when it starts to die out you found your leak.

>

> NEVER buy, let alone use , carb cleaner .

>

> It eats rubber and rubber type materials . But , go ahead and shorten

> the life of your seals and such . The vendors love that and you really

> need the practice of tearing your motor apart .

>

> If you want to clean a carb use brake cleaner . It cleans better ,

> quicker , leaves no residue and doesn't destroy rubber .

>

> If you need to find a cylinder leak , try a propane torch , opened on

> low and unlit .

>

> If there's a leak , the motor will notify you , immediately , without

> any damage .

You may wanna have them edit Freds Guide....im pretty sure he's spreading lies about using carb cleaner to clean a carb....haha

I've been using it for a pretty long time with no problems....clean a carb, check for leaks...

But everyone is entitled to there own opinions for sure ;)

Re: Mikuni 20mm Carb setup help

Dirty30 Dillon /

Yeah, I definitely never buy Carb/Choke cleaner, just good old Brake Cleaner.

Re: Mikuni 20mm Carb setup help

Ok so there is no reason the 180 main jet would cause it not to idle since, like ken said, "the main jet is only used to limit peak fuel flow". so that limits the problem to either an air leak or the pilot jet is clogged from the factory. Is that the general consensus here?

thanks again to everyone who helped me here

Re: Mikuni 20mm Carb setup help

I run the vm20 needle clip second notch from the bottom 17.5 idle jet and a 122 or 125 main jet. Temperatures play a big role in jetting so your bike could run great one hot day then when temperatures changes u have to rejet.

Re: Mikuni 20mm Carb setup help

SOLVED...it was a huge air leak. I had pushed the stator side seal in too far and it caused a gap in-between the seal and the case. Fixed it and now carbi tuning is normal. Thanks to all that suggested to check for an air leak! Was a huge pain in the ass to fix. I had to completely disassemble the engine to get the seal out. Thanks again all!

ricky

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