VM20 Jetting on Hobbit

Started as a PAI

-Engine rebuilt with new bearings, seals, race crank

-DR Kit (no porting just chamfered the ports)

-MLM peoples sideblead

-PAII ramp plate (havent nitched the vario yet)

-PAI vario weights

-Stock clutch springs for now

-Boyseen carbon reeds on a PAII reed block

-VM20 with homemade intake, dremeled the crap out of the PAI intake to match

Any suggestions for jetting on the VM20? I wasnt able to find much searching, probably because there's not an "off the shelf" intake option.

Here is a pic of the intake, came out pretty slick, exhaust header from some puch pipe, fabricated the flanges.

(edited)

HHVMintake_____.jpg

Re: VM20 Jetting on Hobbit

That intake has far too much angle and will be murder to jet.

Re: VM20 Jetting on Hobbit

Mane Layer /

What do you mean by angle? the bend of the intake? elbow? or the angle the carb is mounted?

Re: VM20 Jetting on Hobbit

The carb angle.

Re: VM20 Jetting on Hobbit

Mane Layer /

really, take a look at this pic. I feel like i have seen plenty of carbs mounted this way

(edited)

photo.JPG

Re: VM20 Jetting on Hobbit

The first pic is misleading. That will still throw off the overall jetting but is within the manufacturers max.

Re: VM20 Jetting on Hobbit

I've seen worse angles yeah. Worse comes to worse see if you can invest in a clear float bowl and set it right

Re: VM20 Jetting on Hobbit

I'm using a #195 jet currently with a Filter. For reference 180 ping'd under full load and seized. Had plenty of gas and 50:1 Lucas..

195-200 OK

210-220 bit too rich with the UNI filter.

Maybe the Angle? It would sit at the angle a TM24 would. Using a Weak-ends TM24 intake. (Shimmed with rubber hose inside rubber hose) (edited)

Re: VM20 Jetting on Hobbit

Re: VM20 Jetting on Hobbit

190 is massive. I ran a 120-135 on my Athena, TM24 with a filter. A vm20 would be smaller. However, when I first installed the weak ends intake it had a seriously bad angle, and ran a 130 with no filter just fine. Then, I took some bent hose and leveled the carb. It was so lean I couldn't break 10mph. The angle made that much of a difference. I ended up throwing an oiled filter on to suffice but also ran a 155 without it on at that point. The angle matters. The jet was so submerged in the bowl that it would have had to be practically empty to maintain the same level.

Re: VM20 Jetting on Hobbit

VM20 on my Tomos with 45mm Alukit runs perfect with a 160 jet. Lean with anything under 155. On a Hobbit that is wayyy more thirsty, a VM20 would need more fuel. VM 18 I ran 90-95 with malossi compact filter.

Re: VM20 Jetting on Hobbit

It really isn't WAY more thirsty, definitely not 8-10 jet sizes thirstier. What I am saying is that with an even larger carb, on a hobbit, I was running much smaller jets. I would hazard to say that an air leak might be found.

Re: VM20 Jetting on Hobbit

It should also be considered that different mikuni carbs use different size jets meaning a TM24 and VM20 with the same number jet might have different size orifices in said jets due to the style of jet, Also, the amount of vacuum drawn through different size carburetors will also affect how much fuel is actually being drawn through the hole in the jet, the hole is simply the maximum.

Re: VM20 Jetting on Hobbit

Mikuni uses the same sizing across the line. Different physical jet, but still. You are right though different venturi does affect that, which is why my larger carb would actually need a larger jet number.

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