PW50 VM-12 carb experiment

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
I've just ordered a VM-12 carb on Ebay for trying out with a 49 cc HT engine. I'm not finding much on this forum about this being tried as an alternative to the NanTung clamp-on stock carb.

Carburetor PW50 Yamaha

Reasons for getting this:

Better idle and mixture from low rpm than the stock carb, has an adjustable idle mixture screw,

Pre-enrichment circuit activated by cable/lever from handlebars,

It's also a clamp on and appears to be the same size as the stock intake, it may need some small mods to make work,

I have a small foam Uni filter that will fit this.
 
Last edited:


LewieBike

Active Member
Joined
May 21, 2014
Messages
684
Another reason I'm trying this out is to have a 'realtime' mixture control that can be adjusted on the fly while riding. So it doesn't require my having to stop the bike and mess with either disassembling the carb to change jets, or having to adjust a pumper carb's two Hi and Lo mixture jets. It also allows me to adjust for altitude density as the weather here changes.

I've been riding my little 30cc rear-rack friction drive bike frequently this month, and the need to stop and adjust the pumper carb between fill-ups and weather changes has been a small hassle. Many years ago I owned a couple small 1970's dual sport 2 cycles with cable operated starting enrichment levers, they were handy as a way of trimming the main jet mixture as they warmed up on cold days with high pressure density. In the case of this carburetor's use on the bicycle: I'm thinking of modding a friction thumb shifter on the enrichment valve which can be set as needed for the needs of the engine. I'm also curious about the venturi as it's listed as a 12 mm which is smaller than the NT carb, I'm not looking to be getting the most power out of this bike engine, I'm used to assist engines and this will be set up as more of a motor assisted bike.
 

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
And it looks like Cannonball2 from Motorized Bicycling Kit Forums has already tried this out on a 49 cc HT engine, and the verdict is.. It's a decent carb with excellent idling and low end response, but will need a 0.5 mm shim to interface with the 20mm spigot on the intake.

https://motorbicycling.com/threads/experimenting-with-carburetors.58429/

I'll start a build thread after the engine arrives, having an alternative carburetor for those of us not interested in absolute top speed, ( it's a 49 cc engine it's not going to beat any of the kitted engines..) is a good thing and should be chronicled.



 
Last edited:

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
looks nice. have you looked into air filters?
I've got a small Uni filter I purchased for a small motorbike years back. The carb has arrived, it's got a smaller opening/flange than the NT, and I think the Uni may dwarf it. we'll see.

Now if the brown truck people can find out where in the Midwest my shipment has wandered off to.
 

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
My engine kit has arrived, pretty excited, as this is the only HT engine I've ever worked with. I have dissembled it a bit to deal with greasing and checking the cylinder for swarf and scratches. It's surprisingly decent for a $117, 48cc kit. I was expecting it to have lots of flashing and other issues. Maybe these companies are upping their game a bit?
 

JerboaJohn

Well-Known Member
Joined
Jul 29, 2018
Messages
1,272
I guess some are good with the casting flash, others not so. China runs their dies and molds until they are roached out. my 48cc looks rough but runs great so I didn't mind too much
 

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
For what I paid for it I'm still impressed, the days of finding half decent non namebrand HT engines on Ebay and Amazon seem long past, but there's a lot of little details that speak about this company does care a bit. The fuel tank is spotless except for a small dent, no rust or loose crap inside, the fuel-line is some sort of Tygon with a decent fuel filter with a small magnet on the interior to collect metal bits away from the paper element. The pinion bevel gear has it's key installed, the puller works well. I pulled the head and cylinder, the piston is flat topped with beveled edges there was no visible junk in the crankcase, and the transfer ports and exhaust/intake were nicely radiused on the edges. There was little to none crank wheel wobble that I could see; I clamped a strip of sheet steel to one of the studs to check for runout, the upper needle is metal caged and turns smoothly.

The stock carb did not come with it's o-ring or the throttle cap's upper gasket. This is an issue that would have had an inexperienced builder fighting all sort of running issues. So it wasn't all roses and sweetness. Good thing I'm using the Yamaha PW50 carb.

I've yet to mount it on a bike frame, my chosen bike needs a complete paint strip/derusting and repaint. So this will be a slow build.

It's a 1958 J.C. Higgins lightweight 3 speed with lugged frame. Gear hub was a Steyr/Sturmey clone, but I have a newer Sturmey SC-3 gearhub coasterbrake so to avoid having to mount a dual brake cable lever. Not much extra weight penalty for the extra braking parts in the SC3 versus the Steyr
 
Last edited:

gary55

Well-Known Member
Joined
Nov 27, 2012
Messages
3,940
I'll be curious to hear how it runs. I'm guessing it's 2mm smaller than a NT. It could also be a alternative for the 4 stroke 142's
 

LewieBike

Active Member
Joined
May 21, 2014
Messages
684
Cannoball2 claims it ran great without any jetting changes on a stock 66cc, that it changed that engine's whole throttle response and idling speed.

That it idled very slowly and had more power down the RPM range, sounds like you could pull a lower RPM while cruising slowly.

I've ridden a couple 66cc bikes and not for the skill or lack of maintenance with their owners, those bike were crappy at anything below 13 mph, very stuttery and rich mixture-ish, and didn't hold an idle without some blipping of the throttle.

I figure it will probably need a little needle clip adjustment. It was originally meant for a 50 cc kids dirtbike with a reed case.

I'm not sure how well it would work on a 4 stroke, may need some rejetting.
 
Last edited:
Top