Combustion chamber shape, squish, compression

scootertrog said:
Tiba is right! That OEM Teikei carb Yammie uses on the Vino is just about the worst carb I ever had to work around, with maybe the exception of the Gurtner carb on a Peugeot Speedfight.

Step,
Yamaha sells an 18mm carb (it's a Mikuni) as an OEM part, not sure what bike it comes stock on, but it would be an ideal fit if you wanted to stay with a Japanese-spec carb. It could be used later for a mild-to-decent 70cc kit if and when you upgrade. I've actually swapped out the Mikuni OEM carbs on CPI's with this 18mm, it even used the same slide assembly, lol.

scootertrog
Click to expand...

I'll probably go with a PHBG, though that's not really what I want.
Ideally I'd like a setup like Bib B's, but that's a bit pricey, and apparently
cheap used Tillotsons are no good.

Aren't Vespa carbs downdraft? I've seen pictures, but doesn't look
like anything recognizeable. Also they have two numbers... 14/12..
16/13.. whatever. What does that mean?
 
Mikuni carbs that come from Yamaha Jog Nextzone, SuperZ & ER (Taiwan version) are quite good & easier to tune than the crappy Teikei carb, we have been there & done almost everything to make that crappy carb work but to no avail, it would only run ok if the stock airbox is fitted.

make your life easier & get a PHBG carb, you'll thank us if you did.
 
tiba_karotsu said:
Mikuni carbs that come from Yamaha Jog Nextzone, SuperZ & ER (Taiwan version) are quite good & easier to tune than the crappy Teikei carb, we have been there & done almost everything to make that crappy carb work but to no avail, it would only run ok if the stock airbox is fitted.

make your life easier & get a PHBG carb, you'll thank us if you did.
Click to expand...

A nice sexy black one maybe :). Just kidding.

What are the differences in the ones sold as racing PHBG?
 
StepVino said:
Aren't Vespa carbs downdraft? I've seen pictures, but doesn't look
like anything recognizeable. Also they have two numbers... 14/12..
16/13.. whatever. What does that mean?
Click to expand...


Vespa´s carbs are usualy dellorto SHA carbs, they are not downdraft carbs.

That numbers, 14/12 - 16/13 - 14/14 - 15/15 ... , are carbs throat diameters. A 14/12 has a 14mm hole where the carb attachs to manifold and a 12mm hole where the carb attachs to air filter, same goes for the other number combinations.

SHA carbs are very easy to tune, as the main jet is the only jet one needs to change. No needle, no atomizer, no idle jet, neither air screw to tune, so jetting is fixed from 1/4 to 3/4. Running lean on this carbs is too easy.
 
Thanks eight_ball,
I was thinking of these strange looking things. Does it look like these
might work in downdraft position? Or is this the one you meant?

2nq9t1w.jpg
 
No, that thing is not the dellorto sha carb.. It has some similarities with a tillotson i think, i don´t think that carb is a good carb.

I think the smaller downdraft carb will be to big for your current setup, i´d go with a Phbg carb as all the guys advised.


Diferences from silver phbg vs black phbg:

The black carb or race edition phbg comes only in two sizes, 19mm or 21mm. Silver phbg carbs are 15mm, 16mm, 17mm, 17,5mm, 18mm, 19mm, 20mm and 21mm.

Black carb´s gas bowl has a big screw for quick main jet changes, where the silver one needs bowl´s 4 screws to be unscrewed to change main jet. You can fit this bowl with big screw to the silver carbs too.

Black carb´s slide cap is screwed to the carb for quick needle jetting, where the silver ones cap uses two screws to attach the cap to the carb.

The other main difference is that black carbs come with intake throat mirror polished, where the silver ones are rough.

If you decide to use the phbg carb pm me for jetting.
 
Hi eightball,
I think Dellorto used to make these, but not any longer. This is
a clone from India. Machine finish doesn't look too great, but
it does have a couple of adjustments, plus oddly, the pilot
and main jets are upside down.

I think the flush machined surface is where the air cleaner
mounts, so that would make the bottom of the jets suck in air....(?)
instead of being submerged in gas, like a regular carb. It's
so different that I find it interesting. Might pick a junk one up on
Ebay, just to take it apart :)

I appreciate all the input. Not being pig headed, just for now
still in the process of making changes to the stock intake and
carb, to try reach 60mph. Have quite a few things that are
not optimum, then I need a new belt. After that I'll probably
be taking most of the stock parts off, and go with real parts...
PHBG or PWK, crank, bearings, upgears, clutch, 70cc cylinder.
Hopefully with what I will have learned, that setup might be
made to go 70mph... the next goal.

Already have the cylinder... Ebay special $20 :) NCY 47mm. Looks
like ported same as the 50cc stock cylinder, and ready for lots
of work.

This is how the ported stock cylinder looks now, will try same with NCY
4hc2f0i.jpg
 
WOW that is some prety nice handwork there stepvino, keep going in this direction and you scoot will end up having a reactor engine :p

btw i see a minor scratch mark on the left side of the exhaust hole, you should smooth that out a bit to avoid having a fire flash on the side of your pistons
 
Hi spectro, yeah there's some pretty nasty looking
dremmel slips. I went over them with oiled fine grit
sandpaper, then reinstalled the old rings to take care
of remaining imperfections :).. ran it for a while, then
put new rings in.
 
Yeah, that carb is at least interesting, it would be nice to take it apart to find how it works.

Nice job on the exhaust port, did you tuned the transfers too?
 
Big B, I guess so. When I first started I read, and was told that it
was always ok to make the boost port a bit wider, so I did :).
I guess it's already pretty wide enough when stock?

Here's a pick of when it was stock, compared to when widened.
Couldn't find a better picture of it ported. Ported pic is view from
top of cylinder. Light colored metal in port is just reflection from
light coming in at bottom of cylinder.
2mpeych.jpg
 
Looks like you have some sharp edges, especially on the exhaust port, give them at least a small radius to avoid snagging a ring.
 
StepVino said:
Yes I did the transfers too. Exhaust is at 188 degrees, and transfers at
128 main and about 135 for boost port.
Click to expand...

hmmmm i think i see what other problems you have, i have tried that layout a long time ago with the boost opening first than the transfers, noticed some improvement in acceleration but it ran flat at topend, for best topend let the transfers open first the boost port last, it shouldn't open up higher than 124 degrees.
 
tiba_karotsu said:
hmmmm i think i see what other problems you have, i have tried that layout a long time ago with the boost opening first than the transfers, noticed some improvement in acceleration but it ran flat at topend, for best topend let the transfers open first the boost port last, it shouldn't open up higher than 124 degrees.
Click to expand...


You are right. Also staged transferports give less false "flushing"
i.e first tranfer slightly higher then second.

Boostports have very little fuction. Like tiba said, better to get tranfers open firts and then boost.
 

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