Wallbarrow carb

Walter F.

New Member
Jun 4, 2008
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Connecticut
Was going to say partsforscooters.com - Dean at pipelyne uses a Walbro on his reed cage motor, he could probably fill you in on any questions Walter F.
 

eDJ

Member
Jul 8, 2008
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Wayne National Forest
For those who aren't with a lot of experience with motorcycle carburators I'll list some info
that may help understand them better. Several forms are used. Some have the slide and float
with pilot jet & main jet like many of the HT's. Most motorcycles use these in the various forms
where small motors may rely on diaphram type carburators.

The classic motorcycle carburator uses a slide with a notch facing the intake opening. These
slides have numbers like jets and they contain the jet needle which has rings cut in it to adjust
it. The needle has a number just like a jet and so does the jet it rides in called the needle jet.
Then in the float bowl there is the pilot jet and main jet each with their own number and size.

At idle the air is taken into the carburator at the bottom vents and mixed with fuel via a pilot jet
and an air screw to help regulate the idle. When the throttle is opened the slide lifts and then
air begins to flow thru the venturi where the fuel is sucked up thru the main jet/needle jet, and jet
needle contained in the slide. As the slide moves further upward the venturi changes air flow
characteristics and more fuel is metered in from the narrower tip of the needle. At this volume
the idle circut will technically cease and the carburator is said to be "on the main jet".

See the illustration:



In the illustration below:

At different throttle positions, idle, 1/4, 1/2, 3/4, and WOT (wide open throttle) the graph
below shows which part of the carburator can be best adjusted to assure optimal performance.
At the right hand verticle of the graph is the word: "Effectiveness" indicating how and where
the most effective efforts can be made.



Again this applies to a motorcycle carburator with slide, jet needle & needle jet regulated fuel
supply, float/float bowl containing the main and pilot jets.

The Mikuni, Keihin, and Bing carburators are common on Japanese motorcycles but there are many other makers of similar carburators. Some of better design and with higher quality features.

Mukini carburators are popular and you can check out this tutorial if you are interested in fitting
on to your HT.

Mikuni carburetor operation and tuning

Some motorcycle carburators with slides use a large diaphram attached to them in what is known
as the CV system (constant velocity) where the slide & needle were attached to a large diaphram
which was covered by a dash pot and operated by vacuum. This design was decided to be better
than earlier "butterfly valve" carburators and more sophisticated.

For diaphram carburators such as Delloroto's, CNS, and Walbro's, the system they use works
somewhat differently. Power lawn mowing equipment has moved to diaphram carburation
years ago as it is simpler to produce although not quite as high of quality as earlier float bowl
carburators in earlier times.

In the diaphram carburator the diaphrams can be used to draw fuel from the tank and supply it
to the venturi where a "butterfly valve" will regulate airflow and other diaphram systems can
be used as valves to sense vacuum pulses from the engine to operate the fuel pump diaphram.
The fuel pump diaphram may also cycle the metering of the fuel supply into the venturi via a
regulating needle it works against with it's inward and outward movement.

Delloroto's downloadable ebook here:

Dellorto Motorcycle Carburetor Tuning Guide | Free eBook Download

Simple diaphram carburators such as those by Zama's or Walbro's may be seen on small motors
such as yard and lawn equipment. The HT motors may be refitted with different forms of these
and the link below will explain their inner workings with a well laid out diagram. It's desinged to
help with overhaul and theori of operations.

Engines

The link below contains the first animated illustration on this post and is a good further reading site:

carb tuning

There are many solutions to a problem and rarely is there only "ONE" right way. So it goes with
motorcycle carburation. I'm still with my inclinations towards a good classic & proven motorcycle
carburator design which features a float & bowl, slide, adjustments, and cable control that will
deliver a consistant air fuel mix ratio in all conditions..."where I live". Anymore than that....I'd be
specifying "closed loop" electronic fuel injection.

I hope for those who feel they know so little about motorcycle carburators that this will help. In some of the post members have shown "tweeks" they have made on their stock carb's to notch
the slide above a small hole drilled in the float bowl so that the float bowl will be vented to help it
run better. These little tricks can save an owner lots of money which could be spend on something
which could give better performance. (such as an expansion chamber exhaust system)
 

eDJ

Member
Jul 8, 2008
530
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Wayne National Forest
I think Bikeguy Joe was mentioning to be suspicious about the gas cap and if air is
venting thru it properly. I've seen this happen to tanks myself where when sitting for
awhile insects built nest in the "serpentine" vent system they plugged up or vented one
way better than the other. During the sunlight of the day the warmth built pressure in the
tank which lead to dripping when the vent system failed to work properly.

If when you see this happening on your bike, try opening the cap and see if it sounds like
pressure is released. Small as the tank is it won't make much sound.

Sorry to hear about the welds. Do you have access to light brazing ? One other option is
get a can of "Tank Seal" and clean the thank out well with something like MEK (doing it outside)
and these small cans of seal are a thick liquid like "Pepto Bismol" that you have to coat the inside
of the tank by holding it in different positions to coat it completely, with the cap petcock
removed prior. Those holes can be plugged with a small rubber cork and a folded up paper plug.
When the coating air dries it will seal the tank if the tank is brazed up and mechanically sealed
well enough for the liner coating to fill in and seal the gaps.

I'd like to see those here with HT engines have a TEST thread where everyone removes the cap
from their tank and drains fuel from the petcock into a one liter bottle and time how long it takes to fill it just for comparison. Then do it again with the caps on the thank. Just to extablish some
standard for fuel delivery to the carb. (none seems to exist now)

I would be inclined to drill a hole right down thru the top of my cap just to assure a vent. Buy a
new cap to ride with and use the one with the hole to check with when you're suspicious. And only
run the tank about 80% full to allow room for expansion. (so if you start off to work early on a cool morning with a full tank figuring by the time you get to work you'll have used enough fuel to have a
"head space" in the tank for the fuel, would that space be enough such that when direct sunlight warms the tank (especially if it's a black tank) so the pressure in the tank won't plow fuel past the
float bowl inlet needle. Assuming the cap is venting correctly ?

From reading and writing in all of this, I'm getting more and more sold on that Mukini conversion
myself.

I had one motorcycle (a 3 cylinder 2 cycle) where I had petcock problems and found the internal
seal was broached and torn. That was when I got with a bike mechanic who knew his stuff
and we went thru the petcock and overhauled it and it worked fine aftewards. He pulled the
gas cap down (I'd noticed how is hissed when I opened it) and it turned out that there was a big spider nest in the vent where the bike had been kept in an old barn before I bought it. It was cleaned and blown out to open it and was never a problem after that) The Mikuni's on it had bowl vents with clear plastic tubes where if there was any dripping overflow it went under the bike via those tubes. I've seen carbs which were close to a hot area of a motor where the gas would "percolate" and drain into the intake and cause a flooded condition too. Hard starts resulted from that problem. I can't really imagine that being a problem with the stock carb of an HT in my climate, but the vent hole Egor drills may help insure it doesn't.

And lastly, If you have a carb with a "tickler" check it if you use it and rely on it. Make sure it
has returned to the fully backed off position. I know lots of people who never trusted them not to stick in the down position. I believe they press down on the float to assure the inlet needle
isn't stuck on the seat keeping fuel from entering the float bowl. I know these are just little details
but that's the Devil's favorite hiding place. :-||
 

seca40

New Member
Nov 15, 2008
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Albuquerque
Thanks for all the great reading. I'm still absorbing. I might get a good deal on a Mikuni 22 mm carb that came off a 70 cc honda 2 stroke. It seems about right for my little HT machine although I'll have to make a manifold to fit it. It is the pressure mount kind. Is 22mm too big? This whole project has been a learning experience for me and it has been fun. I never owned a two stroke engine until now. They're pretty neat . So simple.

Anyway, any info on intake runner length would be appreciated. I remember reading somewhere that a longer intake produces more low end torque and Vice Versa. I'm not really interested in tinkering with intake lentgh to produce specific results..weld I'm really just curious if shortening the intake enough to fit a bigger carb ,say 1" or so , will cause problems. Probly not I just wanted to hear some thoughts on this as I am a carb tuning NEWB. I have done the porting that Egor describes in his thread at the other forum.
 
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seca40

New Member
Nov 15, 2008
131
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Albuquerque
Hey Goat Herder maybe I'll see ya on the road sometime huh. Are we the only two here from Burque?
Oh yeah, I left my fuel valve on last time I rode and came home today to a puddle of fuel in my garage. Yuk.
 

eDJ

Member
Jul 8, 2008
530
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16
Wayne National Forest
Seca 40,

It's only 4 mm, but usually a larger carb responds at a higher engine speed. When you say
it's a pressure fit....you mean like the standard carb with the clamp ? If it is like that you could
produce a bushing to place on the intake manifold and seal the larger Mikuni on it.

What's signifigant is that it is a 2 cycle carb. The jet needle which is carried in the carb slide
on a Mikuni from what I've read in the past has a double taper.(for 2 cycle applications)

You'll just have to come up with a similar jetting for this carb to match up to the displacement
of your engine. The info I posted on the Mikuni site and some of the others like the one at the
bottom on Carb Tuning will explain what you need to know about the carb's: slide, jet needle,
needle jet, pilot jet, and main jet. Could help you to match up any of those jets & needles
IF your Mikuni doesn't perform like it should when you bolt it on.

But I'd find a way to make a bushing to shim up the 2 mm all around and seal it well before clamping it down. When you get it to start and run you'll be pulling the plug and reading it from time to time.
I wouldn't do this till my motor was thru it's breakin interval and you can run the 20:1 ratio
mix fuel.

What is the CC displacement of your motor ? Is it close to 70cc ?
 

seca40

New Member
Nov 15, 2008
131
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0
Albuquerque
Seca 40,

It's only 4 mm, but usually a larger carb responds at a higher engine speed. When you say
it's a pressure fit....you mean like the standard carb with the clamp ? If it is like that you could
produce a bushing to place on the intake manifold and seal the larger Mikuni on it.


No, it's not like the standard carb. It has an o- ring protruding slightly from a flat mating surface.
You answered a question for me though. I was wondering if somehow running the 22mm carb on the 18 mm intake would work. Good to know when I design the new intake. I may make something with the appropriate mount for the carb on one side and a stock style clamp on the other. In effect this would be the same as a bushing. Flow wise. I suppose if I'm going to make something I might as well not mess with the stock intake at all. It's just good to know a little bit about what I'm trying to build before I build it. So thanks..bld.

What's signifigant is that it is a 2 cycle carb. The jet needle which is carried in the carb slide
on a Mikuni from what I've read in the past has a double taper.(for 2 cycle applications)

The carb I'm getting came from a Honda ct 70 trailbike. 2 stroke. Right? (fingers crossed)

You'll just have to come up with a similar jetting for this carb to match up to the displacement
of your engine. The info I posted on the Mikuni site and some of the others like the one at the
bottom on Carb Tuning will explain what you need to know about the carb's: slide, jet needle,
needle jet, pilot jet, and main jet. Could help you to match up any of those jets & needles
IF your Mikuni doesn't perform like it should when you bolt it on.


This is the part that has always been a mystery to me, and a big reason I want to try this. Maybe I'll learn something. It's not too late is it?

Reading the bit about putting tape on the throttle and marking throttle positions then doing a plug chop at different positions was like having a light turned on in my head. A little less mysterious now.

But I'd find a way to make a bushing to shim up the 2 mm all around and seal it well before clamping it down. When you get it to start and run you'll be pulling the plug and reading it from time to time.
I wouldn't do this till my motor was thru it's breakin interval and you can run the 20:1 ratio
mix fuel.



I've run one gallon of 20;1 fuel through it. It has 25;1 in it now. I have run very little of that. There seems to be a lot of varying opinion on this subject.

What is the CC displacement of your motor ? Is it close to 70cc ?[/QUOTE]

By all accounts here it is very close to 70ccs. Powerking 80.

So let's say I make an intake and do my best to match the carb (venturi?) and intake port size and shape. My piston and crankcase being what they are should still create enough vacuum to move air and fuel through a slightly larger intake runner. Yes? Like you said it's only 4mm right.
 

eDJ

Member
Jul 8, 2008
530
0
16
Wayne National Forest
I just Googled.......motorcycle junk yards and Albuquerque NM.......and found several sources
for used motor cycle parts in your area there.

When you put the name in the search box of what you are interested in with the word "and"
followed by other search terms or descriptions, it is said to be a "Boolean Search" and often
turns up info you won't otherwise get. The search where you do this uses "Boolean Logic" in
the computer.

Good things come to those who shop around and can wait for opportunity while doing it. Even
creating your own opportunities like this put you over the top. You may find an old bike yard
that will show you a box of old carbs from small motors and make you a price on the lot of them.
If you can sort several you know to be Mukini's of the same size etc....it may be cheaper to buy
that as you can take each of the carbs and inventory the Slides, Needles, & Jets in them and
get an assortment of these things to experiment with cheaper than you can buy new slides,
needles, & jets. Then if you found a booklet on "Selection and tuning of Munkin Carbs" with
graphs and showing which air correction, main, and needle jet to use along with the slide and
jet needle........you'd probably have it in the bag.

I once had a pamphlet on "Selection and tuning of Solex Carburators" and it really served my
needs when I worked on VW dune & rail buggies. It was priceless for me in those times.

So, good luck and happy hunting !

Ps.....at this site there are downloadable e-Shop manuals inexpensive.
Just make sure they cover your series of carb before buying them.
These are at the bottom of the page there.

Carburetor Repair Manuals and Parts Service Sheets

Dellorto Carbs....PDF file $3.99

Mukini VM Carb super tuning manual PDF file $4.99

Mukini flat slide Carb tuning manual PDF file $4.99

This Walbro carb tech page is free:

Walbro Carb Tune up!
 
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seca40

New Member
Nov 15, 2008
131
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0
Albuquerque
Yup, it's hard to be patient sometimes. I WANT TO TINKER DANGIT!!!scratg I do know where there is a MC salvage yard in Albuquerque. Thanks for the Boolean logic. I'll keep you all posted on what I come up with.

Thanks again

Pat.