Wallbarrow carb

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.

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)

Good read these stock chinas have kicked my but lately the float bowls just keep a leaking. Must be the winter cold or something seams as since the dawn of creation on this thing, there was always fuel dribbling from some ware . This is clearly why there is a shut of valve at the tank. I'm going to put one back on mine, but at this point just really bummed by the dribble. During the summer months the cute seemingly innocent screen in the tank fell apart just from focusing my eyes on it, thats all it took. Well OK no big deal got a inline filter using that, the shut of valve almost immediately! Got trash in it meaning a fuel starved motor. That was fun! So the cheap pot metal broke on the valve from the third or fourth examination of this phenomena.[ the peck cock clogged from the lack of the screen] I then about senile threw the part away. Things went lovy for about a month more dribble. Now during this time I played with to tanks all were leaking from foul welded studs. I took one off at work hast fully and flushed it out with water. I went to welding on it at the speed of lite.[shop was closing LOL] The studs welded nicely so I started to seal the filler neck with a bead of steel, first inch around it perfect! The rest has herpes, on a once pretty chrome tank. Back to the carby it went to leaking no prob adjusted the float bowl. this worked on the carb for two months no dribble! I had to paint a tank with engine grade paint to seal it. here we are winter more dribble carby again adjusted away to no avail. Had an extra carb tired that night I just threw it on all looked perfect and lovy. Next day parked at work more dribble. So I adjusted the float bowl on the new carby to days later more dribble. I now have a swamp cooler valve for a cut off it works great. No more float bowls for me right now!!!!!!!!!!!!

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.

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. 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.

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.

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 ?

I must of had trash in the used rubber hose I put on. It had been sitting In my 20 year collection of mechanical rub badge.I'm now back to the way I had it and no leaks. As for the tanks I agree they need a sealer, then good high heat engine paint that gas cannot eat! Drilled a small hole at the top of the cap about 3800 miles ago. CH still runs like a scalded dog! 4200 miles now. seca40 I no about 6 CH here now. As for the swamp cooler Idea if thrash makes its way to it, theres no clogging You can hammer the thrash out by working the valve a good inline filter will take care of the rest. The stock peck cock will clog in a heart beat if thrash gets to it. The cute little screen at the tank just hated the design.

Hey guys a really cool trick with diaphragm carbys when the darn thing gets so stiff with age or varnished gas. This trick works great, but has to be done right. Now if your bummed and want to start that old chainsaw or weed whacker, and this is your case no worries. You will need a propane [can't seem to spell right] torch you will not be lighting the torch, it is a combustible material. The most important thing is something like WD40 or like wise why ?If your messing with 2strocker it needs lube . How this works the diaphragm at this point on a needed overhaul is too stiff to make it pump fuel for this style carby. Get the rpms up by cheating it will run just fine when you wake it up. Got to shoot some lube into it as you do this. Now when the motor is hot it will start right back up. Heat loosened the said diaphragm. I had some grumpy times in my day this is a cool way to a short cut. Least your get the model right and replace the diaphragm. Hope this helps someone. Last the diaphragm is a fuel pump if it is old and stiff , you will never pull a rope fast enough to wake it it up . This is why I speak of this trick. Remember if it is 2 stock you need lube!!!!! It will wake up most of the time! Man if it don't theres more probs. Prop the unlit e gas at the edge of said carb all the while with a shot of lube! It works.

Also anything with a Diaphragm should be ran dry not just drained for this reason! If it will be stored for a long period of time. Good MB iking As for high and low adjustment settings a carb that came for such, designed for said cubic inch displacement has said built in appropriate adjustment less toying, fooling around. Hope this helps my in stinks say to me I like where I'm headed, everybodys welcome to correct me .WE are all goood here lots of great ideas!!!!!!!!! {never store a engine of any without running it dry]