I have been messing with a 40cc Briggs 4-stroke. I installed a CRRC ignition. By messing with the timing and the pop off lever I have a decent running engine. I am getting about 15 lbs static thrust, mid 5500rpms with various 20-22" props. The carb is a WYL142A/ 504. I don't see any refrences to it on the Walbro site. It has a 9mm venturi. I feel the itch to get a WYK carb, those have ajustments.

I have had no luck with getting a butterfly type to work properly despite various tricks to make them work. So I guess I need stick to the barrel type. Should I stick with the 9mm or go a little larger?

You may be having trouble with your pulse source for the pump on some of those carbs?

I'm currently using a real 4-stroke carb, it came stock on the engine. I already beat my head against the butterfly carb wall, getting a correct pulswe and sizing the carb is more wheel re-inventing than I want to persue. I found that I can buy a WYK-194 barrel type carb.

http://www.davesmotors.com/s.nl/c.88...t.A/id.4132/.f

I can't find much information and they don't know at Dave's if the WYK is a proper 4-stroke carb that runs off the intake manifold pulse. I am 80% sure that the WYK is the correct type and works on both 4 and 2-strokes. The one they sell is a little bigger than what came on the engine. It is 1.5 mm larger, 9mm vs 10.5. The thing that I am after is the high and low mixture screws on a 4-stroke carb of about 9mm venturi. Perhaps someone has a better idea than the carb I found at Dave's ? Or, will Dave's carb work properly on the 40cc Briggs?

I was smelling around on Google and I did see that the WYK194 carb is from a Mitsubishi TL33PFD Brush Cutter which is 32.6 cc 2 stroke

Thanks for looking around.

While you were doing that I was converting the carb to have a low end mixture screw..............................not!!

I removed the fixed pin that is in the center of the barrel and fits in the little straw. I then, on the second try, drilled all the way through a 6x48 machine screw with a teenie tiny bit. The bit was about 0.001 smaller than the original needle valve. I used the bit shank to make my new ajustable needle valve. That went in the 6x48 machine screw with loctite. I tapped the brass carb barrel 6x48. Hey it is ajustable now! Not really. [&o] The pin controlls the top end more than the bottom! It does allow me to lean out the bottom though and see how it idles with a correct mixture. It was running rich before, at idle, when the top end was good. The top end can be ajusted to some extent with the pop off lever. This is becoming a tangled web.

I don't know if this is what you are looking for or not, but it is one I was considering when I was playing with my gas four stroke. I got mine to run pretty good on a butterfly carb, but was thinking of buying this one. I subsequently sold the airplane with the engine, so the experimenting never went any further. If this carb is not specifically for a four stroke, you could probably make it work with nothing more than the addition of a small return spring under the fuel pump diaphram. I had to add one to my butterfly carb to make it work on the four stroke as well. (I just used the spring out of the carb that came with the engine originally.)

Anyway, check it out: http://www.davesmotors.com/s.nl/c.88...t.A/id.4132/.f

AV8TOR

Yes, yes!

That is the one I called Dave's about today. I have found internet indications that that serise carb ok for 4-strokes. That is to say it will pump with a neagative pressure pulse only, as supplied from the intake manifold. If someone can confirnm that I will buy one.

Anyone ever called Walbro directly? Are they helpfull?

Like I said, if it has a spring in the fuel pump for the diaphram, it will pump. If not, just add one.

A two stroke has a positive and a negative pulse to operate a fuel pump. A four stroke only has a "negative" pulse if it is operating off of the intact tract. It needs the spring to move the fuel pump diaphram in the opposite direction after each unidirectional pulse.

AV8TOR

Recipee for success...........

-One Zama butterfly carb, about 7/8mm venturi, off a weedwacker featherlite blower
-Make a spring out of a guitar string and it place under the pumping diaphram
-make an intake manifold that works with the carb and gives a "suck pulse" off the intake manifold.

It ajusted just fine for mixture, I picked up some top end RPM, and it now idles down to 1000 rpm with a correct mixture every where in between.

I wish I had a tiny walbro or Zama with normal throttle linkage. This one has a plastic shaft, thumb lever, and detents. It does prove that a butterfly carb can work.

Anyone have a tiny carb, 7 or 8mm in their junque box that they would sell??

I am sure I do. I will look tonight or tomorrow morning.

Congrats on the success,
AV8TOR

Sounds like the carb from a WeedEater 21 2 stroke whacker would fill this need?

From the old pumped walbro list it looks like the first two would be great. The second two might be OK too.

6.5 mm with fuel pump return spring (4 stroke)
WT 366-1
WT 456-1

7.95mm
WT 37-1
WT 424-1

There have been several people to use the Walbro WT-456-1 on their four stroke glow engines for conversion to gas.