Carburetor

[nfk]

Hi folks.
I where looking at cfellows carburetor drawings and i think i`m going to use the design for my engine. (thanks for the plans!)
The doubt i have is how much should i enlarge it...
The plans call for a throttle body of 1/8, and it won`t be enough for my application.
For a 14cc 2T boxer twin engine, what size should be ok?
Should i use another design instead?

Thanks for any help!

Cheers,
Norberto

 

[cfellows]

Don't want to discourage you, but my carburetor design is pretty basic and I doubt it will give you the performance you want.

As far as size goes, Randall Cox built a 6 cyclinder, 1 inch bore x 1.5 inch stroke engine and his carburetor had a 1/8" venturi. It seemed plenty big enough to run the engine at a good clip. Randall's engine was featured in the first 2 issues of Model Engine Builder and was called the Open Column Six.

Chuck

 

[nfk]

Hi
Thanks for your reply.
I have the issue and was one of the first things i note about the size, but glow engines i have, use way bigger carburetors.
I have a few cox .35 and the venturi is 6mm wide :-\
I know venturi size is not linear with displacement, but i`m not sure about going with 1/8.
After assembling, i was amazed about the vacuum generated when the pistons go up, the small hole (3mm) i left where the reed valve and carburetor should go, makes you feel a resistance when turning the crankshaft. (enough to leave a mark on your finger!)
The engine probably won`t see any plain action, but i want it running the best i can. (i will build another with all the bugs fixed, i have a great model for it!).

I will build it with 1/4 and see what happens, i have a load of free materials for it. (nice history, i will tell later!)

Cheers,
Norberto

 

[stevehuckss396]

but i`m not sure about going with 1/8.
I will build it with 1/4 and see what happens, i have a load of free materials for it. (nice history, i will tell later!)

Why not build it with a 1/8 inch and increase if you have to. Might save you alot of work.

 

[cfellows]

A two stroke will usually want a bigge venturi than a four stroke. RPM also makes a difference. A 2 stroke .35 engine running at 6,000 RPM pulls a lot more air than a 4 stroke running at 2000 RPM.

Chuck

 

[nfk]

Why not build it with a 1/8 inch and increase if you have to. Might save you alot of work.

I built it yesterday using 1/8 and leaving some room to enlarge it. (Thanks for the tip!)
The good news is the engine started
The bad ones are that the reel valve is not working as expected or 1/8 is too small...
The valve is 10mm, it should be enough. I`m not sure about the membrane material, i`v used a piece of old x-ray acetate and i`m thinking it could be too thick for my application.
What would be your choice for it?

The engine started easily after priming, but after a few 'pops' it stall.
Repriming makes it to start again, but it stall again after a few pops.

Tonight i`m going to try a different valve material, and if it don`t work, i`m gona enlarge the carburetor to 6mm and try again

I`ll add some pictures to my build thread later.

Thanks for your help!

Norberto

 

[Admiral_dk]

Your imidiate problem isn't the 1/8" venturi, but most likely the height of the fuel in regard to the needle on the carburator.

It sounds as it has a problem sucking the fuel up to carburator and this is a problem that increases with a bigger diameter venturi. That said - I'm sure that 3.175mm. venturi is very much on the small side for a 14cc two-stroke 5-7mm sounds about right - again, start with the smaller diameter.

Your valve material will give you problems when the engine gets hot enough to melt the plastic, but not for a few seconds of running or a minute or two.

 

[nfk]

Your imidiate problem isn't the 1/8" venturi, but most likely the height of the fuel in regard to the needle on the carburator.



Your valve material will give you problems when the engine gets hot enough to melt the plastic, but not for a few seconds of running or a minute or two.

Thanks, it`s the next thing to try tonight

What you recommend for valve material?
This won`t be my last reel valve

 

[mu38&Bg#]

The question of size depends on how much power the engine will be making. If the engine is just free running, the carb need not be large. If it has a load to pull, too small a carb will limit output power.

Greg