Carburetion who, what, where and when

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This is going to be my explanation of carburetion used for miniature engines. Although I have no educational background on the subject I have worked on a great many carbs over the years, everything from the simplest lawnmower types to constant velocity motorcycle and automotive designs.
When I got into miniature I.C. engine building, hit and miss type engines, it was just a matter of making a tube with a reduced diameter (venturi) somewhere in the middle of the tube. At this point there would be an entry for the fuel. Being as hit and miss engines run at a constant speed there was no need for sophisticated jetting or porting. The engine creates an airflow with the downward stroke of the piston and this airflow is accelerated in the venturi area thereby drawing fuel through a jet where the fuel is metered by a needle valve.
When I started building multi cylinder engines that required a variable throttle control I studied the design of the simple RC type air bleed carb. These carbs work like this: 1. They have a venturi area in the carb where the fuel enters. this is machined into a rotating barrel which acts as an air control valve 2. There is one main fuel needle valve and jet which controls the fuel flow at high speed. 3. There is an air bleed port with an adjustment screw (not needle) for idle fuel adjustment. The air bleed port controls the amount of vacuum signal at low speeds. This is how I go about setting an air bleed carb. I first adjust the air bleed screw so the air port is about half open. I set the throttle barrel so that it is approximately 1/2 open. With the main needle closed I turn the engine over and slowly open the the main needle until the engine starts to run. The carb can then be opened up to raise the rpm of the engine. At this point the needle valve might need to be opened ever so slightly. Now comes the part that takes a little feel to get adjusted. As the throttle is closed the engine will do one of two things, it will go rich and start to smoke and possibly stall or it will stall immediately as the throttle barrel is closed. If it goes rich then the air bleed needs to be opened until the engine seems to perk up a little. If this doesn't occur even with the air bleed screw fully open then the air bleed port is too small. On the other hand if the engine stalls form leanness then the air bleed screw needs to be closed somewhat to richen the mixture.
Over the years the carbs used for RC operation have gone from the basic air bleed type to much more sophisticated designs. The most complex of these have a rotating barrel attached to a sleeve which has a very small tapered slot machined into it. Inside of this sleeve is a fuel tube that has a needle valve to control the high speed operation. The sleeve with the tapered slot is also controlled by a threaded adjustment. As the throttle barrel rotates the tapered slot exposes more of less of an opening for fuel to flow. This is further adjusted by the screw thread that it is mounted on. Once the barrel is wide open and therefore the tapered slot is wide open then the fuel is once again controlled by the main needle valve as is the air bleed type carb.
Over the years I have seen many types of carbs used on miniature engines. there are some fellows that have gone with the smallest of 2 cycle type carbs, the type found on weed eaters and like equipment. These are still not an instant success as modifications need to be made. Sometimes the venturis need to be reduced to suit the particular application and in most cases the needles need some type of tweeking because our small engine need a much finer fuel control.
We as builders usually have a very hard time with carburetion. What to use, how big to make the venturis, what to use for the control needle and how fine of a thread to make the on the needle valve.
I started making my carbs by some numbers someone had posted on a forum years ago only to find out over time that these numbers were probably based on full sized carburetor design and didn't work for miniature engines. The one thing I did learn was to make the venturi small to start off with. It can always be made larger. Use the finest thread you can for the needle valve. The amount of fuel these engines use is minute and the slightest rotation can instantly richen or stall an engine.
Another thing to take into consideration is that carburetion is not based on the total displacement of the engine, unlike automotive practice, but more towards the displacement of a single cylinder being as only one cylinder is drawing fuel at a time.
One thing I did find in my searching around the internet is this bit of information taken from Wikipedia.

Bernoulli's principle, which is a function of the velocity of the fluid, is a dominant effect for large openings and large flow rates, but since fluid flow at small scales and low speeds (low Reynolds number) is dominated by viscosity, Bernoulli's principle is ineffective at idle or slow running and in the very small carburetors of the smallest model engines. Small model engines have flow restrictions ahead of the jets to reduce the pressure enough to suck the fuel into the air flow. Similarly the idle and slow running jets of large carburetors are placed after the throttle valve where the pressure is reduced partly by viscous drag, rather than by Bernoulli's principle. The most common rich mixture device for starting cold engines was the choke, which works on the same principle.

This writing is certainly not a definitive essay on miniature carb operation but is meant to give some information on what I have found over the years.
I now use air bleed style carbs on all of my multi-cylinder I.C. engines, from twins to 8 cylinders. Are they perfect, not by any means but they are the simplest to make and adjust and I get very good performance from them.
In closing I'm attaching some pictures of the carb that I use on my 302 V-8 engine. The engine has a 1.00 bore and .90 stroke. After much experimentation I found a tiny OS carb that worked great on the engine so I took the dimensions from that carb and built them into an automotive look-alike carb for my engine. As can be seen from the pictures the main throttle bore is very small and therefore the venturi area is even smaller. The engine can be idled down to about 800 rpm, slower would require a larger flywheel, and will rev to an honest 7800 rpm. The only time any further adjustment is needed on the carb is when the engine starts to heat up and the mixture starts to lean out. Otherwise the needle and air bleed settings stay the same.

302 carb 1.jpg

302 carb 2.jpg

302 carb 3.jpg

302 carb 4.jpg

302 carb 5.jpg
Very interesting post. Many thanks for sharing
After much experimentation I found a tiny OS carb that worked great on the engine so I took the dimensions from that carb and built them into an automotive look-alike carb for my engine. As can be seen from the pictures the main throttle bore is very small and therefore the venturi area is even smaller.

Good stuff, thanks for your insight. What I would like to know is orifice sizing between gasoline & methanol, all other things equal. You mentioned settling on a particular OS carb. So what (presumably single cylinder) OS engine did that correspond to that worked with your 0.71 CI on a single cylinder displacement basis?
Great synopsis of your carburetor experiences, George. We all appreciate your in depth postings. Thanks for a great thread.
The OS carb I started with is a 2A. I can't tell you what it came from but it was small. As far as the difference between alcohol and gasoline I can only go by what I know from drag racing. The amount of alcohol used was about 2-2.5 times as much as gasoline. Gasoline to air ratio is 14.7:1 and can go as rich as 12.5-13:1 under full power while alcohol to air ratio is about 5:1-6:1. Added to this is Nitromethane which has a ratio of about 1.7:1. So with these numbers one can see why carbs set up for airplane fuel which contain both methanol and nitromethane are extremely rich for gasoline use. That's not to say they can't be used but the needles and threads need to be much finer for gasoline.
Thanks George. From what I can determine, the OS 2A carb corresponded to 2 stroke glow engines 20FP, 20FP-M & 25FP. I think they are older models, but that's of no consequence.

So that’s interesting. If a relationship exists between the methanol/nitro 2S carb venturi throat to your gasoline engine on a displacement basis, it works out to 30 - 35% = 0.21/.71 & 0.25/0.71 (CI/CI).

I guess another unknown is the equivalent fuel orifice size corresponding to methanol throat size. Isn’t there some rule-o-thumb converting gasoline auto carbs to alcohol, you enlarge the gasoline orifices by 1.2 - 1.4 to get equivalent alcohol? Not sure if I have that straight, but if so maybe the little 0.25 methanol carb spray bar is ‘jetted’ equivalent to something larger on gasoline basis? Still, that would only bring it up to a .35 equivalent (1.4 x .25CI).

Maybe another sizing factor is ~10% premix synthetic/castor oil contained in typical glow fuel & fuel orifice is nudged up to account for higher blended viscosity? Castor is sticky stuff @ 350 cS, gasoline is ~0.6 & methanol ~0.5. Not sure about synthetic oil, 50 cS?.

Hmm. The plot thickens. Well anyway, I’m thankful you did the hard work to find out what works in the end!
RC carbs have adjustable "jets". Those are the needles. The fuel orifice is not what's in the air flow. While fuel viscosity might change the needle setting slightly, that's partly why the needle is there.

RC carbs have adjustable "jets".Greg
Exactly Greg. I guess what I was clumsily trying to say, or maybe more wondering out loud was: maybe the reason a .21 sized RC type methanol carb 'works' on a .71 gasoline engine has less to do with the (air) throat opening but maybe a proportionately larger (fuel) aperture equivalent of a methanol carb? The needle valve position & spray bar slit collectively control liquid fuel flow rate at a given suction & fuel pressure. But what that value is relative to a gas carb, I don't know.

Maybe a good clue would be examining carbs of a methanol > gas conversion single cylinder RC engine, that way more variables are fixed. Do you have any experience on that front in terms of barrel & fuel size of things?

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