Carburetor information

[cfellows]

George

I am wanting to build the Webster. Don't care about speed control very much. It won't run very much, just for my own entertainment. Would like it to tune up and start easy. So which carb would be a good choice for a first build (and not lots of experience)?

This is kind of what I want to do. Unfortunately, if you don't have some kind of speed control, the only option seems to be to run the engine wide open which isn't a good alternative either. That's why I'm trying to come up with a simple design that allows some kind of speed control, even if I have to tweak the air mixture in between speed adjustments.

Chuck

 

[putputman]

George, thanks a ton for starting this post. Just a lot of good information on it. I certainly hope this post keeps going as there are a lot more information out there to share.

I can certainly understand why your engines perform so well.

 

[gbritnell]

Gentlemen, because I have heard so many people ask questions about carburetion I thought I would start this thread. I have presented information and pictures relative to my experiences with carbs. My presentation is not meant to be the end-all of information. I know what it would take to build the ultimate carb for an engine, the problem is without measuring equipment for air flows, vacuum strengths etc. it's just a hit and miss proposition.
On another note, I have found that if someone writes something that's misunderstood the best thing is to question the reason for the post. Please don't get into a shoving match to prove a point.
BlackVette, the Webster is not a hit and miss type engine so therefore it needs something to control it's rpm. The only thing that will do this is the control of air/fuel into the engine. That being said, I would build a simple air bleed type carb. They are relatively easy to make and can be adjusted to operate fairly well. On Longboy's Webster he uses a factory model airplane carb. In my writings I stated that these carbs are made to flow more fuel than is required by engines running on gasoline. Yes they work but are too sensitive to adjustments. I'm attaching a drawing of the carb I use on my 4 cylinder OHV engine. The bore and stroke of the engine is .75x.875. I'm sure this carb will work on anything from .625-1.00 bore without changing the venturi size.
George

View attachment 4 CYL OHV SHT J.pdf

 

[kustomkb]

Good info!

Thanks for the carb drawing and detailed explanations George.

 

[cfellows]

Thanks for the drawing, George. I had to study it a bit to understand it. And I do have a couple of questions.

It looks like the idle stop screw is what holds the throttle barrel in the body?

Also, it looks like the air bleed hole is blocked by the throttle body at all positions except at or close to idle?

Finally, do you think this design is scalable down to maybe 3/4 or even half that size?

Thx...
Chuck

 

[gbritnell]

Hi Chuck,
You're right on all counts. The air bleed hole is open from the closed position to about 1/8-1/4 throttle. It's only needed for the idle mixture. Like I said, the design of these carbs is for idle to wide open. They don't operate at a constant speed in the mid range. By 3/4 do you mean bore size or 3/4 of the drawing size?
George

 

[stevehuckss396]

Mister Hucks seem to have a problem

No problem Tony. That was not a personal attack of any kind. You said that georges carbs are old tech. Just wondering what the modern stuff is. All we are doing here is trying to figure out the best route to take for carburation. I was trying to get your input.

My most humble apology!

 

[stevehuckss396]

The air bleed hole is open from the closed position to about 1/8-1/4 throttle.

The shores carb is built and tested with a very small air bleed. Then it is to be opened a drill size at a time and tested again until the engine stops leaning out at full throttle.

When I did that it responded to a point and then the throttle started to go soft. It stopped responding to increases in bleed hole size.

The air bleed hole is at the very bottom of the throttle. The hole starts to get covered as soon as the throttle is twisted.

How would raising the airbleed hole affect the tuning if it were uncovered longer. I am wondering if the hole is to low and is being covered to quickly. If i raise the hole, a smaller hole would bleed more air for a longer period. It would lean out the idle mixture more.

Is my logic flawed?

 

[gbritnell]

Hi Steve, no your logic is not flawed. It's one of the comments that I made when I was writing this all out. Not having the ability to measure the exhaust output makes it hard to engineer the carb. The old motorcycle way was to run the engine, shut it off and then read the spark plugs. It doesn't work so well in these sizes. I do know that Rimfire recommends running a richer mixture for the plugs. By that I would assume richer is better than leaner but not rich enough to foul a plug or cause lazy running.
The one thing I have found is that the venturi hole can be much smaller than one would think. The next carb I make is going to start out with about a .200 bore and work up from there. When my Holt is in the idle position you can barely see a crack in the thottle bore so this tells me it's providing more than enough air even at these small openings.
George

 

[cfellows]

George,

My engine has a bore of .8125" and a stroke of about 1.875 for a total displacement of a tad less than 1 cu. in. While the carb dimensions in your scale will likely work fine, I'm concerned that it will look out of scale on my engine.

I would like to shrink the whole carburetor down by about 40%. I'm working on a drawing with a venturi bore of 5/32". Aside from the eyesite and eye/hand coordination of a 66 year old, do you see any problems with that idea?

Thx...
Chuck

 

[gbritnell]

Hi Chuck, why not make the carb outer dimensions to fit the scale of your engine but just do the internal passages so suit the engine's need. I personally don't like the look of a simple air bleed carb on a small engine so usually when I get the engine to run right I whittle away some of the stock to make it look more attractive.
This is what my Holt carb looks like after chopping some of the extra stock off the outside. I even added a dummy float bowl to make it look more the part. These pictures show the governor linkage hooked up also. I would have liked to use the big brass one that I first built but it wouldn't work at all so it was kind of irrelevant.
George

 

[cfellows]

Thanks, George. That carb on the Holt really looks nice.

Based on what I'm reading, it sounds like the venturi bore, for all practical purposes isn't very critical. I know my Plumbing engine will run on my little carb with a 1/8" bore venturi. Yet, based on one of your earlier posts, it would just as likely run as good or better on a carb with a 1/4" bore. Is that correct? For a given size engine, is there any benefit of one size over the other?

Thx...
Chuck

 

[gbritnell]

Hi Chuck,
On a slow running engine the venturi can be made smaller to increase the velocity. The larger the venturi is the less air velocity passing through it. I have read numbers as far as a ratio of engine bore to carb venturi size but have never built one to exactly those numbers. It's a hard thing to assess the proper size for the carb bore. One way you can tell is when the carb is wide open and you can't get the proper adjustment on the needle to make it run clean then the bore should be made smaller. The only problem with this is that who wants to run their engine wide open for 4-5 seconds while they try to figure out the carb settings. It works fine for a model airplane engine because they basically run wide open anyway.
George

 

[stevehuckss396]

I over drilled my air bleed hole on the Peewee. I was going to make a new carb but with NAMES comming I was hoping not to have to spend 3 days getting it done.

I over drilled the hole and tapped it 4-40 and made short screw to go into the hole. Then I chucked up the little screw and drilled the small hole thru the screw and screwed the little bugger in. I'm not back to where i was originally but it runs much better and I wont look like a dumb a$$ at names. Well not because of the peewee. Might make another and go smaller on the hole and see what happens. I think I would rather run real rich and idle and rev nice than run like crap.

 

[Rustkolector]

George,
This is a great thread. I have enjoyed it and have read it a couple times. Some of us like to build slow speed engines that use a throttle type carb (like your Holt), but are best run at constant speed. I have built and purchased a few carbs in my attempts to find the ideal carb and fuel system. What I have found is that a small carb like the OS 2A does well at constant low speed because it has an adjustable air bleed port. Unfortunately, with a .188" throat dia., it is still larger than ideal for a multi-cylinder 1" bore 4 stroke engine. I don't believe they make any RC carbs under .188" with the adjustable air bleed. I built a .100" throat carb for a 1" bore multi-cylinder engine, and it runs well at only 25% open at reasonably low speed. Unfortunately I built it without the benefit of an air bleed for low speed operation, so it is less than an ideal performer on low speed operation. Now I am guessing that a .110" to .125" throat carb with both high and low speed fuel jets would be ideal if it also included an adjustable air bleed port under the throttle for fine tuning. Or better yet, one where the bleed air mixes with the low speed fuel similar to that of larger carbs. Does the Jerry Howell carb come close to this?

With the larger jetting of the few RC carbs I have tried I have noted one practical advantage they offer for 4 stroke model engines. The jets and fuel passages seem to be large enough to allow the easy use of propane as an alternate fuel. It just requires another open turn or two and a demand regulator. It is a very clean burning fuel, but has its own issues as do all fuels. The adjustable air bleed provides the same fine tuning benefits with propane as with gasoline or camp fuel.


Jeff

 

[gbritnell]

Hi Jeff,
I certainly don't profess to be an expert on the subject of carburetion but I've learned so much in the last few months working on my 302 engine. As has been spelled out in this thread I am currently using an OS 2a also. I have it to the point where the engine runs almost flawlessly, from idle to full acceleration. I have the air bleed wide open and I believe I could open up the diameter of the port a little but I really don't want to alter this carb. I can use it as a test carb for other engines. I ran the engine for almost four hours over the 2 days at NAMES. It was getting quite warm and as it did I would have to richen up the needle one or two clicks to keep it fine tuned but it never faltered.

This engine has a bore of 1.00 and a stroke of .900. I can idle it down to 700 rpm and rev it just shy of 5000 rpm. There were fellows stopping by that had Walbros and other types of carbs on their engines and were amazed at the performance of this simple little air bleed carb. As I said and as some others have stated the problems I was encountering were not fuel related but rather ignition problems.

When I first built this engine I made a scale copy of an Autolite 2100 carb for it. This carb was pretty much the standard issue for Ford v-8 engines that used a 2 barrel carb. The one I made had a butterfly for the throttle plate with idle passages, a float etc. The biggest problem was getting the float and needle to operate in that small scale. I didn't have much luck.

Now that I have the engine running so well I wanted to remake the 2100 carb purely to add to the scale appearance of the engine so I took all of the dimensions from the O.S. carb and sat down at my drawing board and designed a new carb.
I had to figure out how to get the rotary throttle barrel into it but have the outside look somewhat realistic. I also had to figure out how to get the fuel from the inlet to the needle valve using the scale dimensions from the full sized carb. It took some extra drilling to get the passages into it and I then plugged them with small set screws in case I wanted to alter them in the future. I also wanted to add more detail to the carb to give it that scaled down appearance so I added the accelerator linkage and the automatic choke mechanism. The choke unit sits over top of the needle valve and kind of hides it. I must step back here and say that before I added all of the extra detail I tried the carb out on the engine. It ran almost as well as the OS carb. By that I mean it would still accelerate clean and would idle 'almost' as well as the OS. I had the airbleed wide open and it could use more air at idle just to clean it up a little. I think that the length of the bleed passage added some restriction to the air flow and that is why it was just a tick off of the OS.

You can see in the pictures the size of the main venturi bore and the barrel is about .04 smaller in diameter. This gives the barrel hole a diameter of about .160. My needle jet is drilled .036 diameter and I am using a sewing needle soldered into my valve body for mixture control. The diameter of the fuel passage from the fuel inlet to the needle valve is .063.

What does all of this mean? I have discovered that most of our carbs are way to big in the venturi department. For an engine having a 1.00 bore I would start out with a carb having a venturi no larger than .200 diameter. You can always go bigger.

The first 2 pictures are overall shots of the new carb with all of the window dressing on it. That's a dime sitting next to it. The next 2 pics show the bottom of the carb with a scale next to it to show the venturi size. The last 2 pictures show a closeup of the needle valve area and how I used a plug to fill the bore for the barrel. I keep it in place with a 2-56 countersunk screw.
You'll have to pardon the dust on the top of the carb as this was the top from the original carb and it's been sitting for quite some time.

 

[gbritnell]

Here's the last 2 pictures.
George

 

[gld]

One issue that has not been covered---atmospheric conditions. Namely humidity. My Snow engine ran perfect at Rapid City last fall and NAMES this spring. Last week I tried to run it and it will not keep runing..Will only run about 4 or 5 seconds and die as if the fuel was shut off. Always starts right back with the same results. My carb is off a Thunder Tiger .15... Only difference is we have very high humidity here now.

 

[gbritnell]

Hi Garry,
You're right about atmospheric conditions but I think they're more applicable to the ignition or should I say combustion chamber rather than the fuel alone. If you are using gasoline it doesn't have that much of an attraction to water but if you are using some type of alcohol blend it can draw moisture with it.
What I have found when the humidity is high is that more condensation forms in the combustion chamber and therefore tends to short the spark out. I have several hit and miss engines that this condition occurs. Usually if I take the spark plug out, wipe or blow it off and put it back in, the engine will start instantly but within a few seconds will stop again. Once there is some heat built up in the plug, head and cylinder the situation seems to go away.
gbritnell

 

[Lakc]

Atmospheric pressure and temperature are the primary inputs in modern electronic fuel injection. Since all we are doing is completing a chemical reaction by burning the fuel to make the power, it is essential to know how much oxygen you have to work with.

Carburetors, even full size ones, have a tough job to do. With gasoline, it has to keep a linear flow of approximately 14.7 pounds of air to every pound of fuel, under full power increase that to approx 9 pounds of air for every pound of fuel, and starting alone can be as rich as 3:1.

Forgive me if I steal some math from the web here, but apparently, a pound of air is approx 2894 gallons at standard temp and pressure, and a pound of gasoline is roughly 1/6 gallon. In the best case, that's a whole lot of air for a tiny amount of fuel. When you talk starting or max power output mixtures, you have much less air to work with. If you want to run on alcohol fuel, you basically just double the gasoline numbers on the fuel side.

I will leave the heavy math to those on this forum who can do it so much more eloquently (and correctly) then I. Anyway you look at it, our small engines move only tiny amounts of air. That does not leave us with much leeway at all to move and vaporize minuscule amounts of fuel.

That said, as much as I love carburetors, and do have a few ideas of my own to try out, I think they are the wrong solution to our problem at this scale. Positive fuel displacement, like mechanical fuel injection, or just plain pressurized gaseous fuels (propane or butane) make a lot more sense in some areas.

While any number of carburetor designs can fulfill the needs to make our beautiful music on the bench at a show, my interests were always geared toward what kind of work we can perform with our engines. That will require a robust fuel system.