Carb talk for Beginners

[Peter Twissell]

I have had to change idle jets to work with modern fuels with alcohol.
My wife has a 2000 Triumph 790 Bonneville. It is set up from the manufacturer as a 'lean burn' engine and needs the choke on for some time even in warm weather. Simply adjusting the idle mixture screw could achieve a good idle, but the engine would fluff if the throttle was opened quickly.
Raising the needle positions would only change the delay before the engine fluffed.
It was only by fitting larger idle jets that I managed to get it to behave.

Back on topic, the target with carburettors is to get the correct gas velocity range.
An engine which is intended to run only for display under no load will be easier to tune with a smaller carburettor, which will operate well working the required gas velocity range at low rpm.
If the engine is intended to produce significant power, a larger carburettor will be required, but it will be more difficult to tune for peak power, smooth idle and clean throttle response.

 

[Ozwes007]

Just a little info I have found over the last 50 years as an engine tinkler ( 16 State and National titles in catergories as. Diverse as GoKarts, dirt bikes, rally, speedway and road racers), the one thing most people get wrong on carburettors is the size of the wells in which the main jet, needle, emulsion tubes etc sit. This size determines how well a particular part of the circuit will work. The fuel starts at idle from the idle circuit , then progress to the joint circuits of idle and main thru to just main. Tuning must be done without accelerator pump interference using very slow rev transmission thru the entire range. Then we add the extra fuel to accelerate quick. Carb sizing is wholeheartedly dependant on what the engine is doing as is the Venturi sizing, if it’s reving its head of all day long at max power the carb size and Venturi size is huge, want the rev range to go from bottom to top and be good and efficient in all that range then you need a smaller carb with smaller venturis. Emulsion tubes blend your fuel and emulsify i so it can be lifted to engine. Wet fuel doesn’t burn! The gas it turns into as the emulsion hits the chamber does. The better the emulsion, the better the power and economy.

 

[Nerd1000]

Just a little info I have found over the last 50 years as an engine tinkler ( 16 State and National titles in catergories as. Diverse as GoKarts, dirt bikes, rally, speedway and road racers), the one thing most people get wrong on carburettors is the size of the wells in which the main jet, needle, emulsion tubes etc sit. This size determines how well a particular part of the circuit will work. The fuel starts at idle from the idle circuit , then progress to the joint circuits of idle and main thru to just main. Tuning must be done without accelerator pump interference using very slow rev transmission thru the entire range. Then we add the extra fuel to accelerate quick. Carb sizing is wholeheartedly dependant on what the engine is doing as is the Venturi sizing, if it’s reving its head of all day long at max power the carb size and Venturi size is huge, want the rev range to go from bottom to top and be good and efficient in all that range then you need a smaller carb with smaller venturis. Emulsion tubes blend your fuel and emulsify i so it can be lifted to engine. Wet fuel doesn’t burn! The gas it turns into as the emulsion hits the chamber does. The better the emulsion, the better the power and economy.

The emulsion tubes have another important function, which is to compensate for the tendency for the mixture to get richer as speed of airflow through the carb climbs. Usually they are fed by a jet or needle such that the level of fuel in the tube drops with increasing flow through the venturi, exposing additional air holes and 'leaning' the mixture.

The biggest problem I've noticed with model carbs is that they usually have no float bowl and so the mixture strength changes as the engine runs. The simple one I made for my Webster engine has other problems too, I really should make a better one.

 

[Jasonb]

In 45 years of making MODEL engines I have not seen a carb design that has more than one fuel needle, at best you may get an air bleed on an RC type engine's carb

Many an model engine has a float chamber, but depends on what type of model you are making, have a look at the 4 stroke tether boat engines of the past or current if made now as the carb design will be the same, Designs by the greats such as Wall, Westbury, Stuart, etc. But they are unlikely to be found or really needed on simple beginner's engines like a Webster as they are a bit complex for the beginner to make from a casting or in the case of the bar stock webster or these below from scratch.

Size of engine is another thing to consider, many of the small aero engines particularly the diesels had a fuel tank no bigger than the float chambers in those pics below so little point in making a carb with a float chamber but as they were flat out from start to finish carb only needs setting for one speed. Come to the RC aero engines and again why have a float on something that will not stay vertical, at the least the fuel level in the chamber won't stay constant at worse it could flow back to the tank. Best option there is to use fuel tank pressurisation via the silencer to effectively pump the fuel to the carb.

Couple of examples of engines I have made with float chambers.

 

[GreenTwin]

I have used the fuel pressurization on an RC 2-stroke aero engine, and it really helped the engine run consistently at all speeds, and especially kept the engine running consistently at a lower idle speed than it would run without a pressurized fuel tank.

For my foundry burner, I also use a pressurized fuel tank, and it too gives extremely consistent and repeatable results when the burner is operating.

I recall reading a factory brochure for the Baker Ball Hopper Monitor, and it mentions the position of the fuel tank relative to the needle on the carburetor, and why that was important (I forget the exact wording; I need to look that up).

So does the bowl with float and needle valve allow the fuel tank placement to be non-critical ?
What exactly does the float and blowl do for engine operation, other than in general making it more even and consistent ?

Does a float/bowl setup require a fuel pump ?

.

 

[Steamchick]

Thanks gb...
I simply sat down and worked out what I needed to do from the evidence - Black plugs mis-firing within 20 miles of being clean, after riding JUST around town at very low throttle settings.
I could tune the idle before with the air screws, but that was so prone to engine temperature, humidity, and the position of moon and stars, I could set it and next time out it would not idle, or run smoothly then stall, or be OK when part warm OR Hot, Or cold. But with trying a few smaller idle jets it it much better for real use.
But the colour-tune plug showed me that from a clean idle, smooth and easy to live with, the mixture was really sooty just after the slide was lifted a tiny amount, just enough to pull away gently, and the Dell Orto manual explained this was managed by the slide cut-away for about 1/8th of slide movement. And my bike was OK above that point - as the needle was set previously. Main jets likewise OK.
I put it down to the early bike magazine explanations that the carbs were tuned to avoid the flat spot when opening the throttle fast to change form "normal speed" to "rapid acceleration" as per requirement of the Rome police when they asked M.Guzzi to develop a bike to suit their needs.
So when I re-machined the slides to give a cut-away that made the colour tune show a bit leaner at the initial carb lift, (I first did one carb until I got a reasonable result, keeping the other standard as a control - then did the second slide to the same condition) I found it was a much better "slow traffic" ride, and easier (less clutch slipping) to vary speed when below around 2000rpm. With an added bonus that I have NOT had to clean the plugs every 20 miles or so of city traffic.
I can get a slight hesitation now if I am too quick with the throttle, from idle to mid-position or higher, but at 70 I am beginning to learn how to ride with the traffic, not well over their speeds...
Maybe I am "not doing what others have done", but a year on, and I went out on the bike, it started, ran OK, went through traffic (slowly) without hesitation, continued to idle as any bike should (A really new experience after owning it for 26 years!). And I postponed the notion of changing to new carbs, or selling... I just ride it and enjoy the new stress-free city traffic experience... (Honestly, I didn't notice the problem until I retired 6 years ago and started doing a lot of across-city riding, when I experienced 20mph limits, congestion, etc. and no room to fiddle through or past the traffic!).
And it is still fun on the open road. (But lacks the "Poke" from 99 octane leaded fuel, that I used to get from the last garage in North-East to sell it... at £1.75 per litre!).
Fuel HAS changed, traffic and necessary riding style has changed, and now the carbs have been changed to accommodate these changes.
K2

 

[Jasonb]

Pat, a model engine with a float chamber would typically have the fuel tank slightly above the carb so there is a constant feed into the float chamber. This flow is controlled by a needle fixed to the float so that as fuel is used the chamber level drops and so does the float which allows more fuel into the chamber which raises the float and shuts off the supply from the tank again.

Another reason you may not see float chambers on models is that all the hit and miss engines that run with Lunks have the atmospheric inlet valve also acting on the fuel inlet so they shut off the flow much like a float does.

Other engines like the vertical IHC and Root & Van De Voort pump their fuel upto the carb constantly and the excess overflows back into the fuel tank so no need for float chambers there.

 

[Peter Twissell]

Another reason why model engines don't usually have float chambers is the relatively small changes in fuel height in the tank.
In a typical model aircraft, the fuel tank is mounted within a few inches of the carburettor. With the model facing directly upward, the pressure difference required to pull fuel up to the carburettor is very small.
One bar of pressure can push fuel up around 40 feet, so if the tank is 4 inches below the carburettor, it requires only 80 millibars to lift.
Holding the model in that attitude, facing straight up, and then opening the throttle is a common test before flying.
I should add that this is applicable to glow engines running on methanol, which can tolerate a fairly broad range of fuel air mixtures.

 

[gbritnell]

Steamchick working on early Triumph engines with Amal monoblock carbs you could buy slides with different cutaway to modify the off-idle performance. When the Japanese street bikes came out with the CV carbs (constant velocity) that solved the problem. CV carbs have a throttle disc for air control along with a slide but the slide is controlled by the air passing through the carb.
As far as model engine carbs go Terry Mayhugh machines a chamber similar to a float chamber. It has a pressure feed line and an overflow return. The level is kept fairly constant. When running my engines I have the top of my fuel level at or just below the needle valve level on the carb. Once the fuel is pulled to the carb the engine will run the entire tank of fuel with no problem. A carb with a float can be made but the size of the float and the chamber becomes oversized to the carb.

 

[GreenTwin]

Back in the early 70's, I had a homemade minibike, and it had a 3 hp Briggs and Stratton engine, with a fairly large fuel tank.

I remember working on that engine, and noticing two pickup tubes on the bottom of the carburetor.
I think one tube was suction from the bottom of the tank, up to a small reservoir built into the top of the tank, and then a short suction tube into the small reservoir.

I think they called that system "pulse-something", where the vacuum pulses pumped fuel from the tank up into the small reservoir, and I guess there was a waterfall overflow which maintained the level in the reservoir.

Those Brigs carbs were simple, and worked well.
Not sure what a modern Briggs carb looks like.

So the pulse system was basically a fuel pump ?

.

(not my photos)

 

[GreenTwin]

Carb on ebay for Famous 4 hp.

.

 

[GreenTwin]

A few old carb prints I found online.
.

 

[Steamchick]

Hi GB. I am a pure amateur on carbs as I have only had bikes for about 56 years, and only changed carbs from "what was fitted" on a half dozen or so. Then spent ages tuning them to work better than what I took off. So as DelOrto only had slides to make the mixture richer, when it was already too rich for constant crawling through city traffic, I decided to use my reasonable machining skills to make my own idle jets and slide cutaways to sizes not available from Del Orto... If what you have is scrap, then it loses nothing if the mods don't improve it. - Then I could buy exactly the same as what wasn't working to start again at the 1970s factory design.. Fortunately, by reading all about carbs (AMAL and Del Orto work exactly the same way - according to their scripts.) I have managed to get a successful result for zero cost. (except the redundant Chinese carbs at £25 each).
My point was that carbs are pretty simple, and a bit of thinking and careful, logical, tinkering can (not necessarily "will") make an improvement. Carbs are highly technical - for cars etc. - but not so for a model engine to idle on the bench.
But different fuels can have weird effects. (like removing lead, which doesn't just reduce Octane number but changes the whole combustion - and thermal energy exchange to the piston). - I have felt it in performance (mph), and seen it in my wallet (mpg). - As a "stupid" engineer, I study, measure, change, check and re-check, etc. until I am happier. - and frankly am amazed at the better riding after the mods I made! It rides reasonably well in conditions it wasn't designed to be used for. And I don't (can't?) ride it like the Rome police of the 1970s... for whom it was tuned. But maybe I should have bought a new Honda...? (NOT!)
Thanks for reading and your feedback. Always glad to have advice from experts.
K2

 

[GreenTwin]

One thing that the weedeater/chainsaw carburetors seem to have with is the omnidirectional run characteristics.
Perhaps that is why they are complex.

And with a clunk in the tank on the end of the fuel line, the fuel and clunk follows the gravity pull.

.

 

[GreenTwin]

A few notes from the Baker 1912 catalog.

Interesting to note that the engine could be configured to operate on either alchohol (larger jet I guess), or kerosene.

My dad had a John Deere Model "H", and he said it was designed to operate on less expensive kerosene in the large tank, and start on gasoline from the small tank that was located at the back of the large tank.
I did not really believe my dad, so I put diesel in the small tank (we used gasoline in the large tank), got the "H" hot with maximum rpm (which was not really that fast), and switched over to diesel.
The "H" did indeed run very well on diesel with the throttle full open, but would not idle on diesel.

So I guess no jet changes required to run on diesel/kerosene, or gasoline.
Kerosene is a bit lighter than diesel and flashes easier, which I found out from experiments with foundry burners.
Diesel is a safer foundry burner fuel.

And note the check valve in the description below, which prevents the fuel from draining back into the tank and leaving the fuel line dry.

I am told that there is a venturi shape in the inlet below, but if there is, it is rather subtle, if visible at all.

.

 

[GRAUBELE]

Here just as an example, the carburetor from the 103 years old Amanco Hired Man (hit & miss engine).
Very simple, a check valve in the fuel line at the inlet.
Just a nozzle needle and the air intake housing.

Dieter

 

[gbritnell]

When I built my 302 V-8 engine I played around with several different designs and sizes of carburetors. What I settled on was, at the time, the smallest O.S. carb available. It has a bore of about .200 (5.10mm). I then measured the critical sizes, bore, air bleed port, barrel diameter etc. and started drawing a reasonable scaled version of a factory carb for this engine. This would be the Autolite/Motorcraft 2100 2bb. carb. My carb has all the looks of the full sized carb but uses the functionality of the O.S. carb. The fuel control needle is hidden under the choke spring body.
The carb works throughout the rpm range of my engine and quite cleanly I might add. The rpm ranges from idle which is about 1500 rpm to 7800 rpm. The idle is high because the engine is limited by the scale size of the flywheel. The engine bore is 1.00 (25.0mm) and the stroke is .90 (23mm) My fuel supply is a remote tank that has the top of the tank level with the fuel control needle valve. Once started the engine will run all the fuel from the tank which is 1.00 in depth. The picture show the most important aspect of the carb, the venturi bore. I didn't use calculations to arrive at this size it was by trial and error to get the best performing carb.

 

[Jasonb]

I am told that there is a venturi shape in the inlet below, but if there is, it is rather subtle, if visible at all.

You don't actually need the classic taper in- taper out shape. The end of the gas inlet protruding into the air inlet will reduce the cross sectional area and cause the air to speed up thus creating a vacuum which will draw in the fuel.

 

[Klrskies]

In the early 80's the world was evolving into fuel injection. My 1983 VW GTI had a mechanical air flow meter before the throttle body that worked fine with the stock cam but when a cam with more overlap was installed the more erratic pulses from the cam change caused the flow meters plate to flutter uncontrollably and lost control of the air flow metering.
The popular fix was to strip off the fuel injection system and install some type of carb instead. I went with 2, side draft Dellortos, similar to weber DCOEs. The venturies and every circuit was tunable. The fussiest component was the idle circuit. it was against the rules to drill out jets, but I would have had to order them and waited a week to get them so drilling was the order of the day. I drilled too big occasionally so I began making the idle jets out brass. That jet size was so fussy. I found that tapered jet reamers were available and worked great for finding the best size jet orifice.
As Programmable Fuel injection came into vogue for modified engines I built a Mega Squirt kit to step into the new age, but I never installed it. It was a whole new world of data logging and re-mapping fuel and timing tables. I wish I could have gotten excited about it as I know there is drivability and increased power from Programmable EFI, but there is a personal satisfaction from fighting fueling issues mechanically. I could make or modify many of the carbs and linkage systems and it bothered me that I couldn't keep that simplicity with Computerized engine management.

 

[CaneSelvatico]

The float in carburetors is only to maintain a constant "head" of fuel (or pressure) so the only variable is the strength of vacuum from the venturi.
In those older briggs @GreenTwin was talking about, the pulse from the engine is used to pump fuel from the main tank into the small one where the level is maintained by an overflow instead of float. Older engines used a fuel pump with an overflow in the same way. The overflow system is the simplest but it need the engine to be on level ground and not move much, whether a carb with float have more control over the level in the bowl over a wider range of movements.
Different is for the modern small engine carbs where the "head" of fuel created by a diaphragm pump, pulsated by the fluctuation in crankcase pressure, and regulated by a diaphragm valve (like a propane regulator). This way the engine can operate in any position, as long as it can suck fuel from the tank.