I was fiddling with tuning today on a little fourstroke and began to wonder why they dont' need accelerator pumps just like large carburators. I then realized they do. This is why we have to idle them way too rich for smooth running to prevent bogging when you punch the throttle. Does anyone contrive a system to squirt in a little extra fuel when the throttle opens? This would allow much leaner idle settings and slower idle while still getting good throttle response. Without an "accelerator pump", the rich idle mixture is certainly a compromise at best.

I've wondered teh same thing myself. I can get my gas engines to purr at idle, but this meand they die when I open the throttle. I know Walbro makes accellerator pump equipped carbs, for weed wackers etc., I wonder if anyone has tried them on model engines?

The idea is good, probably a small piston pump attached to throttle linkage can spray a small quantity into the carb barrel. We might see a commercially produced gadget coming in the market.

Scott,

There are R/C carburettors that do employ an accelerator pump.
One such carburettor is the Webra Dynamix.

This carburettor is definitely not the freshest leaf on the tree, dating back from the late 70s.
The accelerator pump is incorporated into the idle fuel-metering mechanism.

When the "plunger" moves to a more open position, it squirts out an extra ration of fuel.


Other carburettors I know don't employ an accelerator pump.
It seems enough that the throttle opening preceeds the engine, during acceleration - the engine is lagging behind the throttle.

Since the part throttle metering is actualted by the carburettor's speed control mechanism, whether it is a barrel that moves from-side-to-side as it is opened, or a slide valve, the engine will get more fuel than it needs for sustained operation, at any stage, during acceleration.


You are incorrect about the reason the mixture is set too rich at idle.
When idling, the lower speed airflow allows some of the fuel to 'fall out' and to accumulate in the crankcase.
So the idle mixture must be set richer to compensate for that.

To prove this point, most engines respond immediately when the throttle is opened rapidly after a few seconds of idle, but will load up if allowed to endure a sustained idle. This accumulated extra fuel is really something most engines can really do without.

Dar

Is Webra Dynamix still available? Have you any experience of the difference it made.

-ZM

Yes, it is, Zahid.

You can see it [link=http://www.webra-austria.at/motor.php?search=DYNAMIX&folder=CABURATOR&lang=EN]here[/link].

Since I actually never ran an engine equipped with such a carburettor, I don't exactly know the difference.

Several engines I own (MVVS) and saw, without an accelerator pump carburettor, still transition as if a switch was thrown - immediately!

Dar

Thanks for the quick response, I agree with the quick transition of many engines but was inquisitive on the accelerator pump since many automobile engines are lame ducks without it.

-ZM

The HP Gold Cup .61 also has an accelerator pump. I own one and it works pretty well.

http://www.mecoa.com/hp/61/2203.htm

But it is really not required, except perhaps on highly timed engines such as the Webra and HP. Both engines were designed by the same person and both have accelerator pumps. I have an old SuperTigre that is difficult to tune, it is also a fairly hot engine that likes to rev. Wonder if an accelerator pump would help?

The fuel falling out that Dar describes is partly the reason our engines don't need an accelerator pump. When you go to full throttle the fuel puddled on the bottom of the crankcase is picked up which temporarily richen's the mixture. But that doesn't explain the fact that almost all four strokes do not have, nor need, an accelerator pump. I think the wide mixture latitude of methanol and nitro is the main reason our carbs do not have an accelerator pump, nor do they have many other things an automobile carb has, no midrange circuit, no metering rods or jets which richen the mixture when the manifold pressure drops, etc. Hot rodders have long known you don't need these things when running alcohol. I recall the old alcohol fuel injectors were pretty crude compared to sports cars. Just an idle circuit that switched to continuous feed through the main jets when the throttle is opened, not much more sophisticated than our carbs

Same with "SU" and "Stromberg" carburettors (to example Volvo, Saab and english cars) has not accelerator pump and they has meterings needle.

A common "butterfly" carburator has not enough fuel between idle and full trottle when opening from idle to full trottle, therefore it has accelerator pump to give extra fuel there are not enough fuel.

I has Webra T-4 with Dynamix carburator, works well, but it is not big difference between common R/C carburator and Dynamix carburator.

Jens Eirik

Jens,


The carburettors that you talked about effectively enrich the mixture during acceleration, since the piston, to which the metering needle is connected, initially rises higher than its running position, so exposing an effectively larger fuel orifice to the airflow.

This has the same effect as an accelerator pump, without the presence of a pump.

We had a 1970 Volvo 144 that had the SU carburettor.
Fuel enrichment for cold starting, was achieved by lowering the fuel jet, rather than by using a conventional choke.
Most 1971 and later models, had the Zenith-Stromberg unit, with a diaphragm equipped piston.

The fuel control in these CD (constant depression) carburettors, is very similar to the fuel control of the idle needle in most two-needle R/C carburettors.

Yes! I was at Car mechanic school and learned about these carburettors, bot it is long time since i repaired these carburettors before electronic fuel injection came. These carburettors had problems with worned meterings needle and fuel jet caused more fuel consumption (the metering needle are in contact with fuel jet cause there are a small spring inside above needle) and damaged diafragm, oil leaking in damper.

R/C carburettors are similar as these carburettors, but less problems and maybe more reliable cause few parts and keep carburator in good trim.

Thinking these R/C car engines with slidepiston carburettors are more similar as Stromberg/su carburettors..[sm=devious.gif]

Jens Eirik

used to love the old SU and hitachi CV carbs, three in a row looked really cool. only prob was the jets didn't always pop back into place when releasing the "choke"...that and the constant dribbling of gas onto the exhaust header

back to rc.. most weedie engines are so choked down that they don't need the accelerator pump. on the saws with big carbs they do make a difference though. i've have tried blocking the pump circuit and the transition really suffers, especially when the engine is under load.


dave

Oh rubbish Dar...that's completely backwards. Both the SU and Stromberg CD (Constant Depression) carbs use a hydraulic damper to delay the rise in the piston when the throttle is opened. Although it restricts the rise of the needle and therefore the area of the fuel jet, the DEPRESSION over the fuel jet is greatly increased by the rise in airflow and THIS is what draws through the extra fuel needed for acceleration. Changing the viscosity of the oil in the damper alters the delay in the rise of the piston and is used to vary the amount of enrichment.

Ok, I got a good response on my question, and clearly, there is much more knowledge represented here on the subject than what I possess, but.... I still don't understand some of the answers. If we all have to run our rc engines too rich at idle in order to get good throttle response, isn't that the same as saying that they need a pump. If they didn't need the rich mixture for transition, then I could lean the idle down a bunch. What am I not seeing here?

Intake area are small in modelengine and make more vacuum therefore accelerator pump are not necessary. There are enough fuel and the idlescrew in 2 needle carb. works as water crane. Also when full trottle are idle screw full open, reducing intake area then idle screw are reducing to keep same fuel mass in same air mass. Not more or less fuel/air ratio to make effective combustionin modelengine. If you adjusting fuel more or less fuel in same air mass, it make bad combustion in engine. Right adjusted mixture make more power and good combustion.

Jens Eirik

Brian,

You may be right, as I do remember the small, bolt-in damper piston, on the top of this carburettor.
But, this was how the function of the SU carburettor was explained in the car repair manual that we had;
I cannot recall if it was an Autobook, or a Chilton, or other (it was 1979-1980).

It explained that the piston does rise higher and than settles to a lower position.

You can't have a traditional accelerator pump, without having a reservoir
(float bowl). You must have an additional supply of fuel, and a jet to pump
it through to have an accelerator pump.

Other systems are generally referred to as "enrichening devices".

FBD.

For several years in the 60's, Ford Falcons had no accelerator pump and ran just fine using a mechanical choke to start. The choke was not necessary in warm weather. I have one of the Gold Cup .61s and a 120, they appear to have the same carb, this calls for some digging into. My MAICO dirt bike had an auxilliary venturi off to the side of the main venturi, this auxilliary venturi had a rich jet in it, this venturi was opened with a handlebar mounted lever which you did without touching the throttle. I run my Saitos and Enyas as lean as possible on the low speed needle, they do not load up.

Dar...the manual you had was wrong with their explanation. Personally I think the SU/Stromberg's are marvellous carbs given the huge range of needles with different tapers to suit any situation. One needle and one jet controls everything from idle to full throttle. If I remember right I've had 4 cars all with twin SU's.

As for model engines, I think Sport Pilot probably came the closest with the idea of the wide range of fuel/air mixtures that still burn nicely although a slightly rich mixture at idle must help because airflow will increase much faster than fuel flow could react when the throttle is snapped open. This probably applies more to a 4 stroke though much as plowboy said in the first post. A 2 stroke builds up a small excess at idle which would collect as a film all over the interior of the crankcase. There's no way it could "puddle" with all those whirly bits inside there But snap the throttle open and the extra airflow will scoop up that film and richen the mixture just like an accelerator pump. So in a way, by adjusting the low speed needle we're setting how much "accelerator pump" action it gives.

Brian,

If this was completely true, when you shut the throttle to idle and as soon as the engine was down to idling speed, you would snap the throttle back to full, the engine would stumble and perhaps even die.

This, since fuel from the idling circuit (actually the entire part throttle control) would not have sufficient time to accumulate, as the 'film' you described. So no 'accelerator pump effect' would be present.

But most model engines would respond immediately in such a situation, even if the part throttle mixture setting is too rich.
Yet when allowed to endure at idle, unless the part throttle mixture setting is ideal (and even then), they will either bog down, or sputter, possibly dying, when the throttle is snapped open.

I think a better explanation is that in our model engines, the flow passages and the fuel tubes are so short, that engine speed changes are instantaneous, despite the lack of an accelerator pump.

Or maybe just enough fuel accumulates as the engine decelerates, after the throttle is closed to idle, to act as the 'accelerator'...


Yes, those CD carburettors were wonderful. Some later models of the Zenith-Stromberg had a PTFE (Teflon) coating on the needles, to counteract the wear that necessitated their frequent replacement.

I have an old 60cc converted homelite motor with a very old walbro carb with an accelerator pump on the side of it. High engine vacuum holds the diaphram back against a spring. When the throttle opens and the vacuum drops, the spring pushes fuel into the carb throat. Simple.!The early RC engines of the 60s early 70s had comparatively small carbs. Many of todays engines are over carburetted in the demand for more power, causing poor low end performance. Maybe it's about time for an an accelerator pump. On the old Homelite, coming in for a landing, it stays in that smooth 2 stroke mode at idle. Sounds strange.Yet when you hit the throttle it's right there. Guess we'll have to build one for an add on. Wouldn't be hard.

All Ford Falcon's had an accelerator pump. The one you are refering to is probably the one with the acc. pump & power valve combined. Auto engines are diffrent than small engines. At idle there is a very low pressure (vacuum) in the intake system causing the gasoline to vaporize at a lower temp. than at atmospheric. When the throttle is sudenley opened the pressure rises & gasoline will turn to liquid and stick to the manifold runners & ports causing a temp. lean condition. The accelerator pump compensates for this. Our model engines have very short & small intake runners and idle at a very low vacuum. There is also much less mass to accelerate in the little engines.

Ray

My 1960 with the 144 nor my Dad's 62 wagon with the four main bearing 170 had one.