Author Topic: Carburetor with accelerator pump  (Read 1236 times)

Offline Brian Rupnow

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Carburetor with accelerator pump
« on: January 03, 2021, 07:14:57 PM »
I have many different types of small i.c. engines, which I have built over the last twelve years. All of these engines which are throttled, have a common failing. When you rev them up higher, the fuel mixture leans out, and they lose a lot of their power as a result, until they have stabilized at a higher rpm. I have been a hot-rodder most of my life, and all the automobile carburetors I have worked on have had an accelerator pump. This is a small piston type pump that squirts a jet of raw fuel into the carburetor throat as the throttle is opened. This squirt of raw fuel offsets the "leaning out" so the engine doesn't gasp and snort and lose power as it accelerates. I have never seen such a carburetor for small (1" diameter or less) single cylinder engines. Does anyone have wisdom to share on this?---Brian

Offline gbritnell

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Re: Carburetor with accelerator pump
« Reply #1 on: January 03, 2021, 08:07:50 PM »
Brian,
It's not the result of not having an accelerator pump that causes the fuel mixture to lean out but rather the design of the carb. I have built one carb with an accelerator pump and it was quite an elaborate affair. To use the pump the carb must have a fuel reservoir, float bowl, to supply fuel to the circuit. This involves making a carb with a float bowl, float, needle valve etc. Just like a full sized carb.
All of my throttling engines use an air bleed type carb which has a high speed needle valve and an adjustable bleed port to lean the fuel mixture at slow speeds. When both are adjusted properly the engine will idle and accelerate quite nicely. It does take some tinkering with the bleed port size but once it's right the engine should operate over it's entire rpm range quite cleanly.
If someone wanted to go to the trouble of making a carb with an accelerator pump circuit I would think that would take a lot of tinkering in itself to get just the right amount of fuel squirt when the throttle was opened.  I'm sure this would involve a lot of trial and error to get it to work. To the best of my knowledge I have only seen one in operation, other than the one I built, and that was on an engine that a fellow named Lee Root built many years ago. The one I built was from the drawings published in Strictly I.C. from Lee Root's design.
My 302 has an air bleed carb and will idle and accelerate very cleanly and that's with 30 degree fixed timing.
gbritnell
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Offline Brian Rupnow

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Re: Carburetor with accelerator pump
« Reply #2 on: January 04, 2021, 11:01:42 PM »
 I don't think I'm going to do anything with this, but I was curious. I am a bit isolated here, and I thought that perhaps this question had a simple answer and I hadn't heard of it. With most of my engines, it really doesn't matter a heck of a lot. The thing that prompted the thought was the engine I built with a 1 3/8" bore to run my edger. The engine ran well, and it seemed to have lots of power until I went to accelerate it, then it seemed to wimp out. I never did get the edger to run with one of my small engines powering it. I finally shrugged it off and decide that the edger required more power than a small engine could give it.---Brian

Offline Roger B

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Re: Carburetor with accelerator pump
« Reply #3 on: January 05, 2021, 08:02:20 AM »
This is a problem I have been looking at for a while. There are two separate issues here:

1) Increasing the load on the engine.

2) Opening the throttle.

If you increase the load on an engine it will naturally slow down. This will reduce the airflow speed through the carb and hence the suction at the jet, reducing the fuel flow. Most model engines drive propellers in boats or aircraft where the load drops with the speed so this problem is not so evident.

If you open the throttle the vacuum in the inlet drops, this also reduces the suction on the jet and reduces the ability of the air to hold the fuel in suspension so some of the fuel condenses out onto the inlet pipe wall causing leaning of the mixture. This is the main reason for accelerator pumps (or other enrichment devices) on full size engines.

I have been playing around with a constant depression type carb in this thread:

http://www.modelenginemaker.com/index.php/topic,9601.0.html

However as George says you then need a constant fuel level which needs pumps and float chambers or similar.

If you want to build an accelerator pump you will be halfway to making a fuel injection system anyway. My 1”bore 2” stroke fuel injected engine has a 2 mm diameter pump plunger and a working stroke of around 0.5 mm to deliver the full quantity of fuel per stroke.
Best regards

Roger

 

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