Carburetor with accelerator pump

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Brian Rupnow

Design Engineer
Project of the Month Winner
Joined
May 23, 2008
Messages
14,917
Reaction score
8,219
Location
Barrie, Ontario, Canada
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
 
I would try to find some talented person who's built a lot of engines and put them up to inventing such a thing. Someone like that Brian Rupnow guy on Home Model Engine Machinist, for example.

:)

It's gonna take lots of little fiddly bits, especially because you'll need to tune it to the engine at hand. On the plus side, I think an accelerator pump piston can be a bit leaky -- a small enough leak would just make it not work when you're opening the throttle slowly, and that's exactly when you don't need an accelerator pump. I've also seen them that just have a piston running in a cylinder bored into the carb body, with no explicit valves -- the intake is effected by the piston rising above a slot so that gas can dribble into the pump; the output is effected by a simple passage from the bottom of the cylinder to an orifice above the venturi. (I think this was on a carburetor for a 1964 Buick Special that I rebuilt -- probably pointlessly -- when I was in high school. Maybe others.)
 
Quite a few of the small Walboro carbs have accelerator pumps. This is the basic way they function, with a some valving of course.
 

Attachments

  • Zenoah Racing Kit 009.jpg
    Zenoah Racing Kit 009.jpg
    118.2 KB · Views: 377
I saw a carb on a small engine at cabin fever that worked like a real holley carb. Butterfly's, choke plate, and a working accelerator pump. It was cool but to get it working it was very large and looked terrible on the engine it was on. If it were on a 1/3 scale like Georges 302 it would have been about perfect. I did not hear it run so I cannot comment on how well it worked. Maybe someone here knows who it was and knows how to get you in touch with the designer.
 
Could one build a miniature version of a S.U carb? As I understand it they do not require an accelerator pump.
 
I'm thinking the reason you haven't seen it is because it's not required in small displacement, small engines. (With a good carb)
Think "Briggs & Stratton".
I believe they have a "port" opposite the "butterfly" that enriches the mixture when throttle is applied.

John
 
I have a 15cc 2-stroke made by Webra, which has an accelerator pump. The carb. doesn't have a round barrel, which is the usual way of metering the fuel. Instead it has a slide, which slides in a groove across the venturi. Adjacent to the slide, and fastened to it, is a rod which slides into a hole. The hole is full of fuel, so when the throttle is opened, which is achieved by moving the slide into the carb. body, the rod goes down the hole and the fuel in the hole gets pumped into the venturi.
Jack
 
I would look at the carbs. used on model glow plug engines. These R/C engines throttle and have very good response. You could also look at 'Perry' pumps that provide constant fuel flow normally working off crankcase pressure
 
Yes, Perry Pumps are (used to be?) available. Some people like them, but I like to get my engines to operate without them.
Glow engines also use a pressure tap on the silencer to pressurise the fuel tank, all other pipes to the tank being sealed, apart from the feed pipe. If you put a one-way valve inside the tank, this will give more pressure. This is a simple device, consisting of no more than a piece of fuel pipe with a split in the side, the end being sealed. The air from the silencer will enter the tank thru the slit, but will not pass out again, thru the slit.
Jack
 
I've been using OS carbs with an adjustable idle circuit bleed. You can run the correct high speed setting and at idle, bleed more air into the mix using an adjustable spring loaded screw.

Some small versions of carbs are out there but the supply is diminishing. I called Tower Hobbies about the dearth of small carbs, they replied that the niche of the small 2 stroke engines had been filled with electric motors.

John in Wyo
 
Would a vacuum advance help with acceleration? I have been fiddling with my Holt's adjustments and timing has a big influence on acceleration.

Mark T
 
I posted a fairly lengthy reply to your question, it posted but when I opened the thread today it was gone. ????
So I'll try again.
Many years ago while attending the NAMES show there was a fellow, Lee Root, who had a small carburetor with an accelerator pump circuit. Although I had worked on many full sized I.C. engine I was new to the model engine world, at least multi-cylinder I.C. engine so I wasn't quite sure what I was looking at.
At the time there was a publication, Strictly I.C. published by the late Robert Washburn. He and Lee were friends and at some point Lee allowed drawings to be made of his carb. I was working on my 302 V-8 at the time and knew very little about miniature carburetion so I thought that Lee had already perfected it so I would build one. It is a quite complex build requiring a float bowl, float, needle valve, accelerator pump, linkage etc. When I completed my 302 I tried to use it but came to find out that the venturi size was way too large for my engine so I never used it to any great extent. In using it a pressurized fuel source was required to keep the bowl full. Given the size of all the parts and the float it just didn't have the floating pressure to, at times, completely close the needle valve.
Move many years ahead when I had learned more about model engine carburetion from both talking with other builders to using the internet, which was new to me. The next step in the carburetor experimentation was to use a Walbro type carb found on many small gas powered tools. I few of the fellows from the west coast, the Bay Area Engine Modelers, were using this type of carb so I contacted them for information. To use a Walbro on a small I.C. engine requires quite a few modifications plus a constant fuel supply provided by a low pressure automotive pump. The carb doesn't need a float bowl or float mechanism but does need a constant supply. I experimented with this type of carb with no great success, at least on my engines.
I then learned about the model airplane air bleed type carb and started experimenting with those.
At this point all of my engines utilize this type of carb. It's easy to build and once the air bleed is refined to the engine it's running on the operate very linearly.
At wide open throttle the needle valve is adjusted for the correct fuel/air mixture. At idle or lower speeds the air bleed is adjusted to bleed off the vacuum signal so not as much fuel is pulled through the main jet.
My 302 V-8 will idle fairly slow, 1200 rpm. This is more due to the fact that it doesn't have enough flywheel weight to overcome the compression. It will accelerate very cleanly up to 7800 rpm. Not that I have had it there many times but it will rev very well and that's with a fixed timing of 30 degrees advance.
With this in mind I don't see where all the extra work and experimentation involved would justify building a carb with an accelerator circuit.
The four pictures are of the Lee Root designed carb which is almost a shrunk down Stromberg carb with accelerator pump circuitry. As you can see it is an exercise in machining.
The video is of my 302 V-8 taken by a friend at a show. It shows the throttle response with a properly created and adjusted air bleed carb.
 

Attachments

  • IMG_2008.JPG
    IMG_2008.JPG
    250.4 KB · Views: 319
  • IMG_2009.JPG
    IMG_2009.JPG
    263.3 KB · Views: 353
  • IMG_2010.JPG
    IMG_2010.JPG
    264 KB · Views: 354
  • IMG_2011.JPG
    IMG_2011.JPG
    207.3 KB · Views: 355
George--I know that you posted. I read it. I appreciate your answers. 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
 
Just a comment on model airplane style carbs, from someone who's used them but not built them (I fly control line, so I mostly take them off and replace them with venturies):

First, the air bleed type looks to me to be really easy to build, If you can make even a 1-cylinder 4-stroke work, you can build an air-bleed carb with one hand tied behind your back. There's lots of little fiddly bits, but as long as you can make the barrel a good fit into the carb body, you'll be fine.

Second, the world has gone over to two-needle carbs. These achieve the same effect as an air bleed carburetor, but instead of an air bleed, they use a main needle that regulates fuel feed at high speed, and an idle needle that regulates fuel feed at low throttle.

In both cases I'm a bit vague as to how and when the changeover is made -- I suspect that getting the midrange working really well is going to be a challenge with either one of these carbs. I also know that for model airplane usage, you need to adjust the idle to be a bit rich, so that when you do nail the throttle the engine doesn't just stall. If you throttle down and adjust the air bleed for maximum RPM at idle, you're guaranteed a stall when you try to accelerate. You have to juggle the idle speed & mixture to get a sustained idle (keeping the glow plug warm) and still get acceleration with no, or at least minimal, sag when you punch it.

They generally work pretty well pulling fuel from a fuel pickup that's 3" to 6" away from the engine depending on the size of the tank, although this is done in part by undercarbureting a bit so you have a lot of vacuum to pull fuel.

Brian, I dunno if you're already using this style of carburetor, so if I'm filling you in on stuff you already know -- sorry!
 
I have a 15cc 2-stroke made by Webra, which has an accelerator pump. The carb. doesn't have a round barrel, which is the usual way of metering the fuel. Instead it has a slide, which slides in a groove across the venturi. Adjacent to the slide, and fastened to it, is a rod which slides into a hole. The hole is full of fuel, so when the throttle is opened, which is achieved by moving the slide into the carb. body, the rod goes down the hole and the fuel in the hole gets pumped into the venturi.
Jack
In the early days of British motorcycles (BC) there were Amal and other kinds of slide valve carburettors. Top end was controlled by the main jet, midrange and cruise by the taper of the needle, idle by the idle jet and/or idle needle, and the enrichment on acceleration by the cutaway on the back of the slide. I caan explain/understand all but the last! Bill in Boulder CO USA
 
I have a 15cc 2-stroke made by Webra, which has an accelerator pump.

Are you referring o a Webra Dynamix maybe? Oh I had my fun with those over the years. They typically needed positive crankcase or pipe pressure to the tank though. Mind you, RC planes see 3D orientations, G-forces & other demands of the carbs which are probably not fully applicable to most stationary engines.

Some info here
https://www.rcuniverse.com/forum/gl...mix-tn-carb-instruction-sheets-reference.html
 
I have come across "submerged jet" carburettors which avoided the need for an accelerator pump by putting the main jet at the bottom of a small well which fed the carburettor venturi. The idea was that in steady running, the jet metered the fuel flow correctly but a sudden throttle opening would supply extra fuel from the "well" which would refill automatically when the throttle closed. Might be more suitable for a model than trying to make a working pump?
 
I like many other of the responders have worked on quite e few different carburetors in my lifetime, Holley, Rochester, Zenith, Amal, Delorto, Mikuni and Walbro just to name a few. The common factor with these carbs is they need a fuel supply, whether by a pressure pump, gravity or engine impulse. With our miniature engines and the small volumes of fuel they need it would require making the tiniest of parts to keep the carb size from looking completely out of place.
As I had posted earlier none of this is necessary. When running my engines the top of the fuel tank is just slightly lower than the main jet and the natural vacuum of the engine will pull every last drop of fuel from the tank. If the engines were installed in vehicles that had to go up and down then an entirely different means of supplying fuel would be necessary but they don't, they just sit on a table and put on a show for the uninformed so all they need is a simple air bleed type carb.
 

Latest posts

Back
Top