Author Topic: Carb setup for single cylinder engine  (Read 576 times)

Offline AlexS

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Carb setup for single cylinder engine
« on: May 12, 2018, 10:11:40 PM »
Hey guys, I need some advise for selecting a right carb setup for my 55cc single engine.  :smokin2: :thinking:

The engine should be running at low rpm.

I did some calculations and I set the maximum rpm to 3000 rpm.
The output of the calculation gives a idea what the diameter of throat and main nozzle should be.

The engine pump air with a fictional volumetric efficiency of 100% an air flow of 1.37 L/s or 5 m3/h.
This gives a diameter of throat of 4.8 mm and a main nozzle of 0.24 mm.


My question is, what are reasonable dimensions for the carburetor?
And what is useful to start with? This is my first homemade engine, and my goal is to get it running.

I have search for moppert carbs, like Bing 10mm (Puch). Doesn't know what the diameter of the throat would be.
Or a carb from the nitro RC engines: https://www.rcplanet.com/traxxas-carburetor-trx-pro-15-6-0mm-bore-tra4033/
I have also seen that engines run on a petrol vapor carburetor.

Thanks in advance!

Offline lohring

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Re: Carb setup for single cylinder engine
« Reply #1 on: May 13, 2018, 03:20:57 PM »
This is a formula for Walbro carbs:

D = K x Square Root(C x n)
where:
D=venturi diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000

I think the one in the last line is the two stroke vs four stroke factor.  It should be 1 for two strokes and 2 for four strokes.

Lohring Miller

Offline Roger B

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Re: Carb setup for single cylinder engine
« Reply #2 on: May 13, 2018, 03:46:30 PM »
L C Mason in "Model 4 Stroke Petrol Engines" suggests the carburettor bore should be about 25% of the cylinder bore for 'normal' engines increasing to 33% for high performance. Model aero/car carburettors do work, but the needle valve/jet may be a bit big for a 4 stroke engine.
There should be a useable adjustment of the needle valve for each throttle opening, but you will be lucky if you get good throttle response.
« Last Edit: May 17, 2018, 07:19:20 AM by Roger B »
Best regards

Roger

Offline AlexS

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Re: Carb setup for single cylinder engine
« Reply #3 on: May 15, 2018, 07:13:42 PM »
Thanks! So I need a +- 10 mm venturi diameter. So now I'm looking for a Zama or Walbro chinees replica. Like the C1M serie of Zama.
Does anyone of this type carb running on a model engine?

Offline Roger B

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Re: Carb setup for single cylinder engine
« Reply #4 on: May 15, 2018, 08:48:31 PM »
It doesn't need to be too precise. I have a model car/aircraft carb with a 4.2mm venture that works on my 16 mm bore 3cc engine and on my 25mm bore 25cc engine. The 25 cc engine develops more power with a bigger bore carb (5.7mm I think) but runs ok with the smaller one. I also have a smaller home made carb for the 3cc engine (3mm bore I think) which runs ok but again delivers less power.
Best regards

Roger

Offline AlexS

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Re: Carb setup for single cylinder engine
« Reply #5 on: May 15, 2018, 10:05:18 PM »
Yeah I agree with you, it doen't need to be a perfect match with the engine. It would be more trail and error. But some research would be good to see what is a good carb to start with.

Offline gbritnell

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Re: Carb setup for single cylinder engine
« Reply #6 on: May 16, 2018, 01:20:25 AM »
My son has a 50cc four stroke motorcycle and if memory serves me I think the throttle bore on that engine is around .50 inch. At least that would be somewhere to start.
gbritnell
Talent unshared is talent wasted.

Offline Admiral_dk

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Re: Carb setup for single cylinder engine
« Reply #7 on: May 16, 2018, 11:54:10 AM »
A 49cc Puch moped had a 12mm. Bing carb in my youth and that was more than enough for 4000 rpm …
But this wasn't enough for the power hungry ones (like me), so a top trimmed 50cc will need at least 19mm.

Offline dieselpilot

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Re: Carb setup for single cylinder engine
« Reply #8 on: May 16, 2018, 02:03:29 PM »
12mm or 19mm carb for a 3000 RPM 55cc? That won't be very practical. ~7mm max.

Offline Admiral_dk

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Re: Carb setup for single cylinder engine
« Reply #9 on: May 16, 2018, 08:16:14 PM »
I will agree that a smaller carb gives better torque / running characteristics + fuel suction at low revs.

Oh, and nice to hear that you're still around Greg   :cheers:

Offline AlexS

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Re: Carb setup for single cylinder engine
« Reply #10 on: May 17, 2018, 04:01:30 PM »
Thanks guys! But to start with I just ordend a 13mm RC carb. Smaller ones were not available. But later on I could always fit smaller ones, maybe a second-hand 10 mm bing(puch).
Now some time to finish the cilinder head (fit a spark plug) and work out the ideas about the ignition system.

Offline Roger B

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Re: Carb setup for single cylinder engine
« Reply #11 on: May 17, 2018, 06:52:28 PM »
I have used this one from Conrad on my 25cc engine to get more power:

https://www.conrad.ch/de/ersatzteil-vergaser-komplett-force-engine-passend-fuer-modell-15-18er-force-nitromotoren-224577.html

They have others in their spare parts section. (I am guessing you understand enough German for this).
Best regards

Roger

Offline AlexS

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Re: Carb setup for single cylinder engine
« Reply #12 on: May 18, 2018, 11:17:19 PM »
Thanks Roger, Conrad has some nice technical parts for sale. Especially for the model hobby etc.