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small carburettors

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steve williams 310/12/2009 19:40:37
28 forum posts
49 photos
hi guys
i have recently completed scratch building a 23 cc single four stroke engine & after a few problems with the ignition & plug failing (which with your help i have now sorted. my problem is, the engine runs sweet on a small lawn mower carb i fitted to it but of course i want to make my own, i have now made one with emultion tube & air screw and a float cnamber BUT i cant scall down the float & get the needle to shut so the float chamber keeps flooding i could make the float chamber bigger & put a bigger float in it but it will look out of place. so i am thinking of making a carb of a different design & would like to buy a book but can't find one on model carbs any suggestions please.
cheers steve
Ian Abbott10/12/2009 20:19:59
279 forum posts
21 photos
A couple of ideas,
How about a surface vapour carb, simple to make.  There's one on my 1917 Audel's, but it's buried right now.  I'll see if I can find it next time I go digging.  A search on the web might turn one up and I think that I remember one in ME a while ago.
Or, a diaphragm control, like on chainsaws and such.  Might be a challenge to get right, but when it's done it could be a standard for others.
Les Jones 110/12/2009 20:20:14
2102 forum posts
144 photos
Hi Steve,
                 The only thing I can think of that would cause your problem is if the hole the needle goes into is too big. If you think of it as a piston it will produce a downward  force which would be the pressure of the petrol times the cross sectional area of the hole. If this force is greater than the than the upward force from the float then the valve will not seal.
There will be a lower limit to the size of this hole dictated by the required fuel flow.  Making the float lighter (If that is possible.) may improve the situation.
Ian Abbott10/12/2009 20:33:50
279 forum posts
21 photos
There's some interesting stuff out there, here's a few starters.
(Last post should have read "in" Audels) 
This last one are US Patent details for a surface carb plus some other related stuff such as fuel injection.
Ian S C11/12/2009 03:41:07
7468 forum posts
230 photos
If you can get Edgar T Westbury's book Model Petrol Engines there is a chapter on carbs.Ian S C
mgj11/12/2009 09:05:11
1008 forum posts
14 photos

Edited By meyrick griffith-jones on 11/12/2009 09:06:50

Edited By meyrick griffith-jones on 11/12/2009 09:25:20

Edited By meyrick griffith-jones on 11/12/2009 09:26:02

Ian S C12/12/2009 10:03:34
7468 forum posts
230 photos
You should be allright with a float carb if ETW is anything to go by,he says that floats have been used on motors down to 2 1/2cc,although not usually on less than 10 to 15cc.Personally I have made and fitted a very plain(crude) mixer ie just a venturi with a needle valve,only meassurements were the size and spacing of the studs on the inlet'it was /is fitted to a 3hp hit and miss open crank horizontal motor.The motor(full size) when bought had a Ford Model A carb that sort of worked,so we thought aplain venturi would look better,so the one described was made with he idea of making an improoved one,but it worked so well that its still on the motor about 10yrs on.The only thing I would change,I would make it out of brass,the one fitted is steel.The fuel tank is about6"-8" below the carb,plenty of suction.Ian S C
John Wood112/12/2009 15:16:18
116 forum posts
It's amazing how many of us have difficulty in sourcing a decent 'general purpose' carb design for our various engines. Surely there must be someone out there who has the knowledge and skill to design a reasonably straightforward carb which could be used with a range of designs. Preferably without a casting.
I have noticed in many earlier published designs that the carb description is often glossed over with such terms as "...I borrowed a carb from an old engine in the workshop and used that or, "....any reasonable carburettor will do"  or the design is based on a casting which is either no longer available or only available as part of a complete casting kit for an engine.
Any takers? I'm sure David would be most interested in a design for ME.
All the best
Ian S C13/12/2009 09:35:15
7468 forum posts
230 photos
Perhaps,because ETW was a past editor(associate)of Model Engineer David C 1 could arainge theinclusion of parts of Model Petrol Engines or articals that apeared in ME.Also the book was published by Percival Marshall
mgj13/12/2009 09:45:52
1008 forum posts
14 photos
Well I shall make a Wyvern next, and I shall simply take the needle valve assy and helical barrel out of an appropriate sized SuperTigre carb (any will do but I know STs) and graft it onto a float chamber.
Fuel regulated, progessive fuel supply, and progresive air opening. Looks better than a little Walboro.
Very easy.
Ian S C13/12/2009 23:41:03
7468 forum posts
230 photos
One of the plans that came in ME is CENTAUR by E T westbury,there are 2 sheets and My Hobby store has the plans.The engine has 1 1/2" bore and 2" stroke.This is basicly a mixing valve with a throttle valve.Ian S C
Ian S C30/12/2009 14:03:41
7468 forum posts
230 photos
Go to top of page,click on shop,scroll down and you'll find one or two sets of plans for carbies.Ian S C
geoffrey ball17/01/2010 20:56:27
3 forum posts
There is a very useful chapter in Len Mason's book "Model 4-Stroke Petrol Engines" which you may be able to find in your local library. There are no drawings though. Edgar Westbury designed a very neat carburettor for his "Busy Bee" engine which is fully described in ME 3rd July 1952 and in fact I have used an adaptation of this for the 7 cylinder radial engine I am constructing at present.
geoffrey ball18/01/2010 09:06:39
3 forum posts
Correction - should be "ME 3rd July 1952 et al.

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