Author Topic: Matching carburettor to engine  (Read 1082 times)

Offline Trevorc

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Matching carburettor to engine
« on: February 04, 2018, 05:27:51 PM »
Making sure that the engine you have built has the correct carb can be a problem especially when the one specified in the drawings is no longer available. My recent experience with my Hoglet has shown me that the trial and error approach can be very frustrating.
I have a lot to learn about how to match an engine with a suitable carb. I know that Bernoulli has a lot to do with it and i understand some of the principle involved.
However, i would like to ask the question " does anybody know of a publication, magazine, journal or website that covers the subject in sufficient depth that given the basic engine parameters one can work out basics of carb such as venturi diameter, jet sizes etc.
Thanks for your help

Offline gbritnell

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Re: Matching carburettor to engine
« Reply #1 on: February 04, 2018, 07:10:32 PM »
Hi Trevor,
There has never, to my knowledge, been anything written that spells out exactly what size of venturi to use with what size engine bore. The primary reason is  that each engine has it's own intake characteristics and running speeds so therefore a different carb would be required for each application. There are numbers that some people use as a starting point but there are no hard and fast numbers like if you have a 1.00 (25.4mm) bore and your expected rpm is X and you have 2 cylinders with valves of Y diameter then use carburetor 'B'. General rule of thumb is to make the venturi on the smaller size until you get the engine running and then you can always increase it.
I have found that we as builders aren't experimenters. By that I mean once we build an engine and get it running fairly well we don't go back and make new pistons to increase the compression, new cams to change the performance curve or carburetors to see what difference they have in the overall performance of the engine.
If you ask a specific question like I have a single cylinder engine with a certain size bore and stroke what would be a good carb size to use. People can then respond and say I used a carb of a certain size for my similar engine and it runs this way or that.
Going by automotive principals an engine will 'flow' so much air through the intake, ports, valves and exhaust. Using these numbers a carburetor can be chosen to meet these requirements but it's not all that simple. Once the carb is installed the fuel metering will need to be adjusted to that particular engine's running characteristics. We really don't have that option because we don't know how much air flows through our engines unless you can find someone with a miniature flow bench.
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Offline Roger B

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Re: Matching carburettor to engine
« Reply #2 on: February 04, 2018, 07:55:39 PM »
This is a subject I have thought through several times  :headscratch:  :headscratch:
There are some other threads on the subject here:,2093.msg51395.html#msg51395,1668.0.html

My experience has been that if the rest of the engine is ok the carb is not so much of a problem my horizontal engine became 'difficult' but the actual problem was compression and ignition  ::),7701.0.html
Best regards