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Converting A Fixed Speed Briggs Motor (carb) To Variable Speed


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#1 Tennblue59 OFFLINE  

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Posted November 24, 2012 - 04:43 PM

I've got a 10 hp Briggs single cylinder L head industrial gas motor that came off an air compressor. It is set up to run at a constant speed. There is no lever or linkage to adjust the speed. The butterfly shaft coming out of the front side of the carb has an aluminum block on it with setscrews to control max/min throttle settings (against a boss on the carb body), and the back side hooks to the governer. Looks like it is set to run wide open (as set by the set screw). It's a verticle draft carb on a "j" shaped intake.

Anyway, I want to convert this to have a variable speed throttle, run off a cable for a piece of equipment. I don't have access to the motor at the moment, or have the model numbers (or pictures) handy..... Also, this is NOT replacing a similar motor that I can steal parts off to convert it...

Aside from the lack of a lever (and hardware to mount it) to adjust the throttle butterfly, would this carb be any different from one used on a variable speed engine?

Could I buy/adapt parts to convert my existing carb? Also, would the governer linkage be different (seems like it would be...)?

Perhaps the easiest (although not the cheapest..) thing would be to get a new carb with the proper linkage on it... But a new carb wouldnt be cheap...

Any suggestions or has anybody else ever been down this road?

#2 ol' stonebreaker OFFLINE  

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Posted November 24, 2012 - 06:23 PM

A couple of pics would be nice to see what you have. Maybe one of the front side and one of the back.
Mike


#3 bowtiebutler956 OFFLINE  

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Posted November 24, 2012 - 07:29 PM

There is almost always a simple way to covert them, but some pics would really help.

Matt

#4 Farmlife OFFLINE  

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Posted November 25, 2012 - 02:47 AM

I've never heard of or even thought of doing this. Sounds like a great idea actually....i'm interested to learn more....

#5 HowardsMF155 OFFLINE  

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Posted November 25, 2012 - 10:03 AM

I'm not an expert on these engines, but if the engine is governed, there must be a governor lever attached to a speed sensing mechanism, probably gear driven weights, or perhaps an air pressure governor like in a lawnmower engine, and a spring which acts against the natural direction of the governor. A fixed speed engine then would simply have a spring attached to the governor at a fixed point, where a variable speed would have movable end point for the spring. Perhaps you can find that spring ( it will act to pull the throttle wide open when the engine is off) and rig a way to move that fixed point back and forth.

#6 Tennblue59 OFFLINE  

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Posted November 26, 2012 - 08:42 PM

I know (from comparing) that the variable speed engine has some play in the governer linkage to the carb vs the non-flexible linkage on the constant spped engine.

That play is supplied at the "control panel" most engines have on the side of the carb - where the throttle and choke levers are located. My engine doesn't have one of those panels. The cutoff switch for the engine is an actual switch mounted in the housing/tinwork on the top of my engine. Thinking I could buy/adapt one of those panels and graft it onto my engine.

I was not sure though if the constant speed carb is "optimized" for a specific speed - and would be unresponsive for less than full throttle adjustments? For example, a racing carb may be set up to rum best at a rpm band, but be fairly crude running below that band.... Just wondering if my constant speed engine carb is similar - or if it is the same carb as a variable speed engine, just missing the controls and linkage....

#7 HowardsMF155 OFFLINE  

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Posted November 26, 2012 - 09:51 PM

I hadn't thought about it, but a fixed speed carb might be missing the whole idle circuit and use only a fixed hi-speed jet. But any engine that is subject to varying loads will need a governor to match the power (amount of air/gas mixture admitted to the engine) to the load.

#8 Jehtro OFFLINE  

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Posted November 26, 2012 - 10:21 PM

i wonder if it the same as the L-heads that came in the john deere 116's, i always see those blownup on the internet, find a doner unit,

just a thought

#9 Tennblue59 OFFLINE  

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Posted November 26, 2012 - 11:58 PM

Howard, that thought was what got me started with this. Would hate to rig up a linkage, then not have the carb run well (or at all).

Jehtro, I may end up going that way and getting a new carb. But the cheapskate in me would rather use the existing (paid for, working) carb I have if possible. Also, while not "rare", I havent seen a lot of these around used..... Seems easy to find smaller or bigger carbs, but this is a less common size... New would be big $$$$$!

I have a couple control panels from trash engines that I could use.....




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