Flatback carb conversion

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Kurt Bogerman
Posts: 47
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
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Location: Harrisburg, PA

Re: Flatback carb conversion

Post by Kurt Bogerman » Thu Aug 23, 2018 12:24 am

Interesting observation, Rob. Makes me wonder... If the pop off is, in fact, tailored towards direct drive, how relevant are the alky orifice specs with reference to the high stall speeds one might see with a tightly sprung Jim Donovan MaxTorque or an oil clutch? Did the evolution from direct drive towards clutches require different carburetor specs, or was it a just a matter of pop off and needle adjustments?

Though... I guess it really wouldn't make much performance sense to modify a flatback for alcohol and then use it with a slippy clutch, when bigger carbs are available for that purpose.

ted johnson
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Re: Flatback carb conversion

Post by ted johnson » Thu Aug 23, 2018 9:18 am

Most guys just played with hi and lo speed needle settings until they got to where they wanted in the early years. Also, on the popoff thing, for the first few years of karting, the vast majority of drivers had no clue as to popoff. We just fiddled with cutting or stretching the inlet lever spring until it ran right. I never saw a popoff pump and gage until the later years. The magnesium Mac carb and the newer flatback were no more finicky than a Tilly. As far as alky went, we bought a Jim Butler flatback, measured the holes and copied them. Most guys just sent their carbs to Jim or to Russell and had them modified. There were carb modifiers in every issue of Karting World. TJ

Rob Voska
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Re: Flatback carb conversion

Post by Rob Voska » Thu Aug 23, 2018 5:05 pm

Kurt that is pretty much what I was thinking about everything. Hole sizes be dammed we will overcome it with pop off! By the time higher RPM clutches & pipes came along flatbacks were pretty much a thing of the past and everyone had bigger tillys and even bored out tillys.. Ted's right also...... they were living the evolution of the sport & gaining huge amounts of knowledge every year.

Popoff overcame a bad drill job. Bigger carb overcame a higher slip clutch or a peaky pipe. Every engine had different fuel requirements. Now lets throw in guys that had no clue & were reading magazine articles and were at different elevations!

ted johnson
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Re: Flatback carb conversion

Post by ted johnson » Thu Aug 23, 2018 7:16 pm

Rob, my Pop was a McCulloch and PP distributor. He built saw carbs every day for years, and there was never a popoff gage in the shop till the day I shut the kart shop down in late '63. Pop may've bought a gage in later days, but I never saw it. Of course, we ran only direct drive, but we had zero difficulties getting carb settings. I was winning everything in Brevard County in the early sixties, and never had a clue as to popoff! Charlie worked up at Wyatt Enterprises. They were more sophisticated at Wyatt than we were, but they never outran us. TJ

Rob Voska
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Re: Flatback carb conversion

Post by Rob Voska » Thu Aug 23, 2018 8:52 pm

Ted you did popoff even if you didn't know it. You just worked around the problem with spring length etc.....

ted johnson
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Re: Flatback carb conversion

Post by ted johnson » Thu Aug 23, 2018 10:07 pm

We even knew what the variations in spring length and fulcrum arm height did, we just never thought of measuring it in PSI. TJ

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Kurt Bogerman
Posts: 47
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
User Agreement: Yes
Location: Harrisburg, PA

Re: Flatback carb conversion

Post by Kurt Bogerman » Fri Aug 24, 2018 12:32 pm

Thats a lot of interesting background information. I say that sincerely: I enjoy learning about the evolution of the technologies we usually take for granted, and learning from the experience of others. Thank you all for contributing to the conversation!

With reference to my stock MC90: If I wanted to burn methanol and retain the 'correct' equipment, I'd want a flatback. Is there a procedural article on this available somewhere, or are you guys protecting that spare carb of mine from a fate worse than remaining stock? :lol:

ted johnson
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Re: Flatback carb conversion

Post by ted johnson » Fri Aug 24, 2018 1:43 pm

Kurt, I still say you need to talk to Paul Booth. He knows flatback carbs and maybe can point you to articles, or can give you the numbers. You corresponded with him thru REAR back in January, and he posted his email, so I don't think he would mind if I re-post it for you: lindab@casscomm.com Be sure to get hold of him. Ted

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Kurt Bogerman
Posts: 47
Joined: Wed Sep 27, 2017 4:39 pm
Vintage Karting items owned: 1964(?) Rupp Grand Prix, MC 90, Safari Gearbox.
User Agreement: Yes
Location: Harrisburg, PA

Re: Flatback carb conversion

Post by Kurt Bogerman » Fri Aug 24, 2018 1:48 pm

Fair point, Ted. Thank you :D

Rob Voska
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Re: Flatback carb conversion

Post by Rob Voska » Fri Aug 24, 2018 5:28 pm

Not much difference between a FB & a tilly.......

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