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What is the advantage of double pumper carb? Thanks

Dennis,

In the BDC line of large bore carbs for Mac engines the single pump versions can't deliver enough volume of fuel to run 100% alky no matter what you do to the jets and other passages in the carb. If you run straight alky in the single pump carb the fuel pressure will drop down to near 0psi at around 11,000 if you have jets large enough to flow the required amount of fuel. The BDC 16 version with the double pump setup will deliver more gallons per hour so it can maintain reasonably steady fuel pressure all the way up to over 13,000 rpm. To give you an idea of the flow rates... we used to run alky through the single pump version and had to limit the top rpm to around 11,000rpm on the 91 series engines and the fuel consumption was about 3.75 gallons per hour. If we geared to turn more rpm the engines would just run out of fuel when the revs went over 11,000 no matter where we set the jets or how we modified the passages. Using the BDC 16 double pumper carb on the 101 engines we would burn 6 gallons in an hour and the high speed jet was open less than 1 turn so I could flood the engine if I opened the HS jet at top rpm.
In the late 70s I developed a modification to the single pump version that was internal and allowed the single pump to deliver enough fuel to turn the 91 series engines up to 13,000 but it was still not as good as the double pump version. If you run gas then the single pump is all you need and the double pump offers no benefit but for alky the double pump setup is the only way to go.
Hope this helps.
Steve O'Hara

Thanks Steve,
I purchased a 91b off of ebay a few days ago. It's in transit. Probably in need of a total rebuild. I'm not sure what size carb is on it. The pic looks like large BDC, but it's a single pumper. I'll be mounting this engine on a soon to be restored 74 Margay Concept Sprint. I'm slowly gathering input on the engine and kart. I'll also be needing to find specs for the Mc91b.
Thanks again for your input.
Dennis

http://64.224.15.40/Mcculloch_main.htm

go to this site then click on 91 engine chart not sure if thats what you are looking for. lots of neat stuff there

jerry

Yes there are some tips there on how to modify the fuel pump.

Thanks, I started reading on some of it last night. I'll get back to it later today.
I'm also looking for specs for rebuilding the engine, journal, piston, cylinder, etc specs. I want to disassemble the engine, when I get it, and find out what it needs.
Dennis