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nelson .40 q500 carb setup

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Old 03-25-2013, 02:24 PM
  #1
Stevem9
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Default nelson .40 q500 carb setup

anyone have any info reguarding a real carb setup for a nelson. i have an older nelson that was thinking about changing to a carb so it will idle (or will it idle?). i would assue that a carb would take away some top end correct? didnt know where to post this but i firgure some of the fellow pylon guys might have run accross this before
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Old 03-25-2013, 04:05 PM
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Ed Smith
 
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Default RE: nelson .40 q500 carb setup

It is a racing engine. The timing of the engine will prevent a reliable idle.

Ed S
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Old 03-26-2013, 05:00 AM
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Stevem9
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Default RE: nelson .40 q500 carb setup

didnt they used to have one with a supertiger carb on it though?
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Old 03-26-2013, 06:34 AM
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DHG
 
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Default RE: nelson .40 q500 carb setup

Steven,

If you have an older case, the modified ST carb that Nelson & Shadel used to sell will fit. Also, no disrespect to Ed, but I cannot understand where that myth came from that says a high-timed racing engine won't idle. I own two of the modified ST carbs with the larger (11 mm) opening, and not only do they purr like a kitten at idle, the extra diameter makes the top-end rpm identical to the top-end rpm of the regulation racing engines with the unobstructed 9 mm opening. In other words, the spray bar does hurt the rpm, but if you get the bigger carb you can make up that loss.

Both of my big carbs are mounted on Q40 engines. I use them for trim flying & practice. Some day I hope to put them both on a scale Mosquito and terrorize the villagers.

I also have one of the smaller carbs (8 mm), which I've put on a Q500 engine. (The only difference between the Nelson Q500 and Q40 engines is the piston/sleeve assembly, so you can convert one to the other using the same case.) That one is definitely not as strong as a regulation Q500 engine, but I plan to use it for sport. Bench testing shows that the ideal prop would be about a 9 x 7. The 9 x 6 APC sport prop is scary fast (19,000+, too much for anything without the reinforced Q500 hub) and the 9 x 8 won't quite pipe up.

There are only two problems with these carbs: First, on a Q40 engine, the high rpm makes them loosen up and try to fall out. So if you're just looking for a hot sport engine, I'd say stick with the Q500 sleeve and shoot for about 18,000 on the ground. You may still have to re-engineer the hold-down system to keep it in the case. I drilled & tapped for two #6 x 32 cap screws, one on each side of the throat, in addition to the normal pin across the back.

The second problem with these carbs is finding one. If you lurk on the Popular Auction Site That Cannot Be Named, you may find an engine that has one on it. Then you have to buy the engine for top dollar, take off the carb, and sell the engine with a standard plastic carb or venturi in it. Or keep the engine; it may be worth your while to do that since the old cases are harder and harder to come by.

Hope this helps!

Cheers,

Duane
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Old 03-26-2013, 07:55 PM
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Default RE: nelson .40 q500 carb setup

awesome thank you Duane!
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