Holley NH Carb Question

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myersair
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Holley NH Carb Question

Post by myersair » Wed Apr 21, 2021 8:01 am

Are the NH carbs all the same from 20 - 26 ? I’m looking for one for my 26 coupe and I thought the later ones (26-27) were slightly different......em I thinking correctly?
Anyone have a decent core they would sell?
Thanks in advance


speedytinc
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Re: Holley NH Carb Question

Post by speedytinc » Wed Apr 21, 2021 8:19 am

25-7 use a different mixture needle for additional choke function. Early 20's are straight thru ventury. Otherwise the bodies are the same. 20-22 have long throttle & choke arms. 23-27 short arms. 25-27 choke arms have a dog leg to clear the upper choke rod/mixture linkage.


TXGOAT2
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Re: Holley NH Carb Question

Post by TXGOAT2 » Wed Apr 21, 2021 9:49 am

Do the earlier "straight through" carburetors flow better at higher engine speeds? Ford made several concessions to poor quality gasoline. I wonder if a smaller venturi may have been one of them. "Low grade gasoline" in the 1920s meant lower gravity gasoline, which is more kerosene-like than higher gravity gasoline. Lower gravity gasoline is more difficult to vaporize, and typically has lower octane equivalency. Gasoline available today is fairly consistent as far as volatility, and the octane equivalency is well beyond what a stock Model T requires. I don't like gasohol, but fresh gasohol ought not cause any problems with a Model T except during extended storage. If you use it, buy it fresh, and use it up or drain it off. It is definitely persishable.


speedytinc
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Re: Holley NH Carb Question

Post by speedytinc » Wed Apr 21, 2021 11:31 am

If i recall correctly, sway backs flow 29 cfm & straight thru's flow 31cfm. So, yes, slightly. At least in theory, assuming the fuel flow has not reached its maximum limit. If they were such a good carb, why did the design change in less than a year? I believe the swayback mixes /vaporizes the fuel much better.
I tried a straight thru many years ago. Didnt like it. Got poor mileage(not that important). Needed to be richend for hi speed. then it idled too rich. Too much hassle. This may have been a float adjustment issue. Have tried one since. I have since done a bit of research on the issue, as follows.

HUGE CARBURATOR TIP: Nh float adjustment @ 1/4" is an (initial) setting. When properly set, there is no need to open it a bit more for starting or hi speed.
1) Set mixture needle for best hi speed running.
2) reset mixture @ idle.
3) If you leaned the mixture for best idle, fuel level is too high.
4) if you richend mixture for best idle, fuel level is too low.
5) Re adjust float a little accordingly until that one mixture setting is optimal for idle & hi speed.
No need to mess with it again, except for large altitude changes.

I did not know this when i was trying a straight thru.


Dan Haynes
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Re: Holley NH Carb Question

Post by Dan Haynes » Wed Apr 21, 2021 4:08 pm

My experience with a straight-through was as bad as John's. It gave crummy mileage, around 10 mpg and the vague flat spot trying to get it going from a stop was annoying. And agreed about always having to dial around on it to get the mixture right for engine speed. I'll take a Russ Potter swayback NH over the straight-through any day.
"The further a society drifts from truth, the more it will hate those who speak it." -George Orwell


Plankhill
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Re: Holley NH Carb Question

Post by Plankhill » Wed Apr 21, 2021 5:30 pm

I went from Wisconsin to Montana and back a few years ago and got 21 mpg on a straight thru.

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RajoRacer
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Re: Holley NH Carb Question

Post by RajoRacer » Wed Apr 21, 2021 11:06 pm

I'm running 3 straight thru's - no problemo !


Alan Long
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Re: Holley NH Carb Question

Post by Alan Long » Thu Apr 22, 2021 12:38 am

My 1926 Roadster Pickup has always been a tad sluggish and over the years I’ve tried several items to get it “peppy”
I have a test road near home that has a roundabout at the bottom of a hill. This was always a drop back to low half way up the rise. I’ve fitted a “Z” Cylinder Head, Truefire ignition system and taken the fuel line to 5/16. Absolute no improvement.
Finally, after fitting a straight through NH the T goes around the roundabout and up the hill in top gear and actually accelerates!! To me the carby change was the single item that cured my T. Like others, I set the mixture screw to run well at a fast idle and never ever touched it again. The Truefire ignition was changed back to standard coils and roller timer well before the carburettor change.
Alan in Western Australia


TXGOAT2
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Re: Holley NH Carb Question

Post by TXGOAT2 » Thu Apr 22, 2021 10:04 am

A straight thru or other oversize carburetor would probably give better results on an engine with an improved cam and lower than stock combustion chamber volume. A carburetor stove would probably be helpful in some areas and with some types of regulated fuel blends. The specific gravity of gasoline blends may vary considerabley, which could affect carburetor operation, as could differing vapor pressure, BTU content, and more. A custom hot spot manifold might improve driveability and fuel economy while avoiding most of the disadvantages of an intake air stove. I seem to have more problems with downdraft carbureted engines flooding after a hot shut down than used to be the case. I suspect that it results from fuel blends tailored for modern, fuel-injected engines, where vapor lock, carburetor percolation, and evaporative emissions are not an issue.

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