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Old 08-19-2020, 07:56 PM   #1
mcgarrett
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Default '40 Ford carb fine tuning help needed

Stock '40 engine with about 5,000 miles since rebuild. Running correct Ford/Holley 91-99 carb. Can't seem to find the sweet spot. Starts quickly and takes throttle fine, but has slight hesitation off idle and seems to run slightly better when I add some choke even after it has warmed up. Could it be that it needs the float raised to increase float bowl volume to increase flow through the main jets? Just installed a new carb kit with new accelerator pump plunger, gaskets, etc. and all passages were blown out with spray carb cleaner to make sure they were open. Don't know what to do about it running better when I apply some choke, so help is certainly appreciated.
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Old 08-19-2020, 08:06 PM   #2
drolston
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Default Re: '40 Ford carb fine tuning help needed

What main jet size are you running? You might need to go to the next larger size.
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Old 08-19-2020, 08:11 PM   #3
scicala
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Default Re: '40 Ford carb fine tuning help needed

Check to make sure the accelerator pump is squirting two streams of fuel with the engine off and looking down the throat when opening the throttle. It should squirt with the slightest movement of the throttle. Sometimes there is slop in the pump link from the throttle lever too, which causes a slight delay and possible hesitation.

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Old 08-19-2020, 09:04 PM   #4
mcgarrett
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Default Re: '40 Ford carb fine tuning help needed

I'm running #50 main jets. What would be the next larger size? Also, I checked that both nozzles are squirting fuel properly and they seem to be OK.
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Old 08-19-2020, 09:11 PM   #5
Phil Gillespie
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Default Re: '40 Ford carb fine tuning help needed

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Originally Posted by mcgarrett View Post
I'm running #50 main jets. What would be the next larger size? Also, I checked that both nozzles are squirting fuel properly and they seem to be OK.
The stock standard, is a 51 jet for the holley with a 6.5 PV. Some variation dependant upon the sea level you are driving at.
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Old 08-19-2020, 09:21 PM   #6
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Default Re: '40 Ford carb fine tuning help needed

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Thanks Phil. I have some #51 main jets. I'll try switching them out and check the power valve too.
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Old 08-20-2020, 03:07 AM   #7
fordscript
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Default Re: '40 Ford carb fine tuning help needed

I have always found that when you get the idle real 'sweet', it will often hesitate when you give it some gas, so then I richen the idle mixture a little, and it seems to cure the problem. might work for you.
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Old 08-20-2020, 12:56 PM   #8
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Default Re: '40 Ford carb fine tuning help needed

The main jets are not in play at low speed off idle conditions. As noted, try enriching the idle a bit.
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Old 08-20-2020, 08:36 PM   #9
GB SISSON
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Default Re: '40 Ford carb fine tuning help needed

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The main jets are not in play at low speed off idle conditions. As noted, try enriching the idle a bit.
I'm sorry, but do the twin idle screws enrichen the mixture when turned clockwise ? I have twisted these two screws for 25 years to get that sweet spot, but still don't get if they control fuel or air.
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Old 08-21-2020, 02:37 AM   #10
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Default Re: '40 Ford carb fine tuning help needed

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Originally Posted by GB SISSON View Post
I'm sorry, but do the twin idle screws enrichen the mixture when turned clockwise ? I have twisted these two screws for 25 years to get that sweet spot, but still don't get if they control fuel or air.

The idle screws are needle valves. They open counter clockwise on the idle fuel circuit, directly behind the venturi, which sucks the fuel from that circuit. The further they are opened, the faster the idle within the fuel/air optimal ratio, during and after which point the idle speed adjusting screw is adjusted outward, lowering the idle back down by restricting the air intake. This procedure is done individually on the two idle screw valves, repeating until the highest steady vacuum is achieved, all of which is done on a fully warmed up engine.


All that said, I've had experts improve on my adjustments just by ear, not bothering with a vacuum gauge at all.
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Old 08-21-2020, 01:04 PM   #11
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Default Re: '40 Ford carb fine tuning help needed

They control fuel.
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