Author Topic: Choke pull off  (Read 272 times)

Offline dlphil10

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Choke pull off
« on: May 16, 2020, 11:18:48 AM »
Since I put a regulator on, and reduced pressure to 4.5psi, my car runs pretty great and nozzle drip is long gone. I'm tweaking my secondary opening rate now. So far I've been able to reduce the spring tension from 1.125 turns to 7/8 turns with favorable results, but I've found the initial WOT experience to be very lacking until the secondaries do come in.

 I know my setup shouldn't need a lot of secondary action too quick(7029262 carb rebuilt to roughly recipe 2 on a 9:1 pontiac 350 with a 212/220 @ 0.050" cam), so I checked my choke pull off and its fully released around 2 seconds as specified in your book. However, it only pulls off when there is zero vacuum. Is that right? Doesnt seem right that the air valves would only kick in at WOT only.

Offline Cliff Ruggles

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Re: Choke pull off
« Reply #1 on: May 17, 2020, 03:58:46 AM »
4.5 psi is pretty "low" for fuel pressure.  The only Q-jets sensitive to fuel pressure are the pre-1969 Pontiac models.  1969 and later are fine with 6-7 psi or even higher.

1 1/4" turns on the spring is too tight for most set-ups and can ruin the spring.  Hopefully you put a new one in as I've found 50 year old ones seldom work well and need wound up too tight.

The pull-off link should be adjusted to hold the flaps tightly closed but still allow them to fully open when it releases.

The pull-off opening rate is important as it not only dampens the opening rate to prevent stumble/hesitation/bog but not so slow it doesn't allow them to open smoothly via correct spring tension.

All of the early plastic pull-offs being sold doe NOT have enough restriction in them.  That's why I modify the ones we sell here and make them adjustable.

The pull-off should be checked with vacuum pump or engine vacuum as it will be a little slower than just compressing and releasing it by hand.......Cliff

Offline dlphil10

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Re: Choke pull off
« Reply #2 on: May 17, 2020, 08:49:30 AM »
The carb was rebuilt with your HP kit. New jets, primary rods, secondary rods, throttle bushings, secondary air valve spring, pull off and so on.

Anything more than 4.5psi causes nozzle drip with float at 0.25inch. I gained the ability to raise idle well over 100 more rpm after the regulator went on and so far no apparent issues with fuel delivery.

My vacuum testing was done with a Mity vac. Will drilling out the internal restriction and adding a custom one help open it before 0inhg?

Offline Cliff Ruggles

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Re: Choke pull off
« Reply #3 on: May 18, 2020, 04:25:04 AM »
I test all the carbs I build here at 6.5 psi and have a regulator on the engine.  Never once observed nozzle drip from moving the pressure around and have even raised a few of them up to 8-10 psi during testing.

It is an electric pump and I've seen some mechanical pumps dead-headed that caused all sorts of flooding issues w/o a regulator on them.......

Offline dlphil10

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Re: Choke pull off
« Reply #4 on: May 18, 2020, 04:50:40 AM »
Interesting. I've only used mechanical fuel pumps. I had nozzle drip with a new carter pump. Fuel pressure would swing very drastically (4-10psi at idle) and inconsistent pressures at cruise. I changed to an AC delco mechanical fuel pump and it also has fluctuating pressure, but not as bad (4-9psi). Off idle the pressure is solid at 6.5psi.

When I put the regulator on the fluctuations were only a 0.5psi (6-6.5psi), but nozzle drop wouldnt clear our until 4.5psi. The carb wanted no other idle adjustment after the change in pressure.

Offline tayto

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Re: Choke pull off
« Reply #5 on: May 18, 2020, 06:18:58 AM »
just curious what model regulator you are running?

Offline dlphil10

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Re: Choke pull off
« Reply #6 on: May 18, 2020, 07:15:14 AM »
Im running a Holley 12-803 regulator.

Offline dlphil10

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Re: Choke pull off
« Reply #7 on: May 30, 2020, 12:35:13 PM »
Found my old 2 barrel carb and checked its choke pull off. It starts to release at 5inHg, but was super quick. I used a piece of plastic to make a restrictor like you have suggested Cliff. I got it to 1.25seconds. I know thats pretty quick, but it's just about right with 1 turn on the secondary air valve spring.

Since the pull off releases at 5inHg now instead of 1inHg, secondaires are actually useful under heavy loads like pulling hills. Is there a vacuum value that you look for the pull off to start releasing?

Offline Cliff Ruggles

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Re: Choke pull off
« Reply #8 on: May 31, 2020, 03:06:50 AM »
Not really.  I look at the release time using a mighty vac pump.  It needs to be pretty quick for best full throttle performance, but not instant.  A good starting point is about 1.5 to 2 seconds to fully release........Cliff

Offline dlphil10

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Re: Choke pull off
« Reply #9 on: June 30, 2020, 05:33:19 PM »
I ordered another pull off and got it to 2.25 seconds. This combined with 3/4 turn on the air valve spring provides the best results.

I have 2 targets now. A very slight off idle stumble that I've helped by using stiffer power piston springs. I'm trying to locate a stiffer spring that starts to lift at 12inHg, or maybe 13inHg. I figured that would imitate the APT system with later model carbs.

The second is my main concern. It's a few faint pulses in the exhaust at 1800rpm after going WOT from a stop. It happens a little while after going WOT, but then it clears right up at 2000rpm and is smooth until 5200rpm. Could I need larger jets (currently using 72s)?


Offline Cliff Ruggles

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Re: Choke pull off
« Reply #10 on: July 02, 2020, 04:39:37 AM »
The plastic pull-offs sold here are adjustable thru the inlet.  All the ones being sold currently are instant opening.  I modify each one so they can be adjusted per application......