I Just rebuilt the carb on my 68 Dart, it has the holley 1920 1bbl carb.

Its running great except for when the engine is cold, it stumbles when you first step on it, as if the accelerator pump is not working. Its very hard to get the car moving with out killing the engine.

The choke setup seems to be fine, I can mash the pedal down once, turn the key and it starts, stays at fast idle for a bit them comes down to normal as the engine warms up a little.
The stumbleing problem continues for the first 5 to 10 minutes of driving, then gets better. When warmed up the problem is still there, its just much less noticable. Please note: It had this problem before the rebuild as well.

I took a close look at how the accelerator pump works on this carb. First thing I noticed is that the pump is not directly connected to the carb linkage. There is a plastic lever on the side of the carb, it pulls the accelerator actuator back when you let off the gas. When you step on the gas it basically just "releases" the pump actuator. The spring inside the carb pulls the actuator in and makes the accelerator pump squirt.
But heres the problem, looking down inside the carb, and opening the throttle, all I get is a little "slow" stream of gas, not really a "squirt" of gas.
Can I put a stronger spring inside the carb to make the accelerator pump move quicker and give me a better "squirt" ?
Or has anyone else had this stumbling problem ? What did you do about it ?

I assume your motor has a choke thermostat. If it does then this is a common problem on a cold engine.
Long ago, when the Slant Six Club was still putting out a magazine, there was an article out this. The problem is that on a cold engine, the choke is held in the 'qualified' position by the choke pull-off (vacuum operated) when you accelerate the vacuum signal is lost and the choke thermostat (which is still cold) slams the choke plate shut, stalling the motor.
The solution was provided by the Ford Motor Company in the form of a vacuum switch which will store the vacuum signal for a short time. Ford used this in there air conditioning systems. All this switch is is a 'check valve' which will 'bleed' the vacuum off when the motor is stopped so that the choke plate will close. The switch is still available, however, I'm at work and do not know the part number. The cost is under $10.

If you could provide a part number after you get home that would be super.
If not, maybe you could tell me what year and what cars these were used on so I could go to the parts store and ask for it

Thanks
Scott

No.

I remember that article, and that information is not correct at all. It was wrong when it was written, and it's still wrong 15 years later. Remember, there was no vetting process for technical accuracy in the articles that ran in the Slant-6 News. If someone snapped a few pictures and wrote a few paragraphs, Jack would run it. And, that was back in the '80s when Slant-6ers were only just beginning to get together and swap notes, so the body of knowledge wasn't anywhere near as complete and correct as it is today.


The choke pulloff system already contains dampers so that sudden changes in vacuum do not produce sudden changes in choke pulloff position. You need the choke plate to respond to a drop in vacuum by being "less open", because lower manifold vacuum means greater fuel dropout on the cold manifold walls. Putting a check valve that doesn't belong in the system will only do a halfway job of covering up the real problem, and usually creates new problems in the process.

The engine gasps when cold because of too little fuel, not too much. The choke plate does not "slam shut" and choke the engine to the stalling point. That's just not what's going on.

The original poster has already homed in on the problem: Not enough accelerator pump shot. Even with a properly-adjusted choke, a weak and/or slow pump shot will really mess with cold driveability.

SuperScamp: You're right, the '64 and later Holley 1920 accelerator pump is a spring-driven design. No matter how fast you mash the gas, the accelerator pump spring determines the rate at which the pump diaphragm is actuated. Over the years and miles, the actuator spring takes a set. I've fixed a great many hesitation/stumble problems in 1920s by simply stretching the spring. Remove the metering block, remove the gasket/diaphragm, remove the spring, hold it between two pairs of needle-nose pliers, and stretch it just past the point where it stops "resisting" and begins to yield to your stretching. Reassemble everything and give it another try. (By the book, you should use another new diaphragm/gasket, but if you're careful you can probably get away with one or two re-uses of the one you have now -- check the float bowl gasket carefully, you don't want a leak there!)

Thanks SlantSixDan,
I was pretty sure that my problem had something to do with the weak squirt of gas I was getting from the accelerator pump.
I'll try your idea out tonight and see if that will get it working better. I should be able to reuse the diaphram since its brand new.
As far as the float bowl leaking, its already leaking with the new gasket from the rebuild kit. So i'll try to fix that at the same time.

I'll repost after I try this fix and let y'all know the results

At the risk starting an arguement, I tried the 'fix' that the article mentioned and it worked for me. As I live in the 'north country', cold carbureted engines are (were) a way of life. If anyone is interested the Ford part number is: D9AZ-19A563-A. the article was in the Slant Six News #40 June 1990 page 45.

Yeah, you should get a strong, white squirt of gas that goes "Sst!" from the jet. If it's weak and dribbly, you need more spring tension.
Check the bowl carefully for warpage.

While the carb is apart, be sure to spray thru the acclerator pump passage way to be sure it is completely clear and also be sure the check ball seats firmly. You will not get a strong pump shot if fuel "backflushs" past the check ball.

One other thing to check is timing advance, late timing can cause an off idle flat spot, be sure you initial, vacuum and mechanical advance circuits are all working correctly.
DD

For your leaking float bowl gasket, I believe Napa still sells a neoprine ( ) rubber gasket for the bowl. I have one on my 1920 and has been apart many times over the past 3 or so years....no leaks. I found it in a junk yard carb that had the baffle in the float bowl, so who knows how old it is. Like said, a cold engine will not run its best.

I agree with Dan, this information ain't anywhere near correct. Vacuum drop ain't that strong for causing the choke pull off to let go the choke blade. The choke pull off is weak enough to keep the choke where it should be even under acceleration.

1920's accel pump sometimes stores gunk and crap in the accel pump passages. If spraying thru doesn't solve the problem, you may need to remove 1 lead ball and 1 brass cap (for accesing the accel pump passages). Inside the main body passages you'll find that 1 of them (perpendicular to the venturi, sealed with a lead ball) is the jet. You can ream it a few thousands over .025 and another passage is parallel to the venturi, wich contains one brass counerweigt and one check ball.

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

If you can't find the gasket, make your own - that's what I did. McMaster-Carr Supply Company (www.mcmaster.com) is one source for Buna-N (nitrile) rubber sheets. I believe I got the 1/16" thick medium durometer. It's, like, less than $5 + shipping.

D/W

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If it ain't broke, fix it!

Just curious, what do your plugs look like?

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Well, I took the carb apart last night and "stretched" the spring a little and put it back together.

After driving it this morning and at lunch today, I would say that its "better" but not cured of the problem. Now it I step on the gas slowly it will take off without stumbling or stalling.
But if I hit the throttle to quickly it will stumble and even stall.
These problems do go away when the motors completly warmed up.


As far as what my plugs look like, their about 2-1/2" long, white on one end and threaded on the other !

Just kidding,, I haven't check my plugs yet.

You're on the right track. Did you inspect/clean the accelerator pump system as suggested by Argentina_Slantsixer and Doctor Dodge?

I had already taken it apart to do the spring and put it back together and back on the car - Before I read the posts about cleaning out the accelerator pump passages.

Maybe if I get time this weekend to take it apart again ?