I am trying to figure out this "stumble" at the first 1/4 inch of the throttle. I pulled the carb off and noticed the accelerator pump is squirting before the throttle plates are opening. It is flooding the carb at that very first roll onto the throttle. I lowered the accelerator linkage to the 2nd lowest on the throttle linkage and the internal hole on the pump arm. It seems to have helped some, but I am still getting this stumble.

My questions is should the throttle plates be opening before or at the same time as the accelerator pump squirts?

A few people have said it could be a vacuum leak. I can't find it. I have tried the wd-40 trick and sprayed that all around the carb and intake and cannot hear an increase in rpm. I did hook up a vacuum gauge and it is reading 0 at idle, then 19-20 solid when I raise the rpms up to about 1500. Does anyone know if it should have vacuum at idle?, if so then I definitely have a leak somewhere.

 

The accelerator pump should squirt as the throttle plates open. Don't know how it would pump before then as it's the action of then throttle shaft that causes the pump plunger or diaphragm to push the fuel.

If you mean you see fuel dribbling from the pump shooters in the air horn before opening the throttle at all, that's a different deal. You then likely have a leak in the accelerator pump circuit that is drawing fuel into the circuit and that's what you see dripping .... and then when you do step on the gas pedal, that same leak diminishes the fuel that is delivered to the shooter from the accelerator pump. Very likely a missing accelerator pump check ball. The ball fits against a opening on the supply side of the accelerator pump is forced to close when pressure is built in the pump by throttle action, squirting fuel through the circuit to shooters. When you let up on the throttle, a vacuum builds in the same area and the ball relaxes and allows new gas to be pulled in from the fuel float bowl. Some carbs use a spring under the ball, nost just have the ball captured in place by a clip.

That vacuum port that has zero vacuum at idle and then has some just after you start opening the throttle is Ported Vacuum .... it's for the distributor vacuum advance usually.

It would help to know what carburetor you are talking about.

 

Use the vacuum gauge to set up the carb, dig around the net and you'll find the manual for that carb. It's important that the idle mixture screws are dialed in correctly. Follow the steps in order.

Vacuum Gauge needs to be connected to a source of constant manifold vacuum i.e. that is "always on". Sounds like had it connected to the distributor port. Set the idle at factory spec or no more than 550 to 600 or so and see what vacuum it will pull. A stock healthy motor should be _steady_ needle of 18" to 20" at sea level, less as elevation increases, but it has to be checked at normal idle. 1500 is "cheating" afaik.

A bog interestingly can be either too lean, or too rich accelerator pump shot. It should shoot fuel instantly, if not sooner. Somebody following might notice a puff of smoke if too rich. The Autolites had the pump rod adjustment settings dependent on temperature. The top hole is for below 40F, 2nd from top 40 to 80, 3rd above 80. The other end should be on the inboard hole probably.

This is where the Shop manual is good to have, it will have all the settings for your truck. The carbs were setup for all kinds of different applications depending on motor, transmission, gearing and after 50 years anything could have been done to the motor or carb.

It really helps to cover the basics and start from square one. Make sure pump is putting out proper pressure and volume, and check that fuel level in the bowl is correct. It will never run right or tune otherwise.

 

No fuel dribbling, just good solid squirting. It is almost like the accelerator pump is working too well. and flooding the motor while the plates are just barely opening.

It is a motocraft 2100 on a 360bb

 

The plates are just barely open at idle, and to some extent at transition. The pump shot is there everytime the accelerator is pressed, to prevent a bog or hesitation. The initial cracking of the butterflies drops the vacuum, and the fuel will drop out of suspension and cause the bog so extra fuel is required for a split second. The accelerator pump arm is adjustable for temperature, or duration of pump squirt. 2nd hole from the top is for 40F to 80F.

All the manuals are are online, they are simple carbs to setup. Start from the beginning. It's common for buggered idle and idle mixture settings.

 

Check the vacuum advance. It should start advancing the timing when it gets vacuum from the dist. port that you had your gauge connected to.It might be coming in too late or not at all. Make sure the initial timing is correct.

 

I got it figured. Tedster you were right on. Going back to basics was it. One of the idle mixture screws was giving me a false bottom, some gunk or something on the threads. Once I bottomed both out I rolled them back to a consistent smooth idle and now it has not stumble. Looks like it was very rich and the accelerator pump shot was drowning it.

 

Thanks for posting your discovery. There are a lot of posts regarding a stumble, and I'm sure this thread will help somebody