First a bit of recent history, the truck's 340 has...

A new distributor with electronic pickup, timing set at 5* BTDC.
Rebuilt Carter AFB 4b carb, with 6 lbs of fuel pressure. F/A mixture adjusted by ear.
Integrity of all vacuum lines verified and a new PCV valve.
Plugs are clean and recently gapped.

Engine idles great, drives down the road well. Punch it down at say a 45 mph roll and she hesitates, down shifts and takes off pretty good. I'm 'fairly' happy with the way it runs once rolling down the road.

What I am dissatisfied with is the almost inevitable extreme hesitation when punching it down from a dead stop. When it does catchup the acceleration is anything but impressive. It just feels doggy, like it doesn't have that Bang you would expect from this engine.
Kinda like you have to feather the accelerator........

I know I have had cars act like this in the past I just don't know the fix.

Any idea's?

Thank you
Joe.

 

It almost sounds like a restricted exhaust but before I go there is there any popping or backfire during your hesititation?

 

No, I seem to have solved all the backfiring issues with the new distributor and getting the timing set correctly. Minutes ago I just went out and stomped it right to the floor from a dead stop and it hardly made a noise and just died, like you turned the key off. Start it back up, feather the gas and down the road you go......

 

Just a guess here but I'm thinking maybe a exhaust retriction, collasped pipe, ect. Can you hook up a vacuum gage and read manifold vacuum at idle should be around 21 in give or take depending on how tight your engine is. If its around 10 or so I would check the exhaust.

 

Will do, I get a vacuum gauge asap and get it checked.
Thank you.
Joe.

 

I'm looking at the volume, and duration of your accelerator pumpshot. When you crack the throttle fast, the vacuum signal to the carb drops. The truck picks-up when the vacuum reaches a point when the engine has enough vacuum to start pulling from the main jets. The accelerator pump is used to cover this "flat" spot. It provides additional fuel into the carb until the main jets start to flow. Clear as mud?

 

Are you thinking that possibly the linkage to the accelerator pump is out of wack? Or is it possible that the accelerator pump is that providing enough or any fuel at all? I think I'm following you.

Joe.

 

I would start with making sure it was adjusted and operating properly. It should start to move with the first throttle movement. But it also has to be adjusted so that that at idle, it hasnt started to move yet! If it isnt fully seated at idle, that will reduce the amount avaiable! Get me? Chances are, if you have the idle cranked-up to get it to run, you have already used some of your pump-shot adjustment. And you need to re-adjust it from there.

 

I just read the accelerator pump portion of this article and I think we've hit the nail on the head!

When we rebuilt the carb there were two sets of instructions and at times I was unsure which fit our carb the best. Using the terminology from the instruction sheet:

Under the 'Pump Jet Housing' we found the 'Pump Jet Needle' the pump jet needle that came out of the carb was shaped like a rectangle and had no pointed end, the replacement that came with the kit was noticeably longer and did have a tapered tip.

Also under the old 'Pump Jet Needle' we found a beebe at the bottom of the cavity. Now one of the instruction sheets show a beebe which is called the "Pump Intake Ball" but shows it in conjunction with a 'Pump Intake Seat' and 'Pump Intake Plug' all of which appear to be in a separate location from the 'Pump Jet Needle'. The other instruction sheet shows no beebe at all. In the end I put back the original needle and ball where I found them.

Now I am thinking this was a bad move. Although I still have no idea what the correct way would be?

Thanks guys so very much, I think we are on to something.
Joe.

 

Pull the carb off in favor of a MPFI system. Just throwing ideas out there.......

Once you get the timing just how you want it, you can adjust the fuel/air ratio based on the RPMs of the motor. FYI - this may help with this as you could be starving or flooding the motor. Have you pulled a few plugs to see what they look like?

 

Oh heck, a TBI system found here
is only around $1k I'll run out and get one in the morning

Well on second thought, maybe I should just find out where the beebe goes

 

I am assuming the above are done to correct specs and that you are satisified with everything this is why I was thinking exhaust. If you didnt mention anything about the carb I would go for the accelerator pump. If you WOT with the car off and see a nice fuel spray with the secondaries start to open at WOT you should be ok. (just a quick check)

 

Hey Sk, I sure appreciate any idea's. I trouble shoot electrical control problems over the phone every day and I never , never get the whole story the first time around and I guess when it comes to explaining my car problems I am the same way.

I thought the same thing and did shut of the engine and 'listen' for a good squirt and think I was getting a good amount of fuel in there. But I did not get on top and look. I hate not having a garage, when the sun goes down around here I am done for the night

The whole lack of vacuum during fast acceleration, the accelerator pump compensation and the fact I stumbled in this area during the rebuild all just makes to much sense, I think it is worth really digging into.

Thanks again
Joe.

 

your afb(aluminum four barrel) has air valve secondary’s make sure they are opening up at the right time...to soon would cause a hesitaion..also a worn/bad fuel
accelerator pump seal would too.

 

Here is a link to an article I think is going to be a big help!

Joe.