So I have had a hesitation problem going on a for a little while now. I thought it was the fuel pump and it probably was partly that (not moving much fuel intermittently). So now I am trying to tune the carb (hoping this is the problem) I have a vacuum gauge attached and a tach. As I try to adjust the mixing screws I am finding that the engine rpm has far more effect than the screws. This would be fine except I do not seem to be able to get a steady RPM. It seems to adjust between 300 and 500 rpms but not in relation to the idle screws. my gauge reads around 15" vacuum at around 400 but the needle isn't steady on the rpm or vacuum. What am I doing wrong.

I should say it is a 239 with a tame holley carb and a bubbatronics mech. dist. also the fuel pump is an inline electric and has a pressure regulator

Frustrated John

 

It might be described as just not full power. Up hill at full pedal in third I am not accellerating.

 

This is going to sound like a catch-all, but make sure you don't have any vacuum leaks. Also, you mentioned that the vacuum isn't steady. If the needle jumps around, but stays within a constant range, that is indicative of badly worn valve guides. Good luck.

 

The vacuum is pretty steady though it isn't much above 15". I do have a mechanical distributor so no vacuum lines go to it.

 

Sounds like the carb butterfly is not completely closed when adjusting. Back the idle screw out until it is not touching then see if mix screw changes rpm/vacuum (screw mix screw all they way in and back out two turns is usually a good starting point). Adjust idle after adjusting mix screw. If mix screw don't make a difference, you likely have an air leak. With engine running spray some ether (starting fluid) aroud carb base and gasket line and see if RPM increase. If so a carb rebuild/fresh gaskets are in order. Sometimes a worn throttle shaft will produce similar symptoms if gaskets don't fix it.

 

If I back off the idle screw all the way it stalls. I lowered it as far as I could (about 300 rpm) and tried the mixing screws. It made very little difference until I had it down to one turn out. I tried spraying starting fluid around the vase of the carb but it made no difference. I did rebuild the carb about a year ago and have not driven much since then (maybe 200 miles total) below is a link to a video of it running. Not as clear as I'd hoped since the electric fan was running but when I rev it slightly you can hear it stumble a little. Under load it amounts to poor acceleration.

 

Idle speed should be 550 - 600; the book will say on the low side of that but 550+ works best day-to-day.

In the video it sounds like you aren't getting a decent squirt out of the accel pump.

 

The carb gaskets rely on being soaked in gas to seal like the cork in a wine bottle, if left sit dry they will often shrink and leak even if new. did the engine run properly right after the carb rebuild?
I agree with Ross about the accelerator pump. Do you get a healthy shot of gas into the carb throat when you open the throttle?

I'll be the first to admit that I am not well versed in stock flatties tho, Ross can give you a lot more help than I can.

 

It was running pretty well after the carb rebuild and has been sitting since march when it started losing power while driving. I attributed that to a faulty fuel pump (the pump was making a hammering sound and investigation revealed that this sound corresponded to extremely low fuel flow). I have heard the fuel squirt from the accelerator pump but have not witnessed it. I will look for that tonight. Any idea what the fuel pressure going into a tame Holley 4 barrel should be. Is it possible the regulator is too restricted?

 

Mechanical fuel pump with carb 5-6#, 7 max. Have you checked fuel flow at carb after changing pump/now?
Other possible issues: float set improperly, plugged/restricted fuel line or filter, sticking float needle.
Just because pump is new don't discount it being faulty, gasahol is notorious for damaging mechanical fuel pump diaphragms, rubber fuel lines.
Is your 4BBL carb properly sized for your engine/cam? Too large carb can cause poor acelleration.

 

Here is a link to the thread where I got the suggested set-up I used (post #3)
https://www.ford-trucks.com/forums/1...mp-set-up.html

I have visually checked the flow and it looked pretty healthy. The carb is pretty tame. From what I can tell it is the lowest cfm carb holley makes in a 4bbl.

I am thinking I'm going to have to take it apart again to find this problem.

 

Have you checked the timing, both static and mechanical total advance (since you don't have vacuum advance)?

 

I am getting a good squirt out of the accelerator pump. I took the plugs out of the bowls and the fuel was low on both sides. When I ran the pump the fuel on the primary came up and our of the hole but the secondary never did. Tried to adjust it but no luck. I am again thinking I have the pressure too low but also possibly something clogged between the primary and secondary.

 

Ok I got the bowls to fill but they were pretty far out of adjustment

 

Do you have an electric pump? Is this a 4-barrel carb?