383 hesitation at mid range rpms

[DP78]

I have searched the threads on here for awhile and cannot find anything that helps me with my problem. The car is a 1970 dodge super bee 383 4 speed and a stock rebuild. The car runs good, but at about 2800 to 3500 rpm it hesitates. Sometimes it pulls all the way through the rpms, and sometimes it feels like the car is turning the rpms and not really pulling like it should, with hesitation. Like the engines turning but the car is not moving down the road and suddenly( couple of seconds) it throws you back like a burst of energy, but not always. It idles great and runs good aside from this hesitation problem. Here is what I have done so far to check the issue. Changed carb and still did it. Checked timing, replaced dizzy, mopar orange box, ballast resistor and reset timing, it still does it, but not as bad. Checked coil and its good. I am starting tothink its a fuel issue, but not sure why it always does it around the same rpms. The carb on it now is a holley street 670 and I also tried an eddy 750. Intake is Dp4b that I put on and checked and replaced the vacuum hose. I also replaced the fuel filter. I am running out of ideas to what it could be, I am going to get a fuel pressure gauge to check that next, also only one fuel line from tank to fuel pump, no return line and the return line spot at tank is blocked off. Vents tubes are in place and look good. Any help or ideas would be great. Thanks again

 

[texas69bee]

sometimes it feels like the car is turning the rpms and not really pulling like it should. Could it be a clutch issue???

 

[DP78]

Thought about that but it does not seem like a slipping clutch and no clutch smell either, but I am still learning. Still have that hesitation/ pause, which make me think fuel or ignition. I am open to all options and just chipping away at the problem until it is resolved. Thanks

 

[Coyoteduster]

Given everything that you have checked or replaced, it sounds like a fuel supply issue. Mechanical pump?

 

[DP78]

Yes stock mechanical fuel pump.

- - - Updated - - -

Could bad gas cause this? I would think bad gas would cause it to run like crap at all ranges of rpm

 

[Coyoteduster]

I don't think it's gas. You need to check your fuel pressure between the pump and the carb as you increase the RPM's. My guess is that you'll see a pressure drop in the range that you're experiencing trouble.

 

[lewtot184]

Yes stock mechanical fuel pump.

- - - Updated - - -

Could bad gas cause this? I would think bad gas would cause it to run like crap at all ranges of rpm

a stock pump will put out 5psi, if it's perfect. i've seen them hover at the 3psi range and thats only enough pressure to drive on. also, a lot of guys put a filter on the suction side of the pump; thats a big no-no.

 

[DP78]

Yeah filter is on the supply side and I just replaced it. I guess I need to check my pressure and mark the fuel pump of the list if its good. Also I am assuming that it is going to act different pressure wise under load, because I can rev in neutral all day long and its sounds great. As soon as I put it in gear and go down the road at about the 2800 to 3500 rpm range it starts to hesitate. So around 5 psi is where I should be with a stock fuel pump?

 

[lewtot184]

Yeah filter is on the supply side and I just replaced it. I guess I need to check my pressure and mark the fuel pump of the list if its good. Also I am assuming that it is going to act different pressure wise under load, because I can rev in neutral all day long and its sounds great. As soon as I put it in gear and go down the road at about the 2800 to 3500 rpm range it starts to hesitate. So around 5 psi is where I should be with a stock fuel pump?

don't put another filter on the suction side. 5psi on a stock pump would be excellent, for a stock pump, but i doubt it will maintain 5psi. do the pressure test and get back with us.

 

[DP78]

Ok did a pressure test and it was between 4 and 5 psi. Also did a vacuum test and it was between 10-25 cm Hg. So I think my fuel pump is good.

 

[lewtot184]

Ok did a pressure test and it was between 4 and 5 psi. Also did a vacuum test and it was between 10-25 cm Hg. So I think my fuel pump is good.

actually it's not. when the pump takes a heat soak it will be down to 3.5psi, just enough to drive. i do driving tests. i bet a wide open throttle your at zero.

 

[MarPar]

do you have any rubber fuel lines anywhere? could it be getting sucked shut somehow?

 

[DP78]

Yes I have rubber line about 8 in from tank to metal than about a foot before pump than 2 in after pump to metal then about a foot more from metal to carb. All rubber lines are 3/8 and I have checked the tank one and the one before the pump, but I will double check. The pressure gauge is under the hood not sure how I can check that going down the road.

 

[texas69bee]

What about the fuel rod. Is it worn? Should measure 3.2" You will have to pull pump to check.

 

[DP78]

How painful is it to pull the fuel pump?

 

[lewtot184]

those 2 valve stock pumps weren't much 45yrs ago when i was playing with 383's and i'm sure as cheap as stuff is today they haven't improved. going to a better 3 valve pump will cure a lot of problems, but removing the filter in front of the pump should be a big improvement. changing pumps isn't difficult. the only real issue is making sure the push rod is as far up in the block (resting on the heel of the pump lobe), and doesn't drop down while your installing the new pump.

 

[DP78]

Ok so I changed the fuel pump today and my pressure went up to 6.5 to almost 7 psi. I was excited, till I drove the car and it still did the same thing. It hesitated and spit and sputter then started to move down the road at a good speed. Noticed my fuel filter ( which I recently replaced) was only a quarter of the way full. Now it is possible that this little fuel filter that I have is gummed up and needs replaced. Plus I have rubber to metal to rubber to metal fuel connections from the supply side to carb. I am going to try and eliminate all of that tomorrow. I am really stumped on what my issue could be and I am about out of things to replace or check besides internals of the motor. Any other ideas or has someone else had a similar problem?

 

[lewtot184]

is it too late to ask if you looked at the fuel pump push rod during the change? actually, i'm not sure you'd get 6-7psi with a worn out rod. start looking at the fuel line from the pump back to the tank. look for hard cracked rubber, loose connections, and maybe it's time to take the fuel tank pick-up out and see if the socks clogged up. another thought, have you done anything to quicken the opening rate on the secondaries and are the float levels correct?

 

[DP78]

Fuel rod looked good not worn at all. It is a steel line all the way back till about 10 in from tank, then it is rubber which I checked and looks good. Secondaries are vac and have the quick change springs and has the black one in it, which is what it come with. The fuel in the floats is just below half, which is what Holley recommends. It only seems to act up when its under load and I thought about the pick up sock.

 

[mopar 3 B]

Went back the beginning of thread a read again. Did you check the ignition items cold or hot? Almost sound like ignition coil brake down or bad ecu. They will often test OK cold and go to pots when hot.