<center><font size="1" color="#ff0000">LAST EDITED ON Jul-15-99 AT 11:56&nbsp;PM (EST)</font></center>

I have a '76 F-250 4x4 with a 390 and manual transmission. I'm running a Holly 600 4V, single pump, mechanical secondaries on a stock dual plane intake and a low RPM RV cam. At low RPM, I have to be very careful to not give it too much throttle. When I do, it will hesitate and sometimes backfire.

I had the carburetor rebuilt by a fuel system specialty house and tested on the flow bench. I have tried advancing and retarding the timing with no luck. I also put a new ignition module, coil and wires. There is no smog pump on the motor, but it does have the heat riser and the automatic choke is working. The truck has always had very peaky throttle response, but the problem seems to be gradually getting worse.

I think I should be able to bury the throttle and get reasonably smooth acceleration. Instead, there's nothing below about 2500 RPM, then it really goes.

Any ideas?

 

What size carb? Manual or vacume secondaries? What gear ratio? How tall are your tires?

Sounds like you've got a big double pumper w/ not enough gear.

 

Have you checked your vaccume advance for leaks ?

 

<center><font size="1" color="#ff0000">LAST EDITED ON Jul-15-99 AT 10:13&nbsp;PM (EST)</font></center>

I totally agree with Mike. I have a Holley with mechanical secondaries on my F250. It is just a 600 CFM, but that is still enough to bog down a 410 at low RPM past 3/4 throttle. I will probably swap to a Carter or Edelbrock one of these days so that I can start smoking my tires again. They are mechanical also, however they have a seperate plate above the secondaries that limit the CFM to the engine demand.

 

Thanks. I added the carb specs to the original message. I'm running 4:10 gears with 33" tires. As I said, the problem seems to be getting worse. The carb shop said that 600 CFM was the right size.

I do have a almost new Carter on the shelf that I could dust off and try. I just hope that it's O.K. after sitting unused for so long.

 

Yup. No leaks in the diphram or the hose.

Thanks.

 

Have you checked the accelerator pump? With the engine off and the air cleaner removed, look donw into the carb and open the throttle. You should see two steady streams of fuel squirt from the second you open the throttle. If you don't, it's time to replace the accelerator pump diaphram.

 

That looks like a good combination. Try the Carter, if I remember correctly there are no gaskets below fuel level so you should be ok. The only other thing I can think of would be ignition timing.

 

Thanks to all who replied. I really appreciate all of the tips!

It turned out to be the distributor. The mechanical advance mechanism was all gummed up so it was stuck at full advance most of the time. The vacuum advance plate was also prettty sticky.

I pulled the distributor out and rebuilt the top part cleaning it up and getting everything working smoothly and freely. You really gotta keep track of the orientation of the springs and the armature.

The choke was set way too lean, so I adjusted it almost to the middle. At this point I drove the truck and it was a lot smoother and more driveable. After warming it up, I can bury the throttle at pretty low RPM and get smooth acceleration...not that I would make a habit of it. It's really hard on the motor.

After the test run, I found that the flapper valve for the heat riser wasn't working. The engine was getting hot air up from the headers even after warming up. I don't know what difference that will make since I haven't driven the truck since I took the snorkel assembly off. You can replace just the thermostat by unscrewing it from the inside. Now I just have to find one!