<center><font size="1" color="#ff0000">LAST EDITED ON Jul-28-99 AT 01:06&nbsp;AM (EST)</font></center>

The throttle response on my '76 4X 390 is still crummy. At low RPM, I had to be very careful to not give it too much throttle. When I do, it will hesitate and backfire.

It has a Holly 600, single pump, vacuum secondaries on a stock dual plane intake.

I had the carburetor rebuilt by a fuel system specialty house and tested on the flow bench. The truck has always had non linear throttle response, but the problem seemed to be gradually getting worse since the carb rebuild.

I rebuilt the top end of the distributor because the mechanical advance mechanism was all gummed up. For a while, I thought that had fixed it, but it didn't.

I've always timed the engine by eye and ear, but the other night I hooked the light up and found that when I goosed the throttle, it was retarding, then advancing. It has the single hose from the carb to the diaphram setup. When I disconnected the vacuum advance line, the timing retarded. I left it dosconnected, capped the carb fitting, retimed the engine and went for a drive. It never ran so well. Smooth response at all RPMs and gobs of power I never knew was there!

I always thought that there was supposed NO vacuum advance at idle, increasing as the throttle opened. What I'm getting is vacuum at idle, no vacuum when I first open the throttle, then a slow build up of vacuum after that.

It's almost as if I'm getting manifold vacuum where there should be carburetor vacuum. Is it possible to rebuild a Holley so this happens? Could backfiring cause this? I'm going off of the fitting on the peice that's sandwiched between the front float bowl and the throttle body.

Is there something wrong with the carb? The carb shop guy said it's supposed to be that way on trucks. All I know is that it runs like crap with the hose hooked up, and runs great with it disconnected.

Reality check, please.

 

Have you tried hooking the vacuum advance line up to manifold vaccuum instead of ported vacuum on the carb? I hooked mine to the vaccuum "tree" to the rear passenger side of the intake manifold, same place my brake booster line is hooked up.
I run my truck that way because it had a similiar hesitation problem. Moving the vaccuum line cured it.

 

I was advised by a mechanic that the Ford engines like to run off the manifold vacuum and not the timed port from the carb. In fact, Edelbrock kind of told me the same thing, they said to try the timed port and if it didn't work right go to the manifold vacuum. Mine runs great!!!

Slik
(351W)

 

Well, i have a 390 with single point dist.
Edlebrock Manifold and Carb.
From My experience..... Set the timing as it should be with the vacuum line disconnected from the distributor AND the engine at the correct Idle speed. connect the vacuum line to the `Ported ` vacuum takeoff at the carburator.
>>>>>>>
I would NOT use direct engine manifold vacuum.
The higher IDLE timing will cause the engine to over heat in traffic and at low speeds.
>>>>>>>>
I have used an Initial Timing of 10 Deg. without problems.....
Trying to Start the engine against the `Manifold vacuum advance` may cause cranking problems especially when the engine is `HOT` and shut off for a short period of time 5-15 min or more/less
Keep in mind that the `Mechanical` advance is working there also and the TOTAL advance may approach 45-55 deg.... at 3,500rpm
ps: check the springs under the `points assembly plate` advance weights may be `Frozen` and not letting you get the mechanical advance you really should have...
Let me know if you can how this comes out....