Off Idle Stumble - A Holley Carb Review

[Roy]

I installed a GM crate engine in my 79 Vette a few months ago. The engine came with a 650 CFM Holley 4-barrel carb. During break-in, the engine seemed to run fine. I then had a less restrictive dual exhaust system installed on the vehicle. It was about that time that I noticed an off idle stumble, or flat spot, begin.

At first I began adjusting the easy and basic stuff - timing, carb float levels, fuel/air mixture, vacuum advance ports. When I still experienced a dead flat spot on full throttle from idle or low RPM's, I read technical articles on the Holley carberator.

A dead spot from idle or part throttle to full throttle, especially if the flat spot is accomapnied with a back fire through the carb is an indication of an overly lean fuel/air mixture or insufficient fuel delivery during wide open throttle, or "WOT".

A racing buddy suggested that the "squiters" were too small and not providing enough fuel during the transition from idle/part-throttle to WOT. He also noted that the secondary valves were not opening at any RPM. The Holley carb had .31 squirters so I bought the next size up (.35) and installed them in the carb. The engine bogged terribly so I put the .31 squirters back in.

Next, I installed a "quick change" secondary vacuum valve/spring mechanism. The stock mechanism requires a fair amount of time to disassemble and replace the secondary vacuum springs with springs of different tension and operating ranges. I found the crate engine Holley carb came with the strongest spring available that never allowed the secondaries to fully open. If you want to have some inexpensive fun trying to improve your mid to top end response, I would recommend that you buy this quick change secondary vacuum kit and experiement with the 7 or 8 different springs that you can purchase in a seperate kit. You really can dial in good performance in a matter of a couple of hours - if you have the quick change kit. Well, I ended up with a spring that was two steps lighter and faster responding, but this did nothing to help the dead spot off idle/part throttle.

My next step was to switch the vacuum hose from the distributor to every known vacuum port I could find. I thought that I may have a vacuum or timing problem so I spent hours switching the vacuum hose and taking test drives to measure the change in performance. No improvement in idle/part throttle response.

The next item I attacked was the adjustment bolt on the accelerator arm that presses against the power valve actuating arm on the left front side of the Holley carb. You can adjust the length of the bolt by tigthening or lossening the adjustment nut. I tightended and loosened this darn nut/bolt set-up and sometimes it would help and other times it would be worse, but nothing corrected the backfiring through the carb and the flat spot response.

I then read an article that suggested a 50cc accelerator (power) valve would solve the off throttle flat spot. The kit cost $44 and basically is a larger resevoir of fuel to squirt into your carberator when you transition from idle to running throttle. I hestitated installing this unit as the 30cc volume of the stock acceraltor valve should be sufficient for the crate engine V-8. I decided to review other options.

I read about the different profile plastic cams that Holley carbs use and thought I would experiment with these. I swear Holley makes more money on these various carb tuning kits and parts than they do on the carb itself. After removing the stock orange cam from the throttle linkage and examining it's profile, I discovered that it had two mounting holes marked "1" and "2". By the rub marks left on the cam from the actuator arm on the carb, it appeared that the cam could be reversed to have the steeper and more aggresive side of the cam actuate the arm that in turn presses the accelerator arm that presses the accelerator pump valve that squirts raw fuel into the primaries during the transition phase from idle to throttle. At the same time, I thought I would replace the #65 main jets with something a little less restictive, so I installed a pair of #67's.

One last adjustment I made was to eliminate any gap or slack between the bolt on the throttle actuator arm and the accelerator pump actuator arm. I figured that if there was any "slop" or gap between these two pieces, the throttle would begin it's opening without making contact with the accelerator pump valve actuating arm ... this would cause a delay in delivering more fuel during the initial opening of the primary valves and this would produce a lean mixture, backfire and stumble.

After installing slightly larger main jets, adjusting the throttle cam to a more aggresive slope/timing and eliminating the slop between the throttle actuator arm/bolt and the accelerator valve pump arm ... the engine ran fantastic. NO stumble. No back fire through the carb. No hestitation in acceleration from idle/part throttle to WOT.

If any of you are having trouble with flat spots & backfiring with your Holley carb, you might try the simple adjustments that are easy to do and don't cost much. The main problems I had were an incorrect throttle cam and a poorly adjusted contact between the throttle arm and the acelerator pump (or "power valve")actuator arm on the Holley carb.

I returned the $44 50cc accelartor pump valve. The pair of main jets cost $6.44, but I probably didn't need those. The cam adjustment made the biggest improvement and that was free - just flip it over and use a different mounting hole. And finally, adjusting the actuating arm was very helpful and cost nothing.

So before you give up or spend allot of money chasing Holley carb problems like mine, try the simple and inexpensive adjustments before you invest in expensive upgrades. Hope this helps.

[lbell101]

You're a better man than I.
I have this same problem with a Holley right now on my truck. I've chased every possible thing on and in the motor including a carb rebuild. I officially have given up and will buy a brand spanking new Holley tomorrow. If that doesn't work, I will buy a brand spanking new truck!

Thanks for the info, it helps me believe the problem really is the carb!

Larry

[1979toy]

Gee, you don't have every power valve from 5.5 to 11.5 yet?

[GTR1999]

Good information,thanks for taking the time to write up.

[The73vetteman]

Hi,

Can I ask a related question?

I have a '73 with the original 350 motor, but an Edelbrock manifold and Holley 600 installed by a previous owner. I don't think the choke is operating correctly. I can set it by turning the adjustment block on the side, but the actual choke never seems to move, regardless of how warm the engine is.

By-the-way, I disconnected the electric choke, because that didn't seem to do anything either.

Could anyone give me some information on how the choke should work, and any tips on how to set it correctly?

Thanks in advance,

'73vetteman

[motorman]

how did the "gap" between the accelerator pump arm and the linkage get there? people read the instructions that there should be a .020 gap there think that is at closed throttle but that spec is at wide open throttle so you do not bottom out the pump diaphram and rupture it.