I'm working on my '72 base 350 automatic with only a mild cam, flat tops, a light "port" (really more of a clean up in the bowl area) job on stock heads, and new fuel pump. I don't know what kind of rods and jets I have in the carb. It was purchased as a rebuilt by Holley. The number off the carb is 17051923 if you need it. It was originally off a '71 Chevy 350 truck as a service replacement.

The car is fine at idle, and great stepping out of idle. No flat spot at all. Where I do run into a problem is around 2000 rpm or so, when I have my foot only partly into the pedal. As thought I'm cruising, and then I want to accellerate kind of briskly, but not really hard. During the cruising mode, the motor is fine. But when I want to accellerate, again, only briskly but not hard, this is where the car hesitates as though it isn't getting enough gas.

At this point, there are three ways to get out of it:

1) Back off on the gas. The motor smooths out and accellerates slightly.
2) Put it to the floor. It causes the tranny to downshift, the secondaries to open, the RPM's to rise, and the car moves on out just fine with no hesitation.
3) Just hold the thing at part throttle until the RPM's eventually rise, albeit hesitantly, in spite of the hesitation. Once they get up a ways, say another 1K or so, the car smooths out and runs fine.

So it's only at that part throttle condition when transitioning from cruise mode to accellerating where I'm having a problem.

Cruise = fine
Accellerate out of cruise under part throttle condition = not so fine.

Any suggestions?


[Modified by Wendell Allen, 10:22 AM 12/21/2001]

 

Accelerator pump not dumping enough fuel?

 

If it were going to hesitate because of that, I would think it would do that right off the line too. But I don't have any problem flooring the thing from a dead stop. And again, if I'm cruising and mash it to the floor, it moves on out fine.

Besides, under the part throttle conditions, I wouldn't think it would be as much of a concern.

But I gotta admit that I can't rule anything out.

 

My guess is that the jetting is putting you a little lean at cruise. Plus, the spring that controls the primaries may be a tad too stiff.

I can look up the carb when I get home but I would suggest getting Lars primer on Q-jet tuning from http://www.corvettefaq.com and start with that.

 

Sounds like your secondary airvalve is cracking open a little quickly. This will cause a momentary lean-out as you transition due to low airflow velocity through the cracked secondaries. It is common for rebuilders to do a "performance mod" of making the airvalve poip open very quickly, and this causes the symptoms you describe. To fix it, proceed as follows:

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.

Now, you need to adjust the secondary rod hanger height. You've read all about the different letter numbers for the secondary hangers, and how a "Y" hanger will make your car faster than an "M" hanger or whatever. Fact is, you can bend and adjust any hanger to any hanger height you want, so it doesn't make a heck of a lot of difference what hanger you choose to use. Just get it set up right:

With the secondary airvalve held wide open and the secondary rods pulled all the way up, measure the distance from the top of the rear wall of the choke horn to the secondary rod hanger hole in the hanger. This distance should be 41/64". Bend the hanger to adjust - you have to adjust each of the two sides independently. You now have a "performance" rod hanger.

 

Let me chime in on this one. I had a 74 that I put a Holley rebuilt Q-jet on it... It had poor throttle response untill I really got on it. Then it was OK. Light throttle response was terrible. After a few attempts at adjustiing I decided to go throught the carb. Here's what I found when I opend it up.. Simply AMAZING... the pin on the bottom of the power piston had been CLIPPED OFF. I replaced it with a piston I had lying around, Jacked up the part throttle ramp.( there is a plug between the mixture screws, remove this plug and you will see a screw inside the hole. This initializes the power piston.) After that it ran great... I could not belive Holley did that but they did..


[Modified by silvervetteman, 3:44 PM 12/21/2001]


[Modified by silvervetteman, 4:23 PM 12/21/2001]

 

I think its the accelerator pump. You can try moving the linkage to the short throw to see if it clears it up. It may be out of adjustment, there is a specified distance for the plunger to lever arm. You can reduce this distance.
Because it works at a full mash doesnt' mean its working for a partial mash.
The pump itself could be bad and this is why it works on a full stroke but a small movement doesn't. A slow movement may be allowing the cup to not seal while a fast movement opens it to the walls and sealing it.
The airvalve should behave the same as flooring it since its only part throttle I think this would be a " secondary" thing to check. These shouldn't be opening under a slight power increase, well below what the primaries can deliver. You have to make that determination.

A simple way to isolate the problem is to tie the choke lock out tang so it blocks the airvalve when the engine is warm ( this prevents the valve from opening while the engine is choked). A twist tie or something to keep it out. If the hesitation goes away its the airvalve. If it doesn't its the accelerator pump.

I would modify Lars method only slightly. If your airvalve is out of adjustment it is just about there now. tighten the spring by the tinyiest amount, less that a 1/16 of a turn. and try it. The proper adjustment method Lars mentioned is getting it adjusted to the fine tuning point. If your already close to the proper setting ( I think you are) you should adjust it from this point. This is the starting point for further adjustments. By going back to zero you will still have to make several adjustments to get it right. This is the course adjustment.
If you haven't done this adjustment before- Handling two tiny tools in a slightly difficult location at the same time. You might loosen the locking screw before your ready for the tension to hit the screwdriver, Then you've lost the starting point. If this happens then follow Lars directions

This adjustment is a drive till you get it right thing. If it bogs the spring tension is too light allowing the airvalve to open with less air pressure on it.(opens too early)
If the tension is too high the airvalve is opening later than you can get away with so the car accellerates less than optimum. (opens too late)

The Secondaries have their own accelerator pump but it is a resovoir of fuel drawn through a couple of those holes you see.

 

the seconary "accelerator pump" holes are fed by tubes that are pressed into the carb top and these tubes can fall out of the top down into the carb body and you no longer have the secondary "accelerator pump" function causing a flat spot when the air flaps open. also on the pri side GM has a spring that goes into the pump check valve to make sure it seal better so there is no leak down of the accelerator pump circut causing a lag in the pump action.


[Modified by clem zahrobsky, 10:02 AM 12/22/2001]

 

I forgot to mention. I had an accelerator pump problem once. Took it out and there wasn't one. The pump cup had disintegrated. I don't know why, it may have been an additive but I didn't remember using one. The bottom of the pump well had a black scum gas in it from the disolved rubber/plastic.
It was 1 year old.