Hesitation on Acceleration

[mdenno]

Looking for help on hesitation problem. I have a 72 with 454 and Rochester Carb which was just rebuilt. I have hesitation / engine cut out on hard acceleration I had the problem before doing a tunup. Put new plugs, wires replaced points and condenser with replacement module. Had carb rebuilt and still have the problem. HAve tried playing with timing, advance weights are free and when reving you can see timing advance. Car runs fine when slowly accelerating. Put your foot to the floor and it dies let up and its OK.

Anyone have any other ideas?

 

[petes74ttop]

Hi mdenno, heres a few things to check:
Blow out the fuel lines.
Make sure fuel pump has proper pressure.
Check for new fuel filter.
Make sure accelerator pump in carb works and is adjusted properly.
Tighten screws in carb.
Check for vacuum leaks at base gasket and around manifold.
EGR valve could be bad.
Egr passages in manifold pluged.
Choke not adjusted properly.
Fuel air ratio not adjusted properly.
Other vacuum leak.
Hope this helps.PG

 

[Space Cowboy]

Checkout your inline stone filters. If they don't have that 'coppery' looking color to them, change them out. I had the exact same problem, and the stone filters were clogged.

 

[wallyknoch]

Give it some more initial lead.

 

[Evolution1980]

Sounds like a fuel delivery problem. Either too much or not enough getting dumped when you open it up.

You said that if you bury the throttle it will hesitate and/or die, as in completely shutdown?

I had hesitation on mine that was completely cleared up by replacing the accelerator pump. My car wasn't getting a good shot when I'd stomp on it and it choke, hesitate, then come back to life.

Was your carb rebuilt to stock specs? I'm guessing you had the carb sent out and then sent back to you to install on your car. ???

 

[Vettehead Mikey]

Sounds like the secondary air door is opening too soon on the carb. Simple fix.

 

[zachh]

Or, you have a flat cam lobe. I have an exhaust lobe going bad on my Trans Am, and it does the same thing as your explained. I would check cam last, chack the cheapest stuff first. Im just addin a point on petes74ttop's list.
zachh

 

[minifridge1138]

I had the same problem.
Actually, in my case it was 3 problems.
1) Gas was too low an octane. Use premium.
2) Timing was too far advanced. I don't know about '72, but '82 is stock for 6* btdc. I'd start at about 6, advance 2* until ping, then retard 1*.
3) Accelerator pump was not set correctly. You can richen the mix or replace the accelerator cam. I richened the mix, but I may have gone too far.

I bet that #3 is your big problem. Once your accelerator pump works, you can advance your timing until it pings under heavy acceleration. And higher octane fuel will allow you to be more advanced before it pings.

Good luck, and I hope that helps.

 

[jims427400]

I too had the same problem on the tripowers after rebuild and it turned out to be the secondary linkages were opening too soon. It was an easy fix once I figured out the problem..

 

[Evolution1980]

Well, so far, mdenno, it sounds like you've got a couple people thinking it might be your accelerator pump and another group thinking that your secondaries might be opening too soon. I'd say those would be two good places to start!

 

[mdenno]

It was doing this before I decided to try a tune up Put new fuel filters in, had the carb rebuilt, new plugs, cap, rotor plugwires, have been playing with timing for a couple of weeks and can't seem to get it to go away. Thinking I should try new fuel pump next?? Previous Post indicates secondaries might open too soon? How do I figure that out and correct it? Its a rochester Carb

 

[Vettehead Mikey]

Previous Post indicates secondaries might open too soon? How do I figure that out and correct it? Its a rochester Carb

Here is an extract from an article written by Lars Grimsrud:

".............you want to get the secondary (air valve) opening rate set up. This is determined by the spring windup.

It is a very common “speed trick” to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.

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."

(from this starting point)............"you can start unwinding the secondary airvalve spring. Relax the spring tension in 1/8 turn increments until the car stumbles on acceleration, then tighten up 1/8 turn again. You have now determined the quickest secondary opening rate that your engine can handle, and your secondary mixture is set."