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Old 12-17-2012, 09:02 AM
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Hesitation under load

Hey guys, I am currently working on a 1969 GTO and have run into some hesitation issues. It idles and revs fine in park but under load it hesitates. If I bearly give it gas is goes but if I punch it at any speed it hesitates more and more. I've installed a new Edelbrock 600 Electric Choke (had the same problem with a 750 Holley), new fuel lines and filter from the pump up. I am stuck and dont know which way to chase this problem...thanks.

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Old 12-17-2012, 10:55 AM
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you have already checked the timing and advance curve? fuel pressure/volume?The plugs and wires are good? Cap is clean?
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Old 12-17-2012, 11:50 AM
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What gear ratio you are running? What is the stall on the converter? Which 750 Holley you had before. DP or VS? How big is the engine? How do you have distributor set up? Is it full manifold vacuum or ported vacuum? Where did you set the timing?

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Old 12-17-2012, 12:32 PM
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Quote:
Originally Posted by 1969 GOAT View Post
Hey guys, I am currently working on a 1969 GTO and have run into some hesitation issues. It idles and revs fine in park but under load it hesitates. If I bearly give it gas is goes but if I punch it at any speed it hesitates more and more. I've installed a new Edelbrock 600 Electric Choke (had the same problem with a 750 Holley), new fuel lines and filter from the pump up. I am stuck and dont know which way to chase this problem...thanks.
I'd be willing to bet that your problem is a lack of pump shot and/or the secondaries are opening too quickly. If that is the case, it's an easy fix.

Bill

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Old 12-17-2012, 01:23 PM
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Fuel pump is dieing!!
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Old 12-17-2012, 08:36 PM
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I had a similar problem once, years ago before electronic ignition. My car ran fine on level ground but would stumble going uphill. The cause was a wire inside the distributor that would ground when the advance moved it around just right.
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Old 12-18-2012, 06:58 AM
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I appreciate all the feedback, I have adjusted the carb and reset the timing. Cap is new as well as the plugs and wires. Ran a little better but still hesitated and only when I punch it does it realy hesitate. As long as I just keep it cruising its fine but I try and burn rubber it starts coughing. I am thinking my next challenge is replacing the fuel pump and maybe even the relay assembly. Maybe then an electric fuel pump? It feels like maybe the fuel is not getting there fast enough, or enough of it. Thanks again.
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Old 12-18-2012, 12:21 PM
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Quote:
Originally Posted by 1969 GOAT View Post
It feels like maybe the fuel is not getting there fast enough, or enough of it.
Go back and read hcomptons' link. Especially the "The Idle, Transition and Accelerator Pump Circuits" and "Power Enrichment System" parts.

Bill

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Old 12-18-2012, 12:43 PM
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Did you adjust the linkage at the front of the carb? Which adjustment hole is it in right now?
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Old 12-18-2012, 12:44 PM
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Quote:
Originally Posted by 1969 GOAT View Post
I appreciate all the feedback, I have adjusted the carb and reset the timing. Cap is new as well as the plugs and wires. Ran a little better but still hesitated and only when I punch it does it realy hesitate. As long as I just keep it cruising its fine but I try and burn rubber it starts coughing. I am thinking my next challenge is replacing the fuel pump and maybe even the relay assembly. Maybe then an electric fuel pump? It feels like maybe the fuel is not getting there fast enough, or enough of it. Thanks again.
I suggest the Thunder Series Carburator. They have a secondary air valve that can be adjusted.
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Old 12-19-2012, 06:59 AM
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Carb is all tuned and dialed in. Very good reaction when I hit the gas. The hesitatioin comes more at higher RPMs as I took for a test drive last night. Had to baby the gas on the freeway and when I got off, stopped and went to accelerate the hesitation was very bad. Just seems as thought gas is not getting to the carb fast enough. I am going to install a new fuel pump later today. I'll keep you posted. Thanks again for all the feedback.
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Old 12-20-2012, 12:59 PM
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Just had the same problem with a QFT 750. Upped the squirter size from a 35 to a 40. No more hesitation. Don't know if you can do that with an edlebrock but you can with the holley.
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Old 12-20-2012, 10:29 PM
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.. 600 Carb.s usually come default setup for a 302"/305" or base level 350", hard to adjust to a GTO engine... the 750 should have been pretty good out of the box, though, but a little lean...

.. Same problem with two known good carb.s kinda points to fuel pump and/or ignition advance problems... can you check the fuel level in that Eddy carb.? I know you can easily on the Holley 750...

.. Don't just check idle ignition timing, rev the engine up slowly and watch what the timing advance does... it may go to 50 degrees BTDC or more under no load/ light load cruising with vacuum advance hooked up, but hopefully only up to about 36-38 degrees under load or with ported vacuum disconnected... if points ignition, dwell has to be correct... Centrifugal advance weights under rotor can rust/freeze up and not move, bushing/bearing in weights can wear out and let weights flop around, bushing can be too tight fit in a brand new distributor preventing weight movement. The weight springs can break.
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