Carb? Major bog / hesitation on stomp

[GT1966]

OK, I've got a 4100 from Pony Carbs 1.08 bore on a 1985 302 block. .030 over, balanced and decked for ~ 9.3 :1 compr. ratio. The cam is flat tap hydraulic 216 Adv. Duration @ .050... (pretty mild / decent idle) and World Products Windsor Jr. heads, and Edel. Performer RPM intake.... when I give it full throttle, it nearly sends me first through the windshield, then it snaps me back a half second later when the motor gets its act together and it pulls awesome! I'm wondering if this is the carb's power valve / accelerator pump issue or is this a timing / spark curve issue? If I feed the fuel to it rather than stomp it, it works great through a solid power band. Per Jon @ Pony, I've got the timing running max vacuum, so what's my next move? I want to install an MSD distributor (presently running Pertronix in an orig. 289 distributor), but I want to close one chapter before introducing another variable. Thoughts?
GT1966

 

[SQUEEZE&STROKE]

It sounds like too much fuel to me...have you tried blocking off the power valve? Or jetting down the secondaries?

 

[85_SS_302_Coupe]

I'm not real familiar with the 4100, but if it's like a 4150 and has an accelerator pump in the secondary fuel bowl, take it off and see if the rubber check valve is in there and if it's still in one piece. I had the exact same symptoms you're describing....it would pull like hell until the secondaries opened and then fall flat on its face and give me whiplash, then pull like hell again. It turned out that the accelerator pump check valve was torn and the diaphram wasn't holding gas.

You can probably go out and just press down on the accelerator pump arm and see if gas squirts out the shooter...if not then that's your problem.

 

[Rick James]

It sounds like a lean stumble more than a rich/bog.

 

[85_SS_302_Coupe]

It sounds like a lean stumble more than a rich/bog.

Exactly, that's why i'm calling it the check valve.

The accelerator pump provides fuel when the carb goes between circuits. When he's mashing the gas, the carb goes from idle/cruise circuits to the main jets, and there's a short period between those where no fuel is being shot out, which is where the accelerator pump comes in. If the accelerator pump isn't squirting, you're going lean for a split second and the car bucks really hard like he's describing (like my car was doing also). Not enough jetting isn't something that's going to cause a violent bucking like this, it'll just stop pulling as hard but it will still pull. A hard bucking/hickup like this is a complete lack of fuel, and that points me to the accelerator pump.

 

[GT1966]

Thanks, people. My hunch is that it is more than just not enough advance dialed in to the ignition timing. I think you are on to something with it being a lean condition.... which makes sense when I feed the gas gradually, I don't experience the "buck". When I get some time this weekend, I'll review the accel. pump operation and see if that's the culprit.
Thanks for the help!
CT

 

[93 LX]

I have the same issue on my Capri. I just need to get larger squirter's but haven't had the time.