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Last weekend I was driving old red and noticed that if I gave her the onions (punched it) it would do its normal thing but after backing off the gas but still accelerating the truck would stumble. I tried it a few more times and it did it every time. If I'm just driving it (normal acceleration, etc.) it has no problem OR when I'm giving it the onions it doesn't appear to miss a beat.
I was thinking gas (vapor lock / filter / fuel pump not keeping up / etc.) so I checked my vapor lock mitigation setup (see thread on that) and it was working but I noticed the filter didn't have much gas in it so I though maybe it was the issue.
Replaced the filter today and drove it around - more fuel in the filter but still had the same stumble. I tried just doing a "above normal" acceleration (not enough to trigger the 4 barrels but close) and noticed that it would start stumbling until I let off the gas. Once I backed off I could either start accelerating normal OR stomp on it and it would response as expected.
It feels like a fuel supply issue but i don't know, transmission kickdown getting in the way, spark issue that isn't present at normal or extreme use scenarios?
Timing - (OEM setup) I need to see what the actual timing mark is ( PO put/marked a 6 on the balancer and that is what I've been working around - just not sure that's 6 degrees or something else) but adjusting it based on engine going from rough to smooth + vacuum reading is where I have it. If I take some timing off it'll roughen up and backfire on cold start. I got a better timing light coming tomorrow and can get you a better answer on this.
Hopefully you have a dial back coming that's what you need.
I would make sure my timing was in the ballpark before I dove into carburation. you really need to know what your mechanical is at 2500-2800 , if you're not in the low to mid 30's you can turn it up or down until it is. then hook the vac advance up and see where you're at. it varies from 50 to 60 but I'm wondering if you're not a bit high there .are you running ported or manifold vacuum ? at moderate throttle you can have too much advance and get the symptoms you describe. at WOT your vac advance drops out and you're ok. depending on where your mechanical is you may have to limit the vac advance.
Never touched one of those carbs I only run standard Holley so I can't help you there.
Got a chance to work on this a little last weekend.
So the "6" was actually the 10BTC mark - PO marked the TDC, -10, -20 and -30 (Once I got a good look at it, made sense - appreciate the PO doing that)
I was originally at -10 and at -11 it seems to be as good as it gets from a balance standpoint. I can still feel the engine start to hesitate a little but it catches up after a few seconds.(Normal acceleration and WOT are still good). If I go to -12 or more it'll hard start but runs smoother and does not stumble...
I'm wondering if this is happening when I'm giving just enough peddle to have the 2bbl wide open but not kicking in the other 2bbl's. I also need to test with the kickdown disconnected to make sure it's not a contributor.
So is your light a dial back ? if so all you need is the TDC mark.
Either way what you need to do is check your curve. we'll assume your marks are correct. don't worry about your initial until you get the mechanical set because that's what makes an engine perform or self destruct.
So with the vac advance unhooked rev the engine to 2600-2800 rpm . what is your reading ? turn the distributor until you get in the low 30's for a starting point. now hook your vac advance up and what do you have at 2800 ? then check your initial and see if it's in the ballpark. until you do this you haven't checked your timing.
That depends on your engine, fuel and altitude. if you're not getting and pinging (detonation ) on hard acceleration you might turn it up a bit until you do then back it off until it stops. this is peak for your engine.
once you have that established check your vac advance with the mechanical all in. you may need to limit it a bit that could be your problem. I usually run a total in the low 50's but I have seen good running engines at 60. the more the better right up to he limit.
You're probably set up to run ported vacuum so be sure that's what you're on.
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