Could it be an over-rich condition when the accellerator pump duration overlaps the main circuit activation, which may happen to be at 3,600?

I've had issues with a bog/flat spot footbraking and been working on shortening the pump duration. Don't have an A/F meter yet.

Bringing the rpm up to 2,200-2,500 may start the main circuit, but it also reduces the volume and duration of pump shot delivery over what you get from idle.

A rich transition would do that, but I cant believe he used the same carb from gas to e85....on gas it would puff a cloud of black smoke during the occurrence....kinda easy to see on a pipe rack and freaky odds that two carbs would have the same problem.

I think it may be a common problem which could exist in both carbs. It's sometimes difficult to convince yourself that "less" is better as opposed to "bigger is better". It's not always obvious.

I recently replaced a well used engine with one having basically the same parts/combo. That changed the carb tune-up to the point that I originally thought I screwed something up in the build! Must have been the ring seal.

But as they say...."Leave no stone unturned." If expected fixes don't work, you have to start with the unexpected ones.

Does over-rich E85 cause black smoke as bad as gas?

If adding timing helps, it's going lean at that rpm. Could try smaller hi speed air bleeds.

TDC WAS accurate upon assembly. After I left for DC my son and a friend had to change balancers(from Chevy to Mopar front) so possibly there was an issue there I was not here so not sure. It happened on both gas and E85 with 2 different carbs. I have been overly cautious with the timing but cannot and do not believe it is actually at 48, am assuming that something has happened there. BUt the plugs are showing it wants more timing still. Definitely has me baffled for sure. Oh yeah did it with 2 different E85 carbs as well, so pretty sure it is NOT a carb issue.

Total w.a.g. on my part, but if it isn't a carb or ignition issue, I'd be suspecting the intake runner volume in conjunction with the small(er) engine size and moderate compression has a reversion issue at that low RPM. The slower burn rate of E85 vs gas would make it worse, too.

I've had that issue with e85 a couple times. One time it was cured by running 180 water temp. The lower the compression the more temp it wanted.

Chased another on e85 and wound up having someone wack the throttle while using 2 spray bottles with e85 in them, and shooting extra fuel in at the same time. Came right up, so we were already at max on the pumps, so fattened up the idle circuit a bunch and went small on the bleed too. Like just a dart was saying

I've been using vp C85 exclusively for a while now, and I'll never go back. Runs like gas, great cooling, and zero corrosion. Makes way better power than pump too. Straight up on the break in everything so far

Fun stuff Al.
If I could not use a spark plug type piston stop to verify TDC, and the plugs are a reasonable heat range.
I would keep putting timing in it until it quit improving or it slowed down and check the plugs to verify.

But I have also been known to color outside the lines.

If you don't have time to verify TDC.. add two degrees at a time until comes up on the brake cleanly. With the two step out of the equation equation I don't think you'll get 85 to detonate with that engine combo just my two cents.good luck 🚦👍

Happy TG ALL. Here's my 2 cents...You have a small motor with "big" heads and low compression (as you admit, it's using pieced together parts) I'll bet the biggest contributor is a dip in the torq curve not allowing the converter to drive thru to it's highest stall at the line. I think it's something a camshaft change could fix, might help advancing the current one. Among other 'minor' issues contributing to the problem that have already been discussed.

It's the converter/s, my I like the idea of no 2 step, go as high as the converter will go. My small block Chevy buddies do that all the time to get them to "go".

Sounds like you have narrowed it down to an ignition problem. First, I would drill a 3/8" hole in the cap inline from plug 1 to coil lead. Use a timing light to verify rotor phasing. I suspect it is off and spark is jumping to the next cylinder in line. If it does show OK, an easy patch would be wire up a timing retard to kick in just above launch rpm and crank up your base timing to get it up on the chip. To be safe, pull out the same amount to the retard module, that you are adding to base timing.

Al,
It sounds like the same problem I had with the 572 predator when I had a converter that was too tight for the head flow. Several things would help it besides that, but it would be cam, compression, can timing etc. I think you need a loose 8" converter. Also, put a piston stop in #1 and find TDC. If you are really running 48*, you already know that can't be good. Where does it dead stall? Is it lazy there too?

i am not sure if it will run 5.2s . my 499 only went 5.0s at the best.

hope you get it fiqured out.

I figure it's the carb

Do you have a compression gauge with you? Might be interesting to pull the plugs and spin it over and see what the cranking compression is. My guess is that it is super low.

I was going to say fat mid range, but you have tried two carbs. I had a fuel injector that was super fat in the mid throttle settings and caused similar grief. Will it flash when not fully up to temp? I am thinking of a test when it is as cold as you can without hurting parts. That might give you some info. You could also go way tight on plug gap for a test, to see if the ignition is going away (.020?)

Electrical

Got an A/F gauge on it? That could eliminate a fuel issue.

x2. If it isn't the carb (worth putting a wideband on it for a pass just to check), my next guess is just a combination of E85, the altitude, 11:1 CR, aluminum heads and a roller cam. On E85 I would be surprised if you could get it to detonate at all. Might be worth advancing the cam a couple degrees to get cylinder pressure up and pick up midrange torque to pull through the converter more cleanly.