I think your timings are pretty close.
You say it comes on hard at 1500, so I'm assuming you have a low-stall TC in that 904. And since you called it a 904, I'm also guessing it has the 2.45 low-gear. Taken together that makes 3.73x2.45=a 9.14 starter gear. With the lo-stall, and a 273 Commando, you don't have a bunch of take-off torque, so there will not be a lot of TM in the convertor.
To get moving without a bog, the secondaries will need to stay closed a bit longer. Longer than what? Longer than you might want them to.
Also, the accelerator pump will need to overcome the sag, but note that the richness from this pump should be seen on the A/F gauge as fleeting, not remaining for more than a second or so.
Now, getting to my pet, the T-port sync.
The more idle timing you run, the faster the idle speed goes. So what do you do: you close the curb-idle screw.But this reduces the T-port flow, which is your low-speed fuel delivery system. So, in compensation, you crank open the mixture screws,right. Then, your vacuum gauge is telling you the idle vacuum went up, so your thinking becomes "that worked, so I'll try more idle-timing". So then you crank it up to 20*. And the idle speed again rises, and again you back out the speed screw, and increase the mixture screw opening. This works just fine at idle, cuz you are trading Transfer delivery for idle-port delivery.
But think about what is happening.
The Transfer slot gets to be nearly closed. It becomes easier for the engine to pull air around the throttle blade thru the transfer port. You know,maybe 98% of the slot is above the blade. So then the engine pulls air thru the slot, and it pretty much dries up. But no worries the mixture screws are doing the job, right.
But now, along comes DR and steps on the gas. Well,right off; the transfers have to wake up.During the wake up phase, you just increase the pump-shot to overcome the sag, which you call a bog. Then the transfers come alive. Suddenly there is lots of fuel there and POW! she takes off!.
But now while cruising, at low-rpm, The transfers are supposed to be the go-to circuit. But your idle discharge ports are on steroids and chugging fuel all-the-time, and so your A/F gauge reads that .
Of course I have exaggerated for literary effect. And you may not be having this scenario; but it is something to consider.
If your mixture screws are too-far from stock, it may be pointing at this.
The cure for my 367, was a two-stage advance curve. It starts very early, allowing me to run a very low idle-timing, to get the T-port synced in a good place. But then it comes to nearly a full-stop at 2800rpm. From there-on the timing slows right down to be all in at about 3400.
With this curve, I can run 87E10 at full-timing with aluminum heads and a
Scr of 10.9. I have a stick-car and they are, Or at least mine was, very sensitive to,too much low-speed timing, with street-gears.
If you had a hi-stall,say a 2400 or better, You might not be having this problem.
I call it a tip-in sag.
The usual cure in my experience, has been to screw in the curb idle screw for more transfer-port fuel, to reduce the idle-timing to get the idle-speed down, and then to tweak the mixture screws closer to stock. After the tip-in sag is normalized, I can tweak the accelerator pump leaner or later or both. Then I attack the power-timing curve, finding out where the detonation limits are at about 400 rpm intervals. Then I mod the dizzy to make it all happen.
Then I gets me a 20/22* Vcan, and I try to make that baby work.
If you do it right,you can get the VA to come in and stay, down to about 10 or even 8 inches of spark-port vacuum. This is a already at a pretty good amount of power delivery on the mainjets but not yet on the power valve.
Say you had to reset the Idle-timing to 8* to get the T-port synced up. And say all of the 22* in the can could be made to all be in at 10 inches. Effectively, your engine would be seeing 8 plus 22 =30 degrees of Part-Throttle timing; on the mains and accelerating briskly! Then as the rpm increases, the mechanical timing increases and at some point you may be running up to 36+22=58* of PT cruise timing.That would be pretty sweet.
I call this at about 3600 in first and doing 30 mph. It could happen again at 3600 in second or 50 mph. By third gear maybe not.Remember this is PT timing. It will be your job to determine how soon you can bring in the VA;but this may also reduce the amount. If you have to limit it to too much,sometimes it is better to swap out that 22* can for a 16* or something else.
If this PT timing increases your engine efficiency from stall to say 2800, say getting you 15 or 20 ftlbs, you can run a smaller throttle opening to drive normally, and decrease your in-traffic fuel useage. 20 ftlbs at 2800 would be about 11 horsepower@40 mph in second gear. More power on less fuel!
I know, another novel,sorry
