Accelerator pump tuning

[Gunner]

Originally Posted by elmariachi

How far out are the mixture screws at present?

I haven't touched them since backing them out a quarter. I think that puts them at around 1-1/8.

[elmariachi]

Originally Posted by Gunner

I haven't touched them since backing them out a quarter. I think that puts them at around 1-1/8.

In my opinion, that is nowhere near enough. If it were me I'd:

1. Remove the carb and reconfirm the position of the transition slots. On a Demon, if you have to even slightly tinker with those to get your car to idle, you will introduce a bog or off-idle stumble. Use the Idle-Eze to get the idle right (as much as 2 turns out) and as a last resort use the secondary idle screw if needed, but don't touch the primary.

2. Roll the mixture screws out to 1.75 and go drive it. With the leaner primary jets you should be able to start at 1.75 and then begin backing down closer to 1.5 as a final. Forget the itty-bitty mixture adjustments, roll them way out and see if it makes any difference.

Have you replaced the plugs in this car since you started? Do they look like mine below or do they have a black soot ring on the thread base?

[Gunner]

The morning drive showed more improvement in the bog than anything else - the more aggressive pump cam. I didn't get much secondary action but it's all relatively low-speed surface streets around me and I could only boot it once or twice without attracting the wrong attention.

I set the butterflies to the recommended positions before installing the carb - square slot on the primary, just-closed on the secondary. From above, it doesn't look as if they've shifted or changed any.

I'll try the richer initial setting but all indications are that it's running too rich across the board already. Plugs vary from clean-black to slightly sooty.

These are all on *very* short test drives, all I can manage at the moment. A freeway run takes over twenty minutes, time I can't spare much right now. I'm trying to tinker a bit at a time, which is not the most effective or efficient method.

The pump cam (blue, switched from stock pink) has had the most positive effect of all the changes. It's not perfect, but it reduced the bog when cold and seemed to have eliminated it on the homeward (warmed up) leg.

And, FWIW, I finally found the calculation for power valve choice. Most references are as irritatingly vague as everything else: "Go up/down a few sizes on the jets" "try a different pump cam" "go to a different power valve." According to the BG manual, I should be running about a 6 - meaning 5.5 or 6.5. It's got the stock 6.5 in it, so it's not something I'm likely to change out except in final dyno tuning.

Fine, fine, I'll go spin the idles out for my next test drive.

[jhv48]

You'll smell it immediately if you open the idles that far. Probably way too rich.

My last Speed Demon ran best about 3/4 turn out, contrary to what the guide said. And I had the t-slots square on the primaries and the secondaries were barely exposed. Idle eze was completely closed also.

It's what runs best, not what the gauges say that counts. Let your plugs, nose and seat of the pants to be your guide!