Are you still running the Ford-O-Matic arrangement where the bird starts in second gear? And .... do you usually set your mixture screws for best vacuum? or just use a "turns" setting? My outfit always ran OK with a manual upshift from low - but had the hesitation when it had to start out in second.

The Demon carbs seem to have a good fuel air curve adjustment range of "rich/lean" based on float setting - on a hunch I raised the primary bowl slightly above the center and it made quite a change.

 
 Steve Metzger       Tucson, Arizona

No, mine's not a Fordo, it's the 3-speed manual. The hesitation doesn't happen exactly when you pull out, it comes around a little bit later in the range under the load of acceleration, more pronounced when you're accelerating up a hill. So a different power valve will open up at different vacuum...vacuum is highest at idle...decreases as you accelerate...so if looks like a lean condition, wouldn't I want to go one number higher so you get fuel delivery sooner?

I did set the mix screws for best vacuum back when I first installed the Demon, but found that to be a little lean. I know it's a rich idle now but I can live with that if everything else gets ironed out.

I have had the same hunch regarding the float setting. Right now it's right in the middle of the glass. These floats are real easy to adjust so it's worth a try to elevate it slightly.

This is from a chapter on performance carb tuning from a book Holley Carbs, Manifolds and Fuel Injection:

"...use as an example a car equipped with a camshaft that provides such low manifold vacuum at idle or part throttle that the power valve opens, giving richer mixtures, or perhaps flutters open and closed due to vacuum fluctuations. In this instance, a power valve that opens at a still lower vacuum should be installed...this ensures that the power valve will not open until its added fuel is needed."

In the past I've figured that you take your idle vacuum reading, cut it in half, then deduct 2 more inches for lopey cam that might cause variances in the vacuum reading.

If I recall right, the last time I checked the vacuum, it was 19 or thereabouts at a low idle. That was with a Holley carb. But without taking another reading, I don't think there should be much difference if any, right?

Acceleration hard enough that the secondaries might be opening? You might need a different spring for the secondary diaphragm. The wrong spring can cause just that.


Lawrenceville, GA

No, it occurs before the secondaries kick in.

This morning I tried a couple different secondary springs. I had been running the standard spring, and I went up in one measure of stiffness, and later went down one. You could feel the corresponding change in when the secondaries open, but the hesitation occurs before that and is still there.

I'm running a 6.5 power valve now, and am thinking about changing to a 7.5. Or should I try a 5.5?

You shouldn't feel the secondaries open at all. This leads me to believe that your secondaries have something to do with it. Secondary spring kits are available which have about 7 springs to choose from.



From my Holley instructions:



"Many people have the misconception that opening the secondary throttles sooner will provide increased performance and quicker drag strip times. Others think they must “feel” a kick when the secondaries engage. Still others believe that they should disconnect the vacuum diaphragm and make the secondaries open mechanically.

Before going any further, lets discuss these points in a reverse order. First, if we could make our vacuum operated secondary carburetors perform better by opening the secondaries mechanically, it would be to our advantage to do so since all that vacuum actuating hardware is expensive and requires much time and money to calibrate. Mechanical secondary carburetors all utilize a secondary pump shot to prevent bogging when the secondaries are opened. Secondly, those who “feel” a kick when the secondaries engage are actually feeling a flat spot during initial acceleration because the secondaries have already begun to open and have weakened the fuel delivery signal to the primary boosters. The engine is struggling to increase speed and what they actually feel are the secondary nozzles “crashing in” as the engine finally reaches he speed where it provides the proper fuel delivery signal to primary and secondary venturi. Third, opening the secondaries early causes the situation described above. The secondaries must not open until the engine requires the additional air. This allows torque to increase along the peak torque curve. Performance is compromised less by holding the secondaries closed a little longer than by opening them a little too soon. If the opening rate of the vacuum operated secondaries is properly calibrated there should not be a “kick”, only a smooth increase in power should be felt."


Lawrenceville, GA

On your original setup it sounds as if it is an ignition timing problem. Check the advance curve, throw in a bit of timing during the hesitation portion of the drive.

To add to my previous post, there is a real easy way to eliminate the secondaries as causing the problem, using the paper clip test. You clip a paper clip onto the actuation rod and slide it up against the vacuum chamber. Go for a spin, get the hesitation to occur. If the secondaries open at all, the paper clip will have moved down the rod. If the paper clip hasn't moved, the secondaries didn't open and you know without a doubt they are not the cause.


Lawrenceville, GA

Thanks Charley. I have read that passage too. If the power (probably a better description) that I'm feeling is indeed caused by the secondaries opening early...then I don't know what there's left for me to do, because I've tried all the different secondary springs that came with the carb, across the board.

I was thinking the same thing. Timing is already at 14* BTDC, which should be enough initial ignition for this engine. So after I put everything back together, I went to a stiffer spring inside the distributor, which would "lessen" the timing in the advance curve. As I thought, this made the hesitation worse, and bigger across the rev band. So the issue is at least partially timing-related...which is not news, seeing as just about everything you can mess with is somehow related.

However, with that, I have now also tried all possible spring matchups that Mallory provides (except for single stage curve combinations which they advise to avoid for street use), and have concluded once again that the best combo is the one I had at the beginning of this process...so in essence I'm kinda back to square one.

I think in general what I'm dealing with here is a combination of a somewhat lumpy cam (although I do have good vacuum) coupled with a non-vacuum advance distributor and a pretty big carb which combine to give me some low-end throttle issues.

About the only thing I can think of to try now is the paper clip test that Charley suggests, and then maybe a different power valve. The 6.5 that's on there is pretty standard; given the circumstances surrounding the hesitation, any suggestions as to whether I should go to a 5.5, or a 7.5 ?