I think what PETE is suggesting here is "try another mech advance distirbuter setup" - NOT BUY and replace. A full mechanical dizzy either borrowed or bought and built (pretty cheaply as a back up) is a very good way to check out that hesitation. ex: you could buy a plain jane dizzy from napa and have it setup for full mechanical; cheap test.

BTW I run full mech. on my old bird in a factory 57 tach drive with pertronics.

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

Pete 55Tbird (12/22/2010)

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A distributor with no vacuum advance may be your problem. Try a factory distributor and see what you get.

Pete, I have often thought of doing that. But unless I pay big bucks for an original from a '57 Bird or something like that, I'd lose my cable-drive tach.

Originally I had a Mallory from the early 1960s and you're right, it was well-worn and pretty erratic. I got this unit new from Concours in Vegas, paid the big bucks...they're specially fit for early Birds and they have the tach drive built in. It's only got a few thousand miles on it.

Yeah, when I bought this distributor it was advertised as "tuned" for TBirds, but for my purposes it was still way off. So I bought the advance curve kit from Mallory and, over time, tried all different spring combos and degree advance tabs. I ended up using 24* of mechanical advance, adjusting the spring perches as well in order to get all the advance in pretty early in the curve. Sure, I do burn a ton of gas. The car really gets up and goes at higher rpms but my wife drives the car too, and most of our driving ends up being cruising miles, which obviously is not the strong point of this unit. I think I'm now at an impasse...not much else I can do unless I go to another distributor.

None of my 3 yblocks have active vac advances and I had each distributor recurved so don't have to chase those gremlins, recurve is only $45. On the tire dyno the engine with the dual 500cfm carbs had a wobble in the power curve with vac advance and the curve smoothed out with it disconnected, considering the over supply intake of this engine I figured that was a good enough test for me. When the dual carb engine is at running temp it has no hesitation or bog, it runs up smoothly instant wide open from 1500 to 5500.



I didn't care to go into it with my carb guy but seem to recall the vac advance was intended to help fuel economy, might give that issue more consideration cause when this engine is being worked it drops fuel like a water bomber. Those mech advance pieces are, by necessity, cheezy little suckers and I make sure they've got correct new springs and are oiled.

Dan in Seattle

I don't know how old your Mallory is, but I have seen older ones where the advance mechanism has worn out to the point that the mechanical advance is very erratic. Sometimes non-existent. Might be an opertune time to rebuild it.

Frank/Rebop

Bristol, In ( by Elkhart) 

joey

When you describe the issue you want to fix you said"

Here's the thing. The engine idles fine at 650-700 rpm and basically runs strong, but under gradual acceleration there is a s-l-i-g-h-t hesitation until the secondaries kick in."

      My understanding is with a carb with vacuum secondaries that the secondaries will NEVER OPEN with gradual acceleration as the manifold vacuum will always stay too high. So can you attach a manifold gage that can be seen from the drivers seat and take it for a spin and confirm that the hesitation occures prior to the point that the manifold vacuum drops enough to cause the secondaries to open. If that is the case then forget the power valve - not involved ( vacuum was too high) and forget the secondaries, same reason

You seem to have two seperate problems, the rich mixture at idle and a hesitation on mild acceleration.

A distributor with no vacuum advance may be your problem. Try a factory distributor and see what you get.

I think it's timing more than anything else. Taking it for another test drive, I pulled over and, what the heck, just advanced the distributor probably another 3-4 degrees, and it made a big difference. Probably I  need to get past the idea that it "should" be timed at the lower degree settings you hear people reference.

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 ?

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

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.

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