Hesitation When Accelerating

[patrickt]

Originally Posted by zzmac

If the vacuum secondary spring is seated properly would this more than likely mean it needs a stronger spring (if it's the spring)?

That's one possibility, and that's the easiest answer (might not be the right one, but it's the easiest). There also could be a sticky mechanism involved, so that the vacuum mounts up and "pops" the secondaries open instead of smoothly and gradually opening them. It could also be a clog on the air bleeds over on the secondary side or any other transitioning orifice that's involved. I always instantly suspect a Holley has a clog even if the Holley just came out of the factory box because I've seen so many clogs and they can be caused by pretty small pieces of crap. It's probably not a clog, but I always suspect them nonetheless and clean them out good with the poisonous carb cleaner that works, not the watered down crap they sell in CA. Then I blow out the air bleeds, transition slots, idle mixture holes, all of that with about 30 psi of air from my compressor. Only then do I start thinking that it might be something else. The chart below is the spring rate for Holley vacuum secondaries. If you put the black spring in there, that's pretty much like having your secondaries tied down shut, so I bet the black spring would stop your bog. But you'd only have half a carb again. If you then walk up the chain until you find the spring that opens the secondaries the fastest, but without creating a bog, then that's the winner. BUT, if the problem is something else, like a sticky mechanism, then you're going to have to fix that to get everything to work just right.

[patrickt]

Out of curiosity I pulled the spring specs for your 770 just to see what it originally came with out of the box (knowing that the carb guys have probably monkeyed with it). Of course Holley would list a stock spring color that is different than every other color in its rainbow of choices in the vacuum secondary spring collection.

[jhv48]

Or, you could contact Roush and ask them what spring they put in the carb that they installed on your engine. Then compare it to the one you find. If not the same, you’ve found your problem.

[Grubby]

If the spring is too strong there will NOT be hesitation. The car will simply be low on power.

In a light car like a Cobra you can start with the lightest spring. Then if you have hesitation, work up.

I worked on one Vac Secondary Cobra that the secondaries were not opening on. The car drove fine, but didn't feel that strong. I put the lightest spring in and the secondaries opened without hesitation.

It is a cheap guess.

John

[patrickt]

Originally Posted by Grubby

I worked on one Vac Secondary Cobra that the secondaries were not opening on.

A really free flowing air cleaner also reduces the velocity of the air going through the carb thus reducing the venturi vacuum. That's why on a lot of threads guys bitc* that their new expensive free flowing air cleaner makes their car run worse. See the Note at the bottom of the Secondary Throttle Opening Ranges in my post above.

[patrickt]

Originally Posted by patrickt

It could also be a clog on the air bleeds over on the secondary side or any other transitioning orifice that's involved.

John, are you reading these posts carefully?

[Gaz64]

Originally Posted by Grubby

Everyone is focused on secondaries. It could easily be a transition circuit. A tip in stumble is usually caused by idle air bleeds.

It would have to be WOT or near it to be the secondaries.

Start at the beginning and tune the carb by the book.

John

Originally Posted by patrickt

John, are you reading these posts carefully?

Originally Posted by zzmac

Okay, so I closed down the secondaries and took it for a test drive. No bog.

One step closer. :-)

So seems like the engine runs fine on only the primary side, so a bog sounds like the secondary spring is too light.