Hesitation When Accelerating

[eschaider]

Duplicate post

[zzmac]

Originally Posted by patrickt

Leaks and stripped bolts don't count, so I pronounce the carb fixable. Zip tie the secondaries closed this weekend, take it for a spin to see if the bog disappears, and report back.

Will do.

[patrickt]

Now, let me show you what I have on my vacuum secondary shaft that tells me how far my secondaries are opening. Some guys put a paper clip on the plunger that is coming out of the vacuum housing, but I prefer a zip tie. If you look at this picture you will see that the zip tie is fastened around the secondary shaft tight enough so that it holds its position but loose enough that the shaft can easily turn within the tie itself. It's kind of tightened up to a "gentle handshake" level. The shaft will rotate counter-clockwise as viewed from this side when the secondaries open. The zip tie is trimmed so that it can be pushed over to where it is touching the throttle screw. This is the position that tells you the secondaries are closed and have never opened any amount at all. On my pic below, you can see that I snapped this pic after a "lightly spirited" run where the secondaries opened up about half way and the zip tie is pointing up at noon on the clock. When the secondaries have opened fully, the zip tie points to about two o'clock. You can easily test this in your garage by holding your throttle fully open and then manually opening the secondaries fully. The zip tie will then rotate on the shaft and remain pointing at the spot on the clock that tells you how far it went when the secondaries were wide open fully. Yours might be one o'clock, two o'clock or more or less. Just remember that with Holley linkage you have to have the primaries fully open before you can manually open the secondaries -- if you try and force it you'll just bend stuff up. Knowing whether your secondaries are opening, and how much, and when they open, is the first step in getting them to open just right. You want to be able to measure the results of something before you start changing things. And the goal, believe it or not, is to get your secondaries to open fully, as proven by your zip tie pointer, but without you really knowing when they did it. In other words, they open so smoothly and evenly that you couldn't detect it by the "seat of the pants" test. But when you check the zip tie you say "wow, they really did open completely during that graduated WOT run."

[zzmac]

Sorry Patrick but I'm not a carb guy and you've got me a little confused. You said to zip tie the secondaries shut and now you're showing me it rotating and the secondaries opening up. Do I shut them with a zip tie or do it like in your pic above?

[patrickt]

Originally Posted by zzmac

Sorry Patrick but I'm not a carb guy and you've got me a little confused. You said to zip tie the secondaries shut and now you're showing me it rotating and the secondaries opening up. Do I shut them with a zip tie or do it like in your pic above?

Sorry I confused you. The two zip tie posts by me have nothing in common with one another. They are entirely different subjects and you can completely ignore, for now, my second post showing you how to tell when the secondaries open by putting a zip tie around the shaft. For now, just wire/tape/even clog the top with towels so the secondaries can't open, then take her for a spin. We're just trying to figure out, for right now, if the secondaries kicking in are causing the bog. Nothing more. If you come back and say "wow, with the secondaries wired shut, the car runs beautifully. A little slower, but beautifully" then that tells us where to start looking.

[zzmac]

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

One step closer. :-)

[patrickt]

Originally Posted by zzmac

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

One step closer. :-)

That presumably tells you that the problem is on the secondary side. It doesn't tell you exactly what the problem is, just what side it's on. Now, you might want to go back to the carb place and tell them "you know, when I lock out the secondaries the problem goes away. What do you make of that?" And see if they say, "O.K., let us take a look at it one more time." They will then check the air flow through the secondary bleeds, the fuel flow, the fuel level in the bowl and, of course, the operation of the vacuum canister and springs that act to open the secondaries. On the other hand, they might just want to be rid of you since they should have done it right the first time.... Out of curiosity, how much slower was your car running on only half a carburetor?

[zzmac]

It was still pretty quick.

[zzmac]

Near the end of my short test drive once again the RPMs stayed at about 3000 for a very short time with my foot off the pedal. Can that have anything at all to do with the carb or would it strictly be linkage?

[patrickt]

Originally Posted by zzmac

It was still pretty quick.

OK, if the guys in the shop are receptive to listening, you might throw in as you're leaving, "some guy on the internet thought the vacuum secondary spring could use some tweaking. Ya think that might be it?" But they still need to walk through the protocol, so maybe just shutting up is a better idea.

[patrickt]

Originally Posted by zzmac

Near the end of my short test drive once again the RPMs stayed at about 3000 for a very short time with my foot off the pedal. Can that have anything at all to do with the carb or would it strictly be linkage?

Could be the linkage between your foot and the carb, and it could be the linkage that's actually in the carb itself making it stick a little bit. Lubricate everything that moves and see if doing that improves the "stuck" symptom. I know the last time I lubricated the linkage in the carb alone, I was struck at how much crisper and cleaner the throttle felt and said to myself "damn, I should have done this a while ago considering it only took about thirty seconds."

[zzmac]

Originally Posted by patrickt

OK, if the guys in the shop are receptive to listening, you might throw in as you're leaving, "some guy on the internet thought the vacuum secondary spring could use some tweaking. Ya think that might be it?" But they still need to walk through the protocol, so maybe just shutting up is a better idea.

They're great guys at the shop.

[NROTOXIN]

Still haven't determined a sticking carb linkage/pedal.......if that is eventually determined not the case, maybe the distributor advance is sticking. Divide and conquer.

[zzmac]

Thanks I'll keep that in mind NROTOXIN.

[zzmac]

Originally Posted by patrickt

OK, if the guys in the shop are receptive to listening, you might throw in as you're leaving, "some guy on the internet thought the vacuum secondary spring could use some tweaking. Ya think that might be it?" But they still need to walk through the protocol, so maybe just shutting up is a better idea.

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

[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.