Hesitation When Accelerating

[zzmac]

Last fall I had my carb rebuilt. After getting it back I noticed a slight hesitation anytime I accelerated, a slight bog and then everything is smooth. I took the car back to the shop and they assumed it was the squirter jet which I think they changed to a bigger one that was originally in the carb. In all, they tried a 35, 37, 40 and we ended up back with the original which was a 25. And the issue kept happening with all of them.

Any ideas on what the issue could be?

[NROTOXIN]

What size holley? I suggest listing some of your engine vitals so people here have something to work with
i.e. ignition timing, which should be sorted out before thinking carburetor.

[zzmac]

Sorry. It's a Roush 402R with a Holley 770 CFM Carburetor w/ vacuum secondaries and electric choke (as per Roush). I asked them if they had checked the timing last year and they said yes, it's fine.

[patrickt]

It can only be a couple of things, but fix your sticking throttle first. If the guys at the shop cleaned out the dirty air bleeds (which is the first thing any carb guy does regardless...) and they checked that the primary throttle setting and mixture screws weren't really out of kilter, then I might suspect secondaries coming in too quickly and causing a bog. Maybe the spring they put in there during the rebuild was too light. Locking the secondaries shut and taking her for a romp would tell you.

[Grubby]

Jets are rarely the problem you have.

There is a specific order to tune carbs. Google it and follow the order. I suspect the issue will become apparent.

The alternate is listen to a bunch of guys on the internet and potentially chase your tale.

John

[incoming]

John, your the man.

[patrickt]

Originally Posted by Grubby

...and potentially chase your tale.

I agree. If you're going to chase a tail, it should not be your own.

[Grubby]

Now that you mention it, chasing tail can be fun.

John

[patrickt]

zzmac -- just to humor me, put a zip tie on your secondary linkage so they're clamped shut and can't open with your venturi vacuum. Then take her out for a run and see if the bog is still there. Yes, the car won't run as fast on only two barrels, but it's the bog we're looking for. When you get back home, just clip the zip tie off and you're no worse for wear.

[jhv48]

Being a vacuum secondary AVS Holley, most likely you’re experiencing a momentary lean condition because the secondaries aren’t opening fast enough.
Easily cured. My guess is when rebuilt, the secondary spring was replaced with a stronger spring than your car needed. Jump over to this site and it will show you how to replace the spring to a lighter one.
https://documents.holley.com/199r9934-3rev4.pdf

[patrickt]

It's easier than you think to not get the spring put in right or to make a hole in the vacuum thing or screw it up another way. It's one of the more irritating simple tasks on older engines.

[eschaider]

Originally Posted by jhv48

Being a vacuum secondary AVS Holley, most likely you’re experiencing a momentary lean condition because the secondaries aren’t opening fast enough.
Easily cured. My guess is when rebuilt, the secondary spring was replaced with a stronger spring than your car needed. Jump over to this site and it will show you how to replace the spring to a lighter one.
https://documents.holley.com/199r9934-3rev4.pdf

Secindries that open too quickly do not enriched the mixture, they lean the mixture because they introduce more air w/o the appropriate increase in fuel volume.

The spring in the secondary vacuum diaphragm retards the opening of the secondaries. Increasing spring strength slows the opening event. Decreasing spring strength speeds up the opening event.

If the secondaries are already opening too fast, installing a lighter spring will only aggravate the problem.


Ed

[eschaider]

OP pay attention to Patrick he is pointing you in the right direction.

When an engine stumbles it is caused by a lean condition not a rich condition. You particular lean condition is not going to be impacted by a main jet. Main jets are for WOT tuning. There are only two items that can affect the off idle / part throttle lean condition, i.e. stumble.

The first thing to check is your idle vacuum. If you have the lumpy idle most Cobra owners desire you are likely idling with a low manifold vacuum. A good manifold idle vacuum would be between 12 and 17 inches of vacuum. If you are down around 6 inches of manifold vacuum this is the beginning of your problem.

Out of the box, Holleys will almost always come with a 6.5 power valve. That means the power valve opens at 6.5 inches of idle vacuum. If your car idles at 6 or 7 inches of vacuum that means your power valve is open at idle creating a rich idle. You want your power valve to open at roughly ½ your idle vacuum. An engine with 6 inches of idle vacuum wants a pwervalve with a 3 inch opening value.

When power valves are open at idle, owners lean down the idle mixture screws to lean out the mixture so the engine can idle w/o asphyxiating every one within ear shot. This is great for idle but terrible for throttle response. As you open the throttle the engine goes lean and stumbles because the power valve is already being used to idle the engine.

There are two fixes, get a different cam and higher idle vacuum — which is generally unacceptable to Cobra owners. The second fix (also cheaper) is get a different power valve that opens at a lower manifold vacuum (½ your idle vacuum) and then properly set the idle mixture screws on the carburetor.

Now as you open the throttle, both the accelerator pump and the power valve will add fuel as manifold vacuum drops below the lower valve's preset opening point. The fuel addition will either mitigate the stumble or eliminate it. If there are still vestiges of the stumble then you need to take the next steps.

The accelerator pump and power valve work together as a team. The carb uses both to supply adequate fuel to the engine early enough at part throttle acceleration to eliminate stumbling or flat spots. Power valves will also provide additional fuel at WOT well after off idle and low to intermediate speed enrichment.

If we assume your accelerator pump has adequate pumping capacity then the metering of the fuel is insufficient for your engine. The first step would be to make sure you are getting the largest accelerator pump shot your linkage provides for. If you are then you need to go up a step or possibly more in your accelerator pump nozzles. As you do you will increase the volume of fuel the accelerator pump adds to the mixture as the throttle is opening early in the engine rpm increase.

If the stumble persists your accelerator pump diaphragm is too small and you need to go up one step in the Holley accelerator diaphragm hierarchy. Something to consider is the larger accelerator pump diaphragms when used with smaller accelerator pump nozzles spread the addition of the accelerator pump fuel shot over a wider rpm band, which can sometimes be helpful.

BTW I'll bet you can't guess why Holley calls them accelerator pumps ...



Ed

[patrickt]

But zzmac if your car has ever run right with this carb then it can run right again. What made you send it in for a rebuild?

[FredG]

.....if it's the secondaries causing the issue, bag the spring and go with the Quick Fuel adjustable unit that replaces it. I love that thing.

Fred

Originally Posted by patrickt

But zzmac if your car has ever run right with this carb then it can run right again. What made you send it in for a rebuild?

[patrickt]

Originally Posted by FredG

.....if it's the secondaries causing the issue, bag the spring and go with the Quick Fuel adjustable unit that replaces it. I love that thing.

Fred

Wait a minute, wasn't it me that told you about that after I put it on my carb in this thread? Theoretical Vacuum Secondary Question

[FredG]

Yes. A while back. No more guessing what is the proper spring to use.

Originally Posted by patrickt

Wait a minute, wasn't it me that told you about that after I put it on my carb in this thread? Theoretical Vacuum Secondary Question

[zzmac]

Originally Posted by patrickt

But zzmac if your car has ever run right with this carb then it can run right again. What made you send it in for a rebuild?

From what I remember (I'm getting old) I had a gas a leak at the back of the carb so I had it towed in. It also turned out that a couple of the bolts holding the carb to the manifold were stripped so they fixed that and they rebuilt the carb when they had it off.

[patrickt]

Originally Posted by zzmac

From what I remember (I'm getting old) I had a gas a leak at the back of the carb so I had it towed in. It also turned out that a couple of the bolts holding the carb to the manifold were stripped so they fixed that and they rebuilt the carb when they had it off.

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.

[eschaider]

If you are supplying an adequate volume of fuel, you should be able to immediately open all four throttle blades w/o any stumble. That is exactly what a carburetor with mechanical linkage does.

When you slow down secondary opening to match an anemic fuel deliver curve you are reducing engine power and throttle response. That is a left handed solution to a simple fuel delivery problem.


Ed