Hesitation When Accelerating

[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.

[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."

[Moriarty]

Originally Posted by eschaider

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.

This was exactly the info I needed for the similar issue I had been chasing. Switched out the 6.5 for a 4.5, readjusted the transfer slot square and viola!

Thanks,
Aaron