[454headache]

Trying to get rid of a hesitation I have at WOT with by dual 1405 Edelbrocks. Here's how my motor and truck is set up-

-454 9:1 compression
-.483 lift 224 duration Roller Cam
-Vacuum Port hooked up to Manifold
-20 Initial Timing (set it here because the engine got the highest vacuum at -this amount)
-18 Mechanical Advance
-38 Total Timing by 2500-3000 (I forget the exact rpm)
-Fuel Pressure and Fuel pump up to snuff and set at 5 psi

-1405 Stock rods and jets
-8" springs installed
-.043 (the largest made) accelerator pumps installed
-Pump shot rod on top hole
-1 to 1 linkage
-Floats checked and adjusted to spec

-TH400
-3.73 Gears 27" Tall Tires
-4500 lbs truck


The symptoms-

When I first stomp on the gas the engine hesitates for a second. Then takes off like a bat out of hell. This only happens when I step on the peddle to the floor and at no other time. From a dead stop, when the engine does this and I lift completely off the gas during this hesitation the engine will idle lower and almost die.

Also, sometimes there is a slight pop pop that I can hear coming from the exhaust, long after I the engine emerges from the bog and is racing toward redline. This also is accompanied by a slight power loss.

The plugs are a half tan half new looking white, but I checked that at idle.

[454headache]

Went for a drive, stepped on it from a stop, and as it bog I killed the engine. Looked at #1 plug, still looks half white and half tan with a small light gray spot on the white area. Stumped, as to whether this means rich or lean bog, I hooked up the advance to ported and the bog seams to be gone on the drive back.

[454headache]

Nevermind, the ported vacuum hasn't done squat except add a nice light throttle ping.

[b00tz]

sounds like a lean spot to me,is your pump shot on the highest hole? (closest to the hood)

if it is try moving it to the middle, if that doesnt work, go to the bottom notch, the lower the notch the richer the shot.

are your idle circuits set properly?

[cobalt327]

You may have too much accelerator pump shot. I'd suggest you try a simple fix first: Put the pump lever into the hole farthest from the carb body. This is to lessen the accelerator pump shot.

If it improves, this will tell you you're rich, and vice versa. There are other ways to tell if you're rich, but this is as easy as any of them for the most part.

You should put a vacuum gage on the engine to see what's up w/that- setting the springs w/o a gage is more time consuming and less sure.

[454headache]

This morning, before the engine warmed up, I put in 3" springs. This made the bog worse, much worse when cold. Which means that the higher springs I had in it were covering up a lean condition. After the engine had warmed up, I took out the stock jets and put in .104's and 7547 rods. This helped a lot. Then I put in 8" springs, and the bog is near gone. I'll probably go richer a bit on the rods and maybe the bog will be gone. I also enriched up the idle mixture a tad.

From my understanding, if lighter springs (3" blue) make an engine bog worse then you have a lean condition. Also, fattening up the primary jets and rod further delays when the secondaries kick in.

[454headache]

Put the pump on the top hole. I think it's helped. Went from 4% to 8% richer on cruise mode as well. The bog is not a bog anymore but a severe flat spot now. I hit the throttle, and the engine no longer wants to die, but doesn't really like to accelerate either.

What I really do not understand is that this flat spot doesn't always occur. Sometimes I have a flat spot/ near bog and other times I get a massive burnout. I just don't get it. My fuel pump is a new electric carter unit that is up to snuff, 3/8ths fuel line everywhere, and new 3/8th inlet/outlet fuel filters (the clear plastic kind). I just wish the engine was more consistent. All I now so far is that it likes the richer jetting/rods and longer duration pump shot.

Maybe a needle and seat change? Or set the floats out of spec to allow more fuel in the bowls?

[454headache]

After more research, turns out that you can not change pump volume with larger nozzles, this only makes the amount of fuel the squirters do put out not last as long. So, changed pump nozzles to the smallest, .24's (increases the duration), and put the rod the bottom hole to give the quickest tip in. Much better, but still not gone. I need to find a way to increase the volume.

[cobalt327]

Is this by any chance a tunnel ram?

You said "1:1 linkage". You mean the primaries of both carbs open together, followed by the secondaries after the primaries are already open a ways- NOT that the primaries AND secondaries of both carbs open simultaneously, right?

[cucumber1949]

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[454headache]

It is indeed a tunnel ram, Edelbrock's "Street Tunnel Ram" Oval Port. I switched over from their low rise dual quad manifold. When I switched, I kept the stock jetting for a bit, and the carbs ran so rich you could cry. Stronger signal at idle?

Yes, 1:1 as in both primaries are linked together to a rod. The primaries and secondaries are not connected to each other other than by whatever method the factory uses to connect them internally.

Other than my flat/spot bog, I love the manifold. The low rise (no heat cross over, as well as a rect. port manifold on oval heads) was very sluggish when cold and would take forever, it seemed to come up to operating temp. The Ram takes even longer to warm up but I can mash the peddle down within a few minutes of driving, and the WOT throttle experience is amazing, not sluggish at all. Though maybe my lazy timing had something to do with it.

There is also no idle or cruising hiccups. Heck, before I switched to 4% richer rods/jets for cruise mode, I would get 13.40 mpg @ 60-65 with sprints to 75-80 on the highway using stock 1405 jetting. The low rise would send my mpg down to 8mpg at 70, at 55 I could only get slightly under 12.

I'm fairly sure I was lean with the stock .100 jets, 70 x 47 rods, and the tunnel ram as I would get a grumble (faint pop) from the exhaust once in a hundred miles or so on road trips. And there was a slight flat spot in power from 2,500-4,000 with more popping. Though that could have been due to the pin hole in my muffler.

To further clarify, the bog/flat spot only happens off idle WOT from a standing or low speed (below 10 mph roll) start. If I take my foot off the gas, only say use 90% of throttle the truck takes off like a beast.

[454headache]

From the carter website-

"All Carter AFB's feature mechanical secondaries, which will guarantee that they open when you floor the gas
pedal. To eliminate any bogs or hesitations as the secondaries open, a secondary velocity valve is used. This
valve allows airflow through the secondaries in response to engine requirements. The opening rate of this valve
can be altered by drilling the counterweight, but this is rarely necessary. It is also possible to eliminate some
secondary bog conditions by removing the small tubes that restrict the "early feed" fuel supplies at the edge of
the velocity valve. This modification requires removal of the secondary cluster assemblies, and like drilling the
counterweight, it is not usually necessary."

I wonder where are these "small tubes"?

[cobalt327]

The secondary auxiliary system port is just below the edge of the air valve (inner wall), much like a Q-jet, if you happen to be familiar w/them.

It is connected to the main fuel well and has an air bleed. Decreasing the size of the air bleed or increasing the size of the discharge port will supply more fuel on secondary tip-in.

It's possible that, if using a smaller diameter primary shooter (to increase the shot duration) along w/the larger pump shot hole on the linkage is not enough, mods to the secondary auxiliary system may be needed w/the TR manifold.

[454headache]

Solved it. I upped the secondary jets from .98 to .101 (12% richer). But what really helped the most and cured the issue was adding molten lead to the air valve counter weights.

I really wish Edelbrock would let you know upfront that these carbs are really mechanical secondary. If I had known that I never would have bough them for such a heavy vehicle.

[cobalt327]

Originally Posted by 454headache

I really wish Edelbrock would let you know upfront that these carbs are really mechanical secondary. If I had known that I never would have bough them for such a heavy vehicle.

While the secondary base plate butterflies are mechanical, the air valve is by demand, so it is considered to be an air valve secondary carb. Concept is like the Q-jet.