Another APT Question

[davis95]

When I built my carb I set the APT at 2 1/2 turns out from bottomed. I tried the tip-in procedure and had no increase in rpm when I closed the choke flap, but had a couple hundred rpm increase when I unplugged one of the top vacuum lines. Does this mean I need to go down even further with the APT screw to lean it out? 2 1/2 turns out already seems a little low according to what I've read.

[Cliff Ruggles]

With the engine running at apprx 2000rpm's, and completely warmed up, turn the APT screw down until you hear or notice an engine speed change.  If you can bottom out the screw, and can not get a slightly rpm drop, or change in engine "pitch"  while it's running, you do not have full control of the part throttle A/F ratio with the APT system.

What is the carburetor number?

Did you install one of our kits?

What jets and metering rods are being used?

Did you verify the height of the metering rods in the jets during the rebuild?

Are the metering rods hangin exactly even on the power piston arms?.....Cliff

[davis95]

With the engine running at apprx 2000rpm's, and completely warmed up, turn the APT screw down until you hear or notice an engine speed change.  If you can bottom out the screw, and can not get a slightly rpm drop, or change in engine "pitch"  while it's running, you do not have full control of the part throttle A/F ratio with the APT system.

What is the carburetor number?

Did you install one of our kits?

What jets and metering rods are being used?

Did you verify the height of the metering rods in the jets during the rebuild?

Are the metering rods hangin exactly even on the power piston arms?.....Cliff

Number is 17059508, I used a Sorensen kit from Advance, factory jets and rods,
No verification on rod height during build (I hadn't bought your book yet!),
The metering rods are hanging exactly even on the arms.
I will need to tap a provision for the APT adjustment so I can adjust it running.

[davis95]

I took the carb down again and installed a plug for the apt. Everything in the carb looks fine. The jets are 73's with M rods. After putting it back on the car and adjusting the apt It runs much better (leaner) than before, but I am still wondering if the apt is working correctly. With the apt screw all the way down should it still make contact with the rod on arm of the power piston or should it be below it? When I go all the way down with the apt screw and then screw it back out, it takes about 1 1/2 turns before it contacts the power piston rod and begins to pull the primary rods out. Is this typical or does it sound like the rod arms may need to be bent down to get the rods further down in the jets before the hanger assembly bottoms out in the center? When I do the tip-in procedure the rpm's stay about the same when I pinch off on the choke flap, but speeds up a couple hundred rpm's when I pull off a vacuum hose. This tells me that the rods aren't going down far enough. Am I diagnosing this correctly? Thanks, Doug

[Cliff Ruggles]

Did you replace the float during the rebuild?

What height is the float set at?

If you did not replace the float, do that first, then re-test.

It takes just over a full turn for the APT adj to contact the post on the power piston.

At the highest metering rod position, the small portion of the metering rods (.036" for "M" rods) should be positioned in the smallest portion of the jets.  Verify this by visual inspection, never assume the hanger height is correct.  At the lowest point, the larger sections of the metering rods should be well down into the jets.  As the APT is raised, verify that it is bringing the tapered section of the rods out of the jets accordingly.

It is not uncommong to have to bend metering rod hanger arms to get the correct relationship between the rods and jets.....Cliff

[davis95]

I did a few things that you suggested to me on the phone a few days ago and it runs much better now. I manually lowered the the main rods (bending the arm and then adjusting with the apt screw) and drilled out the mixture-screw holes to .087. The tip-in procedure now works exactly like it should (actually gaining about 200 rpms when partially choking) and the mixture screws respond on 1/4 turns instead of every 2 turns! I didn't remove the idle tubes, but I did ream them out with a piece of wire. I'll pull them on the next rebuild. Response is crisp and clean. It responds to any slight movement of the accelerator pump. I'ts still occasionally (4-5 times a minute) very lightly popping in the mufflers at idle. Is this something that I can completely get rid of or is this just the nature of the beast? I'm really not sure if this is due to a slightly lean or rich condition. As good as it runs I hate to keep adjujsting things, but it would be nice to get it completely smoothed out if possible. 

[Cliff Ruggles]

Set the idle mixture screws with the engine fully warmed up, for the best idle quality and speed at the leanest setting(s).

Then re-adjust the APT using the tip-in procedure.  You should NOT have to do any further adjustments.

If it still has some light "popping" at idle speed, it may be due to too much spark advance......Cliff

[davis95]

Set the idle mixture screws with the engine fully warmed up, for the best idle quality and speed at the leanest setting(s).

Then re-adjust the APT using the tip-in procedure.  You should NOT have to do any further adjustments.

If it still has some light "popping" at idle speed, it may be due to too much spark advance......Cliff

I've been thinking about going with less vacuum advance. Pretty sure you hit the nail on the head with that one. I'm running a 300 HP 350 with my initial timing at 18 and total at 35. Not sure how much vacuum advance it's getting (haven't checked it yet), but pretty sure it's more than it needs. How much vacuum advance do you recommend for this set up?

[Cliff Ruggles]

What is the static compression ratio?

How much timing does the vacuum advance add in?

What are the cam specs?......Cliff