Since everyone was so great in my last thread:
https://forums.vintage-mustang.com/...om/vintage-mustang-forum/1158124-engine-starts-but-wont-run-2.html#post10170446


I figured I would try again. The engine starts and idles great, timing is 12-13 with full advance at about 35 (all mechanical, vacuum is plugged), I adjusted the idle set screws per the edelbrock manual and by the way I normally do (vacuum gauge) and either way i end up 1 1/4ish open on the idle set screws and about 17" of vacuum. Dwell is in the 28ish range and fuel is 6psi from a regulated edelbrock mechanical fuel pump..

The problem is when I apply light throttle or a light to steadily increasing throttle I get a couple surges on the way up. Its like give it a bit of gas and get a "wa- wa- waaaaah" surge or stutter, not really a hesitation though.

I have never owned an edelbrock before, it was free with the top end kit so I put it on. I am very familiar with dual carb setups and webers where normally I would try to run more timing and see if it covers up the problem but I dont really want to do that here.

Any opinions?

First thing I would check - make sure you are getting a shot of fuel from the accelerator pump. Second thing, is it the right amount (too little or too much)....

 

I get a shot as soon as the throttle moves, and I have tried all three settings for the fuel pump and I feel like the problem is not as bad if I go to the least amount of fuel shot (furthest setting from fulcrum).

 

Just a thought - have you put a timing light on it and watched the curve? Is it steady as the rpms increase? If the distributor shaft has some play, your timing could be jumping around as you apply throttle and gently accelerate after each shift.

 

Distributor feels solid and the curve looks fine in the driveway, it pulls steady as you advance, its only under load of driving it has the issue. Plugs are new and cables are not to old, ive tried two distributor caps no luck there. I feel like im missing something dumb as all the reviews say these things run out of the box.

 

Could even be something simple like worn cap & bug, or plugs?

 

Every carburetor runs out of the box. But to run best requires some tuning.
What’s the cfm of yours?
500 or 650?

 

650, the only choices for the "free" carb when you ordered the top end kit was 650 or 800. I do have the holley I ran on the engine before it was rebuilt, it is a 480 cfm but I would have to change all the plumbing.

 

What’s the cam profile?
The 650 cfm might be a little much but it can be tuned to fit.
Adjusting the secondaries will help.
A different set of metering rods and step up springs would help too.

The carburetor should have come with instructions for making these adjustments.

 

I think Mr. Flash nailed it. Be aware when tuning an Edelbrock carb, you must consider:

- Jet sizes for the primary

- Metering rod sizes for the primary. The metering rods have two steps; lean and rich. Different metering rods have different step sizes.

- Subtract the size of the metering rods from the size of the primary jets to see where you are. Bigger number means richer mixture. Smaller number means leaner mixture.

- The metering rods are supported by tiny springs. At lower RPMs, vacuum holds down the metering rods for a leaner mixture using the larger step on the rods. At higher RPMs, vacuum decreases and the springs push up the metering rods for a richer mixture. I'm thinking it's at this transition where your carburetor is having issues. Therefore, you might want to play with lighter or heavier springs and see what that gets you.

- Jet sizes for the secondary. No metering rods for the secondary, but you can calibrate when it opens. I recall you can bend the secondary linkage, but be sure to check the manual for instructions. You should be able to find it online.

- Sounds like you're actually describing a "flat spot" with your carb. Actual surging is when the RPMs go up and down while the throttle stays in the same place. If actual surging happens at cruising speeds, that indicates a lean condition. When I ran and Edelbrock carb years ago, I kept leaning out the primaries until I got surging at cruising speeds. Then I went rich two steps and it was perfect.

Have fun!

 

it is a pretty tame cam

This is the kit:
https://www.summitracing.com/parts/edl-2027/overview/

Cam Specs:
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Duration: 270 int./280 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.448 int./0.472 exh.

And I am familiar with tuning carbs, just not the edelbrock. I daily drive a 50 year old car with dual webers (IDFs) that commonly have transition issues in the idle to main circuit when you set them up but it feels different than what the edelbrock is doing. I ordered the tuning kit so I will have to play around with it a bit when I get it, from the instructions it looks like its not a huge deal to swap the metering rods.

 

... from the instructions it looks like its not a huge deal to swap the metering rods.

No big deal. Each metering rod has a little cover and a tiny screw. Loosen the screw, swing away the cover and the metering rod will pop up from the spring. You can then simply pull it out. There's a little, thimble-shaped thing into which the metering fits and the spring inserts from below. After changing the metering rod and/or spring, just slide everything back in, swing the cover back over and carefully snug the screw.

 

90% of Edelbrock carb tuning can be done without spilling a single drop of fuel.

 

I dont think you need the calibration kit. My experience Edelbrocks are pretty good out of the box. Here is what I would check.

I suspect a vacuum leak.


Are all the vac ports on the carb plugged.

Is the PCV Valve connected?

I would plug all the ports, vac brake booster and pvc. See if this help.

Try driving with the vacuum gauge hooked up. See if the vacuum changes drastically.

Even though you have good vac at idle there could still be a vac leak.

I had 16-17 in of vac on my FiTech but still found a vacuum leak on the throttle plate. (that is why I switched back to the edelbrock)

Found it by plugging all the ports and putting rags in the throttle body barrels. Connected a hose to manifold an blew cigar smoke into the manifold.

Lastly, have you called Edelbrock?

By the way, did you put a timing light on to see if the mechanical advance is working smoothly?

Gary

 

Why do you have your vacuum advance plugged?

 

recommendation of the guy that assembled and broke in the engine on the stand, I normally run them.

So I did get the calibration kit and its been my first day with a little free time before the rains started I messed around with it.

The first thing i noticed is the stock "metering" spring is set at 5" hg which seemed pretty low with 16" of idle vacuum so I threw the 8" spring in there and that cleared up a bit of the problem (As MrFlash posted) then I went through the accelerator pump settings test driving at each and seemed to have the cleanest transition with the 8" spring and the largest accelerator shot. Then it started pouring and I quit messing around with it.

PCV is hooked up to the port on the carb, the mechanical advance is nice and smooth in the garage, I will check for a vacuum leak.

 

So after spending all morning installing a fire suppression system into our 24hour of Lemons car I finally had time to come back to tuning the fastback. First I tried carb at default with vacuum hooked up, still had issues. So I tried going 2 steps leaner with the tuning kit (with the accelerator shot at the default and 8"HG metering springs) and the flat spots got way worse, so then I went a half step rich from default and it was pretty good, I will probably try a full step rich and see how that feels. But of course with any car that you take apart and rebuild the rear end, transmission, and engine all at once other issues are coming up.

Speedometer doesnt work: Worked before the rebuild (Dave Kee rebuild), works with a drill on the gear, tried 3 different gears 16 and 18 tooth, peeked in with a flashlight and it looks like the driver gears in the right place, but I guess the tail shaft is coming off and im checking the gear.

Rear end is super noisy and it feels like there is way to much backlash. I ordered a pumpkin with 3.25 posi from quick performance and as much as I dont want to I am going to have to pull the pumpkin again and double check their work for backlash and preload.(I really should of checked it myself before it went in).

 

If it is surging in light throttle or cruise driving down the road that is usually a lean spot in the primary jetting.


If its the accelerator pump you will get an immediate stumble on throttling which is caused by the pump shot hitting too late after which the engine will pick itself back up
OR
you will get a bit of initial engine and shortly after that it will stumble because it ran out of pump shot before it got up on the primary jets.


Its been a long time but I think I ran the orange step up springs. I didn't have 17 inches vacuum though.

 

If it is surging in light throttle or cruise driving down the road that is usually a lean spot in the primary jetting.


If its the accelerator pump you will get an immediate stumble on throttling which is caused by the pump shot hitting too late after which the engine will pick itself back up
OR
you will get a bit of initial engine and shortly after that it will stumble because it ran out of pump shot before it got up on the primary jets.


Its been a long time but I think I ran the orange step up springs.

I went with the silver (8" hg) because I have 16-17" of vacuum at idle, and everything I could find said about 50% of idle vacuum. I also used to run 8 power valves in holleys but not sure if they directly cross over in settings.

 

Tuning a carb is rarely done “by the book.”
Most of the time it’s done by the seat of your pants.
(Trial and error)

 

So i am still fighting this a bit, I went through and enriched the "cruise" mode until the lean surge was mostly gone, I then followed the same thing in power mode/secondaries until the lean bog/pop/backfire went away when I mash the gas pedal. And well the carb is now about maxed out as rich as it goes, acts like it wants more fuel, pictures below of the plugs. Also is a picture of where I have it jetted on their tuning kit (currently I am at 17).

Maybe I should try a cooler plug? Or put a holley back on it?

 

Consider installing an O2 bung in your exhaust and installing a wide band AFR gauge. With this, you can see exactly what your AFR is. I installed the AEM model 30-0300 AFR gauge and it is great seeing the AFR as you drive. Assuming no exhaust leak, you see the actual AFR. If it is really lean, first ensure no exhaust leak is skewing the reading after the combustion.


https://www.amazon.com/AEM-30-0300-...em+30-0300&link_code=qs&qid=1565116421&s=gateway&sourceid=Mozilla-search&sr=8-3

 

I think the plugs look good.

What size jets do you have in the secondaries?
I would increase jet size there first if your part-throttle/cruise setting seems good.