1979 ford f250. 400ci. 4 speed.

Engine has a mild cam. Port polished heads. 4 barrel intake. Headers. Double roller timing. Pertronix ignitor distributor and coil. Have an edelbrock avs2 650 cm carb. Cant get to run right or stay tuned. Anybody else have same issue? To much carb? Junk carb? Any suggestions? Carb is brand new. Brand new engine has around 3000 miles on it.

 

Sounds like time to triple check Timing

Having more details of the symptoms would help. I doubt it's the carb, I have an Edelbrock 1405 .. Works great (460ci)

 

Do you have a vacuum gauge? I am finding it to be a huge help in tuning and solving issues. I had my truck with a shop during my build and they swore the transmission was bad. Because it would kill the motor when place in drive. But it was a tune and vacuum issue nothing with the trans. I found another shop it super cheat as far as labor and it close to me. Its myself. I have a Holley 650 on my truck. But my 66 mustang has the same carb as you. It should be super easy to tune to make drivable.

 

Timing has been set and checked. So a little more to the issue, if I sit idle for a few mins its almost like it floods or overloads, have to rev it up to clear it out. If I warm it up, drive ti the gym. Workout an hour, then go to leave, i have to rev it up and clear it out to get it to run. Then it's fine. When I'm taking off ot will go like a bat out if hell, then when I cruise it's okay. If I take off slower it almost gains power in steps it feels like. Make any sense?

 

that size is about right for your engine assuming your compression isn't high.

I personally thing the AFB ( Edelbrock ) carbs are a waste of time and money. some people like them but some people like jazz too so I don't get it. you must have something wrong with the carb, they're not great but they should work better than that. since it's on warranty I wouldn't mess with it I'd take it back.

If you get tired of screwing with it buy a 4160 and be done with it. something in the 600 - range would work good, 750 if you want to over do it some.

 

Have a buddy who has a holley 670. Going to put that on nf see if a difference. It's been rebuilt but not jetted or anything.

 

I am not the original poster, but could you elaborate on this a little bit? So how exactly are you using a vacuum gauge to fine tune?

 

"90% of carburetor problems are ignition related". One reason for that old chestnut is because ignition timing has a direct effect on the level of engine manifold vacuum. If the ignition timing is late the vacuum will be low, and with it the crisp signal through the venturis. And, carburetors are designed and calibrated on seeing a specific manifold vacuum pressure at highway cruise and everywhere in between too. Valve timing and camshaft plays a role too. Very little air actually flows through a carburetor at idle, so a separate idle circuit is utilized that is directly exposed to manifold vacuum, this gets the velocity of the air higher than it would be otherwise for good fuel atomization. Don't have to understand it necessarily, just make sure it's right, and the carb will do the rest.

Basically if the ignition timing is off, or say there are subtle air leaks in the manifold or flange gaskets, you'll pull your hair out trying to get a carb tuned, it will never run right. A mechanic's vacuum gauge is one of the most useful tools for engine diagnostics and tuning. They are good for idle mixture screw adjustment too, which is actually the major system in play up to about 35 mph or so. Idle or actually just off idle, is where most city driving is done. A healthy stock engine in good tune at sea level will pull 18" - 21" of vacuum at idle, this makes for crisp response on acceleration pulling away from a stop hauling heavy loads, say. Important for trucks. If manifold vacuum is off spec for a particular engine/cam/carb/application it will not run right and can't be made to run right.

 

Where is the best place to hook the guage up to?

 

The vacuum gauge is connected to a source of constant manifold vacuum. Some aftermarket carburetors have more than one port, some carburetors have a spacer with a port for power brakes, that will work too. Many intake manifolds have a pipe thread fitting for PCV and things like that. Basically anywhere below the throttle plates of the carb is fine.

 

Okay. What numbers am I looking for? Does cam size make a difference?

 

Yes, the cam makes a tremendous difference in manifold vacuum. A stock camshaft tends to have high manifold vacuum. Altitude also factors. If you live in Denver, the average vacuum will be maybe 5" lower than at sea level.

The general rule is a healthy, stock engine in good tune should pull a steady 18"-21" at idle on the gauge. Steady is more important than the actual number; but the higher the better for a stock motor. Low vacuum can be caused by late ignition timing, late valve timing, exhaust restrictions etc, but if the needle is bouncing all around that points to other defects or issues. So it's a real quick easy check on engine condition and overall state of tune.

A "mild" cam should be fairly close to the high side. A rumpety-rumpety cam otoh, might struggle to pull 10" at sea level. It's important to know what manifold vacuum is at both idle and highway cruise for carburetor tuning purposes, because the power circuits are based on this. Holley uses power valves (economiser) for example. It's the manifold vacuum that holds it closed. Mash the accelerator, vacuum goes bye-bye, and the power valve opens. The situation to avoid there is a power valve that never closes. So a performance engine needs are far different power valve than a factory stock engine. They usually like a different connection for the distributor advance too. Maybe you're not running a Holley, but vacuum based power circuits are common. It's a useful signal for many different things.

 

Ya, larger cams usually end up making less vacuum overall. Affected by elevation above sea level too, the higher you are, the lower the vacuum reading too.

At sea level you are looking for up to 22 inches of vacuum during idle (with a stock cam). Also examine the vacuum gauge's motion to see how it behaves, it can indicate where , if any, a problem exists. There are several vacuum gauge charts on the Google that you should be able to find that show how to read its motion.

example, I have a 460ci with stock cam/manifold. Edelbrock 1405 carb. I live at about 154foot above sea level. At idle I get about 22inches manifold vacuum constant (needle does not dance around)

 

I should back up a bit. Before attempting to tune a carb, make certain the fuel pump pressure output is within spec. Vacuum gauge also measures this, btw. See why they are a good tool to have? Edelbrock has the pressure spec, it is an important spec. Many aftermarket mechanical fuel pumps are not correct right out of the box.

THEN, when you know this is straight, and have the carb installed on the engine, measure the fuel height in the bowl after idling a few minutes on level ground. This is another important spec, the whole carb, every circuit, is calibrated around this being correct, not too high, or not too low, under all conditions from idle to wide open throttle. This is easier to do on some carburetors. Holley has an adjustable external lock nut and sight window that allows this to be done on the fly.

I don't know how Edelbrock does this. Fuel height is usually spec'd within 1/32" inch, so it's obvious they consider it important. Setting the float height on the bench just gets it kinda close, the actual fuel height in the bowl is what counts and should be checked after carb installation on the engine, warmed up, and the float height fine tuned up or down as required to achieve this.

Shut the engine OFF to do this if it isn't a Holley, keep a fire extinguisher handy. Don't do this in the garage, extinguish all smoking material, don't operate heavy machinery while using drugs or prescription medications or within 48 hours of watching TV.

All the usual standard disclaimers apply.

 

And I bet without seeing it run, it runs REALLY GOOD; a stock engine that's all wacked out won't do that.