I have a 76 Ford F150 with 360 FE engine. Has EGR, does not have smog pump. Has PCV. Has Motocraft 2150 carb.

OK, so I thought my harmonic balancer had slipped because when I set the timing to 10 degrees BTDC, the engine would run rough and want to die.

So I set the timing by ear until it sounded good and smooth to me.

HOWEVER... I used a stick and measured carefully through the spark plug hole on cylinder 1, and it turns out my harmonic balancer is CORRECT!

So, what would cause the engine to need more timing advance to run smooth?

It kinda explains some things, like why I was having backfires through the carb (ignition advanced too far).... but I am still not having any luck getting it to idle smooth with this timing setting...

This is on 87 octane gas, and it's fresh, I got rid of the old gas and put in new gas.

The carb was just rebuilt (by me) but maybe I didn't do a good enough job?

Any ideas? What would cause this kind of thing?

 

Is your choke set right? Power valve blown out letting it run rich?

There is a bass-ackward trick that I use to get 'em to idle good, which is usually reserved for motors with a larger cam. That is to plug the vacuum advance into full manifold vacuum. This will crank up the timing at idle, but leave the rest of the timing unaffected. I would DEFINATELY look for other causes before resorting to doing it this way however.

 

How can I tell if the power valve is blown out?

 

Well, if you're getting black smoke out the exhaust, that's a pretty good sign. If you can turn the idle mixture screws all the way in on both sides and the motor still idles, that's a good sign. If you replace it and it suddenly runs better...

 

Well I took the carb off and disassembled it, to go over everything.

It looks like it is fine. The power valve is not blown.

Whatever is up, I can't get vacuum to get any higher than 15 and it should be 20 or 22 right?

 

You can't tell from looking at a power valve ifit is good or bad. I've only seen one in over 40 years that the eye could see that it was bad.

 

could the timing chain have jumped a tooth?....Neil

 

Ok, how many miles on motor? All stock? Did you have vacumm advance hooked up while setting timing? If a high mileage original motor, then timing chain could have jumped. Usually though once it jumps a tooth, it will continue to jump and finally strip out in short order.

Can you adjust idle mixture screws on carb to smooth it out after you set the timing down to 10? What's the idle speed set at? How about when in gear?

Just giving out some things to look for/check. Kinda hard to diagnose over the internet.

As for vacumm numbers...that could be anything. I don't think there is an actual number set in stone for this. It has too many variables.

There is nothing to rebuilding an Autolite carb. Very few moving parts and not much to wear out. It should have come with a new powervalve so that should not be an issue.

 

The motor is a freshly rebuilt longblock I bought a couple weeks ago.

The vacuum advance was unhooked while setting timing.

Idle mixture screws don't help much.

 

You know what, try my vacuum advance trick. Hook it to full manifold vacuum and see what happens. You might be surprised.

 

to me it sounds like it is either the cam incorrectly timed when the engine was rebuilt or you have a vacuum leak somewhere, maybe try taking the egr off or putting a plate over the mounting holes, they are a big source of vacuum problems when they stick open or rust open....

 

Why does it matter if they stick open, aren't they supposed to be open all the time ?

I was told the EGR valve should be hooked up to manifold vacuum... with a ported vacuum switch that makes sure it stays shut until the engine heats up.

So once the engine heats up shouldn't it be open all the time? So why is it a problem if it sticks open?

I do think I have a vacuum leak though....who knows from where though.

 

I think he was concerned that maybe the valve itself is leaking(the diaphram), not the mechanical part that leaves exhaust flow. Unhook the line and plug the port(at the source of vacuum) and see if it changes anything.

How about the power brake line? Unhook it and see if it changes anything.

How's the carb plate gaskets?

The cam timing could be the culprit? Who's to know? I guess you need to nail down all the topside possibilities first. Try spraying carb cleaner around the base of carb and around any vacuum lines. It engine speed changes you have a culprit.

 

Still sounds like a blown power valve. Try running the idle mixture screws in to a 1/2 turn or less and see if it helps. It it does the power valve is blown.

 

It actually was running pretty darn good with the idle mix screws about 1/2 turn out. So...

I'm gonna get a carb rebuild kit with a new power valve in it. I was going to check for vacuum leaks but like a dummy, I took apart the carb first to see if I had it put together right. Now I need new gaskets so I'm just gonna get a whole new rebuild kit, it's like $20 and includes the power valve.

Gotta get this fothermucker on the road...