I have posted about this before, and have done everything that was suggested... still not working right though:

My car is a -69 with a stock 307, 2bbl carb and a Powerglide. It has got about 100 k on it, so it's hopefully not worn out.

It runs good, and has plenty of get-up-and-go, but the problem is that when I'm going up a hill, and hold the pedal down about 1/4" or so, it will start stumbling a little bit. It feels just like if it was starving for fuel, but if I hit the pedal it will accelerate just fine, without missing a beat. The only time it happens is when holding the pedal down just a little bit.

So far, I have changed fuel pump, fuel filter, air filter, carburetor, ignition coil, ignition wires, spark plugs and distributor. All these parts are new, except for the distributor. I know it's good though, as it ran great in another car I had.

Some other things I have done: Checked the fuel line, and blown it out with compressed air to make sure the sock is not clogged up, and that the vent works. Everything seemed just fine, no signs of anything being stopped up. Timing and dwell angle are set to factory specs, but I have also tried different settings without any luck. Checked the throttle linkage, to make sure nothing is moving around.

I don't have a clue what to do next. Luckily, most of the parts I have changed needed to be changed anyway. Still, I'm beginning to wonder if there's something in the engine that can cause this (burned valves, worn cam etc) so my next step will be to check the compression and the valve lift.

Sorry for the rant, but I'm really frustrated with it, and would really appreciate any suggestions! And I have that nagging feeling of having missed something obvious...

Olle,
Everything has to work together for proper performance. If the distributor is not calibrated (proper advance curve and vacuum advance) to work on your particular engine, it could be the problem even if it ran fine on another vehicle.
It really does sound like a lean condition to me though. You said you replaced the carburetor ? Is the new one the proper one for your combination ?

Bill,

The "new" carburetor is actually rebuilt, but exactly the same type as the old one. I have been wrong before, but I really don't think the problem is in any of the parts I have changed. The car was doing this before I got it, and it was all untampered original and well taken care of, serviced at the local Chevy dealership and so on. The reason why I started changing parts was mainly because of leaks and dry cracks, but everything worked before I started working on it. I have tried it after each part change, and it's still doing the same thing.

I'm not sure about the distributor curve, as this happens at any RPM. It might be worth a try to take the distributor to a shop and check the curve though, if nothing else just to rule it out as a possible problem.

It might be running lean at part throttle, but I don't know what can be done about that. The factory jetting should be correct, and as far as I know there's really no adjustment you can make on it except for the idle. And the funny thing is that it will run just fine at part throttle, as long as you're accelerating. Once you get up to speed and trying to maintain that speed up a hill, it will start acting up again.

What about a vacuum leak? Vacuum lines, manifold & gaskets, etc?

Originally posted by Dan72:
What about a vacuum leak? Vacuum lines, manifold & gaskets, etc? That could be it. I was playing with dwell and timing last weekend, and made it run quite a bit better by increasing both about 2-3 degrees higher than specs. While I was doing that, I noticed that the vacuum line to the distributor was a bit loose where it connects(possibly leaking). I think I'll change all lines, hoses etc this weekend, and look at other possible vacuum leaks.

I have had a similar problem in one of my vehicles. I might suggest disconnecting the vacuum advance, plugging it and taking a drive to recreate the problem. In my case it was a bad wire in the pickup module of the HEI distributor and when the vacuum advance moved it would cause a bad connection. In your case it could be the ground-wire underneath the base plate. In any case it is a cheap and easy test.

Advance weights moving freely?

I have tried two different distributors, and both are nice and clean, nothing is stuck, with new points, rotors etc. I really don't think the distributor itself is the problem, but I have been wrong before. Have actually been thinking about getting a HEI, so this might be the time to do it. Would be a nice upgrade, even if it's not the problem.

Try running Hi- octane gas....Can't hurt. I ONCE made the mistake of getting regular and the car ran like crap.

Worked on it today, and I think I have finally found the problem. The vacuum line to the distributor is half hard line, half rubber hose. The hard line has a flare fitting where it mounts to the carb (looks exactly like a brake line), and it seals against the tapered seat in a brass fitting. At some point, someone removed the brass fitting and connected the flared line directly to the carb, so it was leaking pretty bad around it.

I knew that the line was a bit loose, and when I compared with an old carburetor I realized that something was missing. I just put the fitting in there and the line feels nice and solid now. The engine runs well, and the idle is much stronger, doesn't feel like it's about to stall every time I stop at a red light like it did before. Hopefully, this took care of it and it only goes to show how easy it is to miss a simple thing like that. I really appreciate all the help! graemlins/beers.gif

Good to hear!

Feels gd to solve an annoyance that you just can't seem to pinpoint, doesn't it!?!