Bucking/Lugging at low speeds.
 

1970 T/A Stock RA3 motor. Stock original carb rebuilt by Cliff Ruggles. The issue is that when I am driving thru the neighborhood in 3rd gear around 30 mph the car bucks and feels like it is lugging even thought the rpms are adequate. Same thing happens in 4th gear when coasting down from about 40 MPH up to a red light or stop sign. If I throw it in neutral and coast to stop sign, it is perfect. The motor otherwise runs flawlessly. The points style distributor is stock as well. Dwell angle is at 30 degrees and timing is set at close to 12 -BTDC. Any ideas what I should check? Thx.

Fuel pressure?

Will the ignition coil fire a points-style (non-HEI) spark tester?

How long has it been since it got a "tune-up"--points/condenser, plugs, wires, cap 'n' rotor, idle mixture adjustment, etc?

I had a loose timing chain act like that. Put a new one on and it smoothed right out. But... easier stuff to check out before diving into that probably.

Vacuum advance functioning properly?

Sounds like a timing issue, I have too seen Vac advance issues do that.
Happy hunting!

Instead of speed, concentrate on the rpm it happens. See if its always at the same rpm. Then you can shoot it with a timing light, with and without vacuum advance, see if its erratic. Could be bad or broken advance springs, or vacuum cannister.

This car just went thru a complete restoration that took 3 years and basically everything is new. I am just in the test drive stages and have about 45 miles on it. Stock distributor is new from Napa as is the coil, plugs, wires, points, cap rotor etc. I do have a good timing light with tach but don't really know how to check if the vacuum advance is working properly. I do have the rubber vacuum advance hose going to the port on the front of the intake manifold. When I remove the hose from this port and plug it when setting the timing, the rpms do drop a few hundred so I think that indicates that the advance is working.

What does your gearing look like? When I bought my car, it had 3.08 gears to go with the M21 transmission and a big cam. Gear change to 3.73 solved that issue.

3:55
 

I have the 3:55 gears in the stock 12 bolt posi rear end.

Is the Distributor the original?
If so then due to old grease the advance shaft has been known to hang up and not allow the timing to drop back off smoothly upon a drop in rpm.
I would also consider choking the Carb off slowly at idle to check for vacuum leaks .

Also check that there is slack in your throttle linkage at idle and that when throttle is applied that the motor does not roll over far enough to pull the throttle open more.

You say the distributor is from Napa? It may have the wrong vacuum advance unit on it adding too much timing early. Wrong advance springs? Try switching it to a ported source for the vacuum and see if that helps. Better yet disconnect the vacuum source, plug it and take it for a ride.

To do that test the right way if you have been running full Manifold vacuum You'll need to reset the timing first because if you do not use the vacuum advance the rotor will be in a retarted position.

The initial may be at 12, what's the total mechanical advance and is it returning back to fully retracted weights at idle? Agree with the others, sounds like ignition related. If the distributor is from NAPA chances are the weights and springs are aftermarket and... not necessarily correct.

dpoltzer, I'm thinking it is lean-surge...a condition caused by too much timing advance (especially the vacuum advance) in the distributor. Like someone else previously mentioned, try driving it without the vacuum advance (plug the port first) and it should go away. That will tell you that there is too much advance and you either need a vacuum cannister with less (degrees stamped on the bracket portion) or get an adjustable unit. Factory cannisters are getting difficult to find, especially NOS, so that may not be a viable option.

What is you initial timing set at (vacuum advance disconnected, port plugged, idling at 600 rpm)?

Dennis

Just chiming in here with an observation. Doesnt the quadrajet have step up needles like an AFB ? I have had lugging like that down low with an afb and it was the needle tuning. Just a thought. Never had a Quadrajet car.

Initial advance was at 12 btdc, I adjusted it to 10 btdc and took it for a spin but it was no better. I will try it with the advance disconnected in the next few days to see if that changes anything. Thanks guys.

If it's a lean surge condition you can set the idle mixture screws on the very rich side and see if that helps to make a difference in the off idle / low rpm surge condition when taken on a short drive.

If this helps out and seing as the Carb was done by Cliff and should be fine,I would have to say that there would then be a good possibility of a vacuum leak going on somewhere.

What cam this engine have in it?

Clay

dpoltzer, I'm assuming since you said in your opening thread that the RA3 has a 'Stock' rebuild so it most likely has on 068-style cam and 10.0 (+/- a tenth or two) static compression. Steve 25 mentioned a vacuum leak and that is a good thing to double check as well as turning out the idle mixture screws half to a full turn to see if it still bucks. Do you know what your total mechanical advance is (such as 34 degress all in by 3000 rpm) with the vacuum cannister disconnected? You said your distributor is a NAPA rebuild, so who knows how it is set up and that needs to be checked and most likely tuned (recurved for your specific engine).

I should have said the bucking could also be caused by too much advance along with a lean condition in the carb. Lean-surge is totally carb related but can be exacerbated by too much advance in the distributor.

Happy Thanksgiving All!

Dennis

It's natural. My 4-speed car with a stout cam does the same thing.
You just can't lug them that low with a healthy camshaft.

GT.