‘68 GTX 4-Speed Hemi Car Bucking

[Shadetree]

I am looking for ideas on what could be causing my car to “buck” or “jerk” for a lack of better terms. It does it at all speeds, including cruising. It goes away when you give it more than 20% throttle. It seems to be at it’s worst while in the lower gears with zero-light throttle but again it goes away with more throttle. It idles fine but does seem to take forever to warm up and idle on it’s own, even after the choke runs it’s cycle, which is rather long. The car has “bucked” for the 3 years that I’ve had it and the previous owner had the same issue. Some of the things that I’ve done personally- set fuel/air mixture to most vacuum, richened fuel air mixture,
new voltage regulator, new ballast resistor, changed fuel types, changed timing (currently set at 20 initial and 37 total), ran with and without vacuum advance, currently without due to too much timing, changed plugs (NGK GR4 V-Power), changed wires, checked to see if wires have good connection, checked points (super clean), set dwell on dual point distributor, ran the plugs with and without crush washers, changed metering rods & springs, rebuilt the Carter AFB carbs, changed fuel pump. None of that had any positive influence that I could notice. I have included the day I sheet that shows the engine specs. It has 410 gears and I’m pretty certain it has a McLeod Street Clutch. Please be as descriptive as possible when suggesting potential solutions. Thanks!

 

[dadsbee]

4 gear or auto...

 

[Budnicks]

Lean surge (?)
engine way too lean
or something in the ignition system, mechanical or vacuum advance hanging up
or in the actual distributor maybe bad
is it all stock, are the carbs completely stock too

fuel pump (?)
or fuel delivery,
something like the pickup/sending unit or the sock on the bottom inside the tank,
getting clogged & starving it (?)
hole somewhere in the line sucking air (?)
fuel filter (?)

points, condenser, I assume still,
maybe 1 set of points grounding out intermittently,
bad + positive 12vt (9vlt after the ballast resister) wire inside distributor
or cracked or worn/broken wire, on one of them under the insolation on the wire
hanging on by 1 or 2 strands of wire (?)
or bad - ground somewhere, inside the distributor (?)
Advance Plate inside the distributor all wonky, worn out (?)

fried/leaking/cracked tower on the ignition coil (?)


bad engine to chassis/body ground, chassis to battery grounds/rust, oxidation or corrosion (?)

corrosion/oxidation or rust between any of the connections
same thing maybe
in the main 'bulkhead connections' on the firewall
they are know for that after getting wet or just sat for a while



clogged air-filter or fuel filter (?)

the dude driving behind the wheel

sorry
all I got for now

 

[hunt2elk]

I'm guessing you are running lean.

 

[Shadetree]

4 gear or auto...

4-speed

 

[Shadetree]

I'm guessing you are running lean.

I have richened the fuel/air mixture and tried richer metering rods (even too rich) and neither changed the symptom. I may try to go super rich on the fuel/air mixture screws just to completely rule that out once and for all though.

 

[Shadetree]

Lean surge (?)
engine way too lean
or something in the ignition system, mechanical or vacuum advance hanging up
or in the actual distributor maybe bad
is it all stock, are the carbs completely stock too

fuel pump (?)
or fuel delivery,
something like the pickup/sending unit or the sock on the bottom inside the tank,
getting clogged & starving it (?)
hole somewhere in the line sucking air (?)
fuel filter (?)

points, condenser, I assume still,
maybe 1 set of points grounding out intermittently,
bad + positive 12vt (9vlt after the ballast resister) wire inside distributor
or cracked or worn/broken wire, on one of them under the insolation on the wire
hanging on by 1 or 2 strands of wire (?)
or bad - ground somewhere, inside the distributor (?)
Advance Plate inside the distributor all wonky, worn out (?)

fried/leaking/cracked tower on the ignition coil (?)


bad engine to chassis/body ground, chassis to battery grounds/rust, oxidation or corrosion (?)

corrosion/oxidation or rust between any of the connections
same thing maybe
in the main 'bulkhead connections' on the firewall
they are know for that after getting wet or just sat for a while



clogged air-filter or fuel filter (?)

the dude driving behind the wheel

sorry
all I got for now

Thanks a lot for all those possibilities. I’ve tried some of those but didn’t have them listed so I added that info to my post. I’ll definitely be looking into each of the others until I come up with the culprit. Except “the driver” possibly lol

 

[dadsbee]

My hemi 4 gear bucks trying to go slow down our gravel road. Other than that once in the gas she goes like a scalded cat. You're not running too high of gear for your speed are you?

 

[Shadetree]

My hemi 4 gear bucks trying to go slow down our gravel road. Other than that once in the gas she goes like a scalded cat. You're not running too high of gear for your speed are you?

No, it even does it cruising at 55 MPH but the “bucking” feels exactly like what you’re talking about when you’re in too high of a gear for the speed.

 

[Fran Blacker]

37* total a bit much if accurate and what are the cam spec's. dadsbee think mentioned going to slow for the gears your in. I call it cam lope.

 

[kiwigtx]

Sounds like some slop (too much play) in the four-speed gears and universal joints....possibly also the rear gears.

Idle maybe slightly high also if this happens while idling in gear down a driveway at low speed etc.

I know the feeling....it doesn't sound quite right.

 

[Shadetree]

37* total a bit much if accurate and what are the cam spec's. dadsbee think mentioned going to slow for the gears your in. I call it cam lope.

Hopefully this picture is clear enough

 

[Shadetree]

After reading some other threads, the symptoms sound exactly like can surge. Any suggestions on diagnosing and or fixing?

 

[Jerry Hall]

Cam surge, I called it riding the cam back in the day, for lack of a better term. My problem was caused on purpose back then by wanting it faster and faster. To much cam, but I had a high 10 second 383 at the strip. My question is about the dyno sheet. Why did you stop at 6100 rpms as it was still making power?

 

[Shadetree]

Cam surge, I called it riding the cam back in the day, for lack of a better term. My problem was caused on purpose back then by wanting it faster and faster. To much cam, but I had a high 10 second 383 at the strip. My question is about the dyno sheet. Why did you stop at 6100 rpms as it was still making power?

I didn’t own it at that time, so I’m not sure.

 

[Fran Blacker]

I'm waiting to see this figured out. Was surprised with 4.30 gears you'd have cam lope. I had it with 246*@ .050 with 3.54's. Trying to think what it could be, Ignition?

 

[4vnut]

Distributor shaft play? I had a 4-speed 383 66 Satellite that new bushings for the distributor shaft fixed my bucking. Hope you find the problem.

 

[Shadetree]

I'm waiting to see this figured out. Was surprised with 4.30 gears you'd have cam lope. I had it with 246*@ .050 with 3.54's. Trying to think what it could be, Ignition?

Ok, despite having Covid I pushed through some trouble shooting this morning, which wasn’t wise, I know. But, I believe that I may have found some of, if not all of the problem. First, I hooked the vacuum gauge and timing light up and let the car warm up, then I proceeded to back out the fuel/air mixture screws that were previously set to most vacuum plus a 1/4 more out, to 3/4 turn richer each, which to my surprise didn’t affect the vacuum much at all, but Note-The screws are sensitive when turned inward greater than the original setting. I then test drove it and it seemed to help quite a bit but something still wasn’t right, it was still surging or bucking. I then decided to test a drastic change in the timing, so I started by advancing it 8 degrees more and that made it surge worse when I test drove it. So, I retarded the timing 8 degrees from the original setting and that helped tremendously. There were two marks on the harmonic balance, one could barely be seen and the other clearly seen. I wrongfully assumed that the clearly marked spot was correct which was giving it too much timing and causing what I believe now to be a cam surge. I’ll add that I could hear the cam when before I couldn’t. I ran out of energy, but I will feel more confident when I can drive it more but I think that did it. I do have another question now, though. I am now setting at 12 degrees initial timing and my distributor is turned as far left as the vacuum advance will allow before hitting an intake bolt. I find that odd. I would think that 12 degrees would be somewhat close to the middle of the distributors range of motion. I’m curious where other distributors in 426 Hemi Engines are pointing now and if I could possibly be off a tooth. Please post a picture of your distributors direction and any thoughts. Thanks for everyone’s time!

 

[Jerry Hall]

Looks like you are headed in the right direction.

 

[Fran Blacker]

I wouldn't go by where someone else set there's. Sounds like your going in the right direction. Now make small adjustments one at a time.