Stumble on rapid acceleration
Thread Starter
Cruising
Joined: Mar 2025
Posts: 13
Likes: 7
From: Manchaca,TX
Stumble on rapid acceleration
76 L 48 4 speed acceleration to 3500+ rpms shift to 2nd accelerate to 3500 + rpms, shift to 3rd accelerate to 3500+ rpms car starts bucking. Let off and it smooths out. 1st time I had problem checked fuel pressure 3 psi, changed out fuel pump seemed to cure problem. Next time it happend took top off Q jet and raised float level, seem to work ok. Checked timing 36 degrees full advance at 2500, EGR valve is working as it should. Just looking for advice on where to look next. I did have the AC on this time when it started to stumble I didn't think to turn off AC.
Advanced
Joined: Mar 2006
Posts: 68
Likes: 28
How many miles on it?
Worth looking at the secondary metering rod cam.
They do wear out, get loose on shaft and I have seen a few broken.
My H/O had similar symptoms a few years back.
turned out to be just wore out.
Got a complete kit plus extras from Cliff and it took care of it.
Worth looking at the secondary metering rod cam.
They do wear out, get loose on shaft and I have seen a few broken.
My H/O had similar symptoms a few years back.
turned out to be just wore out.
Got a complete kit plus extras from Cliff and it took care of it.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,002
Likes: 4,158
From: US-PNW
Your 36° at 2500rpm is pretty aggressive, but that should reveal itself by pinging.
One of the truisms about engine issues is to look at all ignition components before blaming the carb.
It only happens in 3rd @3500, not 4th at any rpm?
What about 1st & 2nd above 3500?
What brand is your HEI module and how old is it?
One of the truisms about engine issues is to look at all ignition components before blaming the carb.
It only happens in 3rd @3500, not 4th at any rpm?
What about 1st & 2nd above 3500?
What brand is your HEI module and how old is it?
Thread Starter
Cruising
Joined: Mar 2025
Posts: 13
Likes: 7
From: Manchaca,TX
Thank you to everyone who took the time to respond to my question. The car has 22718 miles on it. It is numbers matching and everything seems to be OEM parts, distributor is AC delco. As far as I know everything is as old as the car.
previous owner told me I was 4th owner with mileage each time car was sold. motor has 21.5 -22 in of vacuum at idle so I would think the mileage is correct, although the speedometer is off by 10 mph at 60 mph is really 70 mph. I changed out the fuel filter when I had the top off carburetor (17056207), I will change it out again and pull check valve out of new filter. Weather is not looking good for test drive after work so it maybe next week before test drive. Rain in forecast.
previous owner told me I was 4th owner with mileage each time car was sold. motor has 21.5 -22 in of vacuum at idle so I would think the mileage is correct, although the speedometer is off by 10 mph at 60 mph is really 70 mph. I changed out the fuel filter when I had the top off carburetor (17056207), I will change it out again and pull check valve out of new filter. Weather is not looking good for test drive after work so it maybe next week before test drive. Rain in forecast.
Advanced
Joined: Sep 2021
Posts: 66
Likes: 3
10 mph off would make me question “all original” PS Lars knows his stuff. I used his vacuum reduction timing part and it worked as advertised and setting up my 67 427 435 hp as per his instructions worked great. I had set up the distributor to have 15 degrees initial timing for a total of 36 but it worked way better after following Lars’s guidelines has it running better than it ever has. Thanks Lars!
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Thread Starter
Cruising
Joined: Mar 2025
Posts: 13
Likes: 7
From: Manchaca,TX
Sorry about the delay in getting back with work I did. While I had the top off carb I raised the float about a 1/16” pulled the check valve out of the filter, checked timing 36 degrees at 2500,, new gas cap non locking. This past weekend I drove up to Killeen to an Auto X and the car ran and pulled good, no stumbling. It was my first Auto X had a great time can’t wait for next one. Thanks to all for help.