79 403, been trying to solve this for over a year. Engine is stock with general maintenence.

Car starts great, idles and runs good cold. Good power and revs out fine when cold. Once driving for 30 or so minutes the problems begin. Whats shown in the linked video is how it begins to act. It will downshift and pull through 1st, then getting through second it stumbles, loses power, and falls. if i let off the throttle the engine returns to an idle like nothing happened.
after say 10 more minutes it gets worse, driving off from a light, or trying to pass someone does the same stumble thing, no matter gear or moderate/heavy engine load.

Ive checked over fuel, hei, timing, choke, wires/plugs... Seen this before? Any help? Its been driving me nuts trying to tinker with it, only to drive around for 30ish minutes untill its hot and does the same thing... such a waste of time and fuel. Thanks all.

https://youtu.be/2YoRVdefd6w?si=gK6UpGxQF20HBPbc

 

Years ago my engine would do the same thing. I found it was running out of fuel at the upper RPM, and lowering the float slightly cured the issue. Since yours keeps doing it at part throttle I’m guessing it’s not gonna be such a simple fix.

 

If it is an after warmed up, maybe HEI coil or module is having issue when hot. If you have another set try that if you think ignition is cutting. Also make sure your volts are good(battery in good shape and charging properly)

if it’s fuel starvation, does it matter how much is in tank? Does it do it it full or empty?
- maybe the sock in the tank is collapsing
- double check filter(s)
- might even try removing carb filter, make a few passes and see if changes.
- - I have had a small piece get in needle and seat area and it would not refill bowl quick enough, but if it runs hard when cold this does not sound like the issue
could be vapor issue when hot.
- make sure tank is venting properly, might be enough vent space for a while and then to hard to pull fuel after driving a while

 

I am just now changing the new airtex fuel pump I put in 5yrs ago with a new Carter pump, and carb fuel filter.

I can suck/blow through the fuel cap. Happens at any tank fuel level.

I was thinking of the sock. I cut open the carb filter and there is some debris in the pleats. I hope its not the sock i was in the tank 5yrs ago and it looked fine, tank was clean. Not ruling it out.

I just put a new battery in this morning.

I tried an ngk coil and standard module, one module made the car a no start. I put back in the old oem module. I have a few more to try.

 

It sure sounds like fuel. I am fighting the same thing but plugging in and regulating the big in tank fuel pump to keep up. There could be a pin hole in the fuel pump diaphram. Those fuel filters new are restrictive, full dirt might be enough to cause starvation issues by itself. I had that issue with the 403 in my 88 Cutlass at the 1/8 mile track.

 

^^^THIS^^^

Next time it dies swap the HEI module and report back. Make sure you use the thermal paste on the bottom of the module.

You can get this at Amazon. I'm having difficulty with the link.

https://www.arctic.de/en/MX-4/ACTCP00002B

 

If your running the in-carb Fram fuel filter with goofy check-ball assembly , put a new filter without that crap and try again,, they won't support WOT performance very long, very restrictive in place.

 

Even though it is likely fuel, try running with the vacuum advance disconnected. If that stops the symptom the pole piece with the green and white wire in the distributor is bad.

 

Unless you checked your fuel with a pressure gauge right at the carb, you didn’t really check your fuel delivery

 

I think that entire check valve assembly can be removed from the filter to turn it into a non-check valve filter. I worked at auto parts stores in school and I remember looking at both styles and the filter element was identical.

 

Thank you for all the replies. Good news is the issue has changed. After replacing the fuel pump, I drove the car for about an hour to get it nice and hot. Got on it from a 20 roll and let it wind out 1st, almost wind out second, and at about 4000rpm its started to break up, but kept surging back into it. Bumping higher and higher. So not a total flat on its face like before at high rpm runs, and definitely not just coming off a stoplight like before... Not the complete result I was hoping for, but we're headed the right direction.

The new pump felt wayy tighter moving the arm by hand, so I think that was part of the problem, maybe the check ball in the filter is the last peice of this? I didnt think it would make a big difference... next sunny day (thursday) i am going to cut out the check ball and reinstall the filter.

 

Check the fuel hose at the tank closely for cracking, also try running it with the fuel cap loose in case of a restricted vent. If it is fuel it could also be a fuel tank sock clogging. Keep looking and checking you'll find it.

 

check the wires on the pole piece they used to break and would be held together by the insulation around the wire when the vacuum advance would move the wire would open

 

Nice, and by the way the engine sounds great at WOT before the issue kicks in.

Yes, had this issue with 69.

Lunched what you describe at higher RPM my engine was doing at the track when I first got her. It was a combination of hose leak, lousy air fuel mixture and lousy gas. On occasion 2/3rds of the way down the track the acceleration would seemingly cut in and out, beyond frustrating of course because each occasion was a wasted pass. It happened after the 1-2 shift and roughly around 4000 rpm...