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Car starts to stumble and buck when warmed up

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6:33 AM
Joined
May 21, 2024
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Location
Illinois
So I'm having a weird issue here. Bone stock 2 barrel 273.

When the car is cold, it runs perfectly fine. Great throttle response, idles smooth, cruises good. Im right on the edge of town. So the other day I started the car up, hopped right on the 2 lane road by my house, and it gets up to speed and cruises at 55-60 with no issues

Well today I tried going to work. It drove through town good. But after about 8-10 minutes, it starts to stumble and buck. Trying to accelerate from 40 to 55 takes some patience, but it runs well enough to go 55. Still stumbling but it maintains speed. After 15 minutes i decide to turn back around. Now coming into town I can barely go 45 because of how bad its running under throttle. Driving through town its bucking bad, hard to accelerate or go 30mph lugging the engine at the bottom of 3rd. It's not bucking all the time, it will run good for a bit, then start to stumble, then run good again, etc. After half an hour im driving down my street and even at the top of 2nd gear its running rough. At idle it runs fine but occasionally bogs/slows down. It didnt stall in my experience

This didn't just start happening overnight though. About a month ago, I noticed the car would miss/stumble for just a moment at 55 mph once every few minutes. And its slowly gotten worse since then.

The problem here is I'm not sure what it could be that I havent fixed already. Carb was rebuilt 2 years ago along with new plugs, wires, fuel pump, and a distributor/vacuum advance. New ignition coil last year. Recently put a new ballast resistor on along with a new fuel sending unit. All the rubber fuel lines have been replaced here and there so theres no old rubber lines there. Choke looks like its working as it should

Anyone have some ideas here? My best guess is something inside the carb has failed already after the rebuild. The car sits over the winter and i never used fuel stabilizer so maybe theres already something clogged by ethanol
 
An obvious fuel delivery issue, no doubt at least partially caused by the corrosive crappy gas we have today.
The carb is rich enough when the choke is on, but leans way out once it's warmed up, to the point of the
car falling down under normal acceleration.
Could be a filter, could be the pickup in the tank - heck, it could even be in the carb itself.
Nothing to do but to start at one end - check the fuel pump output to see if there's enough flow, then work
your way towards the carb from there.
 
An obvious fuel delivery issue, no doubt at least partially caused by the corrosive crappy gas we have today.
The carb is rich enough when the choke is on, but leans way out once it's warmed up, to the point of the
car falling down under normal acceleration.
Could be a filter, could be the pickup in the tank - heck, it could even be in the carb itself.
Nothing to do but to start at one end - check the fuel pump output to see if there's enough flow, then work
your way towards the carb from there.
It must be something under the hood then. It runs good enough on cool dense gas. But once it gets warm under the hood and it thins out, somethings going wrong
 
First you have to see if you still have spark and fuel when it is happening that will give you the route to go to.


Also can be the ignition condenser starting to short out. Look at the points they will tell the story.

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Check the easy vitals, then, I am thinking fuel pump.
 
I'd check for vacuum leaks when warm. It's not uncommon for a leak to show up around the intake when warm.
 
I'm going with a guess of the ignition coil.
 
Same here - coil going bad and breaking down. The motor may run fine upon a cold start but once the coil starts heating up it starts functioning erratic.
 
Really it could be any of the new parts having gone bad, so many are junk right out of the box.

Im going with junk in the carb for the win though.
 
My vote is the pickup screen in the sending unit.
I've seen fibers from the sound deadening mat get in there.
When it sits they settle to the bottom of the tank.
The longer you drive it the more they get sucked up and restrict the sock on the pickup.
It sounds like what's happening here.
But there's only 1 way to find out.
 
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