How could this be a carburetor issue?

cadfan82

Car is a 1976 Fleetwood Brougham with Rochester Quadrajet carb. In hot weather, once you get over about 40mph it stumbles and bogs and just won't accelerate at all. Its like the car comes up against a wall, if you press the gas pedal harder the engine will die. I can then start it again but after a few minutes the same problem will occur. It seems to occur once the air temp reaches a certain level, eg on a hot day it can start playing up after as little as 20 miles of driving and others its taken 100 miles before it played up. In those first 100 miles I could accelerate hard with AC running, no issues. As I got it into the garage, the engine then started shuddering which its never done before

Car starts and idles just fine, no black smoke or funny odors from exhaust, I've replaced the fuel filter (old one didn't look too bad),air filter, sprayed carb cleaner in the carb, replaced PCV valve, fuel pump, spark plug wires, ignition coil, ignition module and put heat wrap around the section of fuel line near the manifold and carb.

How could this be the carb? The choke would have gone to its warmed up setting well before 100 miles and engine temp was at its normal reading well before then too.
I don't understand how if this is a carb problem,how it would only show up once it got hot enough-even on a cooler day there is still plenty of heat under the hood. What sort of carb problem could it be? If its not a carb issue, then what else could be causing it?

Thanks

thecardoc3

If it is a ""fuel" problem, you have described a fuel supply issue. This could be the fuel pump failing, as well as the fuel sock in the tank plugging up. When the in tank sock gets restricted, that can make it difficult for the pump to draw fuel from the tank. That causes a vacuum between the tank and the pump which results in the fuel boiling and the symptom commonly referred to as "vapor lock".

It cannot be ruled out that you are losing spark at this point. You might not be losing it completely, but it may be to weak under certain conditions to fire the spark plugs.

cadfan82

So you don't think its a carb problem at all? I've replaced the fuel pump and checked the spark plugs-no issues with plugs themselves.
Could it be a blocked vapor return line?

thecardoc3

When you stated that sometimes it takes a hundred miles to act up, that pretty much makes it very unlikely that it is a carburetor issue.

cmhj2000

Has the carb been torn down and redone properly? You mentioned spraying carb cleaner in it, that does little and still does not address float level, condition of float and inlet etc. I worked on these old QJ for years and they can be touchy

thecardoc3

cmhj2000 said:

Has the carb been torn down and redone properly? You mentioned spraying carb cleaner in it, that does little and still does not address float level, condition of float and inlet etc. I worked on these old QJ for years and they can be touchy

Its very unlikely to be a carb issue, they don't come and go like that. Yes there could be issues where a needle valve (float) sticks and you could get some random fuel levels in the bowl, but that is much more repeatable than the O.P. is describing. Besides a float issue is far more likely to stick open and flood than closed and the O.P. put some effort into trying to suggest that is already ruled out. If I was working with this and experiencing the symptom, supplying secondary fuel, quickly measuring the float level (which can be done without removing the air horn) as well as checking for maximum spark output would confirm what's working and what is not and give a sense of direction.

His description is pretty specific, it has to be hot both from an ambient as well as vehicle operating condition. Both of those factors make vapor lock, or fuel volume to the carb far more likely causes. The reason that spark is still in play is that if the secondary ignition system output is weak, it could have just enough spark while the engine is running richer to fire consistently, but the moment the system starts to get lean the ignition system might not be able to fire the spark plugs when the throttle is opened and timing retards. That actually means both spark demand and fuel volume could be partially to blame. There is a lot more to the "feel" that goes into diagnosing issues like this, but this is the first step to understanding it, and how to learn it.

texases

Have you replaced all fuel filters? IIRC, there might be one at the carb. One check - drive it until it dies, pull off the road (far off the road), remove the air cleaner and look down the carb as you pump the gas. If you don't see gas squirting into the carb, it's a fuel supply problem.

Edit - I checked, there's a small filter INSIDE the carb - did you replace it?

thecardoc3

Here is an article that demonstrates checking the float level with the carb assembled. http://www.underhoodservice.com/diagnostic-dilemmas-servicing-quadrajet-carburetors/