Bogging, hesitation, stumbling when warm

[MeToo]

Car is a 1976 Fleetwood Brougham with carburetor. Some time ago it started hesitating and stumbling on heavy acceleration when it was hot.It would only do this when speed got up near 50 mph or greater. It could even cut out completely. I've changed the fuel filter, sprayed carb cleaner through the carb, changed spark plug wires, changed air filter and checked the plugs,all to no avail.

Now its getting worse and worse, it takes less time driving it before it starts bogging, its like the car comes up against a wall, it now does it with less pressure on the gas and at speeds as low as 20-30mph. Funny thing is last week After 50 miles of driving using only light acceleration all was fine until I gave it some harder acceleration, then it started doing it again and just got to the point where you couldn't do more than20mph without the engine cutting out. As I got it into the garage, the engine then started shuddering which its never done before.

When it does this stumbling, the orange fuel indicator light comes on much more readily than normal, even with light acceleration, when it cuts out, I can start it again just fine and I've even had it cut out when I tried revving it in Park.

I'm hoping its not a carb problem as it looks a pain to take apart. Could this be an ignition coil/distributor problem? It starts and idles just fine when cold.
Car has 61000 miles, has always run just fine until now, I use a 98 Octane gas that the oil company says cleans the engine and fuel system as you drive. Its only driven from the late spring through summer and into early fall. In the winter I just start and run the engine in the garage for a bit to keep it active.
What, besides the carburetor could be causing these issues?
Any advice appreciated
Thanks

[Coupe Deville]

Hello. This could be a range of different things. Have you taken off the distributor cap and looked at the condition of the terminals, and the condition of the rotor? How old Is the HEI module? The pickup coil in the distributor can also cause problems like this. Cutting out, missing, stalling, etc.

-Gavin

[76eldo]

If it's getting worse when it gets hotter I would think electronics in the distributor.

Do you have another cap/coil you can borrow from another car as a test?

The modules do go bad but normally they fry and the car will die.

Any popping or backfiring?  If the plugs look sooty you are loosing spark.  If they are white and burned looking you are loosing fuel and leaning it out.

It will be a process of elimination.

Brian

[dplotkin]

Replace the HEI module in the distributor. I'll bet you the price of the module it solves your problem, just let me know.

Dan

[TJ Hopland]

You could start by just unhooking and plugging the vacuum line to the distributor.  That will keep the vacuum advance from operating.  If its cracked pickup coil wires that should make it more consistent.   Could be consistently bad or good.     An even better method would be if you have a hand vacuum pump would be to hook that up and run it through the range with the engine running.  Other option would be to take it apart and hook ohm meter to the coil wires.   

[DeVille68]

Check your fuel supply system. Maybe the return line is blocked, or the tank is not vented anymore due to some deposits.

[James Landi]

I'm thinking weak fuel pump--- as the engine heats, the mechanical arm pinion provides less gasoline volume to the carb.   Might try a fuel pump test before you do anything.   YOu may not have much gas in the carb

[Scot Minesinger]

If it turns out to be carb, which is likely then don't worry it is not that bad.  Daytona sells a super nice rebuild kit with much easier to install needle/seat valve combo - all for under $60 (that price includes float).  Buy the float, which is not included in re-build kit.  My first carb rebuild went carefully and slow and with help of this forum, but worked just as good as any pro.  Now I rebuild them myself for the time savings.

[chrisntam]

Don't forget the fuel filter may be clogged..............

[gary griffin]

This sounds like fuel issue to me, possibly accelerator pump in the carburetor?  Letting too much air in without sufficient fuel makes the mixture too lean and not achieving combustion properly.  If you are burning summer gas there is another possibility. Not knowing your region or seasonal modifications to gas available to you.

A 1976 car with only 61,000 miles and having used different fuel concoctions throughout the years could be just filthy inside the carb.   Also for cars that are not driven often gas can be a problem.  There are hundreds of formulas for gasoline with variations foe season and region and even elevation. A gas purchased in the summer will not perform in the winter as well as it did in the summer. I store 50 gallons of gas in 5 gallon cans for lawn mower and emergency generator. I always use it up in the fall and early winter in my daily driver and replace it with winter gas which is more volatile.

[fishnjim]

Assuming you mean it's at the proper operating temperature and not overheating...
Definitely want to check the fuel system.   If the carb is squirting a full stream, when you give it full throttle when off, the accel pump is probably OK, just starved.  If the stream is weak it needs attention.   Check fuel filters.   See if there's a small screen in the fuel inlet connector on the carb.   It'll plug easily and you don't see it.   If no screen, then the carb might be plugged up.   Remove the top cover on one of the fuel bowls and check for sludge/corrosion.   Pluggage tends to become a problem as the vehicle ages, rusts, and had many fuel stops.  Fuel gets water in it and water causes corrosion.   With filter plugged or restricted, it flows too slowly for full throttle but will catch up when it idles down or starts up again.   Common problem, I had it, and the carb was also full of trash, corroded, and needed rebuilt.
If the carb comes up clean, check pump output/pressure.   
If the fuel system checks, then move to the ignition system.   But it's real easy to hook up the battery powered inductive timing light and check each wire for proper spark to rule it out.  It should flash steadily on each, if OK.  Only takes a few minutes, if you have one.

[Jon S]

OP has changed the fuel filter.  Might be a good idea to re-check the filter to see if it is becoming clogged.  How was the filter you took off?  Sounds like a fuel issue to me.  Weak accelerator pump normally provides a backfire followed by a surge in acceleration.  It could even be an Ethanol-related problem - vapor lock.

[MeToo]

so I've now changed the ignition coil, ignition control module, rotor, dist cap as well as the fuel pump, cleaned out fuel line and have out heat wrap around the part of the fuel line that runs alongside the AC compressor just before it enters the carb-none of these things have fixed the problem. Pick up coil appears to test fine.

The problem is directly related to outside air temp-the hotter it is the sooner it starts acting up, it can do it after as little as 20 miles or if cooler, after 100 miles.

If its a carb issue, I don't see how it would be outside heat dependent. How could it be a carb issue exactly? On a cooler day, the temp gauge will be at normal operating temp for miles without problems so its probably not the choke staying in the cold setting.

Could it be exhaust related? There are no funny smells or unusual smokes from the exhaust but the front of the exhaust looks weird-photos attached.

Could it be the fuel tank not venting properly or in need of a clean? Again I don't see how that would only be a problem when hot.
What about a blocked fuel return line?

This thing is a mystery and is sucking all the fun out of having a Cadillac.

[Julien Abrahams]

I also would think fuel/carb related. Have you checked the gasket between the carb and the intake? Does the '76 also have an exhaust crossover under the carb? Does it have the heat riser in the exhaust manifold? That could also be stuck closed, causing the exhaust gasses to heat the carb until there is fuel evaporation (i.e. not enough fuel in the bowls when needed for acceleration).
I would also check all vacuum lines, and check for a crack in the intake (I encountered that problem). When it warms up, the crack becomes bigger due to expansion and that causes a lean condition: hesitation, bogging, the works.
Good luck.

[TJ Hopland]

Heat riser is a good thought.   It could be inconsistent.   If its staying shut that will certainly raise temps.   If its there it will be attached to the right exhaust manifold and have a vacuum control line.   On a 76 I don't know what access is like.  On the later cars you can't hardly see the darn thing.   Hopefully a 76 still has a little more room.  It will be around the firewall and heater/ac box.     IF you can see it hook a hand vac pump up and see if it moves.   Usually no vacuum signal = open so if it is moving try running it with the vac line disconnected and capped.   I believe its controlled by a thermal vacuum switch kinda around the base of the distributor.

The vacuum controls varied quite a bit in this era so you really need the diagrams for your exact model and emissions class.  Just that could be the problem, someone routed the lines for a CA car on a Federal system.   Could also be one of the sensors is wrong and doing the wrong thing at the wrong time.  Some years the same thermal switch ran both the EFE and distributor.  If those hoses were crossed odd things could happen. 

[beastly beauties]

Just something else to consider along with all of the above ideas.I have experienced these symptoms on several older cars before. I found in my cases after exhaustive testing of all of the above mention for it to be a blocked fuel filter sock, located in the gas tank pick up tube. To check I had a clear fuel filter connected to the fuel line between carb and tank. I had a started the car and watched the fuel flowing in the filter with my hand on the throttle at carb and watched the change in volume upon acceleration. The sock itself picks up more and more debris sometimes in a very short time creating this blockage. When you shut down the engine and restart it would refill the carb but not for very long. You mentioned exhaust system which would also have the same symptoms. Check that having another person working the gas pedal while you check out what's coming out of the tail pipe with a piece of light cardboard (match book cover) held against the pipe. You should have a pretty strong exhaust flow that pushes the cardboard away. Good Luck,   David Symonds

[Jon S]

Reading the thread I'm thinking carburetor, but unlike the posts above I believe your float is set to high.  If you were fuel starving, you would get a carburetor backfire followed by good acceleration.  You described it as a "bog" which implies too much fuel being dumped.

I've solved those problems on other cars by checking/readjusting the float height and float drop and double checking the Timing.  Start with adjusting the timing 2 degrees more advanced and see if that helps or eliminates the problem.

[TJ Hopland]

How many tanks of fuel have you been through?  I have chased issues before that turned out to be bad fuel.

[Jon S]

How many tanks of fuel have you been through?  I have chased issues before that turned out to be bad fuel.

I've never experienced bad fuel even after sitting for 5 years.  I think of the Country's Reserves - they sit for decades.

[TJ Hopland]

I was thinking bad as in contaminated.   It does not happen as often as it used to but it still happens.   You also have to factor in ethanol most of the time now.    Ethanol sitting under some ambient conditions can absorb a lot of moisture fairly quickly then has interesting reactions with just about everything.   

I'm assuming the rest of the country learned how to handle ethanol from the people that have been doing it for 20+ years now.  Early on there was quite a learning curve when it came to the blending and distribution.  I remember cases that went to court where a station had fuel that was like 40% ethanol and 10% water.   It was not intentional it was just the way things ended up due to the overall handling practices at the time.