Idling Issues
Thread Starter
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Mountain Pass
Joined: Dec 2017
Posts: 119
Likes: 3
Idling Issues
Hello, thanx for looking.
Have a 78 F150 with 351m and stock 2 barrel, the biggest one. It starts right up every time and idles good. As it gets warmer it idles rougher. When it is at full operating temp, it will hardly run.
I have tried adjusting the carb. The idle screw is all the way in as far as the spring will let it go. And have adjusted the idle mixture screws everywhere from 2 turns out to 7 turns out. Currently at 4 turns out and still gets rough. At this point it idles kind of high at start up.
It has the tube that goes from the carb to the intake for fast warm up. Also has a mechanical thermostat.
The engine may get just a little too hot even though it has a hefty radiator that I flushed once.
The carb was rebuilt about a year ago but has been the same. Could something in the carb be gummed up? Is there an adjustment I am missing? Should I open the thermostat up more to let it breath? I think if I do then the butterfly wont close all the way for start up.
Should I dump the carb and get something else?
Have a 78 F150 with 351m and stock 2 barrel, the biggest one. It starts right up every time and idles good. As it gets warmer it idles rougher. When it is at full operating temp, it will hardly run.
I have tried adjusting the carb. The idle screw is all the way in as far as the spring will let it go. And have adjusted the idle mixture screws everywhere from 2 turns out to 7 turns out. Currently at 4 turns out and still gets rough. At this point it idles kind of high at start up.
It has the tube that goes from the carb to the intake for fast warm up. Also has a mechanical thermostat.
The engine may get just a little too hot even though it has a hefty radiator that I flushed once.
The carb was rebuilt about a year ago but has been the same. Could something in the carb be gummed up? Is there an adjustment I am missing? Should I open the thermostat up more to let it breath? I think if I do then the butterfly wont close all the way for start up.
Should I dump the carb and get something else?
Laughing Gas
Joined: Sep 2020
Posts: 875
Likes: 158
Get a can of starting fluid lightly mist all vacuum lines carb even intake manifold. Idle will change, increase if a vacumn leak sucks the starting fluid in.
Give the airlifted a light mist so you can hear what the starting fluid does. You have to be careful not to let the fluid be pulled in the filter or carb as your misting the vacumn lines and carb base area.
Give the airlifted a light mist so you can hear what the starting fluid does. You have to be careful not to let the fluid be pulled in the filter or carb as your misting the vacumn lines and carb base area.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
It sounds from the description that the idle mixture, and the carb generally, is running way too rich. That's why it runs "OK" when the engine is first started, when cold, and as it comes up to temperature it starts to run badly.
A sharp engine tune is just a bunch of different things that all have to set right and be in spec, (Compression, Ignition, Ignition timing, manifold vacuum) before messing with the carb, so it in turn will respond correctly to basic adjustments.
This is one reason why pretty much the first thing everybody always wants to do is bolt on a new carburetor, when truth be told it should probably be the last. It does sound like it's pretty gummed up inside though. Cleaning a carb once a year probably isn't too often, though nobody does.
How do the spark plugs look?
A sharp engine tune is just a bunch of different things that all have to set right and be in spec, (Compression, Ignition, Ignition timing, manifold vacuum) before messing with the carb, so it in turn will respond correctly to basic adjustments.
This is one reason why pretty much the first thing everybody always wants to do is bolt on a new carburetor, when truth be told it should probably be the last. It does sound like it's pretty gummed up inside though. Cleaning a carb once a year probably isn't too often, though nobody does.
How do the spark plugs look?
More Turbo
Joined: Feb 2018
Posts: 710
Likes: 3
From: Oregon
Do you have an EGR valve? It could be stuck open. There is usually a temperature controlled vacuum valve that fits to the EGR, keeping it from operating at all until the engine is warm, but it's also not supposed to ever open at idle. Perhaps it's open at idle for some reason once it's about to get vacuum through the temp valve
Thread Starter
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Mountain Pass
Joined: Dec 2017
Posts: 119
Likes: 3
Thank you for the responses.
I will have to check the choke and vacuum lines tomorrow. However, it does run too rich and thats why i tried to adjust the mixture screws in but any less than 4 turns it would idle rougher. I suppose another rebuild is not out of the question. It also falls flat if i give it more than mild acceleration. Never can give it much throttle, even at speed it will fall flat.
It has an air induction tube that goes from the grill to the air cleaner. Do these choke the engine on acceleration?
I believe the egr has been removed.
New plugs, air and fuel filter
I will have to check the choke and vacuum lines tomorrow. However, it does run too rich and thats why i tried to adjust the mixture screws in but any less than 4 turns it would idle rougher. I suppose another rebuild is not out of the question. It also falls flat if i give it more than mild acceleration. Never can give it much throttle, even at speed it will fall flat.
It has an air induction tube that goes from the grill to the air cleaner. Do these choke the engine on acceleration?
I believe the egr has been removed.
New plugs, air and fuel filter
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
When was the last time it had a good thorough Tune-Up? Compression test? Vacuum test?
Is the Ignition providing a good HOT spark, is the mechanical and vacuum advance ignition timing functioning normally, carburetor float adjusted? Power valve OK?
Typically a stock Autolite/Motorcraft carb is going to end up with the idle mixture screws maybe 1/2 to 3/4 of a turn out, somewhere in there.
Vacuum leaks are the great bugaboo in carb adjustment, and gum or varnish in the fuel passages and air bleed orifices. But, if the engine itself isn't tuned up and providing the right signal, you'll pull your hair out trying to adjust the carb, it simply will not respond the way it should.
Is the Ignition providing a good HOT spark, is the mechanical and vacuum advance ignition timing functioning normally, carburetor float adjusted? Power valve OK?
Typically a stock Autolite/Motorcraft carb is going to end up with the idle mixture screws maybe 1/2 to 3/4 of a turn out, somewhere in there.
Vacuum leaks are the great bugaboo in carb adjustment, and gum or varnish in the fuel passages and air bleed orifices. But, if the engine itself isn't tuned up and providing the right signal, you'll pull your hair out trying to adjust the carb, it simply will not respond the way it should.
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Lead Driver
Joined: Oct 2018
Posts: 7,892
Likes: 2,370
The intake tube would help more than it hurts.
When your engine is up to temperature it should be wide open . start with that since it's the most likely and the easiest fix.
When your engine is up to temperature it should be wide open . start with that since it's the most likely and the easiest fix.
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Thread Starter
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Mountain Pass
Joined: Dec 2017
Posts: 119
Likes: 3
Finally had time to check it out.
The choke pull off is working fine.
I sprayed starting fluid all around the base and every vacuum hose. There was no change in idle speed at any time. The large hose coming out of the back of the carb going to the crank case vent is a little loose but no change when sprayed. I pulled the hose off and the enginge revved up.
I was thinking it could be a vacuum leak since the idle screws are turned out so far but now I dont think that is the case since it revved when i pulled the hose off.
I wonder if it could have anything to do with getting hot?
The choke pull off is working fine.
I sprayed starting fluid all around the base and every vacuum hose. There was no change in idle speed at any time. The large hose coming out of the back of the carb going to the crank case vent is a little loose but no change when sprayed. I pulled the hose off and the enginge revved up.
I was thinking it could be a vacuum leak since the idle screws are turned out so far but now I dont think that is the case since it revved when i pulled the hose off.
I wonder if it could have anything to do with getting hot?
Lead Driver
Joined: Oct 2018
Posts: 7,892
Likes: 2,370
What you described amounts to a rich condition if I'm understanding correctly, vacuum leaks cause a lean condition . while it's idling slowly cover the air horn with a rag and see if it speeds up or slows down.
While your description doesn't really sound like timing I'd play with the timing a little and see if it improves things.
if timing doesn't help my suspicion would be a screwed up carb.
While your description doesn't really sound like timing I'd play with the timing a little and see if it improves things.
if timing doesn't help my suspicion would be a screwed up carb.
Cross-Country
Joined: Oct 2020
Posts: 57
Likes: 0
From: Usa
I am also having carb issues. I was trying to get a fuel filter that screws straight into the carb. but it is not the original carb. The parts store suggested I go "inline".
I bought a grommet. gas line, and filter.
Should I have not done that? Will it work? The parts store couldn't find a filter that "screwed" directly into the carb.
I bought a grommet. gas line, and filter.
Should I have not done that? Will it work? The parts store couldn't find a filter that "screwed" directly into the carb.
