Rough idle when warm
- Thread starter marilyn
- Start date
Daves55Sedan
Well-known member
Could be the pertronix module breaking down under the heat, but could be other things too.
If you increase engine speed and the roughness disappears, I would say that's a fuel system problem, for instance faulty fuel pump, or carburetor could be close to flooding due to high fuel pressure or a breached float in the carb, or inlet needle stuck in the seat.
Older cars like this that are left sitting for several months with ethanol gas in the tank and in the carb float chamber can cause fuel system problems. All gas powered machines nowadays need this attention. If ya aint gonna run it, DRAIN it. Gasoline that is. Cars, lawnmowers, roto-tillers, chainsaws.
All my lawn equipment is gas powered and everything gets drained at the end of the season. They sit bone dry all winter.
If you increase engine speed and the roughness disappears, I would say that's a fuel system problem, for instance faulty fuel pump, or carburetor could be close to flooding due to high fuel pressure or a breached float in the carb, or inlet needle stuck in the seat.
Older cars like this that are left sitting for several months with ethanol gas in the tank and in the carb float chamber can cause fuel system problems. All gas powered machines nowadays need this attention. If ya aint gonna run it, DRAIN it. Gasoline that is. Cars, lawnmowers, roto-tillers, chainsaws.
All my lawn equipment is gas powered and everything gets drained at the end of the season. They sit bone dry all winter.
Start with a full, complete tune-up like we did in the old days. First, adjust the valves. Next, a compression test (it'll never idle smooth with burned valves). Replace the spark plugs. Check the cap, rotor and plug wires and replace as needed. (Look for carbon tracks in the cap and splits in wire insulation and cracked boots.) As engine warms up watch for automatic choke to gradually open. Blip throttle to release fast idle cam and set idle speed to factory specs and adjust timing to factory specs with a timing light NOT a vacuum gauge. Lightly seat each idle mixture screw and back each out 1 1/2 turns. Now hook up a tach and turn one mixture screw in or out to get highest idle RPM. Again, leave the vacuum gauge in the trunk or somewhere. Go by RPM. Repeat with the other screw. Do each a couple times more to get them balanced and reset idle speed as needed.
If everything is within factory specs, it WILL idle smoothly! If it still idles rough, now is the time to start troubleshooting. Look down the carb and see if any fuel is dribbling out of the venturis. This will cause flooding. Now hook up the vacuum gauge and look for a steady reading around 20". If the needle is wavering or low, look for a vacuum leak at carb base or intake manifold gasket. Check for loose manifold bolts.
These are most of the basics. Always start with the basics. As mentioned, stale gas could be the culprit. As gas dries up in a tank, it leaves a varnish-like residue. Fresh gas dissolves this goo and when it burns in the cylinders, it leaves a sticky coating on the exhaust valves which makes them stick open.
If everything is within factory specs, it WILL idle smoothly! If it still idles rough, now is the time to start troubleshooting. Look down the carb and see if any fuel is dribbling out of the venturis. This will cause flooding. Now hook up the vacuum gauge and look for a steady reading around 20". If the needle is wavering or low, look for a vacuum leak at carb base or intake manifold gasket. Check for loose manifold bolts.
These are most of the basics. Always start with the basics. As mentioned, stale gas could be the culprit. As gas dries up in a tank, it leaves a varnish-like residue. Fresh gas dissolves this goo and when it burns in the cylinders, it leaves a sticky coating on the exhaust valves which makes them stick open.
willowbilly3
Well-known member
If it were me, I'd start with a hand held vacuum gauge. That one little, inexpensive tool can tell you so much and would reveal right off if it was leaking valves, carburation, timing or a tired engine.
Daves55Sedan
Well-known member
Actually, I did get a couple days advance warning that my Pertronix was going bad.
One morning I came out to go to work as usual, but the engine didn't want to start right off like it usually does. Acted the same way when I went out to come back home. Luckily, the next day was Saturday and I was about to begin my weekend errand run. The car would not start at all.
I jumped out and did an old school spark check. Nothin. I knew it was the Pertronix at fault.
I had kept the old points and condenser. I tore out the pertronix and re-installed the old points and condenser.
She fired right up like a champ.
Johnnydidd
Well-known member
My 56 Merc did the same thing, I put a 1 inch insulating spacer between carb and manifold works great idles fine hot or cold.
Diagnostics.........most definitely. But for making mixture adjustments and timing? No way! Trying to set timing and idle mixture with a vacuum gauge (or "by ear") is like building a china cabinet with an axe! Sure, it can be done but will look and function like crap. After $5,000-10,000 for an engine rebuild, another $100 or so for a timing light with built-in tach should be a no-brainer.
scicala
Well-known member
"Diagnostics.........most definitely. But for making mixture adjustments and timing? No way! Trying to set timing and idle mixture with a vacuum gauge (or "by ear") is like building a china cabinet with an axe! Sure, it can be done but will look and function like crap. After $5,000-10,000 for an engine rebuild, another $100 or so for a timing light with built-in tach should be a no-brainer. "
I have to disagree on this one. Mixture can be adjusted just as well using a vacuum gage, tach or ear. A matter of personal preference. or if you have a hearing issue.
Sal
I have to disagree on this one. Mixture can be adjusted just as well using a vacuum gage, tach or ear. A matter of personal preference. or if you have a hearing issue.
Sal
JeffB2
Well-known member
It can be done just fine in takes a knack,some guys have it some guys don't, some of us old farts could adjust carb idle by watching the ripples in a glass of water while we tuned
The old time mechanics that actually fixed things are a dying breed that became "parts changers" and then evolved into "technicians" kinda 
in a way.While you can make an engine idle smoothly by ear or vacuum gauge, testing with an exhaust analyzer will usually show that the fuel mixture is excessively rich, washing the cylinder wall, diluting the oil, wasting fuel, and adding to air pollution needlessly. There is a range of idle screw adjustment where vacuum and idle speed stay the same but the mixture can change from OK to rich. At light part throttle loads the engine is still feeding from the idle circuit as it transitions to the main circuit, so a rich idle adjustment will lower fuel mileage.
Use a tach to set the mixture screws to the highest RPM, going back and forth a few times to get the highest RPM. Reset idle speed as needed to maintain specified RPM. Then turn one screw in (lean) to drop about.10 RPM and do the same with other screw to drop another 10 RPM. Reset RPM to specs. This method is "actually fixing things" instead of "sounds good to me".
Use a tach to set the mixture screws to the highest RPM, going back and forth a few times to get the highest RPM. Reset idle speed as needed to maintain specified RPM. Then turn one screw in (lean) to drop about.10 RPM and do the same with other screw to drop another 10 RPM. Reset RPM to specs. This method is "actually fixing things" instead of "sounds good to me".