1979 Other Plymouth Models Rough Idle

Tiny
CARHO50
  • MEMBER
  • 1979 PLYMOUTH
  • V8
  • 4WD
  • AUTOMATIC
  • 85,000 MILES
Heres one, for an old car mechanic. Got a 79 Trailduster, 360 carb engine, started running rough, rough idle, felt like it was cutting out (dying) when cold. Given decent tune up, plugs, wires, cap, rotor. Still runs same. Changed condensor, runs same. When given gas, is fine, runs really strong when under acceleration, but harsh idle, and still acts like it wants to cut out. Dont have any vacumn leaks that I know of, and havent changed anything recently, if that helps. Carb was rebuilt and gone thru last year also, been fine up till now
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Thursday, October 23rd, 2008 AT 3:15 PM

5 Replies

Tiny
JNOVACK
  • EXPERT
It sounds like a choke probblem do you know what carb is on it I can help better knowing the carb they used rochester, holley, carter and stromburg
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Thursday, October 23rd, 2008 AT 4:34 PM
Tiny
CARHO50
  • MEMBER
I do, and thanks for your reply. Its a re-man 2 bbl holley. If needed, I can get the # from it. Thanks for the clue. Can it be fixed, ?
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Friday, October 24th, 2008 AT 9:05 AM
Tiny
JNOVACK
  • EXPERT
Yes it can here is what to inspect on the choke pull-off
A malfunctioning choke can cause a variety of performance problems, from no-start to missing and power loss at cruising speeds. These problems can be caused by the choke being stuck closed, the choke being stuck open, inoperative choke pull-off, or a malfunctioning bimetal choke coil.

A visual inspection may identify the trouble spot quickly. Remove the air cleaner, making sure to label each vacuum hose as it is disconnected. If the engine is cold, the choke should be closed. Watch the choke plate as an assistant cranks the engine.

Check for broken or leaking vacuum hoses.
Check for a cracked housing.

and testing of choke pull-off

If the choke plate is not opening when the engine starts, check for a vacuum leak in the choke qualifying mechanism. If a diaphragm vacuum break is used, look for a cracked vacuum hose or loose hose connections.

To check the diaphragm:

Remove the vacuum hose, and install a vacuum pump.
Apply vacuum to the pump diaphragm.
If the diaphragm does not hold vacuum or if the vacuum leaks down faster than the manufacturer specifies, the vacuum break should be replaced.


http://www.2carpros.com/forum/automotive_pictures/52960_0900823d8004fcd6_1.gif


Adjusting the fast idle cam.

If the carburetor has a secondary vacuum break:
Test the thermal vacuum valve or choke vacuum thermal switch.
Consult the manufacturer's service manual for temperature specifications.
Connect a vacuum pump to the lower port and a vacuum gauge to the upper port.
Apply the specified vacuum and alternately warm and cool the valve or switch. The valve should permit vacuum passage when the valve is warm and block vacuum when the valve is cold. If the engine runs rough immediately after start-up, the choke might not be opening the specified amount.
To check the qualifying dimension: Apply the specified vacuum to the diaphragm using a vacuum pump. Measure the clearance between the upper edge of the choke plate and the air horn wall using a drill bit with a diameter equal to the specified dimension.
To adjust the dimension, bend the choke linkage with a pair of pliers. Operation of the heating element is checked with a digital volt-ohmmeter.
Refer to the manufacturer's service manual for procedures and specifications.
Before making any adjustments on the choke, move its linkage by hand to be certain there is no binding or sticking in the linkage, shaft, or choke plate.
Use an approved carburetor cleaner to remove any gum deposits that interfere with its operation.
If the choke housing permits adjustment of the choke setting, the choke can be adjusted to enrich or lean the air/fuel mixture

you may also have to ajust Choke Thermostat
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Friday, October 24th, 2008 AT 10:59 AM
Tiny
CARHO50
  • MEMBER
Thanks for the lesson. Could all this be gotten around by disconnecting the mechanical choke and making it manual? Heres another symptom. When decelerating. Coasting, it also sounds like its dying. Choke symptamatic?
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Friday, October 24th, 2008 AT 1:53 PM
Tiny
JNOVACK
  • EXPERT
Yes you can get a mechanical choke and yes to last question but if you want more pep or power I put a new intake and 4 bbl on mine wow what a differance a holley 650 dubble pumper
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Friday, October 24th, 2008 AT 4:29 PM

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