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Just getting 4172 out after a month of sitting. Ran a little ragged but smoothed out and it was good to get out and run. Here’s the question, when I got home I noticed the idle was actually pretty good, meaning it sounded good. Then the fans kicked on and the idle got low and rough. I turned fans off and idle went back up. What does that mean? What should I check or do?

thanks for the help,


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...If I were to would be...

The Spark plug ARC is the 'Heartbeat' of the Engine.

It sounds to me that The Ignition voltage may be coming off the Fan Circuit (or visa-versa). That would mean, the Fans are 'Robbing' Voltage from the Ignition Circuit, Reducing spark efficiency.  Or the Battery is low and the Alternator is failing.

Does the Idle reduce/change when the Headlights are turned On and with High-Beams'??

Use a Multimeter and check the Voltage AT the COIL, with the Fans ON and OFF. Engine Running and then, Not Running. You will Learn a Lot!!

Good Luck with it! Let Us know What You Find.


P.S. There is one more Question...Is the Idle Position set with a '12 Volt Solenoid'?

Last edited by marlinjack

MJ- checked volts at the coil and got a reading of 7.6 with no fans….6.1 with fans. Also found the lead coming off of positive side of coil was loose so I tightened it and the idle didn’t seem as big of change. So thanks for starting me there. Are those readings I gave within tolerance? Or should I be looking at replacing coil?

as for your PS.. I have no idea, nor know how to find that out! Hahaha



Hi Stu -

Originally, there was an “Idle Up” solenoid on the carburetor.  What this unit would do is increase the idle speed by ~300 RPM when the Air Conditioning kicked on by pushing on the throttle linkage.

The solenoid is a cylinder about an inch in diameter that mounts next to your carburetor. Mine has been removed for many years, so I don’t have a picture.

I think what MJ is getting at is that this unit could also affect your idle performance, and might not be adjusted properly.

Your measurements show pretty clearly that you’ve got a 1.5 volt drop when your fans are on.

That lower voltage driving the ignition is almost certainly the cause of your idle performance change.  My guess is that the effect I’ll be even greater at high RPMs with the fans on - the engine will start to misfire.

You need to do a wiring upgrade to get the fans on a separate circuit then your ignition.

The easiest thing to do would probably be to put in a new heavy gauge wire To realize that drives your ignition. Then you can just use the existing ignition power wire to control the relay, and this will eliminate the voltage drop.


The original idle solenoid was used to set the idle speed.  It was not used for the air conditioning.  The solenoid was energized when the ignition key was in the run position.

The stock ammeter is a known source for causing electrical problems.  If you are still using the stock ammeter, that could be the source of you problem. On the back of the ammeter, look for loose or burnt connections or wires.  


Idle SolenoidIdle speed adjustment


Images (2)
  • Idle Solenoid
  • Idle speed adjustment

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