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RPM drop Problem

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dennisvh

Joined: 21 Aug 2006
Posts: 3

Posted: Mon Aug 21, 2006 6:44 pm Post subject: RPM drop Problem

I have a 1946 Ercoupe with a 85 HP (started as a 75HP) Continental engine with a 200 RPM drop on the right mag at run-up.  Mechanics have 100% eliminated as the problem both mags, all plugs, P-leads, and switch.  Also, the engine ran fine for 25 hours after a new ignition harness was installed.  The new harness was checked out yesterday on the engine and appears to be ok on all wires.  We don't where to go next.  Any advice will be appreciated.  Thanks, Dennis Hatfield.

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Bob Buckthal

Joined: 10 Jan 2006
Posts: 2

Posted: Mon Aug 21, 2006 8:28 pm Post subject: RPM drop Problem

Dennis,



Something that you think is OK isn't.  You didn't mention timing.  A late

mag will produce a big drop.



Bob



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jrccea(at)bellsouth.net
Guest

Posted: Mon Aug 21, 2006 8:29 pm Post subject: RPM drop Problem

The timing may have slipped on one mag.



dennisvh wrote:



 	  Quote: 	
		   



 I have a 1946 Ercoupe with a 85 HP (started as a 75HP) Continental engine with a 200 RPM drop on the right mag at run-up.

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cubflyr(at)comcast.net
Guest

Posted: Tue Aug 22, 2006 6:39 am Post subject: RPM drop Problem

 	  Quote: 	
		  The timing may have slipped on one mag.


	


That was the first thing to pop into my mind, everything else being checked.

Are the mags new or recently overhauled and reinstalled? Sometimes things

can shift. It would be unlikely for a Woodruff key to be missing, but not

impossible. This would allow a gear to shift. You may have to pull the mags

and check them out, as well as to make sure they are installed with the

timing mark in the correct place on the drive gears. It would be odd for

this to be it as it sounds like it just started after being normal for a

while. Something has moved out of place, most likely in the mags or gear

drive.



I recently put in a new set of ($20 each) spark plugs only to find after

about 20 hours one was faulty. Have them pressure checked. If they are

leaking, you can see blow-by gas products in the wiring harness end of the

plug. 



Michael

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cubflyr(at)comcast.net
Guest

Posted: Wed Aug 30, 2006 11:45 am Post subject: RPM drop Problem

Dennis: did you ever arrive at a cause/solution for your RPM drop?

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margdon

Joined: 04 Aug 2010
Posts: 1

Posted: Wed Aug 04, 2010 2:35 pm Post subject: Re: RPM drop Problem

Hi my guess is a pour spark on two cylinders. Change plugs. Don

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qaz11

Joined: 20 Aug 2010
Posts: 6

Posted: Wed Sep 01, 2010 2:20 am Post subject: Re: RPM drop Problem

Try replacing the idle air control. But maybe try this test before you do. Sitting in the drive way, turn the steering wheel. The idle should go up. If it does not then it may be that the pressure sensor on the power steering pump is bad.

Idle air control failure does not always show up with a check engine light coming on. Some times they do get dirty and others claim that they can be cleaned. But by the time you get into clean it why not replace it.

The other things that may be wrong, there is a part on the throttle sensor that keeps the idle high for a little while before letting it drop down. It is called a throttle damping valve. The throttle position indicator may not be registering a return to idle. The temp sensor may not be telling the ecm what the actual temp of the engine is. But I have never seen these parts fail, the idle air control I have seen fail.



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