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RPM drop Problem

 
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dennisvh



Joined: 21 Aug 2006
Posts: 3

PostPosted: Mon Aug 21, 2006 6:44 pm    Post subject: RPM drop Problem Reply with 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. 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. Question

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Bob Buckthal



Joined: 10 Jan 2006
Posts: 2

PostPosted: Mon Aug 21, 2006 8:28 pm    Post subject: RPM drop Problem Reply with quote

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
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PostPosted: Mon Aug 21, 2006 8:29 pm    Post subject: RPM drop Problem Reply with quote

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
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PostPosted: Tue Aug 22, 2006 6:39 am    Post subject: RPM drop Problem Reply with quote

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
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PostPosted: Wed Aug 30, 2006 11:45 am    Post subject: RPM drop Problem Reply with quote

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

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margdon



Joined: 04 Aug 2010
Posts: 1

PostPosted: Wed Aug 04, 2010 2:35 pm    Post subject: Re: RPM drop Problem Reply with quote

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

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qaz11



Joined: 20 Aug 2010
Posts: 6

PostPosted: Wed Sep 01, 2010 2:20 am    Post subject: Re: RPM drop Problem Reply with quote

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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