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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 454FOTF on July 20, 2006, 12:46:00 am

Title: 2500 RPM miss, need help
Post by: 454FOTF on July 20, 2006, 12:46:00 am

I have a '79 C30 with 454.  I get missing and lose power around 2500 rpm.  My tachometer loses power when this missing occurs and drops to zero.  When I let up on the gas pedal and the rpms come down a little the missing subsides and the tach returns to normal.  I have replaced the distributor cap, rotor, coil, spark plug wires, ignition module, and vacuum advance.  Timing is set at 9 degrees before TDC at hot idle with the vacuum advance disconnected.  Spark plugs all look good and are gapped at .035

Anybody have any ideas on what I should check next?

I have thought that maybe there is resistance in the 12V supply wire going to the distributor or maybe there is resistance in the harness.

Does anyone have any resistance specs that will help me diagnose?  

Title: Miss
Post by: cjtimp on July 20, 2006, 06:10:00 pm

May be vaccum leak too.....................check vaccum

Title: Re: Miss
Post by: 454FOTF on July 20, 2006, 09:50:00 pm

not sure how a vacuum leak could be the problem when the tach drops to zero when it's missing.  I guess there could be a different problem affecting the tach.  I'll the vacuum just in case.  

any other ideas?

Title: Re: Miss
Post by: 123 pugsy on July 21, 2006, 07:29:00 pm

Could there be a problem caused by the alternator when it gets to the 2500 RPM mark???

Only thing I could think of that affects electrical and goes with the engine turns.

Title: Re: Miss
Post by: 454FOTF on July 22, 2006, 01:33:00 pm

thanks for the thought about the alternator.  I'll check the voltage at 2500.  

I wiggled the two harnesses that plug into the distributor and that appears to have stopped the miss so far but my tach continues to loose power at 2500.  Although, now it is only dropping to around 1500 even though I am revving much higher.  

Title: Re: Miss
Post by: roundedline on July 25, 2006, 05:18:00 pm

are you trouble shooting a broken tach, or is the motor dropping rpms.  The stock tach clusters are notorious for being not so accurate.

Chris Lucas
www.73-87chevytrucks.com
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Title: Re: Miss
Post by: 454FOTF on July 26, 2006, 12:33:00 am

It now appears that the miss has been cured by wiggling the wires going into the distributor.  Yes, now I only have the issue with the tach.  The tach works fine until you push the engine past 2500 rpm, at which time the needle starts to drop back toward 1500 rmp as the engine increases speed.  Not a major problem, just annoying.   Is this a common problem from these OEM tachometers?  

I thought the two problems were related before because the miss and tach failure occured at the same time.