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69chvldrvr


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Engine mis-fires under load
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Just finishing my inner fender replacement and started the engine for the first time in nearly a month. While apart I replaced the sparkplugs (was 43xls now 44xls as book calls out), points, cap, rotor. Did not replace wires as they are fairly new. Unknowingly had ignition in on position (probably 1-2 hours) when installed new battery (before replacing points). Engine ran fine before I started project, now mis-fires under load, and gets worse as engine heats up. Any ideas?



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Jeff S. - Watertown, MN

1969 Chevelle SS 396-325hp, 400 Turbo, 3.31 Posi

2003 Monte Carlo SS, Fully Loaded
John D


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Could have possibly burnt the point contacts.

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John D. - St. Louis Park, MN.

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69chvldrvr


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Replaced points after ignition mishap.



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Jeff S. - Watertown, MN

1969 Chevelle SS 396-325hp, 400 Turbo, 3.31 Posi

2003 Monte Carlo SS, Fully Loaded
Lost in the 60s


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69chvldrvr wrote:

Replaced points after ignition mishap.

Is it possible you got a couple plug wires mixed up ?? 5 and 7 are very easy to do that with.

 



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Mitch D.   River Falls, WI

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1966 Chevelle SS 396 M20

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1967 Camaro SS/RS 350 M20
SShink


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May be a dumb question... but did you reconnect the vacuum advance hose after replacing the cap?



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Stan S.-Twin Cities 'South Metro'

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69chvldrvr


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I did indeed. Good question though.



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Jeff S. - Watertown, MN

1969 Chevelle SS 396-325hp, 400 Turbo, 3.31 Posi

2003 Monte Carlo SS, Fully Loaded
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cap? dwell?

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Kevin

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yellowbowtie


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I agree with Mitch. Only because I have done this before. Engine will idle fine but stumble under load

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Ian M  St louis Park
Chris R


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If you dont have a crossover in your exhaust system, you can have a partner rev the engine up in drive with foot on the brake, while you stand behind the car and see which tail pipe the missfire is coming from. You then narrow which bank of cylinders the missfire is on. Works even better for those that know its really one cylinder thats missfiring. 



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Chris - Ramsey, MN.

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Lost in the 60s wrote:

If the wires are on correctly, I would suspect a bad plug. If you can locate which side with Chris' suggestion, pull those back out and see if they are all firing. It will be obvious if one isn't.

I woke up thinking the same thing with the bad plug.  The other thing you can do if you have a temp gun is let the car idle for a bit, then check the exhaust manifold temps just where it comes out of the cylinder.  The one(s) that are cooler aren't firing completely (or at all...).  Then you can swap the wires and/or plugs with a known good cylinder to trouble shoot it. 



-- Edited by SShink on Friday 29th of June 2012 07:42:16 AM

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Stan S.-Twin Cities 'South Metro'

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69chvldrvr


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Thanks to everyone for their suggestions. Will be busy tonight. The sad part is the engine ran fine before I started the project. But as long as the inner fenders were out, and everything was easily accessible, seemed like a good time to clean up the engine compartment and replace parts...One thing led to another...I guess "if it ain't broke, don't fix it".



-- Edited by 69chvldrvr on Friday 29th of June 2012 09:32:29 AM

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Jeff S. - Watertown, MN

1969 Chevelle SS 396-325hp, 400 Turbo, 3.31 Posi

2003 Monte Carlo SS, Fully Loaded
Lost in the 60s


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If the wires are on correctly, I would suspect a bad plug. If you can locate which side with Chris' suggestion, pull those back out and see if they are all firing. It will be obvious if one isn't.



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Mitch D.   River Falls, WI

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1966 Chevelle SS 396 M20

1970 Chevelle SS 396 M20

1967 Camaro SS/RS 350 M20
Tim H


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If your wires are installed correctly then check the inside of the new cap to see if it is tracking or if there is and grease, finger prints ect that could cause tracking. Try putting the old cap back on and see what happens.

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SShink


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Jeff, I'm sure you'll figure it out.  We all go through this phase with these cars depending on how much we let it snowball.  Sometimes the simplest change can cause the most headaches...  banghead

Step back, take a fresh look at things, and sometimes the obvious will jump out at you.  Or, have someone else look at it with you and they will see what you are missing.

These cars are relatively simple mechanical things, and you will learn from the experience, as we will as well since you are sharing the frustrations and asking for input.



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

2001 Mustang GT Convertible 

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69chvldrvr


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Found the culprit....Removed all the plugs one-by-one and checked for cracks, and rechecked the gap. None showed any signs of damage, and the gap was spot-on, except for one. It had no gap whatsoever. Don't know what happened to that one. Gapped it to the correct .035, put it back in...RUNS GREAT!......Thanks to all who gave suggestions!nana



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Jeff S. - Watertown, MN

1969 Chevelle SS 396-325hp, 400 Turbo, 3.31 Posi

2003 Monte Carlo SS, Fully Loaded
SShink


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That's great Jeff!  Sometimes it's the simple stuff.

Are you going to park with us at Car Craft?



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

2001 Mustang GT Convertible 

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
Tim H


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Jeff

How did the wheel wells fit? They seemed to be nice quality ones.

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