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Old 09-24-2010, 09:10 PM
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Constant miss at hiway speed.

Ok, got a 400 pontiac in my 78 trans am. Mods, eagle rods, crank, forged pistons, edelbrock intake, 'modified' quadrajet, stock heads, MSD 6A ignition.

Problem: from speeds 30-65 it has a steady miss, which sounds like its coming from the same cylinder.

All plugs look good, and are firing when the engine is turned over, all plug wires are in good shape, and only about 1500 miles old, cap, rotor are all new.

could the pickup in the distributor be causing this? any other thoughts welcome!
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Old 09-25-2010, 03:56 AM
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if i Google it right,you have the BANDIT's Pontiac right?
Sweet ride Dude!!!!

now about the your problem,looks like you'll need to definitely change the distributer cap
and rotor (the rotor shouldn't be the problem).
see if the CAP has some cracks in it or something like that,because i had a similar problem with mine,it was cracked near the central ignition button and the engine wasn't starting at all,and when i somehow menage to start the engine,it only worked at high RPM but like only 3 pistons were firing and not 4

hope this helps
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Old 09-25-2010, 06:05 AM
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At what RPM does this begin? By "steady miss" are we to assume you mean one cylinder drops out? Or is an unsteady miss thats always there?
In any case, it almost sounds like a valve issue. Weak or broken spring, bent stem, flat cam lobe, etc.
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Old 09-25-2010, 01:12 PM
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It starts at about 1500, and continues up to about 2700. I noticed this morning, that it only does it when its in gear, if i just rev it out of gear it runs smooth.

and yeah, it sounds like one cylinders isn't firing, its not multiple cylinders missing randomly.
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Old 09-25-2010, 01:22 PM
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running headers? you might check for a slightly cooked boot.
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Old 09-25-2010, 01:25 PM
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Has headers, but on a pontiac the plug wires are not even close to the headers, they're WAY above the header, and sheilded by how the exhaust ports run. Hence another reason why I love these engines!

But, checked the wires last night, and theyre all still squishy and rubber-y.
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Old 09-25-2010, 06:48 PM
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Quote:
Originally Posted by BigRoy1978
Has headers, but on a pontiac the plug wires are not even close to the headers, they're WAY above the header, and sheilded by how the exhaust ports run. Hence another reason why I love these engines!

But, checked the wires last night, and theyre all still squishy and rubber-y.
Check the routing of 5&7 these cylds. fire one after the other and sometimes if the wires are routed parallel can cause an induction triggered misfire.
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Old 09-25-2010, 06:57 PM
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Okay , Just speaking from experience . Appearance of a spark plug or plug wire has little to do with it's function . I have had both fail when they (appeared) to be fine . I'd pull the wires off the plugs one at a time with the engine running to see if it changes anything .
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Old 10-13-2010, 08:20 PM
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FOUND IT!!!

Ok, went and got a new cap, wanted one of those corrected caps, cause the dist on this car is way under the firewall and it would make everything easier, but i couldnt find one for an HEI.

ANYWHO! Changed the cap, nothing. Pulled ALL the plugs again, looked great. Took the carb off, one loose screw on the bottom, tightened it down, was a little better.

Still sounded like a vac problem to me, so i started hunting.

I had a buddy pull off the vac caps and hold his finger over them, as i ran the engine through the rpm. He tried a couple with no luck, then BAM! It ran great! I told him that we needed a new cap on that port, but when i got out to look under the hood, it was the line to the vac advance.

right then it hit me lol, *slaps forehead* those stupid two little wires are coming loose when the vac advance moves...the one thing i ALWAYS check when i have the dist out, and i missed it.

Getting a new pole peice this week, and its getting put on when i get back from kansas city!!



Ive heard that GM vac advance canisters need to be limited on the amount of advance they put in, correct? or no?

THanks all.
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Old 10-13-2010, 08:54 PM
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Ya some have way too much advance when a high vacuum amount is applied to them (deceleration) I like to limit them to a max of 15deg tops.

The little wires on the mag pick up are a common fault.
So is cap/rotor phase alignment shift caused by excessive vac advance and poor cap/rotor/mag pick up phaseing alignment.
If the spak occurs when the cap post and rotor tip are too far away from each other, missfire and crossfire occur.
Vacuum advance moves the mag pickup , effecting rotor phasing.
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Old 10-13-2010, 09:15 PM
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When the vac advance was hooked up, it would pop like heck when it was deaccelerating from 65-30, and from my experience that means too much timing advance, correct?
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