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Discussion Starter #1
What could be making my motor break up when I hold it at certain rpm's.Seams that when I romp on it and go thru the gears it responds fine , but when cruising @ say 28-3000 it will start to miss,and only when its held at a stedy rpm. Could use some help ,I checked the carb, checked valve springs, ect. running out of ideas...............thanx guys
 

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Eddie Haskell
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Can you make it do it in neutral?
 

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Did you check the power valve when you checked the carb? If its only at a cruise condition it really sounds like a lean spot.

Also if its a points ignition with vacuum advance problems can occur where the points dwell can be thrown off when the vacuum advance rotates the points plate, if its a vacuum advance deal try pulling the line temporarilly and see if this resolves the problem.
 

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Discussion Starter #4
Thanx guys.
No ,it does not do it in nuetral. I thought about that, and thought it strange that it didnt do it then.Also the dist and box are MSD. I changed the coil to new the 6-AL box is about 7-8 yrs old, but never herd of one going bad realy..............Thanx for any help..........
 

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Butch
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8,208 Posts
Run a main ground cable to each head

It is easy to isolate the head from ground or cause poor ground for the spark plugs, stop and think the spark plugs ground out in the head. A weak ground can cause the problem your seeing and it is free to fix it other than the cables unless you have some
 

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Eddie Haskell
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Did you check the power valve when you checked the carb? If its only at a cruise condition it really sounds like a lean spot.
.

Initially this is what I thought was lean, if he could do it while in neutral he could look in the carb. Make sure the boosters weren't dumping fuel for some reason. Dosen't point to ign as it runs fine at WOT.
 

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Discussion Starter #8
Just checked the boosters and nothing weird going on there.I will replace the P/V and see if that helps. Thanx so far guys...........
 

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does it make a diff if they are aluminum?
Needed even more so on alum headed engines, imo.
I run a ground strap to a threaded bolt boss on the block from the battery and from there run a ground to one head. And then from that ground strap, go to the other head.
 

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Butch
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Needed even more so on alum headed engines, imo.
I run a ground strap to a threaded bolt boss on the block from the battery and from there run a ground to one head. And then from that ground strap, go to the other head.
X2

Even worse on aluminum heads
 

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Discussion Starter #11
Let me ask this.Could this just show up all of a sudden? I've run this car for the last 3 years with this combo.Uncovered it fro mwinter storage, started right up ,even ran it around town a few times. Just seemed to do this over night so to speak.

again ,thank you all for the ideas
 

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If it is an electrical gremlin, it could. Corrosion between the terminal and ground strap for instance.
How about the condition of the dist cap and wires ?
Be sure to also check the distributor's mechanical and vacuum advance mechanisms.
 

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Big Block Killer
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I've been chasing this same issue. I'm trying one thing at a time though...I've retraced the grounds for good connections then added (2) 4-guage ground straps; one from the primary ground on the block up to one head, then from that strap/head over to the other head just as has been suggested here...No results yet, phucking bike fest at the track this weekend
 

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Big Block Killer
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I had this Problem Once it turned out to be a bad tachometer AutoMeter Monster tach with Shift light
Of all the threads I've read in here about people developing a miss, this is the first actual post about it possibly being a tach going bad; that I've seen anyway.

I was talking to a friend of mine about my miss and he told me that I could disconnect the Tach and see if it goes away...I was like WTF....your crazy...I guess not
 

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Pontiac aficionado
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What is your Air/Fuel ratio at this rpm? gotta know this to see if it is a fuel related issue. Also what is the timing at that point and with a light on it what is the timing mark doing? have to know this first before chasing stuff that wastes time.
 

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Discussion Starter #19
Don't know the A/F reading.I'm off to the dyno Weds.and will be reading then along with spacer testing.My timming is 38deg total all in at 3000 climbing to total adv. is steady in gear really no jumping around ,plus when I timed it I use the heaviest spring so I get a much truer reading.
 
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