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Discussion Starter #1
Just after jetting up my 670 holley on my 390, I was driving it hard to see how it ran, and it ran great, then after coming to a stop, it barely idled anymore. Prior to it going bad, it idled with my msd box at 12 degrees at around 700rpm in park and it would drop to 500 in drive without ever dying. I checked for vacuum leaks, checked inside the dist, blew out the idle air bleeds, I even rebuilt some of the carb, blowing out the metering plates and replacing gaskets. After the rebuild, it ran better but still barely idled. the idle mixture screws don't seem to have an affect, but the carb idles normally if I put the choke on, but then it won't accelerate past Idle. It still cruises and accelerates normally and really good, no smoke from anywhere or any noises. Any Ideas with whats wrong? I figure its with the carb considering rebuilding it helped and it idles with the choke, but I already checked the air bleeds real good.
 

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Is it only after running it hard that it does this or is it constant now after running it hard once?

Sent from my VS996 using Tapatalk
 

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Discussion Starter #5
Is it only after running it hard that it does this or is it constant now after running it hard once?

Sent from my VS996 using Tapatalk
i was doing a couple pulls back and forth down the street after changing the jets and it pulled good and idled fine after, then after one pull it started idling bad so I havent driven since, it's constant. I'm going to see if I can find a leak in the intake manifold to cylinder head seal, I think i've done everything I can with the carb
 

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I checked one side of the intake and found nothing, I still have to check the other but I dont think I'll find anything. I'm at a loss for whats wrong, I've had to fix most of these things before and I've seen the symptoms, If it was an intake leak it would probably be eating oil and smoking, but its not. I'd think maybe something with the cam and timing chain, like if it skipped a tooth, but I took it out once with it running like this to see how it accelerated and off of Idle and just above cruising, it runs strong like normal. could the msd box be causing any issues? It's a digital 6A.
 

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If it is advanced weight they could be sticky giving false readings and playing games with the idle speeds.
So will a leak around the throttle shaft bushings. If it’s bad enough the things gets hung up on the throttle blades and does dumb stuff.
 

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Discussion Starter #8
If it is advanced weight they could be sticky giving false readings and playing games with the idle speeds.
So will a leak around the throttle shaft bushings. If it’s bad enough the things gets hung up on the throttle blades and does dumb stuff.
I did check the advance weights, they were sticking but werent the problem. The throttle shafts could be something. I ended up going out and buying a 650 cfm edelbrock to keep my car moving. I runs really well now, I might not go back to the holley
 

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Are you running a heat spacer between the Carb and the intake ? If not you probably should because the problem you described is common without one.
 

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Are you running a heat spacer between the Carb and the intake ? If not you probably should because the problem you described is common without one.
Even with Holley float bowls exposed to air top and bottom? I've never experienced that problem, even on hot Texas days.

Edelbrock carbs are a different story. Their float bowls essentially sit right on the manifold. But the symptoms are lots of cranking required after a "hot soak" due to the gas boiling off on hot days.
 

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5.0 heat shield

Mustang early 80's 5.0 with the Holly 4 bbl used large aluminum sheet metal heat shield. bolt pattern should match.
 
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