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Discussion Starter #1
I was out last Sunday sort of seeing what my car could do. It sounded great and pulled hard until I think around about 4500 rpm, then it kinda started sounding bad, like a bit of "missing" or "crackling" from the exhaust note.

What would be some things to look at here?

Now I know this is pretty much impossible to pin-point the problem via the internet, but I'm completely clueless at this point. Any leads to check would be greatly appreciated...

Here's the motor:

460 from a '73 Mercury.
stock, unmodified D3VE heads.
750cfm Holley vacuum secondary carb.
Torker II intake.
Accel Street Billet distributor (not points) and Accel Super Coil w/ ballast resistor.
No headers yet.
"Aftermarket" cam, but specifics are unknown to me at this time.

I'd like to take it to the local dragstrip to get a baseline time on the car, but I dont want to damage anything by being stupid either.

thanks-
 

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Discussion Starter #2
car didnt seem to hestitate or slow down, it just sounded..... odd.

maybe it's just the exhaust system noise?

can has dual exhaust (no X-pipe or H-pipe) and Cherry Bomb glasspacks.
 

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Does it break up after 4500? Had a friend change complete igntion and fuel system chasing a miss at 4000. It ended up being week valve springs. Need to borrow the tool that checks them on the motor and see what your seat pressure is. Good luck.
 

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Stock unmodified heads with an aftermarket cam? Met bet would be on the valvesprings.
 

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Are you sure it's breaking up? Will it turn past 4500? If it does turn past 4500 does it still sound funny? Might just be the obnoxious sound of glass-packs. Otherwise like Cale said, check the valve springs.
 

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Discussion Starter #8 (Edited)
Put the car on the dyno today.

Air/fuel mixture was good in the mid 12's. Car sounded great. Until about 4600 rpm, then it went rich (11's) and both torque and horsepower took a BIG nose dive. And the car sounded awful at that point, just like before.

We installed new plugs, and got pretty much the same thing.

The shop recommended I ditch the 40-yr.-old Holley because they have no way to jet the top end due to the secondary bowl just having a metering block instead of jets. A 750cfm QuickFuel was recommended. Summit part number QFT-Q-750. 600 bucks for that carb. Dang.

Hmmm..... valve springs are probably cheaper to try first.... I completely forgot about this old posting, and failed to discuss the springs with them.


Un-filtered dyno data:



"Filtered" dyno data:










Here's my setup now, vs. 2 years ago:

460 from a '73 Mercury.
stock, unmodified D3VE heads.
750cfm Holley vacuum secondary carb.
Perfomer intake.
Accel Street Billet distributor and Accel Super Coil.
FPA headers.
"Aftermarket" cam, but specifics remain unknown.
275/60 R15 rear tires.
stock C6 trans.
3.50 rear gears.
 

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What were the HP and torque figures before and after the "nose dive"? With a Performer intake on it and not knowing what the cam is it just may have been starting to be "done". Your not going to go out and turn 6300 rpm with a performer intake. It may be advertised for off idle and up to 5500 rpm but that doesn't mean every motor will be happy going to 5500 rpm. I read through all of the old posts as well. It could very well be poor valve springs not helping a lot. But you aren't going to turn a whole lot of RPM's through untouched D3VE-A2A heads either.
 

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Discussion Starter #10
just added some dyno charts above.

the reason the un-filtered graph has a big colored chunk at the end is cuz the HP and TQ numbers were all over the frickin' place.
 

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You definitely have something goofy going on. Power should have peaked at the higher RPM. I've never seen a dyno HP graph so flat. Torque looked decent at the beginning but did fall off rather hard. I would definitely do the valve springs first. They are obviously old and tired. I'm thinking that they are a single spring as well. Not good at all for an after market cam. I think that will perk things up a lot. Then maybe go for a carbuerator if needed later.
 

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Yes, it is a goofy graph.... and yes, they are single springs.

If you could throw a recommendation at me for springs, that'll be my next adventure.
Without knowing what the cam is in there it makes it very difficult to recommend spings. With the intake being a Performer I'm guessing that it is a relatively mild cam. Look and see what springs are recommend for the Performer cam and maybe start there. I'm not saying that you have to buy the Edelbrock springs but a nice dual valve spring setup that is close to the Performer springs I think would probably be a good choice. A good choice if the car idles smoothly with no lope that is. If you have a substantial lope you may want to get an expert opinion from somebody that is more familiar with the car than me.
 

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Secondary metering plates can be modified or changed. There are also secondary metering block kits Quick Fuel has them. They also have metering plates with changable jets. The later was what I went with due to space limitations on my dual quad set up. My truck I swapped out the metering plate wit ha metering block I had laying around. Possibly the dyno shop is selling the carb or there ....

That rich of a mixture shouldnt cause it to pop I have been there with A/Fs injected and carbed lost some power no popping.
 

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Discussion Starter #17 (Edited)
I may have to go visit my online buddy, bigblok86ranger, down in Beloit.

I dont fault the shop with the dyno. They are very well known in the area, and I was just some goof-ball coming in off the street with an engine full of unknown parts. They probably dyno 200-300 cars in a year's time.
 

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Discussion Starter #19
I suppose if the valves started REALLY floating at that certain rpm, everything woud go to hell in a handbasket...as shown on the dyno chart.

I do understand that my setup is not made to rev to the moon. I told the dyno guy I dont really drive around at 5000 rpm too often.:D
 

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You know it is pretty easy to change the valve springs with the heads still on the car. All you need is some compressed air and the male end compressed air fitting that plugs into a quick connect fitting. That is how I put the inner valve springs back on when I broke in the cam for my 466 build that went into my F100.

OK... I just re-read that. I wouldn't call it easy. But it is doable. Very doable.
 
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