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Discussion Starter #1
so when i got my car out at the end of season i was having a pretty bad high rpm miss..i had just put a new carburetor and fuel system on it so i thought it was fuel..i upped the fuel pressure and re -checked floats and it made it better but still did not pull after 6k and started to miss at about 6500 ( should of been good to around 7300 per cam)

-considering i had to run a bone stock 10 inch paper air filter i figured that might of been my problem-getting choked out at the high rpms.i had since turned it in for the year and was just hoping by putting a scoop on over the winter would fix the problem..

-well, i decided to have the motor freshened up so when i was pulling the motor apart i noticed the exhaust valves were kissing the pistons..every piston has a very very fine nick the shape of the exhaust valve...think i found my problem?
 

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Discussion Starter #3
-i was kinda thinking the same thing...

well it didnt miss down low..cut a 1.38 60ft ..et was 6.48 @ 108..so it was still a decent pass...but with what i have in the motor compared to other guys it should be at least 30's ..maybe 20's....its possible it started to miss a tad bit as early 5k..but idk..but
 

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Definitely sounds like you found A problem for sure...

what's your ignition setup? crank trigger/ distributor/etc?

have you checked for cam walk? <-- this is what gave us a high rpm miss for about 4 months that we couldnt figure out.

have you looked at timing all the way past the point of missing?

just throwing out ideas. definitely get the valve thing fixed ASAP. like, NOW!
 

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I'm with above^^^, you've actually got 2 problems, piston to valve clearnaces issues AND the high RPM miss.

What plugs are you using and what are they gapped at? The upper rpm miss might go away with something as simple as slapping in some new plugs or.... going up or down on the heat range, been there done that.
Not knowing your combo it would be tough to recommend a plug, on my 13-1 496 with 722/722 solid roller I use NGK#8 gapped no wider than .032

More motor specifics like cam specs, compression ratio, carb size, jetting, etc would help with recommendations for your "other" problem.

Cp
 

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I would also check your dist cap for corrision inside on the contacts if so take some sandpaper and sand the crap off of them and try it again or buy a new cap.
 

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Another question cold you run it up above 6500 while not in gear..I had a coil go bad and under load it would start breaking up but could rev it 7000 no problem in park
 

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IMO, I don't think that the exh valves ever so slightly hitting the pistons is causing your miss. That is an easy fix with a thicker head gasket. Did you change the fuel filter when you made changes to the fuel system ? If so, then the fuel side is ok.

Is this the car in your avatar ? What fuel ?
Check the valve springs.

Like others have asked, what does your whole ignition system consist of ?
Did you change the plug gap any while trying to figure out the "miss" issue ?
Do the power and ground wires from the ignition box go directly to the battery ?
What other electrical components are running while the car is going down the track ?
Are you running an alternator ?
Is the block grounded directly to the battery ?
Is there
 

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Another question cold you run it up above 6500 while not in gear..I had a coil go bad and under load it would start breaking up but could rev it 7000 no problem in park


ditto, I had a pair of Accel coil(s) problem on my 4.6 and didn't even know it untill we put the car on a chassis dyno, ran fine down the track, or so I thought:confused: put the stock Ford ones back on and no more high rpm "skip"
 

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Discussion Starter #13
heres the car and setup

1987 trans am backhalfed. 468 bbc.13.1.1 on alcohol with a prosystems 1000cfm xe carb.Canfield 310 heads fully ported with 2.25/1.88 valves.all forged bottom end. cam is pretty mild. dont remember the duration but its a .710/.710 installed @108..the rpm band on the cam card was 3500-7200 on a 110lca..car weighs at least 3200lbs

igninition system-mallory hyfire digitial box. msd pro coil . msd billet distributor
hot wires and grounds are not run to the battery..they are ran to a junction on the firewall and the ground is tapped into a bolt that was welded onto the cage.

heres some other possibilities to the high rpm miss. i am running an electric fuel pump on alcohol-it is a front mounted fuel cell-but i was running a magnafuel quickstar 330 pump..i have since sold it thinking that could be the problem and will be buying a magnafuel 500

the other problem i was thinking could of been the tiny air filter i was running. its a bone stock air filter 10 inches wide by 1 inch tall..i will post a pic of my car doing a burnout and you can see it..
 

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You would advance your cam for more exhaust piston to valve clearance. If your springs are too weak, the exhaust is usually the ones to hit first. Remember, the piston is chasing the ext valve..
 

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Discussion Starter #16
You would advance your cam for more exhaust piston to valve clearance. If your springs are too weak, the exhaust is usually the ones to hit first. Remember, the piston is chasing the ext valve..
Well i am having the engine the rebuilt our local engine shop so hopefully they will figure out what the problem is..i will probably have to opt with a thicker head gasket ..the car already makes enough low end torque and not quite as much hp so i dont wanna advance the cam anymore.
 

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heres the car and setup

igninition system-mallory hyfire digitial box. msd pro coil . msd billet distributor
hot wires and grounds are not run to the battery..they are ran to a junction on the firewall and the ground is tapped into a bolt that was welded onto the cage.

heres some other possibilities to the high rpm miss. i am running an electric fuel pump on alcohol-it is a front mounted fuel cell-but i was running a magnafuel quickstar 330 pump..i have since sold it thinking that could be the problem and will be buying a magnafuel 500

the other problem i was thinking could of been the tiny air filter i was running. its a bone stock air filter 10 inches wide by 1 inch tall..i will post a pic of my car doing a burnout and you can see it..
You really should run your power and ground wires from the ign box directly to the battery. Ask me how I know this.

If you have the room, I suggest getting a belt driven pump and regulator set up to supply the carb with fuel over the electric pump. Bo Laws has a really set up.

That tiny air cleaner sure isn't helping you. It's too short in height for one thing.
 

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Discussion Starter #18
You really should run your power and ground wires from the ign box directly to the battery. Ask me how I know this.

If you have the room, I suggest getting a belt driven pump and regulator set up to supply the carb with fuel over the electric pump. Bo Laws has a really set up.

That tiny air cleaner sure isn't helping you. It's too short in height for one thing.
lol i already got a scoop to put on the car so no more air filter .

i will do what ya guys say about the wiring. no biggie.

how much does a belt drive pump run these days? i know a lot of people have had good luck with magnafuel pumps..i also have the prosytems setup where there is a vacuum port of the regulator to keep fuel pressure down at idle and up at higher rpms.
 
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