Top End Miss

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805moparkid

805moparkid

Slant and AFX Guy
Joined
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ok as most of you know i have had a couple of other threads on this issue...

i figured since we have moved on from the original thoughts i would post a new thread...

ok so here is what i did and what happened...

i installed 4.88 gears, from 3.91's, becasue my trap rpm was only 5 grand and i wanted it up around 6K... so in went the gears and out to the track, had a very bad miss from 5400 and up...

came home and installed a new fuel pump (street carter to holley red)(already had new 3/8" steel line and p/u) and also drilled out the fuel rail... this bumped it up to 5600...

then we thought ignition might be to blame so i installed an MSD box, blue streak cap, and rotor... no change...

so on a hunch we checked valve springs, they were 75seat, 250 open... so i installed new springs @ 300 open, 120 seat... moved the miss up to 5800... which is where we are now...

as asked i checked the cap and the "burn mark" is constant across the entire brass contact, on all contacts...

i have a friend who is going to let me use a wide band setup with data logger to see if anything come up there... also gonna back timing off and see if that helps...

here are the other threads
http://www.forabodiesonly.com/mopar/showthread.php?t=171722

http://www.forabodiesonly.com/mopar/showthread.php?t=177219&highlight=valve+spring+woes

http://www.forabodiesonly.com/mopar/showthread.php?t=168274&highlight=shooting+ducks
 
Do you take the air cleamer off? If so maybe you are getting too much turbulence messing up the vacuum signal on the carb
 
daredevil said:
Do you take the air cleamer off? If so maybe you are getting too much turbulence messing up the vacuum signal on the carb
Click to expand...

i take it off at the track... but on the street i run it (obviously) and have the same issue in the same place...
 
as asked i dropped my timing from 30 to 24 but had the same miss at 5800...
 
Hydraulic cam ? My 408 had the same problem over 5800 when the lifters were set at zero lash.I preloaded the lifters to 3/8 turn and no more missing all the way to 6200...
 
The Shocker said:
Hydraulic cam ? My 408 had the same problem over 5800 when the lifters were set at zero lash.I preloaded the lifters to 3/8 turn and no more missing all the way to 6200...
Click to expand...

nope solid flat... 246 @ .050 .475 lift... its a slant but really thats irrelevant...
 
probably the carb cfm

just tossing these out there as well, i'm not sure if you addressed these yet
-biggest headers you can get? (exhaust backpressure)
-cooler heat range on plugs?
-largest valves you can get in the heads?
 
moparlover said:
probably the carb cfm

just tossing these out there as well, i'm not sure if you addressed these yet
-biggest headers you can get? (exhaust backpressure)
-cooler heat range on plugs?
-largest valves you can get in the heads?
Click to expand...

has a 600dp on it

1 5/8" tubes 2x 2.5" collectors... best other than custom steps, run axle dump axle dump exhaust 2x 2.5 into single three thru a super 44... at the track i have 12" 2.5" extentions

running street hemi plugs autolite 65's

valves are 1.7/1.44 defiantly the norm, thinking if of goin to a 1.8 intake though someday....
 
Its in the valve train IMO.If not the valve lash ,and it has solids ,maybe its spring bind at high rpm .Have you checked that out ? You got me thinking when you said you up'ed the springs /pressure and the miss went up another 200 rpm ...
 
Not sure if this will apply to your case just thowing it out there. i had a similar problem with my car, at the top end of the track it would stop pulling and sometimes regain just before the end but not always..after checking everything I could think of..turns out it was my fuel delivery system, mainly the filter I had between the tank and pump...last week at the track it did it and this time I looked down at my fuel pressure gauge...it was zero.....checked the plugs in the pit, very lean, I was running out of fuel when I needed it most, not because of the pump but the filter had debri in it....just a thought.
 
The Shocker said:
Its in the valve train IMO.If not the valve lash ,and it has solids ,maybe its spring bind at high rpm .Have you checked that out ? You got me thinking when you said you up'ed the springs /pressure and the miss went up another 200 rpm ...
Click to expand...

yes i set the installed height on every valve... so no binding... besides this only started with the gear change, i changed springs in trying to fix it... made it better but not gone... lash is 16/20

moper said:
Does it miss, or just nose over?
Click to expand...

miss... feels like a couple of small pot holes... you know that surge feeling i mean...

RangerRick said:
Not sure if this will apply to your case just thowing it out there. i had a similar problem with my car, at the top end of the track it would stop pulling and sometimes regain just before the end but not always..after checking everything I could think of..turns out it was my fuel delivery system, mainly the filter I had between the tank and pump...last week at the track it did it and this time I looked down at my fuel pressure gauge...it was zero.....checked the plugs in the pit, very lean, I was running out of fuel when I needed it most, not because of the pump but the filter had debri in it....just a thought.
Click to expand...

has a new filter and made sure the pump had no junk in the screen on install but cant hurt to change filters...
 
The one slant I fooled with had a .530" Isky solid in it, full length headers, 4 bbl, much of the same stuff as yours. I did run double valve springs, which I'm not sure is necessary with the small, light valves in a slant, but I ran a 750 cfm. carb. I pinged it off 7K a bunch of times. I don't know how high it would rev before it would skip, I never had it happen. Can you borrow a bigger carb to try? Everyone thought I was nuts running that big of a carb, but it ran really well.
 
Abodybomber said:
Are you running uncorked when it happens?
Click to expand...

it does it thru full exhaust and open headers...

BBBCUDA said:
The one slant I fooled with had a .530" Isky solid in it, full length headers, 4 bbl, much of the same stuff as yours. I did run double valve springs, which I'm not sure is necessary with the small, light valves in a slant, but I ran a 750 cfm. carb. I pinged it off 7K a bunch of times. I don't know how high it would rev before it would skip, I never had it happen. Can you borrow a bigger carb to try? Everyone thought I was nuts running that big of a carb, but it ran really well.
Click to expand...

its starting to cross my mind...
 
ok if i tried a 750 should i go vac or mech? im thinking being its so big...

also whats a good model to go with? prefere dual inlets, not the cross over tube...
 
A vacuum carburetor will have best results with an automatic transmission.
 
No stick?Still with a [email protected] solid,gonna need some stall.Never had good luck with big cams,high stalls with a vac sec.Still more than one way to skin a cat.
 
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