Top End Miss

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I ran a 750 Vacuum with the lightest spring I could, so I can't say for sure it was using all of it's cfm rating, but it ran really well. I had a very low vacuum reading that went to 0 when in gear, so I had to plug the power valve and jet WAY up. I used Autolite 65 plugs in mine too, 4.86 gear.
 
BBBCUDA said:
I ran a 750 Vacuum with the lightest spring I could, so I can't say for sure it was using all of it's cfm rating, but it ran really well. I had a very low vacuum reading that went to 0 when in gear, so I had to plug the power valve and jet WAY up. I used Autolite 65 plugs in mine too, 4.86 gear.
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well i wonder if that might be my problem?? i havn't check vacuum latey but last time i think it was around 11 or 12... have a 6.5pv though...
 
You may want to stick a vacuum gage on the intake and watch it at the point where it misses. If it shows any vacuum try a larger carb. The power valve is spring loaded open, so unless it simply isnt flowing enough (usually a problem with alcohol carbs) it will be open at WOT. I'm more inclined to think it's a valvetrain issue. Now that you have the springs in, it might be a good experiment to stick .060 shims under the springs and see if it rises again.
 
Sounds like you're on your way, but I'll throw a couple of other ideas out there in case you're not sick of trying stuff! LOL

Have you done anything with the coil? I saw you changed box, cap, and rotor...

I would recommend checking the coil wiring also.

I would really closely check the grounds and such in the car, especially if the battery is relocated (and if it is, make sure you still run a ground wire from the block to the body in the front where the negative cable used to be).

I had a problem with a high rpm miss that I chased all over. It turned out to be the ground wire from the relocated battery was either not heavy enough gauge, or bad wire. Changing it for a bigger wire, extra clean frame hookup (after I tried everything else) all of a sudden the car would go as high as I wanted.

Of course it just might also just be the lobes are too aggressive for that kind of RPM with a flat-tappet lifter, no matter what the spring pressure is, but that would be a fall-on-its-face thing, rather than a miss or "breaking up".

Good luck, sounds like an awesome combo.
 
You may want to stick a vacuum gage on the intake and watch it at the point where it misses.


was gonna mount one under dash for watching on the run... but will do and get back on that...

If it shows any vacuum try a larger carb. The power valve is spring loaded open, so unless it simply isnt flowing enough (usually a problem with alcohol carbs) it will be open at WOT.

thats what i thought

I'm more inclined to think it's a valvetrain issue. Now that you have the springs in, it might be a good experiment to stick .060 shims under the springs and see if it rises again.

the springs are as tight as they get... a racers trick i learned it to take coil bind (1.100) add valve lift .475, and add .050 for clearance for installed height... so they are as tight as they get...



Sounds like you're on your way, but I'll throw a couple of other ideas out there in case you're not sick of trying stuff! LOL

to far to stop now!! lol

Have you done anything with the coil? I saw you changed box, cap, and rotor...

already have a new blaster 2 coil... only maybe a yr old... like a knuckle head i broke one of the studs off the other one by over tightening, live and learn...

I would recommend checking the coil wiring also.

MSD leads

I would really closely check the grounds and such in the car, especially if the battery is relocated (and if it is, make sure you still run a ground wire from the block to the body in the front where the negative cable used to be).

stock battery location... but i did replace the engine to firewall ground... have a solid steel mount on drivers side and negative cable goes right to the block...

Of course it just might also just be the lobes are too aggressive for that kind of RPM with a flat-tappet lifter, no matter what the spring pressure is, but that would be a fall-on-its-face thing, rather than a miss or "breaking up".

well that could be it but it never did this before... ive always shifted at 6K... just wanted a better trap RPM so i upped the gear and got a miss lol...

Good luck, sounds like an awesome combo.

i have alot of fun with it... on and off the track... its very mellow on the street, have half a mind to do a water injection kit and go for 12.5:1 comp on pump gas...
 
We had problems with MSD rev limiter, they admitted that they had put in newer chip, and didn't tell anyone. Digital 6 al.
 
green1 said:
We had problems with MSD rev limiter, they admitted that they had put in newer chip, and didn't tell anyone. Digital 6 al.
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yea ive heard about this issue... granted mine is a soft touch but it was doing this before i even installed the MSD...
 
805moparkid said:
havn't tried that... may i ask what that would tell? lean i assume? will try maybe tonite if the police stop circling... just kidding
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at full throttle the ignition sees alot of load and it all comes thru the main feed in the bulkhead connections. so find out where that is and take it apart and see if it is corroded. i had this problem the same as you and this is what caused it. it certainently doesn't hurt to check it out.
 
bOb shingler said:
at full throttle the ignition sees alot of load and it all comes thru the main feed in the bulkhead connections. so find out where that is and take it apart and see if it is corroded. i had this problem the same as you and this is what caused it. it certainently doesn't hurt to check it out.
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i will check it out... but being that im now on the MSD box the original coil hot is just the signal for the box, an a direct hot lead to the battery for the box...

could it still be an issue?
 
Is the MSD getting at least 12 volts at the small red power wire?
 
Might try 19/23 on the valve setting and see if it get better.....
If it doe's then I'm with it needs more valve spring pressure. like 350 or 375..
Sounds like its more effected by the rpm.. I think the valve train is hanging in the air when the lifter go's over the top of the nose on the cam.

just my 2 cents
 
It's sounding more and more like it's the valvetrain. Might need more spring... I'd suggest backing the lash off and see if she revs higher. It's not the cam size holding it back, so I'd think giving the spring a chance to chance the frequency of the harmonic might help.

What cam is it again? And what size/length pushrod? and what spring?
 
moper said:
It's sounding more and more like it's the valvetrain. Might need more spring... I'd suggest backing the lash off and see if she revs higher. It's not the cam size holding it back, so I'd think giving the spring a chance to chance the frequency of the harmonic might help.

What cam is it again? And what size/length pushrod? and what spring?
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ok so i had erson springs it it orginally, no idea the pressures on them...

these new ones were crane 354 lbs springs
http://www.summitracing.com/parts/CRN-99839-16/

have smith bro's pushrods in it, part number NH58A-CB... so 9.750 long 5/16" x .083 thick

cam is 282/282 adv 246/246 @ .050 106LSA installed at 102

why would i just suddenly have spring issues after replacing a set that were not good enough to begin with (but made over 100 passes with no issues)
 
something i think i should make clear...

when it first did this at 5400rpm it would still keep reving to 6...

it just breaks up...
 
It wasnt right if it's always been missing. Or am I confusing what you're saying?
You had a set of too weak springs. It didn't rev, but didnt break up? Or the weaker springs pulled clean to 6K? Did it ever pull cleanly tp 6K?
 
moper said:
It wasnt right if it's always been missing. Or am I confusing what you're saying?
You had a set of too weak springs. It didn't rev, but didnt break up? Or the weaker springs pulled clean to 6K? Did it ever pull cleanly tp 6K?
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its never missed untill i installed the 4.88 gears...

it always was able to pull 6 grand with no issues, until the gear change, before that, never missed, never broke up

since the gear change from 3.91's to the 88's it always breaks up above 5800, no matter jetting,timing, exhaust setup, air cleaner on or off, slicks or street treads

i havn't changed shift points, fuel mixture (100oct leaded), anything else... just a gear change...
 
Hummmmm....gear change alone would not make it pop out the exhaust if thats the case.

Hummmmm
Fuel system good..
Ignition system good..
Valves good..we think..
Only thing I can think of you could have messed up back there while changing the diff. is
Yu pinched off the fuel line enough to hinder it...Or yu pinched off the exhaust...
Other than that, I just go blank.... Unless a grounding or a power issue accrued?

Boy what a stumpier!!!!!!
 
racerhog said:
Hummmmm....gear change alone would not make it pop out the exhaust if thats the case.

Hummmmm
Fuel system good..
Ignition system good..
Valves good..we think..
Only thing I can think of you could have messed up back there while changing the diff. is
Yu pinched off the fuel line enough to hinder it...Or yu pinched off the exhaust...
Other than that, I just go blank.... Unless a grounding or a power issue accrued?

Boy what a stumpier!!!!!!
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valves are good as i stuck a valve a couple of months before this... had to replace the exh valves and do all bronze guides... so the head is set...

exhaust is two peice headers, dual 2.5" collectors, dual 2.5" into custom y pipe, 3" out into super 44... at the track just run dual 2.5" dia/12" long extentions

i'll look over the fuel line but i was using a lift so not alot of banging around was done...

i already cleaned and di-electric greased the bulkhead back in 09 when i had my fire lol... and added a 10ga ground from the block to the firewall... also steel motor mount...
 
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