I have only been running 1/8th mile events since putting in 4.88 gears (with 31x10.5 slick) but tried to get my '75 Colt down the 1/4 last night during T&T. I made 2 runs before putting it on the trailer. 1st run was a 12.68 (1.8 60ft) shifting by ear and watching tach. All shifts were around 5500-5800. Valves started floating around 6200 in 3rd so I backed it down just a bit in order to make it to the stripe.

2nd run I tried to get more from each gear by shifting by tach. Had to shift by 6000 on 1st shift (valvetrain instability) and made it to 6200 on 2nd shift where I got some more vibration/instability. Barely made it thru the traps by the time I was pushing 6200 in 3rd. Ran a 12.8 on that run (same 60ft, 1000ft, lost .2 in 1/8th).

Had to trailer it due to oil seeping past front & rear intake seal (RTV only - gasket on the way) on high-rev pass. Motor has 509 cam w/bowl-ported 2.02 J heads, roller rockers, double springs on a stock 360 short block. MSD-6A, aftermarket dist, Demon 750, custom bigtube headers, 2800lbs w/driver, 4000 stall converter (727 footbraking).

I know the shortblock needs to be decked and/or aftermarket pistons to up compression to match the rest but I expect more out of the combo as is.

Any comments and/or suggestions would be appreciated.

Whats the spring pressure and whats the gear in back
Who's springs

Since it lost .2 at the 1/8 on your second run when you carried your shift points out a little further, I'd say you'd be best served by putting a taller (lower numerical) gear in the car to keep the revs down. If it nosed over that bad shifting at 6000 as opposed to 5500 then it doesn't want to see a lot of RPM.

That being said, I think the motor should be able to see more RPM's before it really starts to take a nosedive. Some of the Stock Elim guys run their 340's with "stock" heads over 7k and they don't get to use the same good parts you do. As the above post says, check your valve springs.

Several things to check, the Tach, the valve spring pressures as well as the lifter preload It sounds like the lifters are pumping up or the tach is off, which I have seen more than once. Try setting the lifters with 1/8 to 1/4 turn of preload after backing the adjusters off enough to get some clearances on the pushrod and then tighten to zero preload and add 1/8 to 1/4 turn hot Makes some runs at 1/8 and then try 1/4 and keep experimenting until you find what your motor likes

4.88 gears out back w/31x10.5 slicks leaving at around 2000 on foot brake (have bad left foot - can't stand on it). Will check the springs soon hopefully.

Re: tach - after having just recently obtained the car and started racing it, I noticed that the tach wasn't working. Replaced it w/known good tach w/same results. My voltmeter wasn't seeing pulsing on the negative side of coil w/factory ECU controlling ignition so I went to the MSD-6A and used it's tach output which I knew to be pretty stable. My gut tells me it might be reading high and a bit jumpy even while on the MSD's tach out.

I would like to go right to 4.56's or even 4.30's but everyone I talk to says the 4.88's should be doing the job w/my combo - hence the post.

If you haven't set the lifters do so now and make sure there set correctly, as Jared said many SB Mopar will rev to 7000 RPM with hydraulic lifters easily I dyno tested two SB 340 stocker motors several years ago and found that 1/8 turn preload hot worked very well, this was on a set of the latest "trick" hydraulic lifters that the manufacturers suggested using .002 clearances on

Wow I didn't realize how expensive the good valvespring testing tools were. Is there a common place to go borrow one? There aren't many professional engine building shops around here.

I plan on checking/setting my lifter preload this weekend as that only requires a clear head and some elbow grease.

I did notice that the MP 292/509 cam is listed as having a 2800-6200 RPM range. I'm guessing that is part of my problem.

It appears that I was leaving money on the table - there is lash to be taken up on several rockers. More interesting to note is that I found spacers (as expected) between the rockers on the passenger side but NONE on the drivers side! I'm guessing this was a major part of my valvetrain instability issues around 6200 rpm. I put spacers on last night and hope to attend to the lash-setting this afternoon. Hopefully I can get everything together for T&T Wed night.

They are aftermarket adjustable (5/8 + hex) roller (shaft & tip) rockers. I was thinking that while on the base circle snug is when I can't roll the tip w/my finger or spin the pushrod anymore. I'll probably start with 1/8 of a turn from snug to start with.