[red78ta]

Around 5200 it starts to break up & backfire through the carb. My combo should be good to 6500 but will be happy with 6000. Have tried numerous timming setting & advance curves including locking out the advance totally. Only slight changes in reaction but still present. Doesn't matter hot or cold. Seems like a timming issue but am running out of ideas on where to look. Getting VERY frustrated ......... could this be a valve float issue even though the cam spec says valve float should not occur till 7200? fresh built 400 (about 2500mi. now) stock crank & rods, cast pistons, bored 30 over, 6x-4 heads shaved for 9.5:1 comp.(this is a street/strip car), Crane cam (P/N 284281) adv duration 296/304 (234/242 @ .050 tappet) with 473/488 lift, stock 1.5 rockers, Valve adj. to 0 lash w/Polly locks, Tomahawk intake, holly 4150 750 cfm carb. & Pertronix HEI.

[screamingchief]

What valvesprings are you using?

What sorta fuel system does the car have?

[SlowSpitzer]

What kind of ignition, how old?

[red78ta]

Not sure on the springs, checking into what the builder used. Have a new Pertronix HEI & running a high volume Holly mechanical pump w/factory steel tube supply line & steal braided hose pump to carb.

[SLICK1]

Originally Posted by red78ta

Around 5200 it starts to break up & backfire through the carb. My combo should be good to 6500 but will be happy with 6000. Have tried numerous timming setting & advance curves including locking out the advance totally. Only slight changes in reaction but still present. Doesn't matter hot or cold. Seems like a timming issue but am running out of ideas on where to look. Getting VERY frustrated ......... could this be a valve float issue even though the cam spec says valve float should not occur till 7200? fresh built 400 (about 2500mi. now) stock crank & rods, cast pistons, bored 30 over, 6x-4 heads shaved for 9.5:1 comp.(this is a street/strip car), Crane cam (P/N 284281) adv duration 296/304 (234/242 @ .050 tappet) with 473/488 lift, stock 1.5 rockers, Valve adj. to 0 lash w/Polly locks, Tomahawk intake, holly 4150 750 cfm carb. & Pertronix HEI.

I know it may sound stupid, but do you have a rev limiter on the car? And where is it set? I chased what I thought was a gremlin the early part of last season, changing fuel pumps, changing caps, coils, and even adding a scoop for additional carb clearance, only to find out I still had my rev limiter set at 5,600 for my stocker 400 from the prior season! I can laugh at it now, but it was'nt funny when I was spending that Loot! Your problem seems different than what was happening to me, but this is just my ~~>

[red78ta]

no rev limiter on the Pertronix module & the MSD module I used had the limiter turned all the way up so this is not the issue either. Thanx for the input though. Also had a Jet Perf. Q-jet stage 2 on the car (same condition), also acted the same w/stock distributor & the MSD module.

[wrench]

That Crane Cam is a Hydraulic Cam right? Shouldn't you have some lifter preload (1/2 turn. 020"?) instead of the 0 lash?

[Brian Baker]

Find out what springs you have, and what their installed height is. I'd bring tis issue up with the builder and see what he thinks.

[Rob B]

Which MSD Module? I was chasing a high end miss and discovered that I need to change my chip 400 rpms higher than I originally had in it for my rev limiter. I basically put a 6800 chip in my 2-step (hi-side) and it solved the problem. Low side limiter is dead on to the chip in it.
Your also using Stock rockers or some kind of aftermarket rockers??? High high are you wanting to rev the motor??? I would think around 5400-5700 would be the perfect rpm for the power band of that camshaft.

[red78ta]

Rob - The Module I bought has an adjustable limiter which I set all the way up. Acts the same way with the new Pertronix dist. & module. I do have the stock rockers, am looking into what springs the builder put in. The cam sheet says it's power band is 2800-6800 with float occuring at 7200. I will be happy with 6000, think the power may drop off sooner though because I kept the compression down a bit (9.5:1) for streetability.

[george kujanski]

How can the cam sheet know where float occurs? it's dependent on all of the valvetrain components you are using, spring tension, etc. no?

George

[70judgeconv]

that sounds like a awful long power band. I could see 2800 - 5800 not 6800. You would need some pretty stiff springs for that and would not really work with stock rockers. I would check cam card again for a misprint.

[screamingchief]

The cam sheet says it's power band is 2800-6800 with float occuring at 7200.

HA ha ha ha,,,yeah,,,that's crane cams for you,,,a fella would be very hard pressed to find a hydraulic cam equipped pontiac that would be reasonable to turn 7200 RPM's,let alone 6800 on a regular basis!

Really 6800-7200 RPM for any hydraulic cam equipped engine is a bit of a stretch IMO.

Cam "descriptions" by some companies leave a bunch to be desired,that's for sure...

They can be confusing/misleading for the less knowledgeable folks out there too!

As for the springs,just make sure they are'nt those crane "single w/damper" #99840-16 springs,those are junk,and even crane recommends their dual #99838-16 springs for that cam.

HTH.

[red78ta]

Here is a copy of the spec sheet -

Camshaft Specification Card
Part Number: 284281 Grind Number: H-296-2 (REPLACES CCH-296-2)
Engine Identification:
1955 1981 PONTIAC 8 FAIR IDLE, PERFORMANCE USAGE, GOOD MID-RANGE HP, BRACKET RACING, AUTO TRANS W/3000+ CONVERTER, 3800-4200 CRUISE RPM, 10.0 TO 11.5 COMPRESSION RATIO ADVISED. BASIC RPM 3000-6500
Engine Size Configuration
265-455 C.I. V

Valve Setting: Intake .000 Exhaust .000 HOT

Lift: Intake @Valve 473
All Lifts are based on zero lash and theoretical rocker arm ratios.

:Exhaust @Valve 488
Rocker Arm Ratio 1.50

Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 35 BTDC 81 ABDC 296 °
Exhaust 83 BBDC 41 ATDC 304 °

Spring Requirements: Triple Dual Outer Inner
Part Number 99838
Loads Closed 112 LBS @ 1.650 or 1 21/32
Open 296 LBS @ 1.190
Recommended RPM range with matching components
Minimum RPM 2800
Maximum RPM 6600
Valve Float 7200

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 10 BTDC 44 ABDC 107 234 °
Exhaust 58 BBDC 4 ATDC 117 242 °

Remarks:
MAINTAIN .040 MIN. BETWEEN ROCKER ARM & RETAINER AFTER VALVE ADJUSTMENT WITH VALVE CLOSED.

[red78ta]

Update ........ tried closing plug gaps from .55 to .35 to test ignition strength & ran to 6200 with no noticible miss or break-up. Down on power though because of smaller gaps. So I am looking at ignition issue. I guess the next step will be to verify voltage to the distrubutor, everything else is new & high perf.

[Brian Baker]

Peak power in your 400 with that cam is going to happen before 6000 rpm. Your thinking you're losing power by tightening the plug gap is likely all in your mind, but it looks like you've found your problem. I've never run a gap larger than .045" with an HEI (with or without an MSD box) because based on timeslip results, it didn't make a difference with a larger gap, so why tax the ignition system? Sounds like a bad coil (or bad ground to coil).

[red78ta]

That's what I was thinking also. My Chilton's lists the factory gaps at .60 & I have not added an ignition box yet. Wanted to try & find where the issue was first. Probably going to add a box for more power & try opening the gaps alitlle more. Thanx for all your help diagnosing this problem.

[Brian Baker]

It's been my experience that the MSD-6AL does not add any power whatsoever compared to a properly set up HEI. My timeslips showed no difference in MPH between the two. The MSD does however provide a rev limiter (which I think is a must), and it also cleaned up the idle quite a bit, providing better idle characteristics with a 238 @ .050" cam over that of the HEI alone.

[pinballDan]

Have you ever put a timing light on the engine and ck the advanced to see if it is not retarding as the rpm goes up. I had a builder not get the fuel pump centric on the cam right and the cam was walking in and out causing a problem like this.It was retarting the timing.

Dan

[red78ta]

Thanx Brian, knew the idle & off idle would improve but did not realize it would not make much diff. in the upper rpm's. Think I'll probably stick with gaps around .45 then.

Dan: I have & it does go up, we have done that to check/set the total timing. Interesting info. though.