Technical > Spud Miller's Cave

Flat under low end Load

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tdietz:
Ok guys, 175" WB FED, 410 SBC timing around 36 degrees, MSD Pro-Billet Dist, MSD Blaster 2 coil, MSD 6 ignition box, MSD starter saver, and MSD 2 step rev control.  Engine is fuel injected on Alcohol (down nozzles), Kinsler 450 mechanical pump.

The car on launch runs flat and breaks up.  Our first pass of the day our 60' times were 1.415ish and our normal 60' times are roughly 1.16ish.  It is flat then all of a sudden will pick up and our MPH on top end is normal 166MPH.

We tried leaning the barrel valve, then we tried richening the barrel valve, nothing made a difference.  And by the end of the day the engine would clean out until I hit high gear (powerglide) and then it would break up again. 

I'm pretty certain this is ignition, but where or what part of the ignition should I be looking at?  Our tach is the digital shift light with the tach built in, and it seems very erratic as well....

dreracecar:
Dump the ignition system and put a Magneto in it.
It takes a lot of battery to run a MSD ign and unless you have an alt on the car, the battery cant keep up with it.
  Mags do take HP to turn(very little) verses a standard ign system, but it also takes HP to turn an alt and push the extra weight of a battery, so its a wash, but the Mag gets the nod for its simplicity and reliability.

tdietz:
That's all great and I understand you are probably correct.  BUT, I didn't have issue last year and I don't have 600-800 for a Mag.  I'm sorta stuck at this point with what I got.  I can't afford to just dump all of it and go to a MAG. 

For the previous 2 years we've had no issues.  It developed late last year and we attributed it to weather.  But, now I'm almost certain it is ignition related.

H.G. Wells:
Can you bypass the two step? take it out of the equation, borrow a coil if you do not have a spare.
If neither of those helps, I think the box itself may be suspect.

Assuming you have looked inside the distributor? No tracking in the cap or anything noticeable?

ricardo1967:
Tim,
In in lieu of temporarily replacing ignition components individually to aid troubleshooting, sometimes tightening the plugs' gaps is suffice to flag a weak ignition system. Another thought is checking rotor phasing.

No argument that magneto has its advantages, but there are tons of drag cars running CDI ignition systems, w/o alternator.

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