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Old 07-10-2007, 06:43 PM
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Spark gap closed

Hey everyone, need your input on something.

Heres the specs offhand.

355 sbc
Mahle forged flatop pistons, Mahle rings
Scat lightweight rods
stock crank
World S/R 67 cc heads, straight plug, milled to 57cc, no port work done
block is zero decked
standard felpro gasket
cam is a custom grind solid .500/.525 lift 234ish duration
500 holley 2bbl carb, stock other then choke stuff removed, 76 jets, 3.5 pv
stock intake/exaust manifolds
total timing 34, 12 initial, Even 1 degree more then that and it will put little aluminum bits on the plugs
110 octane sunoco
3200# dirt circle track car runnin 683 gears behind a glide and pulling 6500 to 6700 rpm max
dynoed at 370trq @ 3600rpm, 320hp at 5700rpm, 20+ nights on the engine.

The problem,
I pulled dist cap off tonite because the driver said it had a miss in hotlaps that went away. Found a broken advance weight, the large part was still under the cap, the smaller piece was laying in dist, rotor still moved freely. After more checking found the number 8 plug closed up to .008. Plugs were new last race and set at .045. Other plugs looked fine as far as coloration and gap. The closed one, in the pictures, color looks ok other then the ground strap is brass/bronze colored on the back side.
Questions, comments?
Thx, Mike
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Old 07-10-2007, 07:05 PM
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Mark where the gap is on the porcelain of the plug with a Sharpie and index 'em.
http://store.summitracing.com/egnsea...x+washers&x=26
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Old 07-10-2007, 07:11 PM
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Thats good advice but they already are. I run them all with the strap staight up to no more then 45 degrees off. This particular one was about 45 degrees off. the best I could get it without using indexing washers.

Thanks

Mike
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Old 07-10-2007, 08:48 PM
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Ok, thinking about this more I can only come up with a few things.
1: too much advance. I just finished checking it with the broken advance weight removed. Timing was perfect, right at 34 all the way up to 3500 rpm. So it couldnt have been an over advance situation that caused the strap to heat and close.
2: Fuel pressure/volume. Checked that as well and pressure is right at 7psi idle to 6000rpm with no load. Spoke to the driver he said he noticed no fluctuations in fuel pressure while racing. So that is ruled out.
3 Too tight of valve lash? Specs are .016 intake/ .018 exaust. I havent checked that yet and cant until it stops raining here, no room in the garage for the car. Could too tight of valve lash cause the closed gap?

I suppose the broken piece of advance weight tumbling around in the distributor could cause a spark to jump to the wrong cylinder and a misfire/detonation but wouldnt it take numerous times to cause enough heat and rattling to close the gap. Also I would think that the driver should have noticed something like this unless he was deaf. Hmmm

Mike
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Old 07-10-2007, 09:09 PM
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Only one thing will close a plug gap that was set properly the first time, and that is something in the cylinder hitting it .

Vince
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Old 07-10-2007, 09:17 PM
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dont say that
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Old 07-10-2007, 09:58 PM
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Quote:
Originally Posted by 302 Z28
Only one thing will close a plug gap that was set properly the first time, and that is something in the cylinder hitting it .

Vince
thats what i was thinking id tale a flish light and look at the top of the piston
Shane
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Old 07-10-2007, 10:02 PM
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Ill tear it down if the need be, a few bucks in gaskets is better then 6k in boat anchor. I hope its nothing tho, hate to see it turned to scrap iron. Ill see if i can find some sort of scope first if I can. Not much for parts shops around here to buy specialty tools


Mike
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Old 07-11-2007, 07:37 AM
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It's quite possible that whatever it was passed right through the combustion chamber and out the exhaust.

Vince
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