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[Roadsters] Tailpipe backfiring when shut off

To: "Datsun list" <datsun-roadsters@autox.team.net>
Subject: [Roadsters] Tailpipe backfiring when shut off
From: "Mike H." <twobeaners@earthlink.net>
Date: Tue, 6 Nov 2007 17:57:04 -0600
Hello list,
Well I haven't updated my stroker project since July, so here it is:
I built the 1600 stroker using the stock head (un-ported, stock valves), stock
intake manifold (also un-ported, un-modified), 4:1 headers, and stock 1600
carbs with ADQ needles, Boone EI dizzy, standard U20 flat top pistons and U20
rods, and last but not least, the NISMO stroker crankshaft ($$$). The only
other upgrade to the engine was the Isky DR-66 cam, a little more lift and a
little more duration than the stock cam. I cc'd the head's chambers, and
several online copression calculators indicated this engine would have
approximately 9.4:1 compression. I also rebuilt and added the 5-speed to
complete this upgrade. The rotating components were then balanced at a local
speed shop, I was told to within .2 of a gram. It sure runs like it! It is
smooooooooth! Well worth the $175! IMHO.

After putting 1500 miles on the engine I have routinely cruise at 4500 rpm in
4th and 4000 in 5th,and recently started winding it further to 5000 during
3rd, 4th, and 5th gear shifts. It fairly screams and really seems to "like"
shifting at this rpm range. Must be a "torque curve" thing or something!
Anyway, I'm starting to wonder how far it will eventually wind out in 5th? My
problem is I'm already pushing close to 90 in 5th at 4k. How fast can one go
to "test" his engine? ! :-).

Everything is wonderful, except:   It backfires loudly (one loud bang out the
tailpipe) when I turn off the ignition. I've checked mixtures, timing, and
valve clearance. Compression is good and leakdown testing tells me there are
know valves leaking.
Any suggestions, opinions, or comments?

Thanks to all the listers who have provided help and advice to my project.
Mike Hudson   '67 - stroker / 5speed
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