Well here it is folks. at least for the moment the
problem is solved. Drive dog on the dist. is
slightly sloppy but only 1-2B0 of timing float at
speed. I shortened the 24" plus leads of the
XR700 electronic ignition. Mine is mounted right
next to my Sports Coil. The leads are now only
5-6" long with new crimped on ends. This was to
reduce possible pulse trigger flutter do to
induced EMF (electromagnetic field) pulses as the
coil field collapses (Sure the coil is
electronically shielded - its metal, but the coil
+ and - leads to the XR700 module weren't). I had
suspected the long leads bundled and tie-wrapped
were acting like an antenna and inducing signals
onto the low voltage sensor wires coming out of
the dist. I was right! An oscilloscope on the
ignition showed a missed or intermittent miss fire
on cylinders randomly at speed.
Manifold vacuum tests show 19 inches at idle and
stable ( less than 0.5 in. float) sudden open
throttle=1-2 inches sudden closer=27. At 4000 rpm
cruising vac is reasonably stable at 13-15 inches.
I replaced the 19B0 advance plate in the dist with
a 10B0 plate, and lightened one spring to bring the
centrifical adv. in sooner, while limiting the
overall total adv possible. (returned the dist.
innards to std 1973 specs -original configuration
before I started all of this)
At 1000 rpm I now have the timing set at 12-14B0
BTDC. (at 800 rpm it drops to 10-12B0) A vacuum
tester applied to the vac. adv. unit shows 4B0 adv.
at 5 in of vac. and 11B0 at 15 inches. Cent.
adv. starts to kick in at 5B0 by 1800rpm and 20B0
at 3800 rpm
the vacuum take off on the rear carb shows 14
inches of vac on sudden opening of the throttle)
The K&N's are gone (Bummer they look so nice)
Stock air boxes are back with new filters. I can
only assume the restrictions of the air boxes help
in accelerating the air mix through the carbs
increasing and stabilizing vac. I use standard
engine oil in my dashpots with the RED springs. I
haven't changed the needles back yet so I expect
when I check the plugs tonight it will have been
running rich. It still has the AAA needles in both
carb's. BUT THIS MONKEY FLIES!
Zero stumble on take off and cruising (even cold.)
I scared the heck out of a muscle-car friend of
mine who is use to brute power, but had never
experienced the affects of climbing to 80 mph in
an LBC. As we went out for a test run, turning
left onto a highway on ramp, I put my foot down in
the turn. Wound it out to 4800 rpm smooth as
silk. I kept my foot to the floor the whole way
through the gear range. I was doing 55 when I put
it in fourth. My passenger was white knuckled that
I was continuing to quickly climb in speed and was
doing 65 before I got the end of the on-ramp. I
carried it all the way to 85 before backing down
when he screamed are you F____ing CRAZY! He'll
never laugh at my LBC again. ha ha!
Thanks again everyone for possible clues as to
what was going on here. Truly I think it was the
timing issues.
If it truly were miss firing a cyl. once in a
while, this would make it feel like a stumble,
only noticeable when cruising.
I plan to go back and do some flow testing and get
the K&N's and the right needle combos back in
place. I'll keep you posted.
-----Original Message-----
From: MYLDN@aol.com <MYLDN@aol.com>
To: Paul Tegler <wizardz@amdyne.net>
Date: Tuesday, September 29, 1998 4:42 PM
Subject: Re: Timing, Carbs, SU's vs DGV
>well paul your not alone my 73B does the same
thing, and im also running all
>the same stuff. so if you get a good answer
please forward to me.
>Sean Muldoon
|