[6pack] TR6 - 2500 RPM 'miss'

[Dennis Culligan]

List - 

    Since Mike asked his "miss" question, I'm going to ask mine.

   A long and involved question to try to solve a long standing issue.  Back
in 2001, I spun 

a bearing in my 1976 TR6 motor and had to have it re-built.  Rather than do
it stock (heaven 

forbid!), the motor was bored .30 over, the head was milled to about 9:1
compression

and the cam was re-ground to specs similar to the 150HP cam found in PI
cars.  The carbs

and the exhaust system were left bone stock.  The car is equipped with a
Pertronix ignition 

in the dizzy and the dashpot oil is checked regularly.    

  After a break-in period, I noticed a slight stumble at 2000- 2500 rpm
under load (note that 

the motor idles fine (a little lumpy due to the cam) and will run smoothly
well up over 2500 

rpm while stationary).  I let it slide, assuming it would smooth out once
the motor was fully 

broken in. It didn't. Even though the miss suggests a lean condition, the
plugs were fouling 

pretty regularly so I tried a hotter plug (went from NGK BP6ES to BP5ES) and
eventually 

(2005) had the ZS carbs re-built by Joe Curto.  I mentioned to Joe the
changes I had made to 

the motor and he suggested staying with the stock TR6 needles but said if
they didn't work 

out, I might try the needles used on a 6 cylinder Jag.  I've not yet tried
that change. 

     I am now looking to fix this miss.  I had a tune-up done last week
(valves checked, etc,

etc) and the stumble continues.  It starts at around 2000 rpm and often
continues almost 

up to 3000 rpm in all gears - above that it smooths out and the motor runs
like a beast 

(3000 rpm is about where the cam really kicks in).  I'm assuming this is a
fuel issue but am 

at a loss to figure out which way to go.  I'm considering moving back to the
cooler spark 

plugs (my plugs are almost white and I've NEVER had the "sandy colored"
plugs that one is 

supposed to have) and am planning to purchase a few sets of needles to play
with (including 

the Jag needles mentioned above) but there are not that many available for
ZS carbs .  We've 
already tried shaving the stock needles to increase gas flow at the low end
but it didn't help, 

and no amount of leaning or richening of the carbs seems to make a positive
difference.

   I fully realize that swapping out the ZS carbs for a set of SUs might
make this easier to sort

out (and I have access to a pair if it comes to this) but I'd rather try to
sort out the ZSs. Ideas

are welcome, and thanks.  

Dennis Culligan, Highland, NY/ 1976 TR6 CF57948U