On Fri, 1 Mar 1996, David M. Hall wrote:
> Now, just the normal moderate power/speed. Also, under normal
> operating conditions, while cruising at around 60, my car tends to
> surge. (Power, lack of power, Power, lack of power at about 4 times
> per second. More noticable down hill, less noticable up hill, or under
> acceleration.)
>
I've got the same problem... not as bad now as in the past before I
started my Great Needle Adventure.
> I believe that I have my carbs correctly adjusted for the fasted lean
> idle. I have a fairly free flow tuned exhaust system, Dual SU Carbs,
> some type of hot (PO) cam, Crane Electronic Ignition, Vaccume advance working
> correctly, static timing about 20 BTDC.
STATIC timing 20 BTDC? Zowie! That's a little much, isn't it?
Over-advance has given me a lean-mixture indication in the past.
> a. Should I richen up the mixture a bit?
perhaps
> b. Are my needles wrong?
Almost certainly. A cam change and an exhaust change will certainly
change the optimum needle profile.
> c. Is it OK to run with the choke on?
Yes and no. Yes, if you remember to take it off when you don't need it.
Too much gas in the mixture causes excessive ring wear, pollution, plug
fouling.
> d. What's the best way to get the performance increase I noticed while running
> with the choke on?
>
The absolute, positive, best way is to get new needles profiled on a
rolling road dyno.
The problem is that each combination of bolt-on modifications and exact
engine is going to result in a slightly different optimum. In the case
of a header system, K&N filters, and a long duration cam the flow pattern
is going to be SIGNIFICANTLY different than stock.
You can play the game of swap needle (consulting the profile charts or
the Haystack program to move in the right direction) but it is not cheap
and quite frustrating on HIFs. Only a few profiles are available in
biased needles due to emissions considerations.
Right now I have some fixed needle in an adapter in the rear carb, and an
AAE biased needle in the front. Yes, it is confused, but I ran out of
stock and patience at this point.
I'd check that advance with a light against the book value before going too
far. I played the "over-advance it, increase richness to compensate"
game for a while when my rings were in the process of segmenting
themselves. It seemed to help acceleration around town but didn't affect
top-end (and it sounded AWFUL). When I rebuilt the engine the temptation
went away as I had enough power to get out of my own way.
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire E Stock | '88 RX-7 C Stock
"1st Law of OleoHydroDynamics - Little British Cars Leak. - JMT"
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