On Mon, 16 Oct 1995, JOHN WHITEHEAD wrote:
> Situation:
>
> Car: 1973 MGB, dual HIF4's, Crane Hi-perf. CAM, Timing 11 deg. adv.
>
> Carb's rebuilt 6/95 (were working great until we ate rod bearing)
> Engine rebuilt 8/95 (New pistons/rings, Cam, oil pump, ... etc.)
>
> Engine now has approx. 1000 miles the since rebuild. When check on
> an emissions analyzer the idle mix is very rich. Upon spark plug
> examination after a good run plus appear to be on the lean side.
> Can the current stock HIF4's be adjusted to compensate or must I
> change the jets & needles to correct this problem??
>
You're going to have to switch needles (this is going to be fun, eh?)
The problem is that your higher performance cam has altered the charge
flow at higher RPMs.
You need the Haynes SU book, or similar comprhensive reference which
lists all the needles and their actual diameters at the measurement
points. You want something which is skinnier in the cruise (middle to
high RPMs) range.
This can be relative to the base (idle) diameter. The AAE, for instance,
seems to be thicker at the base but relatively less thicker in the middle
to high RPMs than the ABD. This means you will have to richen the
mixture overall a bit to get good idle mixture, but that will make your
cruise band more acceptible.
One problem with switching needles for HIFs is that there are a lot fewer
varieties than for the HSs (you'll need the spring loaded needle). The
reason I think is that emissions restrictions discouraged experimentation.
There's a program on the autox.team.net server which is called
SUNEEDLE.EXE (for MS-DOS). If you can figure out how IT works it will
substitute for the reference and a headache-creating comparison session.
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire E Stock | '88 RX-7 C Stock
Home Page: http://www.widomaker.com/~trindle
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