On Thu, 11 Mar 1999, doug russell wrote:
> 1. The carbs backfire like crazy. Here's the story. After setting the
>needle
> height to be even with the bridge in the carbs I backed them down 12 flats.
> Then conducted an air flow test where I lifted the pistons (at the same time)
> ever so slightly (about 1/8") and got a slight rise in RPMs, indicating a
>rich
> mixture, so I went up by one flat until the idle did not change when slight
> lift was applied. So far, everything is by the book but I can't get the
>carbs
> to stop backfiring. When cold, they backfire like crazy but quiet down
> substantially when the engine is warmed up - but still backfires especially
> when accelerating slowly (a heavy foot almost eliminates the backfiring).
> Also, when the choke is applied, the backfiring stops. Sounds like it's
> running too lean right? So I make the mixture richer and still get
> backfiring. Anyway, I trust this is something easy that I'm overlooking for
> which you all can suggest a remedy.
(Note: my experience with HS carbs are HS2s on a T****ph Sp*tf*re. I have
had an HIF6, two HIF4s, and now an S&K side draft on the MG).
Yup, this is classic over-leaning. I'm assuming by "backfire" you mean
through the carbs, not the exhaust.
1) How many flats did you go from 12? How much richer did you go to try
and solve it? 12 should run pretty good, and I don't think I've leaned
more than 3 or 4 flats from there without having other problems.
2) Are you using the choke when cold?
3) Is there oil in the dashpots? Lack of damping oil can cause problems
on acceleration, and may even let the pistons bounce and cause
4) Are the needles screwed into the dashpots properly? (don't laugh, I had
a hell of a time tuning a rear carb which didn't raise the needle out of
the jet!)
5) Given the manifold gasket problem you also mention, how about an inlet
air leak? This would cause leaning that would be unresponsive to carb
changes.
6) Oh, my (admittedly dim) recollection of the "raise the piston"
approach was that you wanted a momentary rise in RPM and then a drop...
not *no* rise. (and not "no drop" either). If you raise the piston too
much, you get anamolous results... 1/8" sounds rather much, I think I use
1/16".
Why is my memory sketchy on HS tuning, even though I have them on the
Sp*tf*re? I use peak RPM to tune those, much easier, and close enough for
government work.
--
John M. Trindle | johnt@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire H Stock | '88 RX-7 C Stock
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