[Healeys] Another backfiring question

[Michael Salter]

It seems to me that most of the responses to this issue with "backfiring"
through the carbs at higher throttle settings of engine speeds fail to
address the question of carburettor needle profile.
The needle profiles recommended for our cars were developed by dyno testing
at Morris Engines. The fuels used for these tests would have been somewhat
different from what we get today and this almost certainly has a
significant effect on the needle requirements for engines running on SU
carburettors.
Data is a bit hard to find on the subject but what reading I have done
indicates that there have been very significant changes in SG and oxygen
content over the years.
I would surmise that there is a very good chance that one of the results of
these changes would be the necessity to use reprofiled needles to maintain
consistent mixtures throughout all operating conditions.
A pretty good idea of what mixtures the engine is ingesting can be acquired
by installing an O2 sensor and gauge like this.
<https://www.amazon.com/AEM-30-0300-Wideband-Sensor-Controller/dp/B0184TSI84/ref=sr_1_9?dchild=1&keywords=02+gauge&qid=1611939439&sr=8-9>
After taking readings with a needle height gauge through the problem areas
the results obtained can then be used to select an improved profile needle
from the huge selection that SU produced.
People have been known to put needles in a drill press and use emery paper
to decrease the diameter of SU needles to "adjust" mixtures through the
affected areas of the range rather than buying dozens of sets of needles to
experiment with then purchasing needles that duplicate the "modified"
profile.
Not a project for the faint of heart and to do it properly you need a
dyno....

M

On Fri, Jan 29, 2021 at 11:18 AM Bob Spidell <bspidell at comcast.net> wrote:

> I dunno if it can cause high RPM backfiring (I think it could), but the
> airflow on all (2-3) carbs has to be the same--or, at least, very close--at
> all throttle positions. This has to be checked off-idle, obviously, but if
> it's good at, say, 2,500RPM it should be good at all speeds; if not, you
> have to disconnect the throttle connect shaft to set them. I suppose a
> crack Brit-car mechanic can test with the recommended 'hose-in-the-ear'
> technique--my ear hose is collecting dust somewhere--but I use a Uni-Syn.
>
> To paraphrase the TV commercial we've all (probably) seen a thousand
> times: "If you don't have a carburettor synchronizer, get one."
>
> Bob
>
> On 1/29/2021 6:13 AM, healeyguy--- via Healeys wrote:
>
> Len and Listers
> Pushing the pin up to lift the piston is the start of the "test the
> mixture setting" exercise. You begin the push, ever so slightly,  and
> listen to the idling engine. The idle goes up.  What do you do next? You
> continue to raise the pin  to see what happens. If the idle continues to
> rise you know you are in a rich condition. If the idle stops going up and
> starts to drop off the mixture is pretty close. If the idle does not
> increase from the start you usually assume you are in a lean condition.
> This gets the carb mixture close at idle which in theory means the carb is
> going to operate throughout the RPM range.
>
> There is one other assumption here, that being that the carbs is in the
> same condition as new mounted on an engine operating as new. That is when
> allot of other stuff comes into play.  The carb float bowl items,
> needle/seat and float level  must be set correctly. Float must not be
> leaking thus making it sink. Carb must have the correct main needle and jet
> and they have to be installed and adjusted correctly. Flutter at high RPM
> may indicate that the piston spring in the suction chamber is weak or the
> incorrect spring installed. Ignition timing and condition and setting of
> ignition points.
> The list goes on and on......
> P
>
>
> -----Original Message-----
> From: Leonard Berkowitz <DrBerkowitz at hotmail.com>
> <DrBerkowitz at hotmail.com>
> Subject: [Healeys] Another backfiring question
>
> Hi all
>
> Just finished repairing and reinstalling the carburetors on my BJ7. I
> thought I had everything adjusted properly. According to my Haynes Manuel
> the engine speed should increase slightly when you lift the piston a very
> small amount. Mine does that.  My dwell angle is where it needs to be and
> the car starts and idles nicely. Also sounds beautiful when I punch the
> accelerator linkages. The problem is that when I get it up to 60-70 mph I
> get what sounds like backfiring into my carburetors. Also sounds like it
> doesn’t have a whole lot left in the tank, so to speak. Does that sound
> like I am running too rich or too lean?
>
> Len Berkowitz
>
>
>
>
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