[Healeys] Another backfiring question
banjojohn
banjojohn at cox.net
Fri Jan 29 12:31:59 MST 2021
One other rabbit hole to go down is, what about valve float? At high rpms, if you have a weak intake valve spring, not closing the valve fully before ignition, it could pop back through the carb.Just another idea to muddy the waters.John O'Brien'61 bugeye (Lucy)'65 BJ8 (Madelyn)Sent from my Sprint Samsung Galaxy S10e.
-------- Original message --------From: Michael Salter <michaelsalter at gmail.com> Date: 1/29/21 11:10 AM (GMT-06:00) To: healeys at autox.team.net Subject: Re: [Healeys] Another backfiring question 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. 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....MOn 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>
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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