<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"></head><body dir="auto"><div dir="auto">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.</div><div dir="auto"><br></div><div dir="auto">Just another idea to muddy the waters.</div><div dir="auto"><br></div><div dir="auto">John O'Brien</div><div dir="auto">'61 bugeye (Lucy)</div><div dir="auto">'65 BJ8 (Madelyn)</div><div dir="auto"><br></div><div dir="auto"><br></div><div dir="auto"><br></div><div id="composer_signature" dir="auto"><div style="font-size:85%;color:#575757" dir="auto">Sent from my Sprint Samsung Galaxy S10e.</div></div><div dir="auto"><br></div><div><br></div><div align="left" dir="auto" style="font-size:100%;color:#000000"><div>-------- Original message --------</div><div>From: Michael Salter <michaelsalter@gmail.com> </div><div>Date: 1/29/21 11:10 AM (GMT-06:00) </div><div>To: healeys@autox.team.net </div><div>Subject: Re: [Healeys] Another backfiring question </div><div><br></div></div><div dir="ltr"><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif">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.</span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif">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.</span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif">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.</span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif">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.</span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif">A pretty good idea of what mixtures the engine is ingesting can be acquired by installing an O2 sensor and gauge like <a href="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">this. <br></a></span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif"><font color="#000000">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.</font></span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif"><font color="#000000">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.<br></font></span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif"><font color="#000000">Not a project for the faint of heart and to do it properly you need a dyno....</font></span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif"><font color="#000000"><br></font></span></div><div style="font-family:comic sans ms,sans-serif;font-size:small" class="gmail_default"><span style="font-family:arial,sans-serif"><font color="#000000">M</font></span><br></div></div><br><div class="gmail_quote"><div class="gmail_attr" dir="ltr">On Fri, Jan 29, 2021 at 11:18 AM Bob Spidell <<a href="mailto:bspidell@comcast.net">bspidell@comcast.net</a>> wrote:<br></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote">
<div>
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.<br>
<br>
To paraphrase the TV commercial we've all (probably) seen a thousand
times: "If you don't have a carburettor synchronizer, get one."<br>
<br>
Bob<br>
<br>
<div>On 1/29/2021 6:13 AM, healeyguy--- via
Healeys wrote:<br>
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<div id="gmail-m_-5427403641000448200yiv9115048286">
<div style="color:black;font:10pt Arial,Helvetica,sans-serif">Len and Listers</div>
<div style="color:black;font:10pt Arial,Helvetica,sans-serif">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. </div>
<div style="color:black;font:10pt Arial,Helvetica,sans-serif"><br>
</div>
<div style="color:black;font:10pt Arial,Helvetica,sans-serif">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. <span style="font-size:10pt;background-color:transparent">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.</span></div>
<div style="color:black;font:10pt Arial,Helvetica,sans-serif">The list goes on and on......</div>
<div style="color:black;font:10pt Arial,Helvetica,sans-serif">P<br clear="none">
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<div style="font-family:arial,helvetica;font-size:10pt;color:black">-----Original
Message-----<br clear="none">
From: Leonard Berkowitz <a href="mailto:DrBerkowitz@hotmail.com"><DrBerkowitz@hotmail.com></a><br clear="none">
Subject: [Healeys] Another backfiring question<br clear="none">
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<div id="gmail-m_-5427403641000448200yiv9115048286yqt58392">
<div id="gmail-m_-5427403641000448200yiv9115048286">
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<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
Hi all</div>
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<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
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? </div>
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<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
Len Berkowitz </div>
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