[Healeys] Another backfiring question

Fri Jan 29 15:28:29 MST 2021

I'm betting that as the problem appeared after carb work that's where the
problem most likely resides.

M

On Fri., Jan. 29, 2021, 2:32 p.m. banjojohn, <banjojohn at cox.net> wrote:

> 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.
>
> <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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