Paul,
I had not seen your SU treatise before. I admit that I am as mystified you
about your observed results.
There seem to be two potential effects of having the air box vs not having it
in the carbs. You mention the slight overall vacuum it creates and you allude
to a change in resonance that it may create. I have never experienced this
problem, but I do not have much experience.
I certainly do not believe that SU carbs REQUIRE a less than atmospheric
pressure at the mouth of the choke. But one thing this does is make the carb
act like the choke is on all the time and it raises the fuel level in the jet a
bit.
So step one in checking things is to verify that the fuel in the jets is
correct.
It would be interesting to see the air-fuel mixture when you are experiencing
the stumble. It would appear to be due to a lean condition.
The idle mixture adjustment on the jet has little effect off idle. So the
needle profile is most important.
When I was determining the proper needle profile for my TR7 with SU carbs, K&N
filters and 4-1 tubular exhaust manifold, I started with the stock needle that
was a bit too lean.
I made a device to measure the piston lift (therefore the needle position in
the jet) and used a manifold pressure sensor as well as RPM sensor and a
tailpipe wide band oxygen sensor.
The piston lift was used simply to try to create a formula using RPM and vacuum
to predict the piston position.
With that info I could relate needle "station" to mixture over a variety of
real driving conditions. I then hand-profiled the needles until I had a decent
mixture over the full range. Then I took those measurements and tried to find
the best match for a standard needle profile. And that is what I am using now.
I suspect that if you were to do the same thing then you would not need to run
the original airboxes. But in your case your performance is great as is your
mileage. So why change anything.
But for other people it may be better to find the ideal mixture than to
tape-off large sections of their K&N filters!!!
-Tony
Sent from my 1837 Babbage Analytical Engine
> On Oct 16, 2018, at 2:00 PM, triumphs-request@autox.team.net wrote:
>
> Message: 6
> Date: Mon, 15 Oct 2018 23:23:23 -0400
> From: Paul Tegler <ptegler@verizon.net>
> To: Jim Henningsen <trguy75@gmail.com>, triumphs@autox.team.net
> Subject: Re: [TR] 61 TR3A SU H6 Carb Issues
> Message-ID: <e3b34234-608c-d56b-dac2-9a628a0fcc73@verizon.net>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> ok....what do you mean by 'carbs aren't running smoothly'? ?
>
> Engine is not smooth?.. sputtering?? not running smooth while cruising?
>
> An old adage.... (well two actually)
> "..most carb problems are electrical..."? and "..most ignition problems
> are the carbs"
>
> Seriously though.... what method didyou use to balance the air flow at
> idle? One of those little flow meters?
> NOPE NADA STOP right there.?? Think about it...you're sealing off the
> air to get the bubble to rise so of course the idle changes....so the
> balance tube in the manifold sucks from the other carb, bypassing the
> fuel pull.
>
> I was finally convinced by an old mechanic and have learned from tuning
> more SU and ZS carbs than I care to mention.... use a 1/4" fuel line as
> a stethoscope ...one end in your ear..the other just up against the edge
> (not in) the carb throat. blocks minimal air... and believe me you can
> hear as little 1-2 cfm readily, but you can also hear the gurgle of the
> fuel flow!?? Years ago I actually gave away my flow gauges.
>
> Eons ago..I got so fed up once I bought brand new NOS SU carbs to
> replace mine after three rebuilds of the orig units. BUT...STILL had the
> same issue...smooth speed cruising and it ran like crap. Pull the choke
> out and it ran fine, but had to push it in at closed or low speed
> throttle. Spent a year+ and hundreds of messages on various LBC
> lists....still to no avail.? Than one day at a car show.... I saw a guy
> with an identical setup.... open big round K&Ns and a end plate but had
> his air filters nearly 3/4 closed off with duct tape.? In the end, the
> problem was incorrect vacuum effects ...not enough air velocity when the
> piston was mid/down as there was no restriction from the air filters.
>
> hhmmm.... ok...
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