The following is an excerpt from a question I had asked the group some time
ago adn recieved only two sugestions. I still have the problem.
Any sugestions/information would be appreciated.
I race a 1300 Triumph Spitfire with dual 1.5 inch SU carburetors. I have
played around with the needles and the carburetors which are currently
equipped with 0.100 jets and either KV or RC needles; the jets set down
between 0.500 and 0.100 inches. The carburetor bodies are unmodified
except for thinning of the butterflies. We run the float bowls down
0.25 inches using Triumph Spitfire float bowls because with higher settings
we get fuel dumping into the engine on right hand turns. We have good
paower down the straights. The EGT guages run about 1100 to 1300.
The pugs are slightly brown to grey on a good cut.
We have observed in the garage that the SU pistons only seem to open about
0.5 inches under no load at about 4500. One opens about 1/8 inch more than
the other. At these lifts there is very little difference between the size
of these two needles. It seems to us that at least at maximum RPM the
piston should be at or nearly all the way open.
We have put the carburetors on a flow bench and set the butterfly to flow
the amount of air required for the 1300 cc engine at 8000 RPM. The
piston-rise is comparable to that observed with the engine in th garage.
When the piston is manually raised at this throttle position, themeasured
airflow increases.
The piston position is also different if the piston is raised by the
airflow or lowered from a higher position. Under conditions of constant
flow it equilibrates to a higher position ( about an 1/8 inch) after
being manually raised and dropped than when it is raised due to vacuum.
This suggests to me there is a lateral loading on the piston due to the
airflow into and over the part of the piston exposed to the airstream, and
this may indicate that the piston shaft and guide may be worn excessively.
We have checked the falling rate of the piston with clay in the two holes
the provicdes vacume to lift the piston. One is about 5 seconds
which is on the low side of the published tolerance (5-7 sec). The other
is about 3.5 seconds which is much too fast (this is the carburetor that
does not open as far as the other). This suggests to me these old
pistons and the suction chamber are worn.
I am considering having the pistons and chambers Teflon
coated and adjusting the coating thickness to a fall rate of 7 seconds
or so, or perhaps to that needed to raise all the way at a flow equivalent
to 8000 RPM.
It seems to us that the piston should rise to, or almost to, a point
where it is completely out of the airstream at full throttle, otherwise the
engine will not be receiving a full charge, also it will not be taking
full advantage of the needle profile.
1) How far do the pistons normally rise in your engines?
2) Does anybody know what the correct level of fuel in the jet is? It
seems to me that while the gross level of fuel is controlled by the float
bowl setting, the fuel level in the jet is also controlled by the reduction
down to 0.100 at the top of the jet. This control is due to capillary
forces. Thus it
is difficult to adjust the float bowl setting accurately. I assume an
ideal setting is the static pressure of the fuel rising just to within the
capillary restriction of it own accord, unaided by capillary forces.
What is the correct setting as the jets are moved around? Should it be the
stock 1/8 plus whatever the jet is dropped? What are the consequences of
having too high or too low a setting?
3) Are our pistons and the accompanying suction chambers, as I have
described, them worn excessively? Can they be recovered with Teflon
coatings or do they need to be replaced?
4) How high should I try to get the pistons to rise. Should I consider
using a lighter spring ( I am using a 2 oz. Spring)? Or should I run
without a spring? Should I lighten the piston? Should I modify the two
vacuum holes in the piston to increase the vacuum?
Thank you for any help you can give me.
Yours
Martin D. Matthews
I sent tehse questions to Burlen. Tey said they would give it to someone to
answer. I reminded them once ane now about a year later I still have no
answer.
My feeling is I should modify them till they rise all the way at 8000, or
at least 3/4 the way.
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