To the Carb experts and others
Before the questions, a solution to a problem: maybe others have used
the same method, but I havent seen it anywhere (I think) so here it
goes. A friend of mine bought an Unisyn air flow synchronizer to be used
on our TR6s. The Unisyn didnt work; it was impossible to get a reading
from it unless the engine was revved to over 4000 rpm. I wanted to
measure the air flow, so I simply used a piece of chocolate bar wrapping
(Ryder or Rider?) I had in my pockets. By putting the wrapping over the
carb inlets I could measure how much each carb was able to suck the
paper into it, and used that as a reading of air flow. The method
revealed that the carbs was not in tune, and I was able to improve on
the situation. However, I dont know how accurate this method is, but I
guess that with some other material you could create your own air flow
synchroniser - easily and cheaply.
Now to the problem. My 74 1/2 TR6 will not go properly (it hunts and
stutters) when the timing is correct (???), but will run smoothly when
the timing is advanced. I recently changed to electronic ignition
(Lumenition), but that has not had any effect other than raising the
idle. This is a short history:
(1) The vacuum retard is non-operating. The car has apparently changed
carburettors at some point or another. It seems to be later spec carbs
on it, the one used for the 1976 TR6. The one speced on my car should
have had vacuum pipes going from the top of the carbs to the vacuum
swithc connected to the heater hose in front of the engine. (I think
this system was used on '73 and '74 TR6s.) My carbs has no vacuum pipe
connectors on top of them.
However, the front carb had a plugged vacuum outlet beneath it, so it is
possible to get the vacuum retard to work. In fact, right now I have the
retard connected directly to the carb bypassing the vacuum switch on the
water hose.
Q: What exactly does the switch do, and is it important to connect the
vacuum via this switch?
(2) Without the vacuum connected the car would idle at about 1100 RPM,
which is correct I believe. However, with the Lumenition ignition system
it rose to about 1400. The static timing had been set to 10 dgrs BTDC.
The car run almost well, but hunted under acceleration at any RPM below
2000. Over that it pulled evenly without protests. This was also the
case before Lumenition. (The change of ignition system has had little,
if any, effect on the actual symptoms I am describing).
I should mention that in the past it had the problem of sudden powerloss
(it would flatten on about 4000 RPM in 2nd). But this problem has
disappeared for some reason or another.
The dynamic timing with this static setup was way beyond anything. close
to 30 dgrs, or outside the numbering on the flywheel. It seems that the
timing was too advanced.
When the timing was set to dynamic 10 dgrs BTDC (still with no vacuum),
the idle dropped to about 900 and was smooth. No problem with the idle.
However, with this setup, the car will hunt through the whole RPM band.
It sounds and feels as if something is wrong. (The change it dynamic
timing put the static timing at 4 dgrs BTDC).
Q: Any ideas what the reason for this behaviour is, and does anyone have
a table of what the dynamic timing should be at different RPMs? Why
should the car be smoother with the dynamic timing so advanced?
(3) The vacuum now connected directly from distributor to front carb and
the static timing reset to 10 dgrs. (Have not had the change to check
the dynamic timing yet.) This gave approximately the same behaviour as
when the dynamic timing was retarded earlier, now being done by the
vacuum.(No I have not tried to disconnect the vacuum yet).
Q: So it seems that with retarded timing (by vacuum or other means) the
engine hunts and stutters under acceleration through the whole RPM band,
but with advanced timing it only stutters below 2000 RPM. Any
suggestions, or does it mean I have to disassemble the carbs? Or just
simply advance the timing?
Hopefully...
Lars
'74 TR6 CF27324UO
Lars.Johnsen@lili.uib.no
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