'68 Trophy mainjet - Page 2

 


 This sounds like the effect of an over rich idle mixture setting on the
T120V due to the half cocked carburetter tuning instructions in the
owner's manual, which failed to _continue_ the process with _BOTH_
cylinders firing and equally applied adjustments to both carberetter
idle stop settings and pilot air screw adjustments (you landed up
screwing the pilot air mixture screws out another whole turn! - taking
the british mpg figure up from 50 to 75[1]) and closing off the ill
advised 'balancing' pipe between the inlet manifolds. Doing this last
(pinching the rubber tubing closed) made the tickover go from a 'lumpy
beat' (eight stroking effect) to a sewing machine like sound without any
change in tickover rpm (750 to 800, afaicr) without any effect on the
performance.

HTH

[1] I eventually raised the mpg figure to 100 mpg (cruising speeds of 50
to 70 mph on british A roads) after fitting CD electronic ignition[2]
and a fatter polished down needle to each of the Amal concentric carbs,
along with modification of the slide cutaway. This was about 30 years
ago.

[2] I homebrewed a twin coil variation of a design given in a "Wireless
World" magazine article and used the original contact breakers as the
trigger. Not only did it give a much hotter and better timed spark (you
_had_ to open the plug gap from 25 thou out to 40 thou to get the full
benefit, particularly in the case of allowing an even less overly rich
idle mixture), you could also use a harder grade spark plug that would
otherwise become unusable with the conventional ignition cct due to the
fouling that would provide an ht leakage path that interfered with the
conventional spark generation system.

 Since the contact breaker points were no longer being eroded by the 4
amps/300v loading of the conventional CB/ignition coil arrangement, they
didn't need the urgent attention every 3,000 miles that the conventional
setup had required. Basically, the CD ignition system was a
win,win,win[3] situation (aside from the crappy capacitors I was using
which used to fail every few thousand miles, forcing me to switch back
to the original setup which made me realise the benefit when it made the
engine harder to start and the need for more revs and clutch slipping to
get moving;-)

[3] I could have added a fourth 'win', this being the improved generator
voltage stability due to the ignition loading going from a high load at
tickover dropping with increased revs with the conventional ignition cct
to a low load at tickover rising with revs using the CD ignition cct.
The practice at that time was to use a permanent magnet alternator (with
it's rising output with revs characteristic) and hope the battery would
act as a regulator without boiling dry too quickly. The conventional CB
coil ignition aggravated the situation whilst the CD ignition helped
mitigate it somewhat, thus helping to extend the life of the battery
(and the running lights;-).

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

 
I think your best bet to get a clear understanding would be to do a dyno
tune.

They put exhaust gas analyzers in the pipes and tell you exactly where
and when things go awry.

Clay