Sorry Norman, I hate to say it....but i spoke to David last night about this and he said he will not be posting on here any more.............Greg.
Ignition Timing & modern Fuel
- Thread starter Martyn Goodwin
- Start date
28 BTDC fully advanced is where I am at right now with my subjective testing.
Extensive road testing and plug checking gave the following optimum carb results FOR MY COMET, which uses a standard head, single plug, standard valves - nothing trick. And on 98 RON fuel (Caltex)
Amal 930 premier carb
240 main jet
105 needle jet
needle in centre groove.
Once I was happy with the carb settings I then commenced further road tests - my test loop is approx. 80 miles and involves mixed terrain - to validate the ignition timing.
The fuel we use today is very different from that which was on sale in he 1950's - todays fuel burns much much faster than the old stuff. It is my understanding that the optimum combustion situation is for maximum combustion pressure to happen around 12 ATDC. So I assume the factory setting of full advance being 38 TDC with slow burning fuel resulted in max pressure at the desired point. If we now change to a faster burning fuel without altering the ignition timing then the max combustion pressure will happen sooner, and as that max pressure gets close to or even before TDC then detonation will be happening which actually robs he motor of power, damages the piston and beats up the big end bearing (hind sight is a wonderful thing!). You my recall I managed to destroy a piston though detonation, using 98 RON fuel, a compression ratio of 7.3:1 and an igniting setting of 36 BTDC.
I also appreciated the post from David (post #73 in the thread on "New Amal Premier 900 Series carb") where he shared his experience with ignition on his Comet racer.
So with all this in mind, and having sorted the cab settings, I started a new set of road tests to check out the timing. I initially set the max advance to 34 BTDC but found that the fully retarded position was the at TDC (this being controlled by the sweep of the ATD mechanism) and as many others had told me - best starting happens with the fully retarded position being 4 BTDC; so I gently bent the ears (all 4 of them) on the ATD to reduce its sweep and thus changed the ATD range from its original 34 degrees to 30 degrees. This set my initial road testing setup to fully retarded 4 BTDC and fully advanced to 34 BTDC - and then I went for my 80 mile test run.
Following that I reset the timing to retarded at TDC and full advance at 30 BTDC and then did another test run over the same course - my seat of the pants experience was that the bike performed better, got up to speed quicker (especially in the 80 to 100 kph range) and pulled harder on the hills.
My next test was again over the same course but this time the retarded position was 2 ATDC and fully advances 28 BTDC. The motor was a wisker harder to start, two or 3 kicks instead of just one, but it was acceptable. This time I noticed a further improvement in performance, comfortable cruising speed increased (last time it was around 98 to 100 kpm) to 106 to 108 kph and performance on the hills also improved. There is one longish steepish hill in my course and on all prior runs at some stage on that hill I needed to drop down to 3rd gear to maintain my speed (around 90 kph). This time (with 28 BTDC full advance) I sailed up that same hill in 4th gear with no loss of speed - infact in some places on the hill where it used to hold speed it was now gaining speed.
I have noticed though with the fully retarded position now being 2 ATDC that , in addition to the extra effort in starting, when the motor is idling that the exhaust pipe, close to the head gets rather hot ( logical if you think about it) so I have decided for now not perform any more testing but to leave things as they are just for now. My next move will be to have the ATD modified to reduce its range even further by welding additional material onto the ears. If I reduce the ATD range down to 24 degrees that will alow me to then conduct further testing initially with fully retarded at 4 BTDC and Advanced 28 BTDC followed by another test at retarded 2 BTDC and advanced 26 BTDC. If needed after that I can do a retarded TDC and fully advanced at 24 BTDC test run.
As soon as my subjective testing detects a drop in performance I will go back to the settings immediately before that and leave the fully advanced position there plus then trimming the ATD ears to give a fully retarded point of 4 BTDC
BUT with family pressures this may be now put on hold till next year (xmas humbug)