Thanks Greg - I backed off the air screw , and took it for a 10 mile spin . A bit spluttery at low revs ,but once warmed up , it ran well ( a few "coughs" but nothing major. Plug is a nice colour , not too white . Got up to 50mph ,which is no mean feat on the congested streets of Dublin !! . Once home, I restarted it , and bang , big kickback and the bloody carb blew off . Not sure what to do next !
Misc: Ignition Comet timing
- Thread starter Piston Pete
- Start date
Remembering that 70% of carburration problems are electrical, is there any possibility that the advance characteristics of your ignition are faulty?
If you "backed off" the mixture screw, as in turned it anti clockwise......you effectively leaned off the mixture, thus making it more prone to backfire........on most all carbs with the bowl mounted under the main carb body, the mixture screw is an "Air" screw.......so out = more air and in = less.......put the screw back where it was, and wind it "In" not more than 1/4 turn......that will make a big difference, possibly too rich......but rich won't make it backfire........Mikuni's are quite sensitive to adjustment, so a change to the mixture screw should get a result.
Dear Pete,
Is the BT-H magneto the same one you used before your starting problem arrived? If it is, have you double-checked your ignition timing?
I use around 34º advance.
Since I built my Vincent from bits I have only ever used an electronic BT-H and it has never backfired when starting in the 50,000 miles I have completed.
I should point out this is a twin rather than a Comet but the above still applies.
Is the BT-H magneto the same one you used before your starting problem arrived? If it is, have you double-checked your ignition timing?
I use around 34º advance.
Since I built my Vincent from bits I have only ever used an electronic BT-H and it has never backfired when starting in the 50,000 miles I have completed.
I should point out this is a twin rather than a Comet but the above still applies.
Greg, Could you explain why a Norton box would make a difference? I’m assuming the K/S raitio, but not sure how that compares to a Burman. My Comet is an absolute pig to start…. there are several factors at play. The biggest being the ignition system because of the way it’s designed.
I've built a 600 Comet and 600 Norvin singles, not including my Norvin racer......both these used the Norton gearboxes, plus belt drive primary drives........ I found on starting that easing the piston just over TDC with the decompressor and then kicking with a full stroke of the lever would not start the engine. It would either simply fail to start or backfire........This led me to believe that the kickstart ratio was different.......So instead of easing past TDC..... I set up the kick start lever at the 9 o'clock position, squeezed the decomp lever and pushed the k/s lever to the bottom of the stroke......let the lever return full up, then full kick down.......start and run every time ........If the K/S ratio was different, my guess is it's not turning the engine over as far compared to the Burman box.......I did some checks at the time but can't remember what I found.......other than with the slight change in starting procedure, it worked pretty much every time thereafter........One bike used a Royal Enfield decomp valve in the second spark plug hole and the other used the later TPV head with 12 volt solenoid decomp valve in the head, and a simple horn type push button on the bars......both these worked very well........much easier than the stock Vincent decomp set up.
With the Norton box it would also depend on what 1st gear ratio was fitted, as the K/S turns the mainshaft via this.
So a low 1st gear pair would turn the engine faster, also a good reason for altering the overall gearing via the gearbox sprocket and not the engine sprocket, for instance my Dominator has a 34 tooth belt pulley whereas my Norvin has a 44 tooth one (chopped engine Norton box) so as you can imagine it only kicks over slowly and what seems to be one revolution.
So a low 1st gear pair would turn the engine faster, also a good reason for altering the overall gearing via the gearbox sprocket and not the engine sprocket, for instance my Dominator has a 34 tooth belt pulley whereas my Norvin has a 44 tooth one (chopped engine Norton box) so as you can imagine it only kicks over slowly and what seems to be one revolution.
Thanks Greg,
I have no experience with the Burman box, but the ratio in the Commando box doesn’t feel right for my setup. The gearbox is mounted higher up than usual, the folding kickstart lever is a little longer than stock, my right leg is a little shorter than optimal, and the ignition system compounds the problem.
It has an optical encoder that records the # of degrees relative to TDC, which is fine except for the fact that it counts in both directions. In other words, if the piston comes up against compression and rolls backwards, it counts the number of degrees just the same as it would rotating forward. So… next time it fires way too early and kicks back. The cure is to rotate the engine past TDC, decompressor in ( hopefully without it rolling backwards) so the ignition system resets itself. I have a twin plug head, but was only using one plug because I need to reprogram the advance curve. I now have the second plug installed in hopes that it helps starting. If it works better, then I’ll reduce the total advance a bunch.
To save my knee, I’ll start it initially with my treadmill come paddock starter.
I have no experience with the Burman box, but the ratio in the Commando box doesn’t feel right for my setup. The gearbox is mounted higher up than usual, the folding kickstart lever is a little longer than stock, my right leg is a little shorter than optimal, and the ignition system compounds the problem.
It has an optical encoder that records the # of degrees relative to TDC, which is fine except for the fact that it counts in both directions. In other words, if the piston comes up against compression and rolls backwards, it counts the number of degrees just the same as it would rotating forward. So… next time it fires way too early and kicks back. The cure is to rotate the engine past TDC, decompressor in ( hopefully without it rolling backwards) so the ignition system resets itself. I have a twin plug head, but was only using one plug because I need to reprogram the advance curve. I now have the second plug installed in hopes that it helps starting. If it works better, then I’ll reduce the total advance a bunch.
To save my knee, I’ll start it initially with my treadmill come paddock starter.
No idea what 1st is, but it’s a stock Mk lll box and it’s a Newby clutch and belt drive.