Discussion in 'Norton Commando Motorcycles (Classic)' started by jimbo, Jun 3, 2019.
that's what I thought
Go up one size with your needle jet and drop the needle to the center notch and try it.
You have lowered your compression ratio -you will need larger jets but not over 300 mains. Jim
I will look into it.
I replaced the busted sprag . Found the timing scale plate in the primary cover misadjusted, after degreeing the rotor . The MK3 scale has slots, screws and is adjustable( moveable too) .The scale marks are 2* each not 1* , so I was off about 8* then 4* .
not sure comp ratio is much lower than before the rebuild because of the different pistons, and the shaved rebuilt head.
Well that much timing will make a difference in plug heat...
jimbo, A little tip - Hold down the "Alt" key, then enter 0186 on the numeric keyboard and magically a degree º symbol will appear.
Combustion temperatures and pressures are a balance between comp. ratio, ignition advance and fuel/air mixture. If you raise the comp.ratio, you need more fuel. That is the reason you get more power when you raise the comp. ratio. (It gives a slight advantage ). Main jets are only in use when the throttle is more than 3/4 open. If you are using full throttle and close it slightly and the bike goes quicker, you don't have a 'weak main jet', you simply have one that is too rich. Slightly over-rich main jets is good practice because engine damage through lean-ness usually only occurs when you are using full throttle. When you use full throttle, you often lose vacuum and the motor can starve causing detonation and burning of components such as valves and pistons. If your carbs are jetted to give maximum performance, you usually need to feed the throttle on in a controlled manner. If you grab a big handful, the motor gasps. The most important thing when tuning a motor is to get the needles and needle jets right. The mains should be rich enough to give some relief when you are fanging the bike.
I am still confused, yesterday when I reduced the timing by 2 marks , or 4º it got worse( but still 4º too advanced because of the shifted scale ), blowing out my sprag
With my bike, I know what size main jets I should use to stay safe, so I never do plug chops - even though if I did, I might get slightly more power. On most race circuits, you are never using the main jets long enough to have a major effect on the result. If you feed the throttle on in a controlled manner, the bike usually accelerates much faster than when you get the bike upright and whack it open. If you whack the throttle open, you lose vacuum. A bit depends on the size of your inlet port.
bingo! I retimed the ignition to spec after adjusting the timing scale to read correctly. It still backfired, I was going to lose another sprag. So I just backed off the timing until it didn't do it. Now it runs like it did before the needed rebuild ,starts ,idles and does not destroy the starter gear train. I am not even going to look at the timing setting. Might have to change the 280 mains though.
Just a thought - have you checked your primary case timing marks against a degree disc from true TDC? I've found a few which have been more than a couple of degrees out.
Yes I did that.The MK3's primary case scale is adjustable and was off. So I adjusted it to read correctly. But when setting the Lucas Rtta to spec it was still too advanced, kicking back wanting to destroy my sprag again. I retarded the timing until the bike was happy with it.
might try a TriSpark as it is the closest(widest) curve to a oem points ignition. I think the bike was stronger with more advance but I will not sacrifice the consistent starting for that.
Trispark seems to have the best rep for low rev stability & not kicking back. Latest units appear to have got through the reliability issues of the earlier version, but I'll stand to be corrected
I believe elektronik saches has the same low revs stabilization (I could be wrong?) And one of its programmable advance curves is the same as points ignition aar
It also has separate igniter
I believe some of trisparks problems derive from living in the timing cover
Some people swear by them some people swear at them
The elektronik saches looks promising ( curve plots against the oem points?) , anybody have the curves for all these different systems for comparison?