- Joined
- Aug 21, 2006
- Messages
- 748
Ludwig's comments regarding timing and octane and recklsrider's followup comments have piqued my interest in ignition timing and the relationship to fuel octane. I have always wondered why the full-advance timing on a Triumph should be different from a Norton and recklsrider's information suggests that this is due to the stroke versus bore relationship. This makes sense to me but I am not an expert in this area. If anyone has additional information about this, I'd be glad to hear it!! I have noticed other discrepancies in ignition timing that have also interested and intrigued me. For example, my 1961 Matchless G12 with points ignition calls for a full advance of 35 degrees BTDC while my friend with a similar machine with magneto ignition calls for full advance at 39 degrees BTDC. Why the discrepancy? When I timed my G12 with a Boyer ignition with a strobe to 35 degrees BTDC, it didn't rev worth a damn. Moving the distributor body up a few degrees (possibly to around 39 BTDC - didn't strobe again to check) made it run much better. Seat-of-the-pants dyno tells me that 35 BTDC was too retarded. Why?
Commando Story:
When I installed my Boyer ignition, I originally set timing on my Boyer ignition on my 850 Commando based strictly on the static timing hole. Bike ran like stink on 92 octane gas (US octane measurement) so I never bothered to strobe it. After a top-end rebuild this past winter, I thought I'd get a wee bit more scientific and got my timing disc out on the end of the crank and made some marks at TDC and 31 degrees BTDC so that I could set my Boyer with a strobe. Turned out that the little timing scale below the timing hole in the primary was off a fair bit reading about 36 BTDC at a true 31 BTDC measured with the timing disc. So now I knew where 31 BTDC was supposed to be. Got my engine all back together and timed it with a strobe to 31 BTDC (36 on the scale in the primary) and 5000 rpm. Filled up the tank with 89 octane gas and went for a 100 mile ride. The bike felt sluggish and didn't want to rev like I remembered it used to. Also seemed to run hotter. When I got back, I measured 44 miles per gallon when I used to get 50-55. My first thought was crappy gas and posted a message on here asking about people's experience with ethanol in different graded of gas etc. As I thought about it more, I came to realize that it probably had nothing to do with fuel. My ignition was likely retarded too far. How could this be? I set it exactly as prescribed to 31 BTDC.
So I timed the ignition back to where it was using the static timing hole. Bike runs like stink again on high-test gas. Put a strobe on it and the ignition advances well past 31 BTDC at 5000 rpm. I am guessing it tops out at about 35-38 BTDC or so. So my question is, why am I seeing poorer performance at the recommended ignition advance settings? Is running the bike at these higher advance settings on premium gas endangering my engine? The bike seems to run great with no evidence of knocking and I am getting 55 mpg. My gut instinct is that the bike is running great so don't fix what ain't broke. However, the scientist in me is not happy when I can't reconcile the more accurate timing measurements afforded by the strobe and real-world performance. Thanks for any insights you may have!!
Tobin
Commando Story:
When I installed my Boyer ignition, I originally set timing on my Boyer ignition on my 850 Commando based strictly on the static timing hole. Bike ran like stink on 92 octane gas (US octane measurement) so I never bothered to strobe it. After a top-end rebuild this past winter, I thought I'd get a wee bit more scientific and got my timing disc out on the end of the crank and made some marks at TDC and 31 degrees BTDC so that I could set my Boyer with a strobe. Turned out that the little timing scale below the timing hole in the primary was off a fair bit reading about 36 BTDC at a true 31 BTDC measured with the timing disc. So now I knew where 31 BTDC was supposed to be. Got my engine all back together and timed it with a strobe to 31 BTDC (36 on the scale in the primary) and 5000 rpm. Filled up the tank with 89 octane gas and went for a 100 mile ride. The bike felt sluggish and didn't want to rev like I remembered it used to. Also seemed to run hotter. When I got back, I measured 44 miles per gallon when I used to get 50-55. My first thought was crappy gas and posted a message on here asking about people's experience with ethanol in different graded of gas etc. As I thought about it more, I came to realize that it probably had nothing to do with fuel. My ignition was likely retarded too far. How could this be? I set it exactly as prescribed to 31 BTDC.
So I timed the ignition back to where it was using the static timing hole. Bike runs like stink again on high-test gas. Put a strobe on it and the ignition advances well past 31 BTDC at 5000 rpm. I am guessing it tops out at about 35-38 BTDC or so. So my question is, why am I seeing poorer performance at the recommended ignition advance settings? Is running the bike at these higher advance settings on premium gas endangering my engine? The bike seems to run great with no evidence of knocking and I am getting 55 mpg. My gut instinct is that the bike is running great so don't fix what ain't broke. However, the scientist in me is not happy when I can't reconcile the more accurate timing measurements afforded by the strobe and real-world performance. Thanks for any insights you may have!!
Tobin