Assuming you find TDC to your satisfaction, using a stroke of 3.503" and a rod length of 5.875", piston positions down the bore were calculated at the crankshaft angles you requested and are reported in the table below.
Timing Protractor Accuracy
- Thread starter Brooking 850
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Are you taking into account the fact that the cylinder bore is not directly over the main bearings?
Brooking 850
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After a lot of measuring and messing about , the protractor is 4* out, at 20* actual it reads 24*, at 28* it reads 32* and at actual 30* it reads 34*.
There is a buckle in one end of the protractor plate, looks like it was put in wrong and under tension.
If it was straightened out at one end, all would be well.
At least I know now and I have a new piston stop!
Regards Mike
There is a buckle in one end of the protractor plate, looks like it was put in wrong and under tension.
If it was straightened out at one end, all would be well.
At least I know now and I have a new piston stop!
Regards Mike
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Four degrees? Wow, that's pretty bad! Engine must have been assembled on a Monday or Friday! 
Had no idea the bore was offset from the crank centerline. How far is the bore shifted and in which direction, fore or aft? Thanks.Are you taking into account the fact that the cylinder bore is not directly over the main bearings?
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Back in the 70s, I had access to a computer with a high level language in which you could programme an algorithm and produce a table of conversions. I was porting two-strokes at the time, so knowing the rod length and stroke I developed the conversion algorithm from crank degrees to distance down the bore. With a two-stroke, it is much more important to get the timings right. With a Commando, I always set the ignition timing, then jet to it. So it is not so important if the timing is out by a couple of degrees. The three things work together - comp.ratio, jetting and ignition timing. The ignition timing is usually chosen to suit the knock rating of the fuel. There is a fair amount of flexibility because the errors are corrected with carburation. Because I use methanol which has unlimited knock rating, I usually add about 4 degrees advance to the timing recommended for petrol - at standard comp. ratio. If I was using 12 to 1 comp. ratio, I would not add anything. At 14 to 1, I would be retarding by about 4 degrees. With petrol, two degrees difference in the ignition timing is probably not a large error. The problem comes when you later get the timing spot on and don't re-jet to suit it.
Nater_Potater
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'Funny; that's exactly just how far off mine was er, is. Yes, the protractor looks a bit wrinkled, almost pigeon-toed. As long as I time it to 32', everything's fine.
Nathan
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I know this will drift this thread somewhat to port...or maybe starboard...but:
I'm trying to understand what possible advantage there is in determining engine timing (valve or ignition) by measuring piston/crank/rod movement and doing some sort of conversion to degrees as opposed to using a degree wheel and actually measuring degrees directly.
Ignoring the fact that the crank moves several degrees while the piston does not when at TDC, the measuring method seems to me to add the possibility of error in the measurement itself or in the conversion. Am I missing or not understanding something?
As far as ignition timing is concerned, as mentioned, the best way to set timing is based on engine performance. Using a factory timing setting is fine - it will always work. But it's commonly not optimum for a specific engine. The old method of advance under load until pinging/back off two degrees works fine. Better, of course, is to use a dyno and determine timing by where the engine makes it's maximum power. As noted, both methods will yield a different timing number based on fuel. I have my Commando timing set based on regular fuel because it operates in Mexico. Fuel is easily available even in rather remote locations but often, in those areas, only regular (87). If it was in TX, for example, I would set it based on using 93.
Setting the timing based on fuel/the bike's performance has another advantage - the "error" of the timing scale makes no difference at all. You set the timing based on performance and then observe where the timing mark falls on the scale and that's the timing for that bike. Using that method, unless you are degreeing in a new cam or something similar, NONE of the other stuff matters - there is no need to find TDC or care anything about it OR the accuracy of the timing scale/mark on the rotor.
I'm trying to understand what possible advantage there is in determining engine timing (valve or ignition) by measuring piston/crank/rod movement and doing some sort of conversion to degrees as opposed to using a degree wheel and actually measuring degrees directly.
Ignoring the fact that the crank moves several degrees while the piston does not when at TDC, the measuring method seems to me to add the possibility of error in the measurement itself or in the conversion. Am I missing or not understanding something?
As far as ignition timing is concerned, as mentioned, the best way to set timing is based on engine performance. Using a factory timing setting is fine - it will always work. But it's commonly not optimum for a specific engine. The old method of advance under load until pinging/back off two degrees works fine. Better, of course, is to use a dyno and determine timing by where the engine makes it's maximum power. As noted, both methods will yield a different timing number based on fuel. I have my Commando timing set based on regular fuel because it operates in Mexico. Fuel is easily available even in rather remote locations but often, in those areas, only regular (87). If it was in TX, for example, I would set it based on using 93.
Setting the timing based on fuel/the bike's performance has another advantage - the "error" of the timing scale makes no difference at all. You set the timing based on performance and then observe where the timing mark falls on the scale and that's the timing for that bike. Using that method, unless you are degreeing in a new cam or something similar, NONE of the other stuff matters - there is no need to find TDC or care anything about it OR the accuracy of the timing scale/mark on the rotor.
Brooking 850
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MM, as I had my road bike apart , I decided I needed to check that my timing marks were correct in relation to each other and as it turns out they weren't.
The motor has just had rings and big end shells after a head gasket failure, the Vv's have had a touch up and and anew set of Ex Vv guides, so on initial start up, I would like to think I was very close to the best timing setting as possible so as not to damage the engine.
I wouldnt want to go for a ride looking for the best performance on a new motor hoping I had got it right first time.
You could do all sorts of damage , especially right at start up.
I do use a degree wheel and had posed the Q to the forum if anyone knew the actual distance down the bore a flat top piston with a standard conrod was just to confirm my findings.
Wasnt trying to convert anything, so yes you did miss something.
Glad that I checked.
If you read my first post, you will see that I am using a timing disc/protractor.
Regards Mike
The motor has just had rings and big end shells after a head gasket failure, the Vv's have had a touch up and and anew set of Ex Vv guides, so on initial start up, I would like to think I was very close to the best timing setting as possible so as not to damage the engine.
I wouldnt want to go for a ride looking for the best performance on a new motor hoping I had got it right first time.
You could do all sorts of damage , especially right at start up.
I do use a degree wheel and had posed the Q to the forum if anyone knew the actual distance down the bore a flat top piston with a standard conrod was just to confirm my findings.
Wasnt trying to convert anything, so yes you did miss something.
Glad that I checked.
If you read my first post, you will see that I am using a timing disc/protractor.
Regards Mike
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Optimising jetting and ignition timing on a dyno is a good idea, but it might be impractical. Dynos are usually situated in closed workshops, not outside in the cold. If you determined the optimum timing for the motor on the dyno and then jetted to it, you might be somewhere near very good. But the two work together, so it becomes a dog chasing it's tail. What has been said about advancing the timing until the motor runs with distress, then backing off - is the way we used to ride 1942 side-valve Indians. In that situation, there were three variables - hand throttle, hand advance and the jetting was also manually adjustable while riding the bike. It did not really matter if you got it wrong - the bike would stagger on regardless. I always used to adjust the advance, then the jetting to get best power but if you lean-off too much, the effect is similar to having too much advance.
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As far as not damaging the engine on startup, when I professionally built engines, our method was to set the initial timing roughly at TDC just using a visual or quick mechanical method, prior to start-up. TDC is normally quite retarded from the correct timing and setting at TDC allows a very easy start-up on any engine with no strain on any systems, very low possibility of backfire, etc especially with high compression engines. Then RPM was immediately brought up to 2800-3000 RPM to ensure proper cam break in. We maintained that RPM for 20+ minutes. During that run-in, we adjusted timing to the factory setting, adjusting the throttle setting to stay in the 2800-3000 RPM range. Optimum timing for the specific engine was determined/set after break-in.
I recommend that method - TDC at initial start up - on the Norton for engine rebuilds for the same reason - it makes kickstarting much easier and is easier on the elec starter if you have one.
I recommend that method - TDC at initial start up - on the Norton for engine rebuilds for the same reason - it makes kickstarting much easier and is easier on the elec starter if you have one.
Brooking 850
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MM, yes but your engine wouldnt be at optimum if your factory setting was 4 degrees out agreed?
Regards Mike
Regards Mike
Brooking 850
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Im building a portable dyno, so will finalise the settings once that is completed and see how much difference there is to my initial settings
Regards Mike
Regards Mike
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True if you are setting per the scale. My explanation was based on setting the engine for max power and then just looking to see where the mark ended up and then using that scale mark - whatever it happened to be - for future timing resets. But I completely agree that if you want the timing scale to accurately show the timing, you have to go through the steps (or similar steps) to what you are doing. As I noted, I am surprised that the mark is 4 degrees off - that's quite a bit.
Of course, as others have pointed out, there can be enough slop in the chancase attachment to change the alignment a bit anytime the case/cover is removed/reinstalled. The locating pins SHOULD ensure the chancase cover doesn't noticeably change position in relation to the chaincase itself but the chancase has some slop as well when mounting to the engine.
I apologize if I came across as being at all critical; I did not intend that.
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Depends on what fuel you are using. If it has a high octane rating, you can use more advance as long as you jet to it. If it was 4 degrees retarded you might get a slight drop off in performance, but if you use the bike for commuting, more flexibility might be a good thing. There is usually quite a large range that the ignition timing can be set within and still be coped with by re-jetting without much noticeable change in performance. The 29 degrees advance for the Commando is probably an arbitrary one-size-fits-all, because everybody tunes their carbs when they change fuel. Theoretically for best performance, there is probably a correct amount of advance to get the most out of fuels of each octane rating. Using a dyno to optimise might get you there, but there are two variables - which one do you change first ?
Brooking 850
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MM , all good, another reason to do the mock up on the bench with the inner and outer primary covers was to check on how big of a difference the sloppiness of the covers does make to the timing marks.
If you slacken the inner cover bolts then remount the outer cover , there is close to 2* mismatch, although with the inner cover bolted up tight again and the outer cover loose, its about the same 2* so in reality the outer cover would have to be put on without the dowels to really make a mismatch which Im sure the average owner probably wouldnt do.
Al, its not my race bike so I use pump fuel, either 98 or 95, very rarely 91 although have had to from time to time.
Yes if it has been running a bit retarded then it wouldnt matter too much on the fuel range.
28* is recommended factory setting for the Commando according to the manual.
Regards Mike
If you slacken the inner cover bolts then remount the outer cover , there is close to 2* mismatch, although with the inner cover bolted up tight again and the outer cover loose, its about the same 2* so in reality the outer cover would have to be put on without the dowels to really make a mismatch which Im sure the average owner probably wouldnt do.
Al, its not my race bike so I use pump fuel, either 98 or 95, very rarely 91 although have had to from time to time.
Yes if it has been running a bit retarded then it wouldnt matter too much on the fuel range.
28* is recommended factory setting for the Commando according to the manual.
Regards Mike
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KiwiNeill
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I have this measurement for my Mk 3 but used its crank timing slot to get 28 degree setting rather than the better positive stop/degree wheel method,which is very accurate if done correctly. Deck height is minus 0.005"
In Austria at the mo but will check next week. You are right to check the timing cover protractor, mine was 4 degrees out, critical on e start 28-29 degree settings.
Possibly due to accumulative differences in alternator rotor keyway and protractor rivet location variations.
I have this measurement for my Mk 3 but used its crank timing slot to get 28 degree setting rather than the better positive stop/degree wheel method,which is very accurate if done correctly. Deck height is minus 0.005"
In Austria at the mo but will check next week. You are right to check the timing cover protractor, mine was 4 degrees out, critical on e start 28-29 degree settings.
Possibly due to accumulative differences in alternator rotor keyway and protractor rivet location variations.
@KiwiNeill don't forget that the degree scale is slotted and screwed on the MK3 so you do get a little bit of wiggle room
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I don't use positive stop to find TDC. I pulled the electrode out of an old spark plug and find TDC by rotating the crank while watching a soap bubble, caused by air pressure in the cylinder. I don't like positive stop because the spark plug hole is on an angle, so any rod inserted through it is on an angle to the piston.
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