Ignition Timing Questions

I am planning on using a Trispark ignition on my 750 Commando. I have read that the timing plate inside the primary is not that accurate. So, I plan on using a dial indicator in one of the spark plug holes to establish TDC.

Question # 1 How many degrees BTDC is a 750 Norton set for proper advance?

Question #2 Using the dial indicator, how many thousandths of an inch do I have to reverse the piston travel to have it in the correct timing position?

Question #3 Am I making more of this than I really need to,and is there an easier way? On my Triumphs there is a hole in the rear of the case to install a timing tool that is used to set the timing to 28degrees BTDC.

Thanks Much ,PhillySkip

funny Thing . Most motorcycles are 3/8 in. before T.D.C. .

As you noted, the most accurate way to check the timing scale is with a dial indicator. There is no inch-measurement involved but you need a piston stop which can be made with a disgarded spark plug. Without a piston stop you cannot come up with a truly accurate measurement. Do an internet search on finding TDC, there are a ton of explanations/examples of using the degree wheel/piston stop. If you don't have a degree wheel, you can print one out from the web and paste it to a CD.

FWIW, my '73 commando's scale was within 1/2 degree which is about the width of the mark on the rotor.

You already have the timing, 28 degrees BTDC.

Set it with the degree wheel and go.

My guess is that unless the timing scale has been damaged and straigtened, it'll be within the width of the timing tic mark.

Link for downloadable degree wheel

http://www.acidrod.com/files/degreewhee ... ctions.pdf

The timing tick in the primary is probably more accurate than the "spoke in the plug hole" method. Even with a dial indicator, you are never sure you are dead straight into the cylinder. iMO using a degree wheel is the way to go.

Set it statically using the alternator rotor timing mark, or degree disc if your worried about timing mark accuracy (fair point - if you don't check you don't know, mine was approx. 1deg out), to approx 28 deg BTDC, or ignition manufacturers instructions. Then strobe to ignition manufacturers instructions. e.g 31 deg BTDC @ 5,000 rpm. Once set on the degree disc, measure piston height, remove degree disc, check piston position and check position of pointer to rotor.

The way I do it w/o any factory dial left [nuts off crank and clutch took their toll] put in safe piston stop that won't break off or bend in plug hole [they sell these devices if not DIY], then with degree wheel on crank go both ways to stop then split the difference on the degree wheel to set to O-TDC. 31' is sort of safe comprise for all non Combats, but lower CR slower burn engines like closer to 32' while the higher CR faster burn like it closer to 28'. Just following cook book makes for a pretty reliable meal that filling but with fine tuning its rather tasty too. The electronic curve per unit ain't completely the same either so a bit of fine tuning is worth attempting. I only bother to look with a light after all the above. I mark TDC and best initial adv.point or at 30' on the correct side of alternator rotor for next time in, for me always seems to be a next time, ugh.

Thanks to everyone for their input. I'd like to get this "right" from the start. I don't like putting holes in my pistons.

Using a piston stop and rolling the engine foward to the stop and backward to the stop and the splitting the difference as Hobot says
is undoubtedly the accurate way to do it.

Just make sure you remove the piston stop as you are turning the motor, as a valve will hit the piston stop and bend the valve with a Norton . Jim

I've used a TDC gauge to get it approximate then removed the plunger and put a soap film over the gauge and watched the movement of the bubble till it reached its highest point, then double checked with a dial gauge then set up a timing disc glued (temporarily) to the end of the crankshaft and turned the rear wheel backwards (engine in top gear) until I had 28 deg. BTDC (This is the setting recommended for a Trispark whereas for the Boyer it is 31 deg. BTDC I don't know why its different and would be very interested if anyone can explain this). I then checked this position with the reading on the plate in the primary cover and found it to be accurate. I followed the instructions for static timing the Trispark and moved the stator until the indicator LED just lit up. The bike seemed to start and run OK, did a hundred miles or so then decided to check with a timing light and found it was actually 19 deg BTDC. I then played around with it (as whinged about in a previous post) and eventually settled on about 26 BTDC. A ten degree difference in timing seemed to make little difference in performance and this is with high compression pistons and a Megacycle cam though I suppose it would show up on a Dyno.

Oh cool edward, never thot of a bubble film guage, just dumb bump stops, best done with head off in my case. If ya really want to find true functional all slack out TDC, I dare ya to do a leak down test where jugs get 100 psi. I was able to balance this act a few times to see my degree wheel indexing was off 1' in points fired Trixie Combat, which felt best at 29', until actual TDC discovered the hard way. Budge a .5' off TDC to know for sure. i use a magnet to stick on degree wheel though DynoDave made up a few screw on adapters.

Might call Trispark to get their logic of 28' as that's the Combat spec, which would be safe for lesser models but take extra spunk out their response. If yours is really happy at 26' then my respects on a good fast burn combo. Only way to have found that was trial error with or w/o a time light before or after.

I have an old spark plug with the electrode pulled out and use a bubble of detergent to find TDC. Then I set the pointer on zero on the degree disc, move the crank to the chosen timing before TDC and lock up the Boyer while peeking at the magnet through the hole. It doesn't matter what the timing is as long as it suits the fuel - I add 4 degrees for methanol. The main thing is that you set the timing exactly the same way each time, because you tune the carburation to it. You can strobe it to check it, however I don't bother to do that because methanol is more forgiving than petrol unless you are running extremely lean. If you get the timing wrong, it can give the same symptoms as lean mixture. If I was using petrol, I'd set the timing at 28 degrees BTDC as recommended in the manual. If you cannot set up the timing exactly the same each time, you are buggered. My problem is that I've lost the sheet on my Boyer, and next time I will be relying on memory as to how to set it up.
If you want t o use a dial micrometer and use the depth of the piston instead of the degree disc, you will probably have to generate a computer list of conversions of degrees to distance BTDC using MS Excel. The trigonometry involves rod length and stroke and you will need to convert degrees to radian measure. From memory the factor is 57.296 (Rohan will check this for you) . I have only ever done this for two stroke engines. However in your case, you really only need to do one conversion - 28 degrees to whatever BTDC.

acotrel said:

From memory the factor is 57.296

Click to expand...

I have no idea what this is or how to use it for this.
Or what is in your memory !?

Commandos have timing marks, and removeable plugs, for strobe timing.
Easy peasy....

As long as you have 8 hands and extendable arms - since the ignition and the timing marks are on opposite sides of the bike.

Think you , or someone stated how many m.m. at that radious , represented ' X ' degrees . Algebraeically . :mrgreen:

To do the conversion between degrees crank rotation and distance from TDC, you need to know the exact rod length centre to centre, and stroke length and draw the diagram representing the crank at about 45 degrees. The calculation involves working out the vertical distances and subtracting them from the total at TDC. The angles usually have to be converted to radian measure using the 57.296 conversion factor (depends on how the computer you use to do the calculations handles sine and cosine) , then you use the sine and cosine of that in the calculations. It's a bit of a mind twister, I did it back when I built my T250 Suzuki racer, and I can easily do it again, however I have to think about it - it is not hard wired in my brain like some other things. You should be able to do it fairly easily in Excel, back then we had to write our own programmes if we wanted a conversion table.
If you can tell me the length of a Norton rod from the centre of the gudgeon pin to the centre of the big end, I will think about it again and do the calculation of the distance from TDC for 28 degrees crank rotation, however it should be in the Commando manual anyway.

You also have to allow for the distance the cylinder is offset, reverse desaxe, to the centreline.

Doing it like that, its easier to put a degree wheel on it during the build, and check how much 28 degrees is in mm.
But since most EI's need to be timed at full advance anyway, a strobe is way easier and guaranteed more accurate too...

Going back to your original post, question three, "am I making too much of this?" answer yes. use the timing marks inside the chaincase and use the time you have saved to ride your bike.

As a side note....When I raced karts we used to set the timing of the MOTOplat ignition by placing a bridge across the top of the jug with a dial indicator affixed in the center. We then rotated the crank backwards from TDC a certain amount on the indicator (I don't recall the spec) then we would line up the marks on the stator with the center magnet and Bob was your uncle!

Easy when the plug hole was vertical.
And don't 2-strokes have fixed timing, no auto-advance to consider. ?

BTW, timing 4 strokes with gear and chain drive to the ignition, the engine should ALWAYS be rotated FORWARD if setting the timing statically,
or all the backlashes can accumulate, and the timing may be way off....

I forgot to mention that when doing the static timing and turning the rear wheel backwards I did go past the mark then turned the wheel (engine) forward to the mark to take up the backlash so this wasn't the reason for the large discrepancy between the static timing reading and that obtained by using the strobe light.