Timing Protractor Accuracy

Al, might pay to reread my post, motor is on the bench with no head on so easy to use a mechnical piston stop.
Regards Mike

acotrel said:

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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A positive stop is exactly what it says! A "positive stop" it makes no difference what angle to the piston it is! It's used to set a timing disc
I think you are thinking of a timing guage that screws into the spark plug hole at the spark plug angle

acotrel said:

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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That sounds halfway reasonable until you realize that any positive piston stop will touch the piston at the SAME angle before and after TDC.
So (assuming the stop itself is not set too close to TDC ) it's actually a very accurate indicator, and more accurate than trying to gauge the volume of a bubble, I'd think.

With the soap bubble method, you are usually looking at the bubble as it rises to the top of the hole where the electrode used to be in the spark plug. At it's top, the difference between it rising and falling is virtually nothing in terms of crank rotation. With a positive stop system, you would need to make something to replace the spark plug, if you want to set the timing with the head still on. With two-strokes, I had a device which replaced the plug and allowed a dial micrometer to be fitted above the piston. I don't bother to do that with the Commando engine. With positive stop, your bore clearance can get involved. In any case, the difference between the three systems probably does not matter. Once you have set the ignition timing, you need to make sure it cannot change, then jet to it. If you find you need to reset the timing, always check your jetting. Changing one usually produces the same symptoms as changing the other. The amount of ignition advance is theoretically determined by the knock-rating of the fuel. If the advance is wrong, it is compensated for, by the jetting. That is the reason that in the old days, the Brits got their manxes going as fast on rubbish pool petrol, as we Aussies ever did using methanol. They used to get everything right, where we depended on being sloppy and letting methanol hide-up our tuning errors. - I do both - I get the tuning right AND use methanol. Tuning for methanol is as easy or difficult as tuning for petrol. Most guys run too rich, so go that bit slower.

If you want to learn how to get your Commando going really fast, start racing a two-stroke on methanol. It is the same game.

As noted, various methods may be perfectly adequate for finding TDC depending on the intended use/accuracy needed. Heck, for some tasks, just putting your thumb on the spark plug hole and feeling when the compression stops as you slowly rotate the motor is accurate enough! Then there is the old standby of dropping a pencil into the spark plug hole and observing its movement. The soap bubble method works and so does the dial indicator. Any of these methods may do the job you need done, depending on what that job is. But they all suffer from the same fundamental issue - at TDC the crank moves several degrees while the piston does not. Using a piston stop eliminates that inaccuracy.

Whether you NEED that level of accuracy for the specific task is a different question but no shop would build/set up an engine using any method other than a piston stop to find TDC/degree-in the cam, etc.

Hey, in a few years we won't have to worry about 95% of what we post about here - electric motors eliminate all the hassles we have been dealing with for over 120 years of IC engines!

Actually, E-motors were some of the very first vehicle power plants in the early 1900's but battery life/maintenance was the issue. Hmmm...still pretty much the same.

Thanks MM, Im not about to go race a shitty old two smoke either, happy to use a mechanical piston stop and a timing disc, its now all done, head is back from the machine shop all nice and flat ready to go on the motor and all to go back in the frame.
Im done on here!!!
Al, get that fire breathing methanol beast out and let me know how it is running, at what ever degree of timing you use.
Regards Mike

Waiting for an opportunity to ride the Seeley again is killing me. There is a ride day on 6th October at Winton, but not much other and I might not have the funds. This time of year at Winton would be ideal. Our race circuit has stopped combining bike practice and car practice sessions - so going there on any Friday has finished. It is a result of the circuit CEO dying on an operating table during a heart op. The car club which owns the circuit has gone bitchy. He was a fairly close friend of mine, so whenever I go there these days, I grieve. The alternative is to go to Broadford which is a dog's breakfast. If I go there I would probably drop the bike somewhere - too many blind and off-camber corners and I don't know the circuit too well - and there is a fuel bill involved in getting there. I should have retired when I had money, but the GFC fixed that. I lost too much. The ride day in October involves spending money I have not got - I might stop eating.

B. - I don't understand what your anxiety is about. Whatever method you use to find TDC, you will usually be within one degree of it. Most people these days use electronic ignition and strobe it. If you are within one or two degrees of the arbitrary 28 degree maximum advance, the adjustment of the needle and needle jets will usually cope with any error. Most guys run rich on the mains and low throttle openings don't usually matter - so only mid-openings. I have never had a four-stroke detonate while on full throttle, but if Harley Davidson say it can happen, it probably does. The Harley Dynatek ignition system has a purple lead which is normally not used. Harleys have a vacuum switch in the inlet port to which the purple lead can be attached. It's function is to drop the ignition system onto a less advanced curve if the vacuum is not there. So if you whack the throttle open, you don't get detonation. Petrol is a nastier fuel than methanol - it has limited anti-knock, so getting the jetting and timing right is more important, but even with petrol, there is a fair amount of leeway. - It makes me wonder whether anyone has ever fitted a vacuum advance to a Commando engine - might be a way of getting better performance at mid-throttle when using petrol.

Leeway with petrol ? - Every 650cc Triumph uses 38 degrees ignition advance and every 750 commando uses 28 degrees ignition advance, both have similar comp. ratios. So either one of the manufacturers has got it wrong, or the Norton cylinder head is hugely superior.

Al, dont have anxiety, just like to know whats what so it takes a lot of guess work out of it.
Motor is ready to go back in the frame
Like a dyno, great tuning tool.

With very little effort , one can make huge gains in power and torque from a Norton head.
Regards Mike

I find it difficult to believe that such simple things as a squish-band and slightly steeper ports makes so much difference. But it seems to be a fact. It's a pity that so many of the old Triumphs are now dead, it might have been worth making better heads for them.

acotrel said:

Leeway with petrol ? - Every 650cc Triumph uses 38 degrees ignition advance and every 750 commando uses 28 degrees ignition advance, both have similar comp. ratios. So either one of the manufacturers has got it wrong, or the Norton cylinder head is hugely superior.

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I once worked on identical stock V8 engines where one liked 40 degrees of advance and the other liked 34 to make the most power. So that's 6 degrees of difference between the same engine by the same manufacturer! 36 was the factory spec. But when the same engines were blueprinted, the timing for both making max power was within one degree, 37 and 38.

Maybe Norton retarded the timing back in the day to ease kick-starting? But Triumphs were kick-started too so that doesn't seem to be the answer. As W Shakespeare said, "It's a Mystery!

The Norton chamber burns much faster than the Triumph chamber.
It's because the Norton can have a good compression ratio with a flat top or slightly domed piston.
A Triumph needs a big dome and that quenches the swirl that makes for a fast burn.

One thing I have not said is that I never rely on dots or similar on cam-wheels and I would not rely on your protractor. What you seem to be doing is trying to calibrate the protractor. It is probably a waste of time. When a sprocket or cam-wheel is fitted to a cam, there is a keyway which is machined, you might hope it is done on an automatic machine which produces thousands of perfect replicas. I used to work in a defence factory where we had every level of the old British trades system. Production engineering is a strength of the Americans, NOT the British and Australia is a thousand times worse. In fact most of the industries which did that have now died of natural causes. Theoretically, you build a prototype and test it, then seal the design. Using configuration management, every subsequent item is supposed to be an identical replica of the prototype. In Australia, when we had a production run, every item was different and a sequence of change happened progressively right through the run.

In Triumph cylinder heads which have been used for a long time, there is usually a build-up of coke on the side of the piston crown away from the spark plug, indicating inefficient or incomplete combustion. Dual plugs are better, but how would you ever be sure that both plugs in any one cylinder are working ? And if only one plug is working, the required ignition timing would not be achieved. I am still amazed that the difference in ignition timing between a 750 Commando and a 650 Triumph is ten degrees.

A 10" wheel helps with accuracy. Flat black paint is good for soapstone degree calcs.

How big is the protractor ? Isn't it on the alternator rotor ?