Timing a Comet using a Degree Disc - what does everyone do?
- Thread starter miked
- Start date
Oh it do make me larf. I used to sell these timing tools from WCS and some folks even moaned at paying seven quid for 'em. All the info on setting one up is in K.T.B.
Do as it says in the book, and once set at your own firing point, throw it in the tool box for future use. On the twin I always used one for each cylinder and marked the tools accordingly, ie, F and R. An added refinement is to drill a hole into the body of the modified plug to let the air out even quicker. Be sure to radius the end of the piston stop and when rotating the engine DO IT GENTLY for obvious reasons. I also made these devices in 10mm 12mm and 18mm and may have some left.
However consider yourself lucky that you have an engine to time. I haven't thrown my leg over anything for 20 months now due to a vital component still not appearing. ET43
Do as it says in the book, and once set at your own firing point, throw it in the tool box for future use. On the twin I always used one for each cylinder and marked the tools accordingly, ie, F and R. An added refinement is to drill a hole into the body of the modified plug to let the air out even quicker. Be sure to radius the end of the piston stop and when rotating the engine DO IT GENTLY for obvious reasons. I also made these devices in 10mm 12mm and 18mm and may have some left.
However consider yourself lucky that you have an engine to time. I haven't thrown my leg over anything for 20 months now due to a vital component still not appearing. ET43
Wow! Thanks for all the great advice - I wasn't really prepared for such a geat response.
It looks like I'll have to find some way of fashioning a piston stop then...
I was thinking it might be easier for me to adapt a sliding TDC gauge as that already has a large bore hole that I could tap more easily.
I don't yet own a copy of KTB, I've been using the Richardson book as my Bible. It certainly contains all the information, but doesn't go as far as detailing how to do anything, which being new to Vincents I would find helpful.
Many thanks for your advice, can't wait to get stuck in at the weekend now!
Mike
It looks like I'll have to find some way of fashioning a piston stop then...
I was thinking it might be easier for me to adapt a sliding TDC gauge as that already has a large bore hole that I could tap more easily.
I don't yet own a copy of KTB, I've been using the Richardson book as my Bible. It certainly contains all the information, but doesn't go as far as detailing how to do anything, which being new to Vincents I would find helpful.
Many thanks for your advice, can't wait to get stuck in at the weekend now!
Mike
There's something wrong with this thread, everyone is saying much the same thing.
Even I do my timing this way.
Where are the 101 different ways to confuse the rest of us??John
Just for you John,
The easiest way to do this, is to remove the inlet rocker cover, turn the engine until the inlet valve is just opening, this is about 40 BTDC. Turn the engine a full revolution and you have 40 BTDC on the compression stroke. Set this as the firing point. If the beast launches you into orbit off the kickstart this is called " too *&^%$%^&* advanced" and you need to retard the setting - I think you turn the distributor thingy anti clockwise to do this but I'm not sure if that's from the points end or the ATD end. It's never taken me more than 18 months to get it right using this method - only cissys use the gadgets described earlier.
H
ps YES! I am joking.
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Oh bugger. I guess you don't want to hear how I adjust the tappets with an elastic band and sticky back plastic, then.
H
A 105° cam starts best at 59-62 degrees i guess
cheers
Bernd
cheers
Bernd
I'm sure all the foregoing is helpful information but even if you set your ignition timing as per book don't forget each and every engine differs in some way. Compression ratio, type of cams, advance characteristics of your ignition system and so on. The only true test is out on the road (or if you have access to a rolling road dyno) to prove whether you've got it right. Once you're happy with it measure your setting with a degree disc and file the figure away for future reference.
I timed my 105's to 4 deg BTDC with equal lift on both valves. Last week I read MPH 648, January 2003, looking for something else. And lo! There was Neville Higgins talking about timing valves "on the rock", a term I'd never heard before. Making a long story short, he timed several camshafts, including a Terry Prince one, some with figures all over the place, some with no figures that made sense, to equal lift at 4 deg BTDC, and all worked. Ruefully I remember that originality is undiscovered plagiarism...
No big surprise though, because the most critical point of the entire four-stroke cycle is when the inlet gas is chasing the exhaust gas out of the pot, aka "the overlap", so while it may make one feel good to get IO in exactly the right place, it is largely irrelevant to performance UNLESS one thus obtains equal lift (you're already ahead of me...) on both valves at between 4 and 6 degrees BTDC. Three other engines I've worked on (Manx Norton, Honda K4 with HRC factory camshaft, and Rudge Ulster with Rep cam) are the same.
(In an ideal world, the incoming charge purges the head of burned gas, and the descending piston (pulling a vacuum) pulls more fresh charge in via the carb, AND recalls the fresh mixture doing the purging down the exhaust pipe, back up the pipe to its duty. Just as the last bit of fresh mixture in the exhaust gets into the head, the exhaust valve closes. The head continues to be charged by fresh mixture from the carb, then the inlet closes, then it all goes bang. If your overlap is in the wrong place, none of this happens. Even if you have set the IO figure to within half a gnat's crotchet of the right place, the trouble is that the "right place" is of no particular significance UNLESS your timing numbers give equal lift blah, blah, blah.)
I did actually check the numbers I got from Gary Robinson AFTER I had set the timing on the 105's, and his numbers came in near as dammit. Makes good cams, does Gary.
No big surprise though, because the most critical point of the entire four-stroke cycle is when the inlet gas is chasing the exhaust gas out of the pot, aka "the overlap", so while it may make one feel good to get IO in exactly the right place, it is largely irrelevant to performance UNLESS one thus obtains equal lift (you're already ahead of me...) on both valves at between 4 and 6 degrees BTDC. Three other engines I've worked on (Manx Norton, Honda K4 with HRC factory camshaft, and Rudge Ulster with Rep cam) are the same.
(In an ideal world, the incoming charge purges the head of burned gas, and the descending piston (pulling a vacuum) pulls more fresh charge in via the carb, AND recalls the fresh mixture doing the purging down the exhaust pipe, back up the pipe to its duty. Just as the last bit of fresh mixture in the exhaust gets into the head, the exhaust valve closes. The head continues to be charged by fresh mixture from the carb, then the inlet closes, then it all goes bang. If your overlap is in the wrong place, none of this happens. Even if you have set the IO figure to within half a gnat's crotchet of the right place, the trouble is that the "right place" is of no particular significance UNLESS your timing numbers give equal lift blah, blah, blah.)
I did actually check the numbers I got from Gary Robinson AFTER I had set the timing on the 105's, and his numbers came in near as dammit. Makes good cams, does Gary.
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Well these are the figures I got from my 105 cams -I timed the inlet as close as I could to 105 degrees ATDC at full lift as recommended by Gary. I then clocked the valve timings and got these results:
INLET OPENS 47 BTDC INLET CLOSES 76 ABDC
EXHAUST OPENS 76 BTDC EXHAUST CLOSES 47 ATDC
EXHAUST MAXIMUM LIFT 102 BTDC.
So a little bit retarded compared to Mk II s !!
INLET OPENS 47 BTDC INLET CLOSES 76 ABDC
EXHAUST OPENS 76 BTDC EXHAUST CLOSES 47 ATDC
EXHAUST MAXIMUM LIFT 102 BTDC.
So a little bit retarded compared to Mk II s !!