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by Mal Marsden - 06/16/22 7:00 pm
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Nick H, Noe, Tigernuts
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Original Post (Thread Starter)
by kevin

people ask me how to do this, so i wrote this up the other day. look it over and tell me where i'm wrong and i'll edit it.

lobe-center method for timing triumph 650 camshafts

the lobe-center method i use for timing the cams is easy. i do it with the head off, no pushrod tubes, putting a dial indicator on the top of the cylinder barrels and using a pushrod to connect it to the tappets. you can also do it with the head on, putting the dial indicator on a valve spring collar, but you have to fight the valve springs to move the cam wheels.

the factory shop manual lists opening degree wheel numbers for the camshafts, specifying an arbitrary amount of lift that you will consider "open," and "closed." that's awkward for me, so rather than measuring degrees when the camshaft is in an arbitrary "open" position, i just measure when it is at maximum open, that is, i write down the number on the crankshaft degree wheel when the camshaft lobe is pointing straight up.

this is what i do:

1. put a degree wheel on the drive side crankshaft end and set it to read zero when the crankshaft is exactly at TDC.

2. install the intake camshaft pinion key and install the intake camshaft pinion just far enough to hold it in place, using the keyway radially in line with the punch mark on the intake pinion. (this is the stock cam pinion position.)

3. install the crankshaft pinion.

4. adjust the intake camshaft and the crankshaft positions and install the idler gear so all the marks line up as they do in the triumph shop manual—intake camshaft pinion, idler gear, and crankshaft pinion. don't put the nut on the camshaft pinion yet.

5. to check the timing on the intake camshaft, position a dial indicator on the cylinder barrels and use a pushrod to connect one of the intake tappets to the dial indicator. doesn't matter which side, but use the same side for everything. rotate the engine and zero the dial indicator on the base circle of the intake camshaft.

6. because you can't tell exactly when the camshaft is at its highest point (the dial indicator needle moves very slowly as it comes to a stop and begins to go the other way) you must figure out a way to tell exactly where the camshaft is pointing exactly straight up. that will be the exact camshaft lobe centerline, which is what you need. what you’re going to do is record two crankshaft positions the same distance either side of camshaft lobe center and then average them to get the middle.

7. remember that the dial indicator is still zeroed on the base circle. rotate the engine until the needle indicates that the camshaft is pointing as close to straight up as you can get. the degree wheel will now read the approximate lobe center, but youare going to use the following procedure to determine it exactly.

8. now turn the engine forward until the dial indicator reading drops to some specific number, say, minus 0.080 inches. count off the total number of degrees on the degree wheel from engine TDC that were necessary to get to minus 0.080 on the dial indicator. note that this is NOT a subtracted 0.080 inches down from where you estimated the cam to be pointing up--it is where the dial indicator actually reads 0.080.

9. now turn the engine backwards until the dial indicator reads 0.080 by going in the other direction. count the total number of degrees on the degree wheel from engine TDC on going the other way.

10. average those two numbers. the exact intake camshaft lobe centerline will be that average number, that is, when the degree wheel is set to that number [/i]after[/i] TDC, the intake camshaft will be pointing straight up, with the piston going down. that is the number you are looking for. this method lets you find the exact center of the cam nose and record it as the lobe center.

FOR EXAMPLE, suppose you turn the motor forward minus 0.080 inches and the degree wheel reads a total of 150 degrees from TDC. then you turn it backwards past zero on the dial indicator (the needle stops and reverses direction) and then continue until the dial indicator reads minus 0.080 again. look at the degree wheel and it might read 52 from TDC. average 150 and 52, and you can see that the cam will be pointing straight up at 101 degrees ATDC, which is a typical stock setting.

if you get some number other that 101 and you want to set the cam to some other number, you can move the intake camshaft pinion one entire tooth (+/- 14.4 degrees) or to a different keyway (+/- 4.8 degrees). if you move the camshaft pinion by a tooth or by a keyway, the stock marks on the camwheels won’t line up anymore and you will have to paint new ones.

to advance the timing, imagine holding the camwheel still and re-fitting the cam one keyway or one tooth clockwise.

to retard the timing, imagine holding the camwheel still and re-fitting the cam one keyway or one tooth counterclockwise.

11. next do the exhaust camshaft. leave the intake pinion, the idler pinion, and the crankshaft pinion alone (if you move them you will lose the intake timing you have just set). watch the intake and exhaust tappets move, and position the exhaust tappet so that it is rising on will be the exhaust stroke. use the same side tappet that you did for the intake.

12. repeat the procedure, but remember that this time the exhaust camshaft lobe centerline will be some number before engine TDC. the exhaust closes late, so the centerline will be with the piston going up. on a stock triumph, the exact camshaft lobe centerline might be around 102 degrees BTDC. on my race bike, i set it to 107.5 degrees BTDC.
Liked Replies
by kevin
Originally Posted by Nick H
Thank you, very useful.
Is the lobe center simply the degree between the given opening and closing figures?

it would have to be if the cam is symmetrical. i believe most all the ones we deal with are, but since i just said that i'm probably wrong.

Is there any simple statement that would explain what is gained or lost by altering from stock values?

summit racing has something on it

search for hot rod publications and supply houses on timing cams. you'll find lts of confusing stuff. triumphs are even a bit more confusing because you can time the intake and exhaust cams separately, like in DOHC machines
1 member likes this
by tridentt150v
Originally Posted by Tigernuts
Spark plug tool is a good idea. Have you got any tips on removing & replacing cam gears with the cams under valve spring pressure?
Adjusters all the way up, rubber bands on the adjusters to the frame or a head bolt etc to keep some pressure on the pushrods so you don't drop one. Use a couple of sockets and handles on the cam wheel nuts to hold in position. As I said, its a PITA and a fiddle, but is still quicker than a pseudo 'top end rebuild'. if you don't need one. And its not the cam gears that are hard, its the intermediate that is the tricky one. Holding everything together while sliding it in place is hard.
1 member likes this
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