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Old 10-08-2014, 07:48 PM   #1
Banzaibob OP
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Location: Kansas City, Missouri
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'59 Triumph TR6 Centrifugal advance binding when installed

This one has really got me vexed. I'm finally getting around to doing the ignition timing on my '59 TR6 restoration. I've got the degree wheel on, TDC is spot on, 35 advanced with the centrifugal advance fully advanced, etc., etc.

I've got a Lucas competition magneto, it bolts on with the 3 bolts in a triangular pattern. There are no damaged components, all threads look perfect. No scraping, no funny sounds, no forcing of parts together. As you know, the centrifugal advance bolts firmly on a taper shaft after you've lined everything up. The pinion that drives the magneto is driven off of the intake cam pinion.

Problem: It is binding. With the magneto firmly installed with all 3 bolts and the mounting bolt of the advance unit even modestly tightened, the advance either returns slowly and stiffly or not at all. The action can be returned by hand and in doing so you cannot feel any hitch or scraping. It just feels as if it is too tight.

Now, when you remove the magneto from the bike and install the advance on the shaft the action snaps back smartly with no binding whatsoever. However once it gets back on the bike fully mounted the advance action either drags or does not return at all.

I have measured until I am blue in the face. There is NO indication of metal to metal contact either on the engine case OR the advance unit. Even modest seating and tightened makes it drag. The thickness of the case at the magneto is approximately 12mm. There is plenty of clearance everywhere. In fact, I am 100% positive that the pinion is NOT dragging on something. There is at least 0.5-1mm clearance around pinion and all housings.

Is there a known problem with binding or dragging advance units if installed wrong? This is my first.

The pictures are of the competition magneto installed and me holding the stock non-functioning magneto as a reference and the advance installed. As of just a few minutes ago I went out and worked the action. It returned from advance but sort of slowly. When it is installed is it supposed to "snap" back quickly?
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Old 10-08-2014, 10:44 PM   #2
motu
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The most common type of magneto advance unit was the 6 ball type, you didn't see those 2 weight types very often. They should just flick back of course. Is your magneto tight? It has to move the mag, and drag in that will make it slow to move. Comp mag had screw in red pickups - are they screw in or held by clips?
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Old 10-09-2014, 08:07 AM   #3
Banzaibob OP
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With the advance mechanism installed on the magneto shaft, I can remove the 3 mounting nuts to the magneto. In essence, the magneto is now "floating" loose on the mounting bolts. In this state, I can sort of rock the entire mechanism laterally and sort of off axis to check for clearance; there is plenty. It is also still binding.

If I remove the magneto from the housing, completely off the bike, and re-install the advance mechanism (essentially holding it in my hand), the advance mechanism does not bind and snaps back smartly.

I should add that I can mount the entire thing on the engine and the advance will return somewhat slowly and sluggishly. I get no sense that anything is grinding or scraping, but it gives me the impression that this will be a long term problem unless I get an answer now. Is it possible that the springs have lost just a little bit of tension?
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Old 10-09-2014, 11:10 PM   #4
motu
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Your tests indicate the magneto itself. Try and compare the tension of the springs to the drag of the mag.
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Old 10-10-2014, 08:14 AM   #5
Banzaibob OP
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I think I may have discovered the problem but I still don't have a solution.

This is a freshly rebuilt Lucas competition magneto so it is for all intents and purposes, new. When I am testing the advance with it installed on the magneto OFF of the bike, that is, holding it in my hand, the fiber wheel snaps back quickly and freely. However, when it is on the bike and making connection with the intake pinion the fiber wheel cannot move; the internals must move. And it must move the armature shaft and ALL of the components in the magneto. Those components possess a lot more inertia than the fiber wheel alone.

Problem: I am assuming that the springs on the advance have to be minimally strong enough to return the magneto to the non-advanced position when the bike is idling and such. Is there an actual weight that these spring have listed? I could go to my local hardware/auto parts store and get something similar if I knew what I was looking for.

Also: I may have to set the advance curve by trial and error. In pouring through the factory and aftermarket manuals, I find NO specification as to what RPM full advance is. My best guess would be 2500rpm but that is based on extensive experience with Asian 4 cylinder bikes. Old British bikes? Who knows?
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Old 10-10-2014, 09:17 AM   #6
LAkevin
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Are you sure it's a problem? I have a Lucas magneto on my dominator, and it exhibited the same issues. However, when I put a timing light on it, I could see that it was in fact advancing. Put a timing light on yours and check it out when it goes from idle to 4 K.
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Old 10-10-2014, 04:48 PM   #7
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Quote:
Originally Posted by motu View Post
The most common type of magneto advance unit was the 6 ball type, you didn't see those 2 weight types very often. They should just flick back of course. Is your magneto tight? It has to move the mag, and drag in that will make it slow to move. Comp mag had screw in red pickups - are they screw in or held by clips?
I'm thinking that the 6 ball type was a BTH unit. Either one will work.

OP, be aware that the Micarta gear on that unit is old, and you may not be able to trust it. I've had them strip the teeth off.
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Old 10-10-2014, 04:53 PM   #8
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Don't forget the impact of engine vibration on the workings of the mechanism.
It can be pretty hard to move unless there is an English Big Twin shaking the bejeezus out of it.....
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Old 10-10-2014, 06:21 PM   #9
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Quote:
Originally Posted by ttpete View Post
I'm thinking that the 6 ball type was a BTH unit.
Back in the day ('70's) I used to prefer the BTH over the Lucas mag, less variation in timing between cyls - Lucas mags were terrible, up to 10 degree out between sides. But the 6 ball was the most common advance unit on all British twins. The fibre gears were flaking then, I'd hate to see them now.
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Old 10-11-2014, 02:07 PM   #10
ttpete
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Quote:
Originally Posted by motu View Post
Back in the day ('70's) I used to prefer the BTH over the Lucas mag, less variation in timing between cyls - Lucas mags were terrible, up to 10 degree out between sides. But the 6 ball was the most common advance unit on all British twins. The fibre gears were flaking then, I'd hate to see them now.
I've hand stoned a few cam rings to get them correct. I always preferred a manual advance mag for a race bike or my personal street bike, though.
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