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#82243 - 08/18/05 5:06 pm great ignorant ignition setting question on a T120 1970  
Joined: Apr 2005
Posts: 13
frenchyT120 Offline
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frenchyT120  Offline
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Posts: 13
I try to set the ignition timing on this bike, but am lost deep between static setting, full advance, pointers,...
what i (think) know) is that the full advance setting is 38 BTC (pistons up)
what i am unabled to understand is the relation between the mark on the cam and the ignition signal ( does the points open front of this mark or 180 ?)
what i am unabled to unterstand too is why the cam opens two times at 180 + or -
the cam is the previous one, so its profile worked before
I tried two plates with the same effect
Did i missed something when remounting points ?

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#82244 - 08/19/05 10:54 am Re: great ignorant ignition setting question on a T120 1970  
Joined: Aug 2001
Posts: 6,878
triton thrasher Online content
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triton thrasher  Online Content
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scotland
What mark on the cam?
I don't understand much of that question, but the spark occurs when cam opens the points.

To set it on full advance, the auto-advance mechanism behind the points plate has to be jammed open, to advance the cam in relation to its drive- timing a running engine with a strobe lamp is better, though.

The cam turns at half engine speed. The pistons go up and down together, but fire alternately because a 4 stroke fires every second time it approaches top dead centre.

Are there any answers in there?


Amateur Loctite enthusiast.
#82245 - 08/19/05 5:04 pm Re: great ignorant ignition setting question on a T120 1970  
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frenchyT120 Offline
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There is a "dash" mark stamped in front of the cam facing aproximatively where the points open a first time

180 from this, half a cam revolution, the points open again??

i agree with you that the cam turns half the engine speed, so for a four strokes and for a specific point, one fire each cam revolution, not two !

I got no strobe, anyway using a strobe suppose the engine works, not yet. I know there are static ways to set the ignition, what i don't understand is the method to achieve

#82246 - 08/19/05 8:56 pm Re: great ignorant ignition setting question on a T120 1970  
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keith100 Offline
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Toronto
Hi

The dash mark on the face of the cam has nothing to do with the timing. You turn the motor until the dash is opposite the nylon block of the points, and the cam should open them. Then adjust how much they are open (14-16 thou). Thats all its there for! Then turn the motor 180 deg. and do the other set of points. Then set the timing as per book.

Hope this helps

Keith

#82247 - 08/20/05 9:18 am Re: great ignorant ignition setting question on a T120 1970  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
Quote:
Originally posted by keith100:
Hi

The dash mark on the face of the cam has nothing to do with the timing. You turn the motor until the dash is opposite the nylon block of the points, and the cam should open them. Then adjust how much they are open (14-16 thou). Thats all its there for! Then turn the motor 180 deg. and do the other set of points. Then set the timing as per book.

Hope this helps

Keith
That's setting the points gap, which you do before setting the timing.


Amateur Loctite enthusiast.
#82248 - 08/20/05 9:20 am Re: great ignorant ignition setting question on a T120 1970  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
OK- it a 1970 Triumph twin- so it has 2 sets of points?


Amateur Loctite enthusiast.
#82249 - 08/20/05 9:22 am Re: great ignorant ignition setting question on a T120 1970  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
OK- it a 1970 Triumph twin- so it has 2 sets of points? one set for each coil and cylinder?

How often does one set of points open in relation to crankshaft turns?

You have a manual yeah?


Amateur Loctite enthusiast.
#82250 - 08/20/05 11:00 am Re: great ignorant ignition setting question on a T120 1970  
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RF Whatley Online content
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RF Whatley  Online Content
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North Georgia, USA
Quote:
Originally posted by frenchyT120:
I am unable to understand why the cam opens two times at 180. The cam is the previous one, so its profile worked before. I tried two [points] plates with the same effect. Did i missed something when remounting points ?
Frenchy -

I think the discussion has gotten side-tracked and not answered your questions. So here's my 2 cents:

1) The mark on the ignition cam is only there to adjust the points gap before setting the timing. It is not used in setting the timing itself. The marks for ignition timing are under a small window on the opposite side of the engine. And you are correct... unlike most cars, the ignition on Brit bikes is set at full advance (high RPM). This is most accurately done with a strobe light.

2) The ignition cam has a long duration lobe which SHOULD hold the points open (at the same gap) for approximately 180 degrees of ignition cam rotation. The ignition cam is part of the AAU (autoadvance unit) that is mounted on the end of the engine's exhaust cam shaft using a taper fit. When the AAU center bolt is tightened too much, the AAU body is unevenly pushed into the taper. The exhaust camshaft, being very hard metal, distorts the taper on the AAU. After this the AAU runs eccentric and the ignition cam will wobble back and forth. THIS will make the points open more than once, exactly as I understand you are seeing.

The only repair for this is to install a new AAU, OR (for approximately the same price) install a Boyer electronic ignition. Even with a brand new AAU you will never be certain of the advance curve (to get great running), so most people on this board choose the Boyer.

Hope this helps! bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#82251 - 08/23/05 4:27 am Re: great ignorant ignition setting question on a T120 1970  
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frenchyT120 Offline
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Many thanks for all your answers

I understand what RF WATLEY wrote and will have a look this way.

The more i think, the more i am close to set a Boyer ignition instead those ***** points
(Mr Turner won't move in his grave for this, i hope ;-)) )

By the way, is the Boyer ignition setting easy? does it need a strobe lamp ?

#82252 - 08/23/05 7:17 am Re: great ignorant ignition setting question on a T120 1970  
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dave jones Offline
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Emsworth, sunny south of Engla...
Wow! what a hectic thread!

Yes the cam has one lobe and opens each set of points once per revolution.

The easiest way to get the machine running is to set the points gaps on the scribe mark on the cam then take the spark plugs out.

Put your finger in the right hand plug hole and turn the engine until air starts to push out with a hiss.

Take off the cover held by three screws on the other side of the engine. Move the engine carefully onwards until the pointer lines up with the next mark on the alternator rotor.

Go back to the points. Hold the cam against the springs and lock it with a wide washer under its bolt and check that the set with the black yellow wire is on just opening (ignition on, move the adjusting screw until the ammeter on the headlamp just flicks back to zero). In case the black/ yellow wire has been connected to the wrong set, check which set is about to open (Nearest the cam ramp). Repeat for left hand cylinder.

Put in the plugs and ride away. No need for a Boyer! Strobe by connecting the light on each plug lead in turn and point at the alternator rotor. Rev. the engine until the rotor mark stops "moving", at which point it should line up with the rotor mark.

There is no need to complicate this by thinking that the marks on the rotor may not be in the correct place or this that and the other unless you intend to race the machine.

#82253 - 08/23/05 3:52 pm Re: great ignorant ignition setting question on a T120 1970  
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RF Whatley Online content
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RF Whatley  Online Content
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North Georgia, USA
Quote:
Originally posted by dave jones:
Yes the cam has one lobe and opens each set of points once per revolution....

Put in the plugs and ride away. No need for a boyer!
But Frenchy seems to be indicating that he's getting two (2) points openings per ignition cam revolution, which would indicate a bad AAU.

If this is not the case, then yes, use the stock system by all means.

bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#82254 - 08/23/05 6:30 pm Re: great ignorant ignition setting question on a T120 1970  
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dave jones Offline
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dave jones  Offline
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Emsworth, sunny south of Engla...
Oh, I didn't notice that! I changed my auto advance because it was a bit wobbly. I remember noticing that on the old one the points would open in the correct place but then come nearly together again before widening once more and then closing. This must be similar to Frenchy's.
The pre '68 ones could be a real problem, apparently. Check out the service sheet on the "British Only" site where it describes how they could destroy your engine! Glad I got a later one!! The later ones are taller (for 6CA points) so easy to distinguish.

The replacement I got was a good secondhand one and cost five English pounds. I got it from a parts dealer here in dear old blighty who picked me out a decent one. Luckily, there are plenty of these around still because they are dear new from a dealer, but nos on US eBay are sometimes a bargain. One went for the equivalent of 12 pounds recently.
Dave


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