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Author Topic: Ignition Timing  (Read 388 times)

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Dennisgb

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on: September 20, 2018, 11:47:25 am
I need a little help with setting ignition timing.

With the piston at 8 degrees BTDC are the breaker points set to open with full advance or static position? I have read a number of descriptions but none of them say where the ignition cam should be.

Thanks in advance  ;D
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portisheadric

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Reply #1 on: September 20, 2018, 12:02:04 pm
Ignition timing at full advance ie with bob weights fully open is 8mm BTDC which on a standard crank is the equivalent of 32 degrees of ignition advance. 
However for road use if you want something a little more on the peppy side then 9.5mm (34 degrees) is a nice figure to aim for on a standard bike.
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cyrusb

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Reply #2 on: September 20, 2018, 12:44:24 pm
Are you sure? That doesn't sound right. Isn't that spec normally given with the weights at rest? Actually it's .8mm or .032" btdc. At least thats how mine runs.
« Last Edit: September 20, 2018, 12:53:39 pm by cyrusb »


Dennisgb

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Reply #3 on: September 20, 2018, 01:19:56 pm
Sorry I meant .8mm.

But...one says advance and one says not  :o
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portisheadric

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Reply #4 on: September 20, 2018, 01:39:23 pm
0.8mm retarded or 8mm fully advanced.  tomatoes tomatoes!

Fully advanced is easier to find on an engine with such a long stroke as the crank can 'wobble' several degrees either side of TDC without showing any signs of perpendicular movement and still thought to be at TDC unless you use a dead stop and timing disc or a dial gauge and timing disc to verify it actually is at TDC before rotating the crank. it also removes any wear from the equation and preferably the gear pinions need to have all their slack removed!
A pencil down the plug hole and 0.8mm retarded is the worst of both worlds. Although it's not that critical Per Se on such a soft stock engine, but when things get modified and you need to run at 38degrees on full advance BTDC  for maximum power a setting of 38.5 shows up as a dramatic loss of horses on the Dyno and that 38* is best guaranteed with something other than bob weights and points!
« Last Edit: September 20, 2018, 01:41:50 pm by portisheadric »
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Dennisgb

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Reply #5 on: September 20, 2018, 02:10:49 pm
0.8mm retarded or 8mm fully advanced.  tomatoes tomatoes!

Fully advanced is easier to find on an engine with such a long stroke as the crank can 'wobble' several degrees either side of TDC without showing any signs of perpendicular movement and still thought to be at TDC unless you use a dead stop and timing disc or a dial gauge and timing disc to verify it actually is at TDC before rotating the crank. it also removes any wear from the equation and preferably the gear pinions need to have all their slack removed!
A pencil down the plug hole and 0.8mm retarded is the worst of both worlds. Although it's not that critical Per Se on such a soft stock engine, but when things get modified and you need to run at 38degrees on full advance BTDC  for maximum power a setting of 38.5 shows up as a dramatic loss of horses on the Dyno and that 38* is best guaranteed with something other than bob weights and points!

That makes sense. I am rebuilding the engine so my plan is to set the timing with the head off on the bench rather than a pencil down the spark plug hole.  ;D

My concern is a brand new rebuild so don't want a chance to start it with ignition advanced.
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cyrusb

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Reply #6 on: September 20, 2018, 02:25:04 pm
As has been said, if you are hot rodding go to electronic ignition. How ever getting your .8 mm dimension accurately is easy if you measure on the piston up stroke. All lash will be taken out, just as if you were going for 8.0mm. It's a good place to start, then take it out and ping time it. enjoy


ace.cafe

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Reply #7 on: September 20, 2018, 08:24:55 pm
It is .8mm btdc static. Fully retarded.

However it wouldn't hurt to also measure it with the advance mechanism fully advanced at 8mm btdc, if you want.
Not all the advance mechanisms are the same, and some might not meet spec.


Adrian II

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Reply #8 on: September 21, 2018, 09:09:07 am
The only way to abolish timing pinion slop/lash altogether is to fit a crank-mounted ignition, for example:

https://www.electrexworld.co.uk/acatalog/STK-100D.html#SID=826

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Dennisgb

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Reply #9 on: September 21, 2018, 10:06:05 am
The only way to abolish timing pinion slop/lash altogether is to fit a crank-mounted ignition, for example:

https://www.electrexworld.co.uk/acatalog/STK-100D.html#SID=826

A.

Agree. I have a Boyer on my Norton. The Bullet is a project bike that I am trying to do on a budget although I bought the piston, bearings, high output oil pump, etc from Hitchcocks because the better parts are not that much more expensive once your in there.
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