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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 07-09-2009, 01:14 AM   #1 (permalink)
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Causes of Weak Spark @ Points? Ig Switch?

Hi everyone,

I'm getting a 1970 TR6 that sat for a while back to running order, and have been troubled by a weak spark at one of the points.

In my restoration efforts, I've used the manual and a multimeter to check the low voltage circuit from the battery through the ig. switch, and to the coils, and was able to carry 12+ volts the entire way.

Interestingly, I was able to get continuity across _any_ two the ignition switch blades without the key in the ignition. A short?

I did however replace the condensers and plugs and regap and clean the points for insurance. I've not yet checked the dwell or timing.

For a while, the bike only ran on one cylinder (left),then after I replaced the condensers and regapped the points it ran on both, but it constantly backfired at idle the first time it ran on both. (That may have been an air-fuel issue...or not.)

Yet the right set of points only sparks periodically and weakly, and the bike won't start at present. I know a weak battery will contribute to this, but the spark has been weak even w/a 12.46 charged battery. I'd like to build a checklist of _specific_ ways to eliminate components and identify the problem.

Advice appreciated!
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Old 07-09-2009, 05:37 AM   #2 (permalink)
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Hi AJH and welcome,
Have you tried to "hot wire" the points? (disconnect std ignition wire from coil , then run another from battery directly to coil)
this will eliminate the ignition switch.
Also if your brake light switch comes from a coil remove that wire.
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Old 07-09-2009, 06:59 AM   #3 (permalink)
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Be careful using a multimeter across the ignition switch contacts.
(on Ohms or continuity ranges)
You are effectively `switching` it. (unless all wires are disconnected).
Pin 1 is connected to the battery via the fuse.
Take the fuse out.
There are lots of low resistance paths in the circuit.
Pin 3 is connected to various lights, which will show low resistance,
with respect to earth/chassis.
The ignition Off connects pins 2 & 3 together, hence that will be a short.
There is a vast difference between a dead-short and low resistance.
A digital meter will measure a few Ohms. (fuse out).
Ignition On connects pin 1 (battery/zener/rectifier/alternator) to pin 2,(various switches and the ignition circuits).
WRT the components, check them individually, not in circuit.
I suppose if I were fault-finding electical problems on this bike,
I`d begin with +ve of the coils, (make sure the the points are open).
This should be open circuit (infinity) wrt earth. (capacitors are OK!)
Then proceed in an orderly fashion.
*Safety note: be very careful around the HT, even with battery isolated...some meters supply 9V on the resistance range.
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Old 07-09-2009, 05:02 PM   #4 (permalink)
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First welcome to the forum.

Second is to add to the advice in last couple of posts. You
may want to take a look at your coils as well. Check with
the ohm meter across the positive and negitive leads, then
from the tower to either the negitive or the tower to the positive.
Both coils should be reading close to the same numbers.

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