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Triumph TR6 - Rotor modification- why?

The thread on poor drivability made me think of the situation I had with my car when I first got it. I replaced the rotor but noticed that the brass piece that contacts the 6 nodes inside the distributor cap had been cut down so it is about 2/3 the size of the stock piece. My car simply will not run right unless I make the same mods to any replacement rotors. I know how to make it work but I dont understand it. I think maybe the rotor mod cuts down on the duration of the spark because their is simply less contact. Does anybody have any experience with this?
thanks in advance
Michael

If you think about it, shortening the rotor's tang has the same effect as rotating the distributor body CW the same amount. Since the rotor spins CCW, a shortened rotor tang would result in retarded ignition if the distributor body were not moved CW the same amount to compensate. Perhaps the PO had difficulty positioning the distributor so as not to foul the tach drive cable and cut the tang to give the needed movement? I bet your car will run with a standard rotor if you retime the distributor to account for it.

Make sense?

Rick O.
Rick Orthen

And if you can't get enough physical rotation of the distributor with an unmodified rotor, simply remove the distributor and re-index the drive dog 0ne or two teeth CW.

Rick O.
Rick Orthen

Rick, where are you from in PA?
MB Mike

MB Mike--15 miles SE of Pittsburgh, in the Monroeville-Greensburg-Irwin triangle.

Rick O.
Rick Orthen

Michael- Was it a Lucas Button or a Bosch? Points or El. Ign.?
Don K.
DON KELLY

Thanks for all the comments. Don, I have used various rotors and I had points originally but then changed to electronic ignition.
Michael

I don't believe the rotor "width" has anything to do with ignition timing. It's just a conduit for the spark that's controlled by the points/electronic ignition. That said, if the timing & distributor position are "off" enough, one edge of the rotor metal can be close enough to an adjacent lead so the spark jumps to where it's not needed. That CAN cause problems. The person that shortened the rotor was probably off a notch on the dist dog gear so he lost spark.

Also, if the rotor is trimmed, it may move out of position too soon to deliver spark of suitable duration at high RPM's. I think it's a bad idea - just my thoughts.

It is a very good idea to take an old dist. cap and drill a big (1/2") hole inside the #1 lead as suggested by the Crane instructions. That way you can use your timing light and that test cap to see the rotor position at the time of spark fire. At all RPM's. It should fire about midway on the rotor.

BB


Brent B

I go along with Brent. A local 'old guy' factory-trained BL mechanic told me to trim the excess metal off the trailing edge of the rotor. The reasoning: spark jumping to two terminals at the same time, degrading the spark to the cylinder that is supposed to be firing. Sounded very logical to me.....and it did help smooth out my idle a bit as well! But....if you are running quite a bit of advance, won't that position the 'blade' on the rotor more in between terminals? I believe mine is set at 12 adv at idle, and ramps up to 32-34 at 2500 RPM.

Rod
Rod Nichols

I don't understand why the rotor has an extra bit on it. And I don't understand why cutting it off would do any good. I like the hint about using a timing light to see where the actual spark is happening through a hole in a spare cap. If someone could make this picture clear in my head, it might make me stop trying to understand SU's for a while, hehe. I may have to just cut a hole and look for myself (yeah right, that'll happen).
Tom

I'm outside of Phila. I'm always looking for people who might have 6s around here and can never find em. And I know exactly where you are since I went to school out there.
MB Mike

Tom -

Try this pdf file

http://www.cranecams.com/pdf/90000700.pdf

and look at Fig. 26 on the bottom of page 14. It shows the hole located on the inside of the cap plug lead terminal. The #1 cylinder is used as "convention", but any plug can be used so long as you don't need engine timing right then.

Brent
Brent B

This thread was discussed between 16/11/2004 and 19/11/2004

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