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Strobe Timing a Comet

Martyn Goodwin

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VOC Member
I need to strobe time my Comet that has a BT-H installed because there is no way (that I can figure out) to check the actual timing provided by a BT-H magneto. I do know how to use a timing light but locating a suitable location to actually check the timing is elusive.

Has anyone strobe timed a Comet before? If yes, where did you find or make your timing marks? I have thought about putting some timing marks on the outside of the magneto drive gear which is accessable behing the ATD cover on the right hand side of the motor but this seems to be of too small diameter for any accurate use.

What has been used and worked in the past?

Thanks,

Martyn
 

timetraveller

Well Known and Active Forum Website User
VOC Member
Clevtrev and some of his French friends came up with a device which fits in place of the big end quill and will hold a CD machined to be a degree disc. It incorporates an 'o' ring and the engine can be run safely for long enough to do the timing,
 

john998

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VOC Member
Hello,
A while ago I tried one of these devices on two of Mark Goodson's bikes, in both cases it looked as though the thread in the timing cover was not lined up with
the main shaft bore with sufficient accuracy to allow it to work.
A great idea though. John.
 

davidd

Well Known and Active Forum Website User
VOC Member
Martyn,

Here is my reply from another post:

"The new BT-H can vary as much as 9 derees ( at least mine does) with the rod in place when setting the timing. If the rod is slipped in the center of the armature (and threaded hole), you with be able to move the crank 4.5 degrees on either side of TDC as everything takes up the clearances. Because I was trying to do repeatable runs I needed to know more exactly where the spark was firing. I checked the timing with a strobe and found that after you slip the rod through the armature you need to move the wheel in the direction of travel to take up all the slack on the rod. On mine, this is where the spark was firing, i.e. not the 4.5 degrees earlier when the rod can slip in or out with ease. I do not think these numbers matter to you, but I mention them to urge you to pick a way of setting the timing that will be repeatable by you with ease. Then you will at least be consistent."

I have used the Grosset device mentioned here and above. I have also strobed the bike with the oil pouring out of the quill. It is messy, but if the engine is warmed up the roller bearings will not be damaged for running without a supply for a few minutes because thew will contain plenty of oil. On my racer I have a dry clutch so I have marked the engine pully and I have a little pointer that I bolt on.

I think if you set the timing as I have mentioned in the quote, you can strobe it once to verify the method you should use statically to get the best result. An old /2 BMW pushrod fits inside the crank end. If you slit the end going in the crank and put a ground nut on the end of a threaded rod that goes through the pushrod tube you can tighten it until it is snug as the ground nut will spread the slotted end of the tube. You can use any tubing that fits of can be turned to fit in the crank end.

Grosset uses a CD disc for the degree wheel. So he machined a little adaptor to thread on the end of the tube to hold the CD. I scaled a drawing of a degree wheel on my computer and printed it on sticky backed paper, then applied it to the CD so I had a full 360 degrees. Grosset just gives you a small range. If you wish a copy of the scaled drawing, just send me an email at daviddunfey@aol.com.

You want to strobe it at the RPMs where it is fully advanced. Without a tach you can just watch and see where it stops advancing as you rev. I think it is quite high on the BT-H compared to the Pazon. There is a graph on the BT-H site that shows the curve.

David
 

Bazlerker

Well Known and Active Forum Website User
VOC Member
Ok, now I am quite confused...I thought the device went into the crank, replacing the big end quill, but from reading this last post it seems to me that the device goes into the armature..Obviously I need someone with as little knowledge as myself to explain it to me
 

davidd

Well Known and Active Forum Website User
VOC Member
Bob,

You are fine. The new BT-h has a 6 mm rod that slides into the armature through the housing to establish where the spark occurs. Unfortunately, you slide this rod through a threaded hole that has qute a bit of clearance, into the armature that has a hole with a certain amount of clearance. This means that the with the rod in place the degree wheel, which is in the crank end, will rotate about nine degrees. Thus, the question is: "upon which one of the nine degrees is the spark firing?" As it turned out with mine, it is firing on the ninth degree or all the way tight moving the rear wheel in the direction of travel.

This is a big issue if you are racing and you need to know exactly where the timing produces max power. On the street it is probably OK to be 4.5 degrees off the mark. However, I think it is good to strobe the bike once to make sure your timing procedure is consistently producing a numer with wich you are happy.

David
 
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