Installing a magneto in 52 K model

All things K & Sportster

Re: Installing a magneto in 52 K model

Postby K Daddy » Sun May 19, 2019 8:26 pm

Wz507, Erik, Ferrous Head and others,
Thanks for your input. Yes, probably I seem hard headed on the issue of installing a magneto with proper timing. Believe me, I am listening, but some comments leave me with more questions.
Back in the late 60's, my brother had a 66 XLCH with factory magneto. It was important to retard the spark using the left hand grip before starting or you could get a nasty kick. I am 68 now and want to protect my knees and other old bones.
With the spark retarded, it would start easily and would smooth out when the spark was advanced.
I don't know the "start" setting or the run setting but I was surprised over the values posted.
I must be wrong, but 10 degrees before TDC for starting seems like an advanced spark, not retarded. And then 25 degrees before TDC seems huge. But I have made note of this and will use it if experience supports it.
And thanks for your input. Maybe there are marks on the flywheel that I have forgotten
How would I find 25 degrees BTDC? As I recall, (and my memory gets worse every day) removing the left side plug will show a line stamped into the flywheel for TDC of the front piston and there are no subdivisions for degrees. So what is the trick to flashing the flywheel with my timing light and identifying 25 degrees?
Thanks for everyone's input. I am keeping a file on all of this so that one day I might approach your knowledge level.
K Daddy
 
Posts: 39
Joined: Mon Dec 10, 2018 10:44 am

Re: Installing a magneto in 52 K model

Postby wz507 » Sun May 19, 2019 8:47 pm

See the link below, and scroll down the page to see all the K model factory timing specifications and an image showing where the timing line should be positioned in the timing inspection hole.

viewtopic.php?f=3&t=896

If your flywheels are stock they will have a dot that indicates front cylinder TDC and a line that should be positioned as shown in the above-cited link to achieve the desired timing of the front cylinder.
User avatar
wz507
 
Posts: 356
Joined: Fri Dec 11, 2015 9:13 pm

Re: Installing a magneto in 52 K model

Postby Ferrous_Head » Sun May 19, 2019 10:24 pm

The reason for the ignition point being BEFORE TDC is that it simply takes time for the combustion process to happen. The combustion process is not an explosion. It has a flame front that moves across the top of the piston until it burns all (or most of( the air furl mixture in the combustion chamber.
This takes time. And with the flywheels moving fairly rapidly it can take 90 degrees or more for this to complete.
We want the maximum pressure where it will do the most good. That's NOT at TDC but at some point well past TDC. Imagine the con rod is just a lever (which it is) and you want to apply maximum force on that lever to turn the flywheel. If you do that at TDC thr force will merely try to compress the con rod and do no useful work. Ideally all force would happen at 90 degrees for maximum leverage and be over almost instantly.
Als, it is not. So we use compromise figures, the better the combustion chamber shape and air flow the faster the burn and the less time you need. The poor old Sportster engine is really not good. hence, 45 BTDC is generally the correct timing point.

I understand exactly about your knees. I'm nearly 68 and have nearly given up kick starting bikes. I have two sets of rollers for that. And early Sportsters ran relatively high compression and did gain from having the ignition retarded for starting. My brother had a big bore stroker with very early cams in it and fixed timing. These were the bikes that threw people over the bars. He used to take bets on people not being able to kick start it. They learned pretty quickly.
He almost never kick started it wither. Hr used to get me to push him to start it.

Buy a set of rollers and use them when you can. Save your knees for when their is a crowd or at least a pretty woman.
"I know only too well the evil that I propose, but my inclinations get the better of me."
User avatar
Ferrous_Head
 
Posts: 384
Joined: Tue Jun 06, 2017 5:36 pm
Location: Australia

Previous

Return to General K / Sportster Discussions

Who is online

Users browsing this forum: No registered users and 17 guests