Misc: Ignition Comet timing

[greg brillus]

I'm glad Chris chimed in on this, as he knows a lot about the AMC boxes that I for one don't........It sounds like your ignition system might be a big headache.......Twin spark I would run no more than 26 degrees full advance pending what fuel grade you have available.......Most belt primary's for singles run a 36 engine/68 clutch wheel giving a 1.888:1 ratio.......I can't remember what the stock Comet ratio is, but it must be close.

 

[Cyborg]

More of a foot, ankle, and knee ache. Folks use this system on twins and seem to have no issues. Watching the belt as it’s being kicked over, it seems more often than not the engine rolls back a bit as it cones to a stop. Makes me wish I had spent the money on a mag, although it may be ok once the bikes gets completely sorted and I figure out exactly what starting drill it wants. Once the monsoon season is over, I’ll use the paddock starter to get it running and sorted as much as possible. Then I’ll try and retrain myself, hopefully without braking anything.

 

[Chris Launders]

The engine running back a bit when it comes to a stop is normal as it comes up against compression which then pushes the piston back down a bit.

Do you have a strobe light, check it's advancing and retarding, timing not critical for this, just the fact it is actually moving.

I had a Comet that started fine when it was cold but was a nightmare when hot, rebuilt the mag, swapped to a Mk1 Concentric, eventually solved it, I sold it as I'd got my hands on a twin (sorry I know that's no help).

The twin has a BTH and is often a first kick starter never kicked back.

 

[Cyborg]

Thanks Chris. I realize the rolling back is normal,,,, it‘s the fact that this ignition system uses the rotation of an optical encoder disk to tell it where it is in relation to TDC. Unfortunately the system can’t differentiate between forward or reverse motion. When it rolls back it still adds that number of degrees, so next time you kick … it has added that number. Say it rolled back 10 degrees… next time you kick it, instead of 4 degrees before TDC, it fires at 14 before TDC. Now that I figured out that‘s how the thing operates, I just have to pull in the decompressor and roll it past TDC, so it forgets about those extra 10 degrees. Unfortunately that’s a bit of a pain because of the tendency for it to do it again, but at least I know what it’s doing. There was no mention of this “ feature” in the instructions or on the website. It took some searching and head scratching to figure it out. I bought the system because it is programmable, so hopefully I can teach myself the proper starting drill. If I actually like riding this thing, I’ll explore making an electric leg.

 

[vibrac]

Greg has written before on kicking with a Norton Box I followed his instructions when I had an Egli Single and it worked a treat

 

[Chris Launders]

Thanks Chris. I realize the rolling back is normal,,,, it‘s the fact that this ignition system uses the rotation of an optical encoder disk to tell it where it is in relation to TDC. Unfortunately the system can’t differentiate between forward or reverse motion. When it rolls back it still adds that number of degrees, so next time you kick … it has added that number. Say it rolled back 10 degrees… next time you kick it, instead of 4 degrees before TDC, it fires at 14 before TDC. Now that I figured out that‘s how the thing operates, I just have to pull in the decompressor and roll it past TDC, so it forgets about those extra 10 degrees. Unfortunately that’s a bit of a pain because of the tendency for it to do it again, but at least I know what it’s doing. There was no mention of this “ feature” in the instructions or on the website. It took some searching and head scratching to figure it out. I bought the system because it is programmable, so hopefully I can teach myself the proper starting drill. If I actually like riding this thing, I’ll explore making an electric leg.

That will make it a bit awkward, could you modify the pickup circuit so that when you turn off the ignition it isolates the sensor, so it doesn't know it's rolled back. ( just thinking aloud)

 

[Cyborg]

That will make it a bit awkward, could you modify the pickup circuit so that when you turn off the ignition it isolates the sensor, so it doesn't know it's rolled back. ( just thinking aloud)

I don’t think that would help… when you first turn the ignition on, you need to rotate the engine past TDC, so the ignition knows where it is. Switching the sensor off would be the same as turning the ignition off, so either way I need to kick it past TDC with the decompressor and pray it doesn’t roll backwards. It makes total sense that the engine wants to roll back when on the compression stroke, but it’s a little surprising how easily and how often it does roll back. You’d think it was a 2 stroke. An electric starter would cure everything…long kickstart lever, short leg etc. I would just need to pull the decompressor lever in prior to hitting the starter button so the thing doesn’t explode. Of course…. when I assembled this lump from the contents of 3 sheds, I didn’t plan for the future addition of an electric starter, so it will take some farting around to make one work. It’ll require a ring gear mounted to the Newby clutch and a starter with a bendix. This is all assuming I don’t just shove the thing off a cliff.

 

[DucATIRadeon]

not much help either, but ive fitted the Pazon (twinspark) ignition on my twin. it don't matter where the crank is or if its on the front or rear compression: kick it and it starts! yes, requires battery power, but brilliant piece of kit; fit and forget!

have 29.something mm amal 389 and 689 carbs on it, 270 main jets and K2 needles in middle (3) or 1 down form middle (2) notch, with K&N conical Airpod filters.

as easy as it is: set nr1 (or rear) cylinder on TDC compression stroke and bob's your uncle. you will need fixed pinion (or locked ATD, u used double tie wraps on the lugs fixing in full retard position).

 

[Chris Launders]

I don’t think that would help… when you first turn the ignition on, you need to rotate the engine past TDC, so the ignition knows where it is. Switching the sensor off would be the same as turning the ignition off, so either way I need to kick it past TDC with the decompressor and pray it doesn’t roll backwards. It makes total sense that the engine wants to roll back when on the compression stroke, but it’s a little surprising how easily and how often it does roll back. You’d think it was a 2 stroke. An electric starter would cure everything…long kickstart lever, short leg etc. I would just need to pull the decompressor lever in prior to hitting the starter button so the thing doesn’t explode. Of course…. when I assembled this lump from the contents of 3 sheds, I didn’t plan for the future addition of an electric starter, so it will take some farting around to make one work. It’ll require a ring gear mounted to the Newby clutch and a starter with a bendix. This is all assuming I don’t just shove the thing off a cliff.

I'm just fitting an electric start to my Norton Dominator, I have a (probably) Norvil clutch in which there is a backing plate fastened inside with countersunk Allen screws, so I've machined a piece of 5mm steel plate with a bore to match the clutch bearing and the outer takes a post 1981 Sportster ring gear.
The plate is aligned by removing the clutch inner and bearing retaining circlip and pressing another bearing into the housing (on top of the existing one) and removing one countersunk screw and drilling the plate and tapping it, replace with a longer screw and repeated until I had four in.
I shall be using additional screws as there are not enough at present to take the torque.

I'm using a standard Sportster starter motor/solenoid unit but have changed the gearbox shell to a AJS/Matchless one to give a better starter location as the lugs are in a different position to the Norton unit, but I've aligned the box so the end covers are in the same orientation as the Norton ones were.

This is it using a borrowed frame and some of my spare parts and some I've bought (so I don't have to take anything off the bike) to prototype it.

Started after Christmas, everything now made ready for paint etc.

PS it has a downdraught head with a single Concentric and should clear the starter motor by over 1/2"

 

[Cyborg]

I have been looking at Harley ring gears and starters and wondering if the bendix hangs down below the starter motor enough that the bendix gear will mesh while the starter motor itself will clear my duel adjuster setup for the gearbox. The top lug on the gearbox might be tilted forward far enough to make room. If all I have to do is notch out the left engine plate and inner primary plate, that would be heavenly. When and if you have time, can you tell me what the diameter is of the ring gear and if you can…roughly what’s the distance between the centre of the bendix and the underside of the starter motor. Definitely no rush… just if you happen to have them on the bench at some point. Please and thank you.

Edit… I just looked at a photo of the Sportster starter and the bendix housing is a lot longer that what I thought. I need something like this one, except opposite rotation.