Experimenting With Timing

cannonball2 · Mar 11, 2015

I finally found an explanation of when the stock CG ignition fires. According to the Grubee site its 15 degrees BTDC. That's conservative for fixed timing, up to 28 degrees works for hand started stuff. So I removed the rotor marked the keys center and filed about a 1/16 on either side of center for advance/retard.
Naturally I went for the full advance expecting the engine to wake up a bit, not so strangely.
I then went for the retarded position expecting a much improved idle and low speed running but a sagging midrange and top. Amazingly the whole range improved! Good bottom end, midrange, and the top reved out better than it usually does.
So The only conclusion I can draw is there is TOO much initial advance in the CDI itself, the key way in either the crank or rotor was wrongly machined/positioned, the rotor is magnetized backwards, and last I just don't know. And I do have the rotor on correctly
I wish some one else with a stock engine would give this a try and see if this is common or just isolated to my engine. The results supprised me.

frank66 · Mar 11, 2015

my dads grubee is a very advanced little SOB. i was thinking an offset key would be nice on it for sure.

more or less spark affects timing.

2door · Mar 11, 2015

There has been a lot of experimentation with ignition timing over the years. Several years ago I played with it a little and I found inconsistencies in rotor (magnet) and crankshaft keyways. Some were advanced, some retarded. The engines with a slightly retarded ignition curve ran better, smoother and stronger, than the ones that were in the advanced range. I found variences of 3 to 7 degrees difference between several engines. My findings were restricted to the mechanical aspect and not the electrical CDI side.

I used an automotive cam degree wheel and dial indicators to confirm exact piston position TDC and BDC. As we all know quality control in manufacturing isn't one of the high points where our engines are made. Indexing the keyway seems to be a hit or miss, in-the-ballpark kind of thing. Slop in the keys and keyways also impact where the plug fires.

Tom

cannonball2 · Mar 11, 2015

Glad you found the same trend 2door. If this was a properly set up point system advancing it a bit over 15degrees would have noticeably improved performance. I can only guess the CDI adds advance after start and too much at that. I agree that tolerances are lax in all aspects of the engines, which makes sorting out the running a challenge. Rotors are pretty cheap($5 or so) so screwing one up wont break the bank. Think I will take a little more off the retard side and see what happens.

Thanks for the input yall!

maniac57 · Mar 11, 2015

Seems to me it would not be too hard to fab a small plate behind the coil to allow adjustment of the timing...it would seem to be easier and more reliable than offset keys and filing the magnet.
Anyone else thinking along these lines?

cannonball2 · Mar 11, 2015

My thoughts were filing the key way was easier than a plate or filing the four mounting holes. There is barely enough boss in the case to keep the magneto centered, its get very close to the rotor as it is. Filing the key would also be pretty easy and it could just be flipped around to change from advance to retard. Doesn't take much off either the keyway or the rotor to make a big difference in timing, the crank holds the rotor centered and its not hard to hold the rotor where you want it when tightening the nut. The rotor or key is a cheap, easy, quick method to do some experimenting. Theres not a lot of room in the engines case to allow a plate or the mag much travel.

frank66 · Mar 11, 2015

if you file the keyway in any manner it will evensually end up retarded because the engine firing forward will jog the rotor to the retarded direction not the advanced direction.

frank

frank66 · Mar 11, 2015

1 degree at the perimeter is 0.0130".
1 degree at the spindle is maybe 0.0035"

--------
diameter times pie divided by 360 degrees.

Trey · Mar 11, 2015

cannonball2- Utilizing the information available here along with continued testing, will help you get some answers.
I'll be following along.

Trey

cannonball2 · Mar 12, 2015

frank66 said:
if you file the keyway in any manner it will evensually end up retarded because the engine firing forward will jog the rotor to the retarded direction not the advanced direction.

frank

I don't think there is enough mass in the rotor to over come the locking friction of the nut and washer. A large cast iron straight keyed flywheel yes. We ran our kart flywheels with out a key, but they were on a taper and were lapped to the shaft with valve grinding compound. That said the desired direction is retarded and if the keyway is reduced a bit at a time until the best running is obtained, its already home.

I have an index mark I make every time I move the rotor. I will keep an eye on it.

Jump in there Trey and see what you come up with, seems nothing is the same with these engines. You may find something different.

Trey · Mar 12, 2015

I'm waiting on you folks to work out the bugs before I get in the 2-stroke boat!

cannonball2 · Mar 12, 2015

Probably never happen--lol! That's what makes them interesting. If you like to tinker, get one.

cannonball2 · Mar 12, 2015

Heres the rotor from my new GT5. Its kind of hard to see the red mark next to the key way. That's the area I will file. Also am checking the poles to see if they are correct, they are. Notice in the second pic the red mark is smaller giving me an idea how much it has been filed. This is to the retard side.
Also notice in the second pic how far away the compass is being influenced by the rotor, nearly a foot. The one in my other engine is not nearly as strong, something to check if one is having constant "mystery" ignition problems. This rotor will suck up just about anything ferrous nearby. Be careful of that, you don't want a jammed rotor from some junk while the engine is running!

I would think balancing one of these would be beneficial. They can be balanced on a model plane prop balancer. Mine is MIA in the attic or I would balance this one now.

Davezilla · Mar 12, 2015

I've been toying with the same idea to mount the mag coil on a plate with slotted holes so it can be rotated a few degrees in each direction so there's no need to file the keyway on the rotor or need to use offset keys. Giving the mag coil the ability to rotate tho will have a limited range since there's not a whole lot of room inside the case, but if it could move maybe 5 degrees in each direction it could make one more tunable as long as the key slot on the rotor puts the timing close to where it needs to be.
The way I would go about this would be to mount the coil solid to a plate then drill and tap the case for a set of studs for the plate to ride on and use small self locking nuts to hold it down once the timing is where it needs to be. The slots in the plate would have to be a very close fit on the studs tho since the magnet runs very close to the stator already and we don't want there to be a chance for the rotor to contact the magnet while adjusting the timing.
I'll take some measurements next time I get enough free time and see if this setup would be possible and able to keep everything inside the case under the stock cover, as well as seeing if it would give enough range to be practical.

frank66 · Mar 12, 2015

i dont see how a different mount is going to help. its allready mounted.

just file the 5 dollar rotors key way wider. leave the tiny woodruf key the way it is or buy a rocket key. since we are trying to retard the engine - no problem since the rotation forces will hold it to the retarded direction aswell as the fastener. done. takes maybe 20 minutes i guess.

i like to follow my own advice so i will do it as soon as i can get some test drives done. currently to much melting snow here.

maniac57 · Mar 12, 2015

cannonball2 said:
Heres the rotor from my new GT5. Its kind of hard to see the red mark next to the key way. That's the area I will file. Also am checking the poles to see if they are correct, they are. Notice in the second pic the red mark is smaller giving me an idea how much it has been filed. This is to the retard side.
Also notice in the second pic how far away the compass is being influenced by the rotor, nearly a foot. The one in my other engine is not nearly as strong, something to check if one is having constant "mystery" ignition problems. This rotor will suck up just about anything ferrous nearby. Be careful of that, you don't want a jammed rotor from some junk while the engine is running!

I would think balancing one of these would be beneficial. They can be balanced on a model plane prop balancer. Mine is MIA in the attic or I would balance this one now.

Balancing will absolutely help! Be sure the crank snout is straight as well.
Doesn't take much to make it wobble.

cannonball2 · Mar 12, 2015

Got a break in the weather enough to ride the GT5 bike. Again a noticeable improvement from retarding the rotor. Next time I order parts I will buy one to sacrifice and use my Dremel to make a larger keyway toward the retard side. I can then get an idea of how far is too far. Its gotta be the CDI adding advance after start up which mkes the total advance too much. That's why retarding improves running.

If you guys want to make a plate, make one that mounts over the mag and holds a set of points. Add a small cam to the rotor open them, a $7 pit bike coil and a condenser and we have reverse engineered the ignition to something that is reliable(not considering the mag) and we can really play with. I like points! They would probably out last the engine anyway.

maniac57 · Mar 12, 2015

Still need a magneto for the juice cannonball. Would need a battery at minimum for points to work.
Good idea though, should allow more adjustment range with only points under the cover if running off battery. Might be worth a try

2door · Mar 12, 2015

The original Russian design used points/condenser ignition. There's an old thread here from a Russian member, Dagger, who posted photos of the early Russian engines. I'll see if I can find it and post a link.

Tom

EDIT: Here's the thread > http://motorbicycling.com/showthread.php?t=6820

BOYGOFAST · Mar 12, 2015

piston ring worn cylinder ports the top of the exhaust port will likely go first the rest will follow until the engine stops running if it does not explode or break a crankshaft minuets before that happens ,only a guess......

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