Change timing by moving the coil?

[watsonr]

Have read until my eyes want to fall out about timing and spark and all that. Now...can you change timing by moving the coil? Always set the coil by a business card and didn't really pay much attention. Wondering if there are any gains to be had.

Thanks,

 

[a. palmer jr.]

Not likely unless you have a highly modified saw. You would have to file out the mounting holes in the coil to rotate it slightly. I'd use the same air gap. Expect it to be harder to start if you move it much either way. Move it in the opposite direction to the flywheel rotation to advance the timing. I'm not advising this idea, however.

 

[bonden]

I have done just that on my modified( Orange)064 saw .

 

[mountainlake]

Get yourself a MS170 Stihl, there's so much slop in the flywheel key you can adjust it 5* either way. On a good saw it might be easyier to file the edge of the key in the direction you want to go. The taper hold the flywheel from turning, the key just keeps it timed. Steve

 

[Terry Syd]

Bonden, how many degrees of adjustment did you get from modifying the mounting holes on the coil?

Re the air gap, if you test your air gap at different clearances you can find a 'window' in which the coil will make the fattest spark on the plug. Lay the plug on the jug and have the area fairly dark to watch the spark. Set it in the middle of the window - you might as well have the hottest spark possible on that cold morning.

Although the factory specifications for my saw indicated it could be set from .030" to .040" - the spark at .030" was so weak the saw would only idle and couldn't hold a load. The middle of the window on the saw for the hottest spark was .039".

 

[67L36Driver]

Bonden, how many degrees of adjustment did you get from modifying the mounting holes on the coil?

Re the air gap, if you test your air gap at different clearances you can find a 'window' in which the coil will make the fattest spark on the plug. Lay the plug on the jug and have the area fairly dark to watch the spark. Set it in the middle of the window - you might as well have the hottest spark possible on that cold morning.

Although the factory specifications for my saw indicated it could be set from .030" to .040" - the spark at .030" was so weak the saw would only idle and couldn't hold a load. The middle of the window on the saw for the hottest spark was .039".

Hmmmm. I would have thought that if close is good, closer is better. BWTFDIK:msp_scared:

On electronic coils (no points) I always bias the coil against flywheel rotation for the most advance. Since there are no timing marks to go by, elongating mounting holes would be by guess and by gosh.

On saws with points, you can vary spark advance slightly with the dwell (points gap).

What one can do is to find T.D.C. and mark flywheel in relation to some datum feature on the housing for timing marks. Measure the flywheel diameter and calculate a circumferential distance that corresponds to the desired number of advance degrees.

B.T.D.T. on my 427 vet. I have 36 degrees BTDC marked on the damper and set my timing by that. Just open the throttle untill centrifugal advance is all-in/all-done. (vacume advance disconnected)

 

[a. palmer jr.]

Bonden, how many degrees of adjustment did you get from modifying the mounting holes on the coil?

Re the air gap, if you test your air gap at different clearances you can find a 'window' in which the coil will make the fattest spark on the plug. Lay the plug on the jug and have the area fairly dark to watch the spark. Set it in the middle of the window - you might as well have the hottest spark possible on that cold morning.

Although the factory specifications for my saw indicated it could be set from .030" to .040" - the spark at .030" was so weak the saw would only idle and couldn't hold a load. The middle of the window on the saw for the hottest spark was .039".

I really think he's talking about the timing of the spark and not the amount of it. To change the timing you have to move the coil in relation to the circumference of the flywheel. Measure the distance around the flywheel and each 360th of that measurement is one degree. If you want to advance the timing one degree move the coil opposite the direction of the flywheel 1/360 of the flywheel and so forth. Changing the air gap will change the timing very little, IMO.

 

[a. palmer jr.]

Hmmmm. I would have thought that if close is good, closer is better. BWTFDIK:msp_scared:

On electronic coils (no points) I always bias the coil against flywheel rotation for the most advance. Since there are no timing marks to go by, elongating mounting holes would be by guess and by gosh.

On saws with points, you can vary spark advance slightly with the dwell (points gap).

What one can do is to find T.D.C. and mark flywheel in relation to some datum feature on the housing for timing marks. Measure the flywheel diameter and calculate a circumferential distance that corresponds to the desired number of advance degrees.

B.T.D.T. on my 427 vet. I have 36 degrees BTDC marked on the damper and set my timing by that. Just open the throttle untill centrifugal advance is all-in/all-done. (vacume advance disconnected)

You probably have 8 to 10 degrees advance at idle and about 13 or so more built into the distributor since it turns half the speed of the crank, making it about 26+10. I used to work on those things too.

 

[Terry Syd]

Yeah, I used to work on them too. I've changed weights and springs to get different amounts and rates of advance on the centrifical and changed pots to change the vacumn advance (some of the pots were even individually adjustable) - the good ol' days when you could fix things with a screwdriver and a wrench.

 

[Mad Professor]

Better to use an offset keyway on the flywheel.

 

[madhatte]

Just did some mental gymnastics and came to the same conclusion.

 

[Blowncrewcab]

The "Optimum" spark timing changes with Load, RPM, Fuel Quality and some other Factors. I Know on 4 strokes they usually like to get 30*-36* advance for Optimum performance. But why do those #'s work so good? I'll try to explain. When the piston comes up to tdc the crank journal is moving lateraly across the bottom of the bore (Not up & down) for approx 15*btdc & 15*atdc. When the coil tells the plug to spark it takes time for the spark to ignite the fuel,let it burn, and create combustion. You want the combustion & max cyl Pressure to happen around the 12*-15*atdc so the max pressure is whats pushing the piston down the hole, not when the crank journal is moving lateraly across the cyl bore,( thats just wasted energy since it can't push the piston down till the crank rotates past the tdc and beyond). Playing with timing (a little) can result in a nice power gain, but can bite back if too much timing by storing that max combustion too long from too much advance, then the heat is absorbed into the piston & cyl.There is a sweet spot where max timing is making max power without overheating. Does this make any sense to anybody? ( It's Friday and I may have been drinking:msp_biggrin

 

[67L36Driver]

Blowncrewcab:
Yup, I understand what you posted. It seems to me you want enough advance to get the fire lit so that the max pressure occurs just a hair ATDC. So that you get near complete combustion and not much push is wasted out the exhaust port.

I'm OK with four stroke cycle but have little real knowledge of two stroke cycle principles.

 

[mountainlake]

I don't know how many saws have timing advance, if they do it would have to be electonic in the coil. If they don't they can only set it as far advanced as getting it started will permit. Like my 044, it will kick back if you don't pull fast enough. Steve