When you get the prop installed and question your placement of the sensor here is a method to quickly check just how many degrees you have it set BTDC.

To find straight line distance between points on arc:
Q&D : {Circumference=(diameter x pi)} * {degrees of arc = timing degrees} / 360
Dia: 1.372" 4.310
Tim Degree: 25

Straight line dist: 0.2993"

Formal calc method:

2r * sin (1/2 degrees)
2r=dia 1.372"
sin (25/2) 0.2182

S line dist: 0.2993"

Close enough for dial calipers!

Just installed a CH Ign on a Ryobi and with good spark (plug out), carb and plug damp with fuel, and just get one mild POP and then nothing. Above method should allow measurement, without removing prop and tinkering with a timing wheel.

This alternate method should allow a quick check of timing anytime you desire.

How many degrees do Ryobi like for an initial BTDC setting? The supplied instructions stated to 'align to the mark on the sensor hose clamp' The mark was rather wide.

Back to shop...

Just found this and it is, as usual, an orderly how-to routine.
http://www.rcuniverse.com/forum/C%25...161183/tm.htm#

We used to use a wood dowel inserted into spark plug hole to set timing on my Triumph " flat tracker" 650 Bonneville . Seemed like it was about .375" before TDC. Be sure it,s long enough to keep it from falling into cyl. Dont ask!!!! Ha Ha
Aloha Les

3.14(Pi) times the diameter, divided by 360 gives 1 degree, multip.ied by whatever the number of degrees you want gives the distance....for 28 degrees the formula comes out to .244 times the diameter..Can't get any easier than that [8D]

Determining crank angle from piston position is not as simple as determining distance between points on an arc, or even determining the vertical (in line with bore) component of some degrees of rotation. You would really need to enter the connecting rod length in your calculations somewhere as you've got a rotating crank joined to a straight line slider mechanism. For example, make a careful sketch, or do some calculations, and you'll see that when the crank is at 90 degrees, the piston is not at mid stroke, but somewhat below that point.

A degree wheel starts to look pretty straight forward.

The easiest way is with a degree wheel. If you don't have a degree wheel, I have one in a JPG file that you can print and glue on a piece of sheet metal or thin ply. It print's out about 4" diameter. If anyone needs a copy, let me know.

Nashcat

How well the arc length method works depends on how large a diameter you have to work with. If you do it on a large flywheel, then precision is not a problem down to 1deg. Off a smaller prop hub, though, you might at best be able to eyeball it between the typical range of 28-32degs.

The degree wheel amplifies the resolution and is particularly useful when you are power-timing the engine, where you may be advancing the spark 1deg at a time.

Our engines are not very sensitive to timing..I think you'll find that a few degrees one way or the other makes no difference at all....IF you use a degree wheel and can time it EXACTLY one degree at a time....We tried timing a 289 racing twin that way, set it at 28, timed it a little less until it started to slow down at about 22..We left it at 23...........[8D]

i think Les was refering to the dowl-in-plughole method as a way to find TDC. the trig method you mention is very usefull for playing around with port timing, especially with a torn down engine. all you need are accurate measurements of stroke and rod length


dave

FWIW This became my instruction guideline, it may help someone else.
1) Put the piston on top dead center, make reference mark on case and hub adapter (TDC);
2) With the spark plug out..Lay it on the cylinder...Turn the ignition on..Slowly turn the sensor clockwise until you see/hear a spark..
3) Put a mark on the sensor mount directly at TDC mark on the case. (case, hub and sensor mount are all marked for TDC;
4) Measure the diameter of the hub close to the magnet. Diameter x pi x desired degree timing degrees / 360 give the straight line distance.
5) Measure that distance on the case, make a mark (Timing mark); Rotate sensor mount mark CW to align with case Timing mark. (on non-keyed prop hub: mark prop hub also)
End results: case, hub adapter, and sensor mount all marked TDC; case marked for desired degrees BTDC. Ability to quickly do a visual check to known reference marks.

On a 1.372" diam case gave these distances: (roughly 1/32" straight line per 3 degrees)
Degrees: IN Dist:
31 = 0.3712 "
28 = 0.3352
25 = 0.2993
22 = 0.2634
19 = 0.2275
16 = 0.1916

Ralph, I agree that these low CR weedies and chainsaw engines dont really react much to ignition timing. The point of advancing the spark timing is to maximize peak combustion pressure - and therefore power - by starting the combustion cycle early. With low CR, the engine just can't generate any more peak pressure beyond a certain point, no matter how early you spark.

Power timing is only really worthwhile if you raise the CR and then switch to a little higher octane.

YUP
Almost impossible on a Ryobi, possible on Homies...And it works [8D]