April 24, 2015 at 1:10 am #14607quote FrankR:
Ah of course! You’re right, it doesn’t matter – as we’re only concerned with the angular relationship between the armature plate and the points cam on the crankshaft.. how far one leads or lags the other.
What matters in this case is the angular displacement of the armature plate from the crank position. If the plate moves, the crankshaft would be moved to reestablish alignment of the marks, or vice-versa.
Actual position of the armature plate is only of concern if we’re measuring timing angle with respect to TDC, which is a fixed point of reference.
Thanks for catching my blunder! Very much appreciated.
US Member - 1 Year
Topics: 2April 24, 2015 at 3:13 am #14615
Once I used a timing fixture, I never used a feeler gauge again. The spark is hot and the motors start and run noticeably better.
US Member - 2 Years
Topics: 27April 24, 2015 at 9:45 am #14630
Franks tools takes all the guess-work out of timing for sure.
US Member - 2 Years
Topics: 46April 24, 2015 at 11:49 am #14631
OK, this covers how to determine when the points open. If you don’t have a timing tool, you can still get this done… I use some kind of stationary pointer that can be screwed on to the block, and get close to the flywheel.
First, make sure the mag plate is locked down and won’t move. Adjust the first set of points using a feeler gauge, then get the flywheel to where that set of points is just opening. Put a mark on the flywheel where the pointer is. Then use a tape measure to determine the circumference of the flywheel. At one half the circumference from the first mark, put a second mark. (This is 180 degrees from the first mark.) Line the second mark up with the pointer, then adjust the second set of points to where they just open.
I was quite surprised how far off the initial settings (just using gap) were on a Scott 10 hp Pennant I had a while back. It was probably 15 or 20 degrees off. The motor ran better, and idled a LOT better with the timing adjusted correctly.
US Member - 2 Years
Topics: 32April 24, 2015 at 11:58 am #14632
Many thanks to all who weighed in on this subject. This thread has been very educational for me, and it has convinced me to purchase one of Frank’s timing fixtures. It will be interesting to see the results first hand.May 5, 2015 at 11:48 pm #15408
So this process should be able to work on nearly all 2 cylinder opposed motors? With that being said, one need to find where one set of points opens, using a Ohm meter, then ensure the other set opens exactly 180 degrees from that.
Am I correct? I have some Mercs, that need this done as I have always used feelers gages to accomplish this, but it is close is not always good enough.
firstname.lastname@example.orgMay 6, 2015 at 5:33 am #15422quote Tom Manley:
Pardon this, but I wasn’t able to send you a PM.
Are you the same Manley that I know from the Minneapolis bike scene, in the 1990s?May 6, 2015 at 7:36 am #15424quote Richard A. White:
Absolutely. Establishing that correct fire-point for the ‘first’ cylinder is important.. but setting the two fire points exactly 180′ off is even more important, if the motor is to run smoothly.
In other words, the relative (cylinder vs. cylinder) timing relationship is probably more critical to running quality than precise absolute (cylinder vs. crank position) timing.May 6, 2015 at 7:43 am #15425
And since we’re on the topic.. I’m still waiting to hear from one of the experts, where an OMC motor should +precisely+ time, with respect to crank position.
In this case, it’s specific to the Johnson TD20 – where (in terms of degrees, mm or inches) should the spark occur?May 6, 2015 at 10:03 am #15427
I am recalling the motor I am working on now states the proper "timing" is .265 BTDC. This is on a 1954 Mercury Mark 20H . That being said, if I set the points to open at that point and set the second cylinder 180 out from that this beast should be dare I say perfect?
email@example.comMay 6, 2015 at 10:06 am #15428
And now since I do not have a degree wheel, or the proper timing template called out for in the manuals, I will find the diameter of the mag, plot that out in CAD, with 2 marks 180 degrees apart, fabricate a pointer and do this properly, then make one for OMC motors too. Gosh this is fun, LOL
US Member - 1 Year
Topics: 169May 6, 2015 at 12:48 pm #15435quote legendre:
I don’t know if I can explain this well enough so I
answer you question but I’ll give it a shot.
Timing relative to load as well as the speed of the motor.
When a timing specifications is given it is made at
a fix point. The components of the ignition will now
make adjustments so the timing is correct as to load
and RPM. Now in the motor your discussing the
timing is controlled by you. There is no timing
specifications because the mag plate isn’t fixed and is
constantly moving. The timing would need to be only
matched to the carburetor. There is probably some linkage
specification in the manual to achieve this but in motors
that don’t have some kind of advance – retard mechanism
your basically trying to get the most advance possible at
wide open throttle. Then as you move the magneto plate
the timing changes along with the throttle position.
May 7, 2015 at 3:02 am #15465quote Tubs:
Correct, and it’s normally quoted in degrees before (or after) TDC.quote :
That’s an oversimplification. What happens, is that timing changes as the speed control is moved up & down the range. The spark advance increases as the control is moved up through the range.quote :
Ok, sure – we understand one another..quote :
That’s a mis-statement. There most certainly +is+ a timing spec – and again, it’s normally quoted in degrees BTDC. On any motor, there will be timing specs for idle and/or full speed operation. Even in the case of manually variable timing as on this old Johnson, there are still lower and upper limits for ignition timing.quote :
Ideally, timing is linked to RPM and load, which is why some engines have features like centrifugal and vacuum auto-advance mechanisms – but you’re correct that on this Johnson, the timing is directly tied to the position of the main carb butterfly. That’s a reasonable compromise, as the engineers figure that at a given throttle position, the engine speed will be X and as such, the timing should be Y. This is very reasonable on an OBM as the load is very predictable – all water acts about the same, driven by the same prop at a particular RPM.
The advance will at first be too high when the throttle is first moved from idle to full, but everything falls into place once the engine RPM catches up, which is (or ought to be!) shortly after the adjustment.
Having said all of this, there are still two hard datum to be known – the correct advance at low / zero throttle, and the same at full throttle. If the factory doesn’t quote it, that’s too bad – but it still can be deduced and measured using the correct timing tools.
US Member - 1 Year
Topics: 169May 7, 2015 at 12:38 pm #15479
Well you haven’t gotten it yet but I try one more time.
If you move 1 piston to top dead center and move the
mag plate to the point where the points fire for that
cylinder the motor is timed at top dead center but in
this design that is a running position you can’t lock it
down there as there are no components (vacuum or
centrifugal) to make the necessary timing adjustments
necessary to run the motor at different speeds.
Giving a spec like TDC serves no purpose as its
necessary to move the mag. plate from that point or
any other point to adjust the motors speed.
Your over thinking it.
May 7, 2015 at 9:18 pm #15490
So, I made me a tool today, Used ohm meter set to audible to stop make the effing racket just as points break.
Reading the manual. it states that you set the gap approximately 1/4 inch past the initial opening. This is for the Phelon ignition. Does that sound right?
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