Merc 20H with conversion Timing

[rjparksjr]

Looking for the suggested BTDC timing for a 20H conversion running on gasoline. Non-race application. Any suggestion would be appreciated. I am using a buzz box and dial indicator.


Kindest Regards,

Bob

[Mike 41P]

I suggest you start with .265 BTDC It will start and run fine Not as peppy as you want, go to .280 If you were racing it you might go as high as .310 in a quest for speed. You also might bust your knuckles and rewind when it kicks back at you with such high timing. Good Luck

[trident]

.265 BTDC on mine... time each cylinder separately.

[rjparksjr]

Thank you very much! I hadn't thought about timing each cylinder separately...makes sense. 180 degrees apart perfectly @ .265 BTDC.

Kindest Regards,

Bob

[ZUL8TR]

Thank you very much! I hadn't thought about timing each cylinder separately...makes sense. 180 degrees apart perfectly @ .265 BTDC.

Kindest Regards,

Bob

Well not exactly at 180, if a perfect engine and all rod and crank dimensions and tolerances are the same for both cylinders and perfect ignition then 180 can be achieved. The way I time these 2 cylinder ignition point mag engines to get both cylinders at 0.265" (in this case) to fire the plug at the same BTDC distance in both cylinders is to gap one set of points to correct gap (pick a cylinder arbitrary I like top). Then use the dial indicator in that cylinder, zero needle out at TDC and rotate outer bezel to zero under needle and lock it (a touchy feely visual thingy looking at the dial gage needle rocking with slight engine crank rocking). Then rotate engine backwards (CCW) to a little beyond desired setting then come back (CW) to desired setting and rotate mag plate to get points to just open (buzz box here) then adjust mag stop and lock the mag plate. Now place dial indicator in other cylinder, zero it out at TDC as in 1st cylinder lock it and set to same BTDC setting as 1st cylinder. Now with buzz box adjust that cylinder points to just open. Final point setting will rarely have both points the same gap but a bit different, not to concern.

Now spark will occur at the same distance BTDC for both cylinders depending on the accuracy of your work. I always check both cylinders again with dial gage and buzz box after rotating engine over a bunch of times. If you get timing to +/- 0.001" on the dial gage with both points just opening you did well. Beer time.

Note piston crown where the dial post touches piston should be free of carbon to get reliable consistent results with needle.

Locking down the timing plate will keep timing setting free of wear on the mag point plate and be more accurate but it will make starting harder with chance of kick back. If not heat racing and just lake stuff you might want to not lock the mag.

[SeaBat]

That's the way I did it when racing in TROA in the early 60's. This is the best way to achieve accurate180 Degree firing.

[rumleyfips]

285. You had to take the bearing slop out to get the points at 180. Wiggle the crank back and forth to get the piston right then set the points.

[Carruthers]

What are the points the be set at on a 20h

[dwhitford]

Start with 0.018'' point gap on the 1st cylinder you set with the timing plate to give you whatever advance you want . . . 0..265'' before top dead center of piston travel, or whatever max spark advance you elect. 0.018'' is the factory-recommended point gap. Then proceed as outlined above tor timing the 2nd cylinder, and don't even think about what the gap is in the points for that 2nd cylinder. It'll be correct for ''180-degree'' timing for the other cylinder, which is what you really want.

And close enough to the ''factory'' 0.018'' to be inconsequential You probably can't even feel the point-gap difference on a feeler gage. . . . Unless you're Maestro the Magician or Superman. Merc made parts to quite close tolerances, so our fussing with thousandths of inches in spark timing was probably just pi$$ing in the wind anyway . . . But of course every little bit helps!

[rumleyfips]

You are probably correct Dave. I remember after spending a half hour setting the timing, checking the second set of points. They were very close .

[raw]

There is a better buzz box than the old style one that requires you to pull the flywheel and disconnect a wire.It is an induction style box used for airplane mags.Tom Johnston in Michigan sells these for about $50 if I remember correctly.This is a must have in my opinion if you own a 20H.Up here in Canada we still race these motors at every race.My father John Webster was pretty good with these motors,he liked a .010 point gap and .290 timing.I use those numbers and the motors run pretty good.Hope this helps.

Ross Webster

[dwhitford]

The smaller point gap enables the coil-condenser setup to charge longer between firings, possibly resulting in a better, more consistent spark ... not a bad idea at the higher RPMs of racing. I similarly used a smaller points gap on my battery-ignition 8800-8900 RPM Loopers for this reason.

[Mike 41P]

Hi Ross! Racing John at Crystal Lake was one of the high points. I used to set gap about 15 on #1, then set advance at 310 and tweak gap on #2 for as close as I could get to 310. Now on my own I am using the OMC CDI at 265. I only ran so-so this year and need to do some testing and plan on going back to 290 first. Anyone interested in the CDI setup can find my post about that.

[ZUL8TR]

Here is the induction buzz and light box I use, great box, accurate and sensitive. When points open it buzzes and light switches from points closed to points open

http://www.aircraftspruce.com/catalo...nchronizer.php

This is the model I use for the 20H, Y80, Y102, KG4 &7H mag point engines.

DELUXE SINGLE MAG TIMING LIGHT 12-16903 $53.75

[rumleyfips]

I was shown, about 1967, a surplus meter and battery setup. The theory was that the meter needle took less amperage to move tan either a light or a buzzer. When setting the stator or the points, the ideal was to float the needle at mid scale. This was supposed to be the exact time the points opened.

[dwhitford]

Yeah, John. Doug Reed & I used our shop's Merc-O-Tronic meter this way, as I recall.

[deeougee]

Central Tool Company use to make the #273 Audio-Visual Timing Tester. They don't produce them anymore but they appear on ebay and other places from time to time. CTC still produces the timing tools though. Excellent quality tools. They have a website.

[Stover Hire]

I set mine at .285 btdc

[Flyer]

Take that one to the bank .
Flyer, Leigh

[raw]

John, Doug, dad had both of those style boxes and you had to pull the flywheel and disconnect one of the wires from the points to make them work properly.The box that TJ sells you just hook one wire to Your kill switch plug and other the to ground on the motor. We use this box on the yamato's with points and it also works great on the 20-H. All of us up here in Canada use this box and nobody has ever had any problems, everyone says they wish they had them years ago because it is way easier to check your timing. Obviously you still have to pull the flywheel to change the points if needed. Most of us throw the factory mag locking system away as they break over time. Once we get the mag plate rotated yo get the numbers you like we drill down threw the hole for the points into the top of the crank case and tap the hole. You then put a longer allen screw threw the mag plate and into the case .I believe this also hold the points down better and they stay put longer. Some guys even drill another hole 90 degrees from the points above the top spark plug . The mag plate will never slip and the points are held down better so the timing should stay put until the phenolic actually wears then you just adjust the points and the mag plate stays locked. Hope this helps.

Ross