The International Cessna^® 170 Association

I found a PDF file that cleared up the timing issues with the 4 cyl and was wondering if there is similar information/procedures about the 0-300D to get the timing up on it. My 1961 172B had an AD on the cylinders complied with approx 375 hours before I purchased it. It has always run a little hot and like the timing was retarded. Any input would be appreciated.
http://www.superiorairparts.com/downloa ... L96-07.pdf

Not sure I understand your question. The AD referenced is for the 0-200 and the 0-200 only. It has nothing to do with the 0-300.

The O300 is already timed at 28 degrees BTC on the left mag and 26 degrees on the right, not sure you would would want to advance it anymore then that.

If you're comparing O200 and O300 engines with respect to ignition timing, you should understand that the spark plugs are wired differently between the two.

In an O200, each mag fires two upper and two lower plugs. In the O300, the left mag fires all the lower plugs, and the right mag fires all the upper plugs.

That is why the O200 has one timing specification for both mags, and the O300 has the two mags timed differently by two degrees.

One must also keep in mind that the O-300 magnetos fire either all top plugs or all bottom plugs and are timed accordingly, rather half top and half bottoms as are many other systems..

That is the explanation I wanted. I know I didn't word the question right, just a little off that night!! Thanks. Has anyone ever timed one on 26L&R? (or 28/28)I'm working on an air boat. w/0-300A. I'm thinking that to run pump gas (reg) it actually needs to be 26 L&R. I know the 4 cylinder 150's would crack cylinders with high timing, but I think that was more the fuel being used back then. 85/87 (not 100LL) This unleaded 10% ethanol fuel is more like the older low octane fuel. I don't want this thing to hurt it's self and the customer will run it on cheap gas. I'm worried a little about the 26/26 deg making the ex temp too high and the boat has no instruments. Any thoughts or comments will be appreciated.

Sixracer wrote:That is the explanation I wanted. I know I didn't word the question right, just a little off that night!! Thanks. Has anyone ever timed one on 26L&R? (or 28/28)I'm working on an air boat. w/0-300A. I'm thinking that to run pump gas (reg) it actually needs to be 26 L&R. I know the 4 cylinder 150's would crack cylinders with high timing, but I think that was more the fuel being used back then. 85/87 (not 100LL) This unleaded 10% ethanol fuel is more like the older low octane fuel. I don't want this thing to hurt it's self and the customer will run it on cheap gas. I'm worried a little about the 26/26 deg making the ex temp too high and the boat has no instruments. Any thoughts or comments will be appreciated.

The old cylinders for the O200 had a design problem at the alum head attachment to the steel barrel and the timing was retarded for longevity. The improved cylinders allowed 28 degrees. It had nothing to do with the fuel.
The "pump" gas as you call it is nothing like the old 80/87 Avgas, if that's what you meant, but the timing would still not be altered for that engine or the O300. The O300 fired differently top/bottom to prepare the flame front to keep lower plugs from fouling. The left magneto fires the lower plugs at 28 BTDC and the right magneto fires the upper plugs at 26 BTDC.

In a parallel discussion see MSB94-8C at: http://cessna170.org/forums/viewtopic.p ... ing#p52329

AS for an airboat only:
(I doubt that two degrees will cause much problem either way on an airboat. Unless you are very careful to get all the gear-lash removed during the timing excersize, as well as sync the points properly (see the overhaul manual) then the timing is unlikely to be all that accurate.) The airboat will not realize any apparent loss of power if you put them both at 26 BTDC anyway, should that be your preference. It won't hurt the engine. The prop you use will have more effect. (You'll need to achieve 2700 RPM to get rated hp.)