Originally Posted by
DaveyMo
Here're the particulars. I can start it at a compression setting of 12.5 turns in and a needle setting of one and a quarter turns out. As I understand it, my goal is to back off on compression after a minute and then find the needle setting that gives highest speed. I then repeat this process (backing off and then diddling the needle for highest speed at each lower compression setting). However after backing off the compression in three or four steps or a total of a half turn out from its starting setting, the speed has crept up very little and seems to max out at the 7,000 or so RPM. In trying to reduce the compression even more, the contra piston fails to cooperate and stays in the 12 turns out position while the T-bar free-wheels on top. As it runs, I'm getting nice, white smoke and just a bit of gray discoloration in the fuel accumulation below the exhaust port.
Dave...
Dave, I think you may have misunderstood the instructions. The goal is not to make it run on the lowest compression setting but to make it run at the correct compression setting without being over compressed.
Once it has heated up you lower the compression until it starts to lose some RPM then increase it until it no longer increases RPM when compression is increased. If you increase it more, compression will drop off. THIS is when you are running it over-compressed. Running it this way will put extreme stress on parts and cause the engine to overheat. By running it at the lower end of the "flat spot" you are lessening chances of running it over-compressed.
Of course as you settle on a compression setting you are also tweaking the needle valve to peak. Over rich will slow down a bit, over lean will provide a zinging sound, too over lean and it will quit running.
One of the advantages of diesel over glow is that you can set it for the best running conditions for various prop sizes. To do this with a glow you must change venturi sizes and vary head shims.
George