You may think your compression is strong enough, but have you actually measured it?
FWIW with a magneto like that, I think you will have to pull it through compression to get it to fire up, but what do I know.
Back to the compression – I have been wrestling with a Blakely or Gray Gearless motor this summer. It would kick when I cranked it but never wanted to fire up. The compression measured at a tad over 40 lbs, which normally is not terrible for a rowboat motor – but it would not start unless I doused the combustion chamber with heavy oil ( heavy as in SAW 75 gear oil). I determined that while the compression value was semi-acceptable, the clearance on the side of the piston ring (single 1/2 inch wide ring) was excessive. Due to excessive material having been removed from the sides of the groove, the clearance ranged from 0.006″ to 0.009″, whereas the usual spec is a max of 0.003″/0.004″. The ring also had a radial thickness of only 0.100″, where as it should have been more like 0.115″. This combination of conditions resulted in excessive leakage of compression around the ring (not thru the end gap) and as a result, this motor would not run as it was.
As an experiment and at the suggestion of the piston ring supplier, I packed the ring groove with heavy grease and put it back together. The engine did start up and actually ran as long as the fuel supply held out, which was only seconds since I didn’t have the gas tank mounted…but once the grease heated up and flowed out of the ring groove, I was back to a the only-once-in-a-while kick scenario again…
When I received the new rings (had to go with a pair of quarter-inchers) I fitted them with the side clearance as recommended by the supplier. This meant opening up the groove width and using a spacer ring to take up the excess. I ended up with about 0.002″ side clearance. The increase in compression measurement was notable, and so was the lack of hissing in the exhaust as compression leaked around the sides of the old ring. The engine started instantly and ran continuously once the side clearance issue was addressed WRT piston ring fit.
You may need to investigate that part of your motor and measure the ring side clearance if you haven’t done so already. And if you have it apart, I’d suggest putting grease in your ring grooves and see how the engine reacts after that. Be ready for some acrid smoke if it does.
Hope this helps.
Best,
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...
