0S 108 Backplate pressure tap
02-15-2015, 11:56 AM
#2
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A plug of some kind. Being a 2C engine you must maintain internal pressure integrity or the engine will run lean. That if not closed off it will suck in air and lean the mixture. In fact, I doubt the engine will run very well, if at all if it is vented to the atmosphere. Put a short piece of fuel tubing and put a plug in the tube to prevent leakage (in or out) of the crankcase.
02-15-2015, 12:03 PM
#3
You can pressurize a fuel tank with a backplate fitting but you'd want a one-way checkvalve to only allow the positive pressures and a regulator to even things out. The pressure will be higher than normal muffler pressure. If not using a pump, I'd thread a screw into the backplate with locktite and grind it flush with the inside of the backplate. I did this on an engine I fitted a pump to and later removed.
Last edited by 1QwkSport2.5r; 02-16-2015 at 05:37 AM. Reason: Fixed typo/add info.
02-15-2015, 05:52 PM
#4
Pressurising a tank from the backplate is only suitable for running wide open all the time as in some CL and usually even then only with a massive intake which is too big to work as a venturi. This type of pressure is about 10 times higher than muffler pressure at full throttle so needle settings get a bit tricky 
. I've always used the short piece of fuel tubing with a tight fitting screw as a blank as mentioned earlier.
. I've always used the short piece of fuel tubing with a tight fitting screw as a blank as mentioned earlier.
02-16-2015, 04:17 AM
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Rook,. a very simple plug can be had by bending and crimping about 1/8" of the end of a brass/bronze fuel tank pipe and attaching it with a very short piece silicon fuel tube. The check valve creates very high pressure and necessitates a regulator.
Last edited by Hobbsy; 02-16-2015 at 04:22 AM. Reason: Add content
02-16-2015, 04:09 PM
#7
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Using a Cline type regulator has a great benefit, you get the same performance from full to empty on every tank. Also, you don't need to run the engine rich at the beginning of the tank so it won't run lean at the end.
02-19-2015, 04:18 AM
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Rook, you could run a Perry pump with but that has head aches of its own. The OS 1.08 has an excellent reputation and doesn't need a pump. Providing that the tank placement is pretty good.
