40dcoe & Two Phase Flow Part 3
Posted: Sun Jun 27, 2004 3:40 pm
Turns out the clearance fit (CF) between the emulsion tube (ET) and the carbie body well plus the shaping of the ET controls the fuel level in the well. The clearance fit is 0.375mm radially and it's the area just below the air bleed holes through the ET. It's just the right amount of clearance so the petrol's surface tension makes it want to cling in that area. As the fuel level fills up float bowl (FB), the CF area will track along when filling. However if the fuel level then is lowered again in the FB, the CF level will not drop. The fuel will stay right at the top brim of the CF. The fuel level in the float bowl can drop by 10mm and the CF area will not be affected. Another experiment was to drain the well and fill the FB with the fuel level 10mm lower then before. When the engine was started and the throttle was blipped for the first time the fuel column was sucked up to the CF brim and stayed there. Looks like all the fuss and bother advice about getting the fuel level perfect is worth about as much as gold-plated doggie poo!
Another neat thing I noticed was when the ET was inserted in the well there was a large air bubble in the area undercut for the main jet radial porting through the ET. It would stay there even with the engine running. Repeated attempts to dislodge by tapping the well failed. It caused no apparent problem as far as the cylinder getting the proper mixture.
The test well was designed with a functional well to accept the ET stack and a same sized dumby one to monitor the fuel level accurately. Noticed in the dumby well the fuel was frothing at the top so there was a layer of the foam about 3mm thick. This was not happening in the ET filled well. Since the test fixture was fastened directly onto the Weber carbie presumably the fuel in the float bowl is frothing too. Not really sure if this is such a bad condition or not. It could be.
Only had a few minutes to play with it and won't have anytime for another month or longer before doing so more testing. Could use any ideas on what to test next.
Oops, almost forgot to add that I did try both the F11 and F7 ETs. Both exhibited about the same behavoir. Just that the F7 one's CF brim is lower and the two phase flow effect was easier to see. The Weber manual's comment for the F7 says it enrichens the low rpm range slightly. I see why now. There is a small volume of fuel I'm guessing about 0.5cc worth which resides in the undercut area above the CF area and when the throttle is blipped it gets sucked in immediately.
-Keith
Another neat thing I noticed was when the ET was inserted in the well there was a large air bubble in the area undercut for the main jet radial porting through the ET. It would stay there even with the engine running. Repeated attempts to dislodge by tapping the well failed. It caused no apparent problem as far as the cylinder getting the proper mixture.
The test well was designed with a functional well to accept the ET stack and a same sized dumby one to monitor the fuel level accurately. Noticed in the dumby well the fuel was frothing at the top so there was a layer of the foam about 3mm thick. This was not happening in the ET filled well. Since the test fixture was fastened directly onto the Weber carbie presumably the fuel in the float bowl is frothing too. Not really sure if this is such a bad condition or not. It could be.
Only had a few minutes to play with it and won't have anytime for another month or longer before doing so more testing. Could use any ideas on what to test next.
Oops, almost forgot to add that I did try both the F11 and F7 ETs. Both exhibited about the same behavoir. Just that the F7 one's CF brim is lower and the two phase flow effect was easier to see. The Weber manual's comment for the F7 says it enrichens the low rpm range slightly. I see why now. There is a small volume of fuel I'm guessing about 0.5cc worth which resides in the undercut area above the CF area and when the throttle is blipped it gets sucked in immediately.
-Keith