Lotus Elan

40dcoe & Two Phase Flow Part 3

PostPost by: type26owner » 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
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PostPost by: type26owner » Sun Jun 27, 2004 5:25 pm

Bingo. At WOT in third pulling up the incline the fuel is frothing enough so the floats are sinking. They sink because the density of the liquid gasoline is reduced by the foaming action and the 26g floats won't float at the same level in a less dense foam as in a liquid. This momentary condition overfills the float bowl and the wells and POURS the fuel down the passageway leading to the auxiliary venturis. As soon as the vibration frequency and/or magnitude is changed (I lift off the happy pedal) the the foaming will be reduce enough so the floats regain the preset level and the flooding stops. The stock fuel level is highly suspect based on my new appreciation on how the emulsion tube operates. Need to consider how to better shock mount the carbies also. Possibly the o-rings I've been installing are too hard. The o-rings are new as of about 3 months ago. Progress! :D
-Keith
p.s. Must be some voodoo going on here or the Bermuda Triangle is wandering off. :)
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PostPost by: type26owner » Tue Jun 29, 2004 2:59 pm

Realized diagnosing the true source of this errant vibration is not trivial. The problem might actually be the stiffness of the two motor and tranny compliant mounts. I'm pretty certain it's not an out of balance issue or the engine leaning over enough to make solid contact with the chassis under high torque loading. Has anyone experienced a similiar flooding effect at WOT with DCOEs on your racecar with solid engine mounts? Our Lotus 41 Formula B car has TJ fuel injection so I can't compare any similiar vibration conditions.

The Weber Manual shows how to brace the DCOEs to stop this problem. Must not be all that rare to have this feature. Only problem is the flimsy steel sheetmetal rear stock airbox cover which bolts up to the carbies will have to be reinforced cause I'll have to attach to it or I'm sure it will soon crack into pieces. :(

Oh hey, I just realized maybe I can fix the problem by retuning the dampened mass. That is I can just add maybe a kilogram onto the carbies and change the natural frequency. That's what the mass of the airbox is already doing. Have to ponder the possible problem issues this could cause some more though.

Of course the brace or weigh solution only works if the carbie vibration is in a vertical direction. Hard to solve something just by guessing alone!

-Keith
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PostPost by: rgh0 » Wed Jun 30, 2004 8:15 am

Keith

Have you tried holding the car at the same revs as the frothing occurs in other gears on the flat to see if the problems still happens.

If it does not happen at the same rpm when the throttle in not wide open then the problem is probably torque related and due to something locking up or touching rather related rather than vibration freqency resonance itself.

Rohan
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PostPost by: type26owner » Wed Jun 30, 2004 3:03 pm

Rohan,
Had the reaction force of the compliant mounts on a 62 Corvette be too low and the exhaust system would contact the chassis where it snakes it's way right through. That made a ton of noise and the vibration could easily be felt thought the fiberglass floorpan and the gearshifter. Not getting any feedback like that from the Elan so I doubt that is the problem.

However, another possible contributing factor is the freeway lane where this keeps happening has extremely rough pavement for an Elan. I only use it to escape from huge vehicles like SUVs which don't respect me cause they've closed the gap and are allover my tailfeathers. Could have the misfortune of the bumps exciting the natural frequency of the Weber compliant mounts. <_<
Regards,
-Keith
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