Oldfeller--FSO
Serious Thumper ModSquad
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Hobby is now "concentrated neuropany"
Posts: 12673
Fayetteville, NC
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Running at highway speed, your vac petcock fuel delivery rate goes lower because the intake tract vacuum level goes down a bit on our engines at full throttle -- so a little less gas (relatively) is flowing into the bowl. The bowl level starts to go down a bit, so the fuel height pressure to the main and pilot jet goes down a little bit as well.
So, a little less gasoline hits the same air volume going past the butterfly in the venturi so your engine power goes down a little bit. So, you crank a little more butterfly open to try to keep up the same speed. So, your fuel delivery is down, your float level is decreasing and you are turning open the butterfly in the venturi more to get a little bit more "go" out of the engine. It is a feedback cycle, resulting in lean conditions in the combustion process in the cylinder.
This increasing "lean out feeback cycle" continues until you can't get enough gasoline to keep your speed up at all, and you feel this and call it "an occasional mild stumble". You may think you need to get some gas and you might stop and get some and this may actually help as more gas in the tank means more vertical pressure at the petcock and a little more gasoline will flow at a given vacuum level.
But, say you are on the way to work and you jest keep on getting down the road.
Lean combustion conditions raises the piston crown temps, higher crown temps expand the piston crown a wee bit more than the cold measured .005-.0055" ring area running clearance can accommodate and the hotter than normal piston ring area begins to run into a mild interference condition with the cylinder wall.
Now, this is a mild case of early chronic lean out -- you won't melt a hole in the top of the piston for a little while yet nor will you pop a valve just yet and your ring grooves widths aren't going to shrink or gall just yet.
Now, how good is your oil? Maybe you won't seize, perhaps you will just get some intermittent friction piston braking when the oil film starts to break down jest a little tiny bit. Heck, feels just like another little stumble to me, crank the biatch open some more and let's go -- I gotta get to work.
You keep on running lean conically because your vac petcock isn't keeping your bowl full and eventually mild galling starts on your piston, generally right up on the front side of the wrist pin support running up towards the crown up past the ring grooves. The rings begin to get pinched or impeded by the displaced metal, and low and behold you start using more oil. You don't notice it for a few days or weeks as you are used to your bike not using any oil, so when you do notice it, to you well you just suddenly started using oil --- "sudden onset oil consumption".
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I can tell you exactly when mine happened -- I was on the new 285 bypass around Charlotte getting ready to take the I-85 exit to go west on I-85 to hit 321 to go up to Toymaker's -- I was winging it at 85 mph keeping up with beltway traffic and getting a mild stumble so I thought I was needing some gas, I flipped to reserve and kept on getting it as I knew eventually putting it in reserve was going to get me some gas. I stumbled/mini seized my way down to 65 mph before I hit the exit, then I rode (stumbling) until I got to the next exit with a gas station.
I could not put 2 gallons of gas in the tank, but I didn't know what that meant back then. Nobody had ID'd the vac petcock as a major pain in the butt back at that point in time, but lots of us sure had odd carb issues we chased with jets and carb tear downs, oh lordy yes ....
But I noticed I needed to top up my oil at Toymakers.
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