Well now, ain't that sumthin’!
2M you rascal, you knew. And 5twins asked the right question as well.
Example scenario.
Ride out to a remote road. Switch off fuel.
Accelerate to 10-20 mph, do the full stop thing, observe the idling character, repeat.
Plenty of fuel to do this several times.
Note any trend in changing idle character.
Turn fuel back on.
Seek out BBQ...
When I read this test carbs were already off and on the O/R bench so I couldn't give it a go.
However, I think it would have gone something like this. (My thoughts prior to actually performing the test)
1st stop, idle still drops.
2nd stop, idle still drops.
3rd or 4th stop, idle maybe still drops.
About now, magically, idle drop problem is gone and idle returns to normal at each stop.
Then after a few more, bike starts stumbling and dies.
Uh oh, what now?
Oh ya, petcocks are turned off, ran out of gas.
My apologies guys for wasting all of your time and diagnostic skills. I really do appreciate all of your ideas and suggestions and you did lead me to confirm my original thoughts that this had to be a fuel level problem. Just didn't think it would go this way.
Also apologies for what's going to be a bunch of pics and words. I guess I could just say "this is what I did" but this might help someone else in the future.
Here are floats as set at 23mm. At approx. 4:10 PM.
Got sidetracked/distracted for a bit.....good thing or I never would have waited long enough for this.
Approx. 4:35 PM. Huh?
Huh?
Approx. 4:50 PM Still huh? I guess the crossover tube had allowed excess fuel to the right carb.
Huh,
HUH,
HUH?
Obviously a fuel level problem. Float or float valve?
Now I realize that this is a static test and not under operating conditions but I wonder if when running the vacuum would pull fuel into the bowls at an accelerated rate and duplicate this condition faster.
On to the bench, off with the bowls.
Tested floats, no leaks.
Float valve springs bouncing back up.
Looked at float valves, all clear. Chrome needle tips look OK. Can see a line where they have been seating but cannot feel any wear with a fingernail scrape test.
On to the test stand.
Just for fun, fuel flow looks pretty similar.
Propped up ruler and shadetree mechanic popsicle sticks to hold floats pretty close to 23mm.
Right side, all good. Left side, drip, drip drip.
You can see the constant drip on the right (in pic) float pin tower.
Held left float all the way up, drip, drip, drip.
Should have for sure totally stopped fuel flow doing that.
Huh? why didn't I replace those float valves? Still in carb parts box #1. Must be slipping in my old age. Doh
New float valves installed. Mikes/XSDirect floats back to factory specs 24mm.
Road test, idle drop at decel problem still there.
Ran 2M’s test. Interesting, went just about exactly as I thought above. (Except I didn’t let it run out of gas….lol)
A few pretty quick in neutral stops and then dropping idle problem disappears as excess fuel is burned off.
Of course I had been tempted to put in genuine Mikuni floats from another carb set when I had them apart, but then I wouldn't have known for sure whether it was a float or valve problem.
Swapped in another set of genuine Mikuni floats after checking them for air bubble leaks.
Set to factory spec height 24mm.
Checked on test stand that fuel flow would totally shut off.
Well, I think we can call this a 'qualified' success. 221kms/137 miles over the last 2 days.
My major idle drop at decel is gone. Very occasionally I will get a 100 rpm drop, but that could well be engine condition.
Conclusions:
1. I guess I still don’t know for sure if it was just the floats or if the valves were faulty as well.
Could have pulled them again and reinstalled the old valves with the new floats to answer that question but at this point, I’m taking a carb break and I’ll leave it unanswered for now. I may test those valves in the future on another carb set.
2. The, finish, feel, quality of Mikes/XSDirect replacement floats is really poor when compared to a genuine Mikuni float. Sloppy solder joints, rough surfaces, ragged edges, much thinner material. They even feel lighter and thinner.
Mikuni float weight = 0.40 OZ. 11 grams
Mikes/XSD weight = 0.35 OZ. 10 grams
Thickness of joiner plate between floats (Mikuni) = .310" (average of a few measurements)
Thickness of joiner plate between floats (MIkes/XSD) = .210" (average of a few measurements)
3.
Use ONLY GENUINE MIKUNI FLOATS.
4. Still have to swap mix screws side to side as per 5twins idea to see if the differences in mix screw settings is related to mix screw wear. Will do that soon.
5. Going to clean and rebuild the 78/79 2FO set from my other 78E project as soon as some more parts arrive, and see how they perform on this bike.
6. Ride her and enjoy her for the short couple of months we have left.
BTW: Replacement Voltmeter arrived and installed. Sure nice to have that back again.
And for the sake of comparative purposes:
Left my brass drifts in the drawer for the time bean 2M. We'll see how things go before I go all neanderthal on these.