Switching off the petcock(s) can be used as a diagnostic tool. At cruise speeds, with fuel shut off, fuel demand will eat 1mm of fuel level in about 4 seconds.

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...
 
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Ah, so 1mm in 4 secs = 92 secs for 23mm plus what's in the lines.....oh boy think I'll put my drilled out grease zerks and clear tubing on.
 
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.

Right 4;13 PM.jpg


Left 4;10 PM.jpg


Got sidetracked/distracted for a bit.....good thing or I never would have waited long enough for this.

Approx. 4:35 PM. Huh?

Right 4;35 PM.jpg


Huh?

Left 4;35 PM.jpg



Approx. 4:50 PM Still huh? I guess the crossover tube had allowed excess fuel to the right carb.

Right 4;51 PM.jpg


Huh, HUH, HUH?

Left 4;50 PM.jpg



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.

IMG_20170829_114548.jpg


Propped up ruler and shadetree mechanic popsicle sticks to hold floats pretty close to 23mm.
Right side, all good. Left side, drip, drip drip.

IMG_20170829_124211.jpg


You can see the constant drip on the right (in pic) float pin tower.

IMG_20170829_130941.jpg


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:

IMG_20170830_211428.jpg


IMG_20170830_211329.jpg


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.
 
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Thank you Daniel. It was an interesting exercise for sure. I hope we're good here and am looking forward to getting my other set done and putting them on on for comparative purposes.
 
Nice bit of testing and documentation, Robin.
I have a hunch that your work will be directly applicable to similar issues I'm having.
Just have to carve out the time to get to it.
 
Thanks Dude. Ya, never enough time is there?

Good luck with your investigations and keep us posted. 'Inquiring minds want to know'
 
I think you are wise to use the real Mikuni parts.
Something to keep in mind when testing carb float levels. Float valves are a flow control valve, and are not an isolation valve. Whenever the engine is not running, the petcock should be closed, because its normal for the float valve to pass a small amount of fuel. Any small amount of leakage is consumed when the engine is running.

A quick static test of the floats/float valve is fine, but to leave the test ongoing over time, with the petcock open, is not a valid test of the float valve's ability to control the bowl's level.
 
There was definitely something wrong with that one float needle and seat assembly though. They should seal better than that even though I do agree, they're not meant to be a total shut-off. You mentioned genuine floats, I feel the same way about float needle assemblies. In fact, my dealer even told me he quit using aftermarket ones because too many bikes came back to him leaking.
 
Thanks guys. I always shut the petcocks off whenever the bike isn't running and am sure glad that I do. The left fuel level actually rose high enough to start to drip out the intake bell into the air box. Glad I had some new Mikuni float valves and seats in the parts box.

I have since read that gggGary has had a problem with Mikes floats as well.
 
My buddy tried the plastic ones for his BS34s from Mike's. They were junk too. A genuine plastic float has a sleeve inside the pivot area so it fits quite precisely on the pivot pin. The aftermarket float didn't have that. It was a very loose, sloppy fit on the pivot pin. That made it's metering very imprecise and it would often hang up. His carbs would randomly belch fuel all the time with those junk floats.
 
Robin this was the first chance I had to read your write up on testing your float level. Really well done! Very clear and informative, and your photos are excellent. When you were describing your idle drop when you come to a stop, I really didn't think there would be anything to it. I was chalking it up to being a 40 year old motorcycle and that was one of those charming quirks ( sort of like the way I can't ever get my digital dashboard to display what I really want in my Italian car, but I digress).
When you did that clear tube check to see you float bowl levels. That's where I would've buckled her up and said good enough. Your methodical approach and thourough testing won the day! Bravo!
What a great site this is! And we are so lucky to have all the resident pros that we have. Keep up the good work!
 
VERY nice job on that docs holder Robin. It seems like such a simple thing but your solution is simple and effective and so much more robust that the old plastic flappy holder thingy which I feel is a bit...fragile....for a 40+ year old motorcycle, IMO.

Pete
 
Thanks Bob. Yes I got lucky by hanging in and not rushing. If I had I never would have seen the fuel level rise so drastically and would still be chasing this one.

And we are so lucky to have all the resident pros that we have.

Yes we sure are. As I have said elsewhere, we have the manuals and we use them but we would never have come this far, this fast, without these guys and this place!
 
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