Help I found something weird... 3LN3

conclusion has been reached.

someone here may not like it. a part may need to be re-designed. new moulds. expensive.
this has been one hell of a puzzle.
i will post various pics here, hopefully...

so. think about it.
the needle and jet should have no bearing on slow speed.
we have no control over the slide position anyway, thats vacuum. we only control the butterfly. a tiny portion of fuel will pass the needle and jet, only under abnormal conditions...what we have here.

the vacuum of the engine should be behind the butterfly, not the slide, and as soon as the butterfly opens, the slide, and needle/jet, should be lifting...

now, my test with the fuel level as it did its thing, showed its fuel level dependent.
i suspected that nut.

i long suspected the bung and seal, yet on thinking... those ports feed directly to the intake. if it was leaking, then the thing should be hydroloacking as the cylinder floods... a whole bowl of fuel should dump through those little holes. it doesnt.
And... why all four cylinders? Why not one?

Theres something common, across all four carbs.
Its not ignition (though clean plugs not covered in rust sure helped!).
its not valve timing...
its (no longer) intermittent.
when i threw some aviation sealant in on a gasket it worked great... for a week then came back....
a new seal kit didnt fix it, but made it WORSE.

threw teflon washers under nuts, and it sort of fixed it, slightly. not.


last night i thought. i thought. then i looked...

sorry maelstrom... JH-003...

its too thin. i really only see "squish marks" on the corners. every time ive pulled it down ive been a bit suss about the amount of fuel im finding in that area...

it may be a manufacturing tolerance thing with these housings. batches? I got about 0.9mm for the recess. Didnt measure your seal though...

i set up some wet n dry on the surface plate, carefully attacked the locating nub with a file to give me a lil bit more room, then ground approximately 0.2mm off the jet housing.

and this morning?

it started. it ran. it rode pretty well. best it has since this developed.

except, i feel its still doing it on one or two cylinders. my grinding was pretty inconsistent. i lost my verniers. (only here cus i cant find my fence clip pliers and im trying to put mesh up for a roof and yeah... try tearing carbs apart then suddenly remembering you have no roof as the rain starts...)

so, this teardown... either i clear off the mill and flycut that surface so theyre all about 0.5mm deep, or shove the old seals ive had soaking in xylene for two years in... nice and squishy now!

mill has to get cleared off, new rear sprocket for the R15, and i gotta modify it... sigh. ***** to find parts for.

so, im going on this theory that the housing is leaking, and thats the bit that separates everything from everything. choke, idle, main, air orifices, everythings passing through that freaking join. and this has been the one change ive made that has not only slightly improved it, or simply made it worse... its made it nearly... almost... perfect.


seriously, i recall this little thing where i could just upload images but all im getting is insert url or upload file with nothing... ffs i hate computers.

 

Stupid stinking effing bleeding...

sort of easy from the phone?

Needles. Perfect though theres a touch of corrsion at the tip. The ones in bike are better.

Needle jets. (Emulsion tubes, not). Perfect.

What i did... major improvement.

Teflon washers. Slight improvement.

The REAL emulsion tube is hiding... under main jet.

Now, some more about my original topic... the slides and inserts!
This is the amount of opening with the little bridge in insert#1 just uncovered. Also equates to idle with air box off...

With the full bypass channel thingy uncovered... this is about where the slide hits when blipping the throttle... and #2 is the same... except you can see the "bridge" here... insert #1.


And so...

This slide stuff aint got anything to do with the flood/bog... but it is a factor in fine tuning, transition, idle....

Comeon guys... maybe i should start a poll?

Have a peek inside the throats, see which carbs have these "bridges", if you are good on levels you can eye the slide cutaway to the needle jet, tell if its sloping left or right...
And how the bike runs/starts!

Maybe we can reach a consensus on the best way to install these things... majority rules?

 

Lol, whattya think i tried?
Who knows where those uploads went?
Not i!
Iunno. From Pc, its finding a usb cable, find theres no thumbnails, opening up in image viewer, cross referencing oddball names, and then hitting "open" and nothing happens!

Phone did its own thing... "surely you only want to see recents from work? What? You dont? Well im going to make you hunt for them then... i dont care that theyre all in one folder. I'M in charge around here!"

Sigh. Story of my life...

 

another ride later...
hates that 30km crawl behind morons. slowing down for corners. that 1/8th throttle below 6k...
get it in a straight line and past the cough...
weeee. this is how theyre supposed to go! i forgot how the clutch loves to slip...

so yeah. its down to one cylinder. no more WOT chugging with fuel off, just one cylinder kicking in and out, occasionally maybe two on what seems every time i back off.

got real keen grinding the first one, was getting pretty bored by the 4th... 0.1mm might make all the difference...

make a lil jig for the mill and wack them all out even, reckon its done. issue sorted.

 

Well... grinding these housings has been the only thing that has worked. And, after eliminating ALL possibilities, there can be only one.

Theres a lot of chugging fizzers that end up in junkyards or going cheap cus people throw new parts in, issues remain, no mechanic can fix them, and then life gets in the way.

Take it or leave it.

 

Back to where it should be. There is no issue anymore.
This thread wasnt about fixing the carb chug.
It was to point out a difference in slides and inserts that no-one, in 30 odd years, seems to have noticed or documented.
Some tuners may appreciate knowing about this. May appeciate knowing what mikunis original intention and reasoning was.
There seems to be no documentation outside of yamaha parts diagrams on these carbs.

I fixed the carb issue. Whether my conclusion is appreciated or not.
The question of slides remain.

 

The housing face is as cast.
The machine punching them out may have upwards of 50 dies.
They wear, they get replaced.
I believe i mentioned manufacturing tolerances. Very well could be a few thousand blocks are oversized in the most critical area. Minkuni really should have machined that face.

It seems to emenate from people pulling their carbs apart.
Unfortunately with the history of grey imports you never know whats been done previously.
Upset seals, rearrange them, alter surfaces on old rubbers, and then a new seal based on "good"tolerances rather than worst case tolerances, and finally.. the problem persists, and one starts chasing wild geese thinking theyve eliminated all possibilities.

Why would grinding the blocks, increasing the squish on those critical seal fix it, yet NOT be the problem?

Idle and main fuel. Idle and emulsion air bleeds, and the choke, all pass through that join.

Whatever. My bike runs. I said my conclusion would upset someone. It obviously has.

 

There are threads galore about this chug. I have read EVERYTHING. I have tried EVERYTHING. when i fail to mention something, consider it absent mindedness.

I did NOT sign up for advice on fixing this chug. Or to start yet another chug thread. Repeating the same things i can read on countless other threads.

I signed up to get an idea of which insert and slide belongs in which carb, something i noticed as i installed new diaphgrams.

All you have to do is pop the tank, the air filter, and push each slide up a few mm and look.
So far... noone has bothered with THIS bit of "help". That is for the benefit of ALL. Not just myself.

Instead, as i accidentally included additional information about "the dreaded chug", this thread was retitled "help" and has subsequently focussed on that topic rather than...

WHICH SLIDE AND INSERT BELONGS IN EACH CARB?

Anyway. I was wrong. Initial assumption, grinding the housings solved the issue.
For three rides. Then back as bad as ever.

So it is NOT your seals, maelstrom. My apologies.
The last possibility... proven again, wrong.

In fact... im thinking it aint the carbs at all.

or, its not the carbs, merely the setup that makes certain other issues manifest as a carb issue?
The design of the downdraft carb, requiring an anti-siphon system in the pilot circuit... thats unusual. And so is a 62cc engine with a 28mm carb. only get away with that by running the CV.

Has anyone done a manifold vac test when theyre having these symptoms?
I sure didnt. Yet i suspect it may very well be the key...




ive bought a lemon.
Heads off.
Its overbored. Probably chinesium.
Leaking intake valves despite good compression. Bubbles

Hmmmmmmmmmmmmm

Low manifold vacuum on an oversized carb of unusual design, that throws unusual symptoms?
Rechecking that fuel level as it clears up, and comparing it to the galleries in that jet block? I got alarm bells going off about how pilot circuits work...

And how they have to deal with siphoning fuel when pilot orifices are well below fuel level... if it isnt dealt with, these would hydrolock...

Instead, the symptom feels like its STARVING for fuel... exhaust smells like it too.

anyway. its a mess inside. Cry.

Sigh. Chasing geese. It went so nice at first.

Whatever!
Skim the head, throw a gasket in and...
Christ, last time i got a neway seat cutter it cost a motza... gunna need new arbor too.
Can i even get them this small?!

These exhausts are freaking gross.

Intakes, meh.

Oil is me though.

Last thought, more absent mindedness...

Last time it was riding good, before it became like this... i had done the valve clearances along with other things. And i only had it a week so no real idea of its habits...

Pretty sure rust aint good on valve seats. Forgot to measure before i popped the head off last night, oops.

Back in a fortnight i guess.