Hype on prepping the carb piston/slide/cylinder

To sand or not to sand? To polish or not to polish? these are just some of the questions that may arise upon the reconstruction of the finely tuned carburator. What is this "clunk" myth all about? What is this carburator piston/slide? What does it do? What elements attack while this part is at work. Some say its coated with plastic, maybe teflon? What does it need protectsion from? Some say annodized some say brass coating. When the carbs are completely together should the slide clunk as you lift and drop? Thank you gentlemen.

 

IMHO the cylinder should be polished with very fine SP like 2000. The piston should drop in and fall to the botom of the cylinder swiftly and cleanly with no sign of hesitation or jitter or stutter...

The piston does have some sort of coating on it -- just clean it with something like alcohol or soap and water -- I have soaked the whole assembly (piston and diaphrams) in warm soap and water and lightly remove unintended grime coating.

I assume the piston has a coating to protect it from gasoline... The piston will gain a "smile" on it on the airbox side -- don't try to scrub it off with something, like, say -- steel wool.

Don't scrub the piston too harshly with anything like, say, steel wool -- just be careful with the piston's coating what ever it is.

Under normal use and over time, continuous gasoline evaporation can leave a residue or thin film on the piston that can inhibit unencumbered rise and fall as required for both quick fuel supply and fuel shut off (engine braking).

Then assmble and test with diaphram in position and spring under hood -- again piston should shut briskly, smoothly without "jitter" or significant hesitation -- if it does any of these repeat process above.

(OR if you are convinced all is clean and you still have jitter or hesitation or travel shut is at VERY different speeds across carbs, then a jet may be clogged.)

 

The concept behind the clunk test, is to make sure that the slide doesn't have to overcome large amounts for friction with responding to changes pressure in the venturi that result from a change in air mass flow, that occurs when the butterflys are opened/closed. These slides are what makes the carbs constant velocity (CV). The operation is well known, and so are the effects, at least qualitatively. If your slides don't go cluck, your throttle response will lag behind the "request" from your throttle by well more than it's designed to, and the response will be different between different carbs.

 

this is what it should do with the carb together, with the top off of the carb it should fall freely.
the "clunk" is no myth. that piston controls the gas going into the engine. what attacks it and why does it need protection? it constantly moves while your riding and it has to move freely.
no it doesn't "clunk" when the carbs are together, it is a sealed cylinder with one small hole that lets it slide smoothly up or down as air escapes or enters
no confusion here, or lack of knowledge, read and learn my friend

 

Just because you don't understand simple things, doesn't make it a myth.

 

well Polock, I'm glad you showed us that video. I've always had a mystical understanding of how the carb should operate under normal conditions (i.e. when it is a "sealed" cylinder) and currently I am working on the 550 carbs. upon reassembly of my 550 carbs, they make that "whoosh" sound depicted in your video link,, all across the board #1 through #4. For the life of me I began doubting myself that the pistons and slide bore were not completely shined. but also thought to myself, these slide bores cannot get any shinier.

So here's what I gather now. Without the carb hats, they all should DROP. With the carb hat properly installed completely sealing the black diaphragm, the brass pistons should all still move freely without any slight hesitation or stumbling and with a swooshing or suction of air sound. The clunk test refers to the brass piston DROP sound then.

I'm satisfied. thanks to the good example Polock. and anyone correct me if I have understood this incorrectly now.

I believe Doug was getting confused because in the church of clean, as well as Rick's carb clean overhaul, it states :

However, in his previous thread people were making it sound like they don't do anything to the brass piston...that's where I could see confusion(s) arise...

 

There's another reason for confusion to arise:

HITACHIS AND MIKUNIS ARE DIFFERENT!!!

This is like the old fable about the blind men and the elephant; One grabs a leg and declares it to be like a tree; one grabs the trunk and thinks it's an arm, like a human, and the one that grabs the tail thinks the elephant resembles a snake. Or something like that.

Like I said, all of my "intimate" XJ carb experience so far has pretty much been limited to the Mikunis but I haven't had that many Hitachis apart. I have had enough to know that the slides are different. I believe Polock is right, the Hitachis seem to have been anodized. The Mikunis definitely have some sort of industrially-applied "plastic-ish" coating on them.

The BODIES are made of different alloys, which corrode differently and respond to corrosion abatement efforts differently.

Yes, your slides should go "clunk." With the hats off. Fully assembled, they go more like "wwhhhhhhhunk." As Mr. P so eloquently illustrated for us.

ACHIEVING said "smooth and free return fall" of your slides is a matter of the condition of the slides, the carb bodies, and your judgement.

The bores need to be clean, corrosion free and if pitted or even "dulled" by corrosion, they should be gently polished.

Slides need to be clean; I use Berryman's and I've never needed to polish a Mikuni slide; I've never found any that were corroded. (They have some sort of plastic-ish coating on them...)

Personally, I don't use sandpaper anywhere in carburetors; it's used for bodywork and sanding sidecovers. Otherwise I'm a Scotchbrite kinda guy. It doesn't leave gouges like sandpaper can (no matter how fine.) I also don't use drill bits or guitar string; just carb cleaner and bristles plucked from a parts cleaning brush. I avoid using anything "harder" than the part I'm trying to clean, and brass is pretty damn soft (as is the alloy the carbs are cast from.) It's way too easy to damage a jet or passage with too aggressive of a tool.

Whether you're working on Mikunis or Hitachis, you have to be careful and pay close attention to what you're doing. And don't sand carb parts unless they genuinely need it.

But keep in mind, there are differences between Mikunis and Hitachis; not every bit of advice that applies to one applies to the other. The principles are the same; the execution different.

 

Fitz, you should really try guitar strings. it's worth it just to see the look the guy at the music store gives you when you tell him what their for.
the ones i got are marked in thousandths so you can figure out what ones you need

 

here is one with the slides while it's running, they move a lot and you don't want them to get stuck

 

Nice video Polock. Bet he was running lean whith out the air box and boots though.

MN

 

The CLUNK Test is:

Piston in Bore with Needle Tip in Nozzle ... Diaphragm Unseated, No Spring.
Raise Piston ---> Let go.
It should DROP.
Boom.
Or, ... as I wrote it "Clunk"

With the Spring, Diaphragm Seated and Hat Screwed-down, ... the Diaphragm needs a Push --> Up
And, "Breathes" down as AIR entering the small hole regulates the decent.

I polish the Bores on BOTH Mikuni's and Hi-tots.

Now, I use 19mm Discs of ScotchBrite affixed too a DREMEL Mandril.
Once the 19mm Discs are squeezed on and tightened, ... I "Shape the 7 Discs into a Ball.

Afrer Scrubbing the Bores with ScotchBrite, ... I BUFF them with Jewelers Rouge's Rouge No.- 5 and 6.

A "Finished Bore is shown in this Photograph.
Bore following Polishing with Rouge's

 

I probably do more than 200 carburetors a year - Mikuni, Keihin and an occasional Hitachi. I have yet to run into a bore or slide that couldn't be cleaned up with carb cleaner and a rag (try to keep the carb cleaner off the diaphragm).

Attacking with anything abrasive should be your last resort, and even then sleep on it first.

 

If you Polish the Bores ...

The Bike RUNS better!

I've had Carbs arrive, here, with the Diaphragm Pistons frozen in place.
Needle Valves stuck in the Main Nozzle.

Way beyond just needing being wiped-off with a rag.
Sometimes the job requires using Dental Instruments just to remove the ravages of time and neglect.


The way I do 'em; EVERYTHING gets Cleaned and Polished.

 

That video helps alot! And I am greatful for all the feedback so conveniently accessable. This clears alot of things up on the functional operation of the piston/slide/cylinder. I now know how much action that piston/slide/cylinder gets, little friction compared to the sewing machine action that I could have been trying to imagine.