DO NOT SAND the brass CARB piston/slide OR ELSE...

This supposedly is coated with some sort of Teflon plastic which when removed may or may not pose a threat.

 

It's supposed to be bare brass. Sure that coating isn't something PO left behind?

 

Some people get obsessed with trying to remove the "Smile" that exposure to microscopic particulate leaves on the Intake Side of the Diaphragm Piston.
I even tried to Polish them out once.
No dice.

Brasso didn't even brighten the Piston a little bit.

If you scratched-up the Piston; refinish them with 1000+ Finishing Papers until you can Polish them with Compound and Rouges.

Put the effort into applying a Mirror Finish to the Carb's Piston Bores.

 

Some of the carb pistons we have run into are plated with something. Not sure exactly what it is. In the case of another brand bike the pistil was polished and once the coating was removed it corroded up quickly ( one riding season ) the owner had to find another carb from a bone yard to get another piston. We never attempt to polish pistons just clean them up as best as possible.
Polishing the carb bore where they go is very important as Rick shows how to do above.

 

It's also important to remember that there are differences between Hitachis and Mikunis.

I haven't had the misfortune of having too many Hitachis apart; but speaking to Mikunis, the slides are definitely coated with something that shouldn't be sanded.

 

or ELSE what?

just posting to ask the question and see where the discussion goes. not posting to ask, as a complete arrogant dip$h!t, so don't think stupidity of me. i'm curious about this since I have sanded my hitachi's all the way up to 1200~1500 grit followed by polishing with mothers.

3000 miles later this riding season I still have a persistent, annoying idle that tends to creep with these carbs (and by creep I mean slowly rise) but I've been so accustomed to it that normally I can control it just by down shifting as I pull to a stop. wondering if anything has to do with the pistons/slide bore so I thought I'd just ask..

or ELSE what?

 

I have sanded my Hitachi's slides with no ill effect to them at all. The creep has more to do with how clean your carbs are in general. Now I had to remove and thoroughly clean my carbs three times then sync, vac sync, colortune and re-sync a number of times to get ride of any creeping. These carbs need to be spotless and setup right to function properly. And that takes a lot of work.

 

Don't know about mikunis but hitachis are anodized aluminum, if you sand enough there's silver under there.
The color is a stain imbedded in the anodize, don't try to get it off

 

If the Piston Binds after the Bores are Poloshed.

Number the Bottoms with a Magic Marker.

Then, ... just EXPERIMENT finding an arrangement of the Pistons that let then all Pass the Clunk Test.

If you have one that Binds in all four, ... Use a WHITE China Marker and draw a line on the Piston from top to Bottom.
Seat the Piston in the Bore.
Turn the Rubber Locating Tab a Full-turn and extract the Piston.

The Places where the Binding is happening will have Tell-tale White streaks where the Bore needs to be scrubbed or the Piston is contacting the Bore too tight.

 

you can wipe the piston a few times around with scotch-brite or 1500 wet/dry paper, anodize is tough but if you get serious you can hurt it.
make sure there's no grit on it when you put it back in

 

This was a makuni issue as pointed out thanks to Fitz on my bike which is the same 83 SECA 550RK. I was having a time with reinstall of carbs with RPMs gone wild. I had not done a through breakdown and did not remove the throttle seals. Several days in chemdip may very well have comprimised the throttle control area with seals involved.

 

If you're talking about the butterfly shaft seals, yeah, that's definitely a bad thing. If those seals were in a dip for days, then those are probably pretty well hosed, and will likely need replaced.

Metal ONLY into a carb dip.