2026-05-07 Check for open Carb Passages using Compressed Air

Wonder if this technique is valid or El Toro Poo-Poo?

Was hoping for some comments?

Anyone?

I try to post and keep this forum somewhat active, but feel like the guy in the forest trying to determine if a tree falls and no one hears it?

d

".......feel like the guy in the forest trying to determine if a tree falls and no one hears it?"

You're preaching to the choir, Don !     The main reason my posts are limited and scarce.

As far as your "test'.  What ever makes you comfortable with the results.

I don’t know.  Which passages in particular?
I guess the method might reveal a complete blockage, but not be a good indication of a partial blockage.  Or am I talking c***?

I always find your posts interesting and enjoyable, Don.

My pet hate is people who ask questions on the forum, receive a reply or comments, but then they just disappear - or don’t even bother to say thanks.

Round and round and round we go  -  it's tiring, so I'll spend an hour of my time to try and put this into terms to explain why it's imperative to do the cleaning / testing properly if you are to have any success.

The Mikuni BSW32 CV carbs are a nightmare due to the complicated Idle Circuit. Too many things going on in this circuit even for a professional carb rebuilder, of which I consider myself to be one after working on carbs for over 4000 hours as an auto mechanic. Carters, Solex, Holley, Mikuni brands and numerous models within each brand. The fundementals are simple but the engineering design of each can be complex.

Scotch has spent 100's of hours working on, understanding, developing tooling, and developing a process to clean out these  BSW32 CV over complicated carbs and has a tried and tested method to clear out and flow test the Idle Circuit. It's the only way to clear and test the circuit and the process must be followed to the letter otherwise you'll bugger it all up.

3 areas of constriction are the Pilot jets, the transition ports and the Idle Air Mixture Jet..
The Pilot Jets are a #42.5 which is a  .425 mm (.017") fuel jet which can be removed
The Transition ports are .018" in diameter and are milled into the carburetor throat so not removable
The Air Mixture Jet I haven't measured but it would appear to be on or about the same size as the Pilot Jets . Let's just call them tiny, and they are pressed into the carb body, so not removable.

The problem with the Idle Circuit in these carbs is the fact that the carbs being relatively small, the jets are small and any debris that enters the carb via fuel or air and finds its way into the Idle circuit must be smaller than .018" in diameter If it's to pass through the Idle Circuit or it will get lodged in the Idle circuit and cause havoc.

The other issue that crops up is due to gas in the carbs sitting long enough to allow the gas to breakdown and turn into Shellac and block the passages. Lack of regular riding and allowing the fuel in the carbs to evaporate and or breakdown.

Scotch's process addresses both the debris blockages and the possible Shellac buildup by flushing the circuit from the areas of restriction in reverse flow and flushing out the contaminants through the larger passages using a Lacquer Thinner solvent to dissolve Shellac and push out any debris with a low pressure flow process.

Blowing compressed air through the passages without following a process where reverse flow is maintained and the required removal of the Pilot Jets, can cause any debris which might be floating around in the Idle Passages to be force lodged in the transition ports  and or the Pilot Air jets and may not be clearable once its lodged.

If you choose to work on these carbs, don't go at it arbitrarily. Follow the process or pay the consequences. Also, when working on the fuel supply to the carbs bear in mind that any debris bigger than .018" can be a problem if it manages to get into the carbs. "Cleanliness is next to Godliness"

I'm done.
KB

KB, Thanks for the thoughtful post and for your time.

I'm about to make another post about something unusual I'm seeing, carb wise, on the A3.

Fully agree with using the 'procedure' which I have done now on 6 or 7 racks..

I have used the 'Scotch' tool on all these racks using lacquer thinner, syringes, compressed air. 
But all this will be included in the next post.

I stumbled upon the topic in this post accidentally. There was just a bit of dust on the A3's lower slide area, so I sprayed with O'Reilly's "BrakeKleen" then used 40 psig of compressed air to fully dry the slides.
O'Reillys cleaner doesn't evaporate as quickly as CRC BrakeKleen or Suspension Cleaner.

That's when I noticed the pooosh of gas through the backside ports which made think this process might be a go no-go test to determine if the idle circuit was clogged or not.

Again, Thanks for Replying,

Don