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How to F&%#-up your idle jets 3 years 10 months ago #20816

  • scotch
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I received a set of carbs several days ago, that the owner claimed couldn't be adjusted to get a smooth idle or a sync. Went through my established routine disassembly, flushing etc. and did the bench static adjustments that have always yielded good results. Other than some debris that flushed out using "The Tool”, I had no reason to suspect any concerns once installed on my engine.
The carbs would not idle smoothly - in fact so roughly that the back-lashing made the engine sound like it was going to toss parts. Despite vacuum syncing perfectly on the bench, the vacuum on the running engine was barely 3"hg. per gauge. Back to the bench and then the bike 3 more times with no better results. EVERYTHING was CLEAN! It was easy to find the offending cylinders by blocking the idle air mixing intakes one at a time. Got it narrowed down to the right hand carb. Block both idle air ports and the engine smoothed out significantly. Running lean!
Completely mystified, I put them aside for several days while I went through all the scenarios. I sat down one evening and spent several hours "Googling" related topics to no avail. Then went to three familiar sights: Mikuni, Keihin and Dellorto. I found a common theme related to jets, from these reputable manufactures. They flow test every jet for quality control and indexing and,…… their jets have very specific high tolerance details milled into their design to assure the correct fuel-flow pattern. This was the light-bulb moment!
Despite visually inspecting the idle jets and determining they were clean I removed them and put each one under a microscope.
Long story short; If you want to render these jets totally useless go ahead and poke some piece of wire through it. Those “Jet Cleaners” ……..nothing more than Oxy/Acy tip cleaners – just as bad.
If you have to clean debris from a jet and lacquer thinner and compressed air won’t do the job buy some propane/natural gas pilot jet reamers and do it right. Or….just replace them with new jets.
The orifice is factory milled and reamed to guarantee a very specific flow pattern and should be shiny smooth.
Sticking a piece of wire into it and twisting:

The end of a piece of wire will gouge and lift material that with totally screw-up the flow pattern.
You have to look closely but the jet material has been shaved and lifted.

This is the end of one of those "Jet Cleaners" Same results as a random piece of wire cut with typical wire cutters
The end-burr will destroy the jets characteristics.

A couple of very fine Gas appliance pilot-jet reamers.

The solution: 6 new jets !
I've rebuilt 12 set in the past 3 years with no problems setting them up. Lucky, now I think. What have I learned: Holding an idle jet up to a light source doesn't guarantee the jet is good. From now on, every jet goes under the scope ! ;)
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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How to F&%#-up your idle jets 3 years 10 months ago #20817

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Nice catch Scotch. Another fine example of the "craftmanship" that goes into previous owners attention to detail when working on these bikes. Breaks my heart.

Your findings are disturbing. I would be interested in hearing how things work out after a new set of jets. I have seen jets with an overbore through 80% of the jet and only the last .025'-.030" of the bore actually "sized" to the jet size. Punching out jets with number drills was a common practice back in the old days and Mikuni frowned upon it. The only time that practice worked so to speak was when someone was changing to alcohol for fuel.



Just throwing this pic in here as an example. The entrance and exit angles of the jets are just as critical as the size of the jet. Change those angles and turbulance will cause the jets to not flow properly.
Laquer thinner should soften up any build up on the jet bores and then it's just a matter of getting the junk out with compressed air.
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How to F&%#-up your idle jets 3 years 10 months ago #20818

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That does look like someone had been grinding coffee beans there! LOL A miniature grinder, possibly a Guinness world record? I just put one of these on the tip of my finger, and it does indeed look smaller than a grain of salt. Definitely can't catch any angles unless being a Hollywood Antman, or microscopic scotch! haha Looking at the density and business of those lines, I can't shy away from kind of doubting the cut thin wire theory. Looks like that would have to be done by someone going through an epileptic seizure... I think there is more to those lines than just that.

Looking forward to updates, what happens after the new jets installed? Have a set of carbs on hands here and hoping for no such trouble when I fire the bike for the first time.
Just bought the hand saw and some 2x4's. This time I am on a mission. Not to fix the carb that is, of course, but to get this brick of a bike out of here.. by the end on next week. No matter what!!! Enough is enough. Time for the first start up!!!!

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How to F&%#-up your idle jets 3 years 10 months ago #20819

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I can't find anything more technical or detailed regarding the entrainment of air into a flow of idle fuel .
Noun: entrainment
entrainment (plural entrainments) - Any of several processes in which a solid or liquid is put into motion by a fluid.


This process is obviously critical to the correct concentration, size and dispersal of entrained air (bubbles) in the idle fuel..
With air (bubbles) now of uneven size and distribution the flow of fuel is impeded. Hence the Lean condition which could be temporally corrected by blocking the mixing-air ports. Fair to say in this case it's not a "Blockage".
A "Tip Cleaner" is not intended to be forced through an opening. A size that allows the cleaner to pass easily, allowing the spiral cutting edge to "Rub" away foreign material, is the idea.
A good spray pattern

A spray pattern being affected by a disrupted flow

Better pictures of how little it takes to make getting a decent idle, impossible.

Same jet refocused (very narrow depth of field) to another area
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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How to F&%#-up your idle jets 3 years 10 months ago #20820

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I see, makes sense scotch.

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How to F&%#-up your idle jets 3 years 10 months ago #20827

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With some better lighting I discovered these jets had more problems then just being "burred" in the orifice. Some serious erosion/corrosion that likely had as much of a detrimental affect. All contributing to flow degradation. This damage was hidden by the fuel residue and natural Patina. Damage was identical on the inlet and outlet sides.
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !

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