Topic: Idle issue - tips on cleaning carb passages

[Maltboy]

I have a problem with my CB750K5 carbs.  It's only running on the 2 and 3 cylinders at idle (all 4 carbs are synced).  I have good spark to all four cylinders, and all cylinders fire the instant the mains kick in.  All 4 Keihin idle jets are all clear and flow the same.  I disassembled the carbs and did a shade tree test of the idle jet circuit and found out the passages on 1 and 4 have flow, but not as much as the 2 and 3 carbs.  I tried soaking them in carb cleaner, heating and flushing the passages, and pressurized air.  It seemed like they were a little better, so I put them back on, but the problem persists.  I'm thinking the passages are still a little clogged, and that's restricting fuel flow enough to keep it from firing. Soooo....

What do I do next?  Do any of you have a sure-fire solution to clean stubborn passages?  I am thinking about removing the ball plugs and reaming the passages, but I don't know how to remove and reinstall the plugs without trashing the carb body.    Do any of you have experience doing this?

Sorry for the book, but I wanted to make sure I included everything.

[seanbarney41]

What did you use for carb cleaner?  What brand?

[Maltboy]

I have a 20 year old gallon can of that evil berryman carb cleaner.  It's brown smelly stuff with an oil layer on top to keep it from evaporating.  They outlawed it years ago.  I soaked the carb body in that stuff for an hour or so.  After I did that I used Gumout spray directly into the passage.  Then I heated the passages and sprayed more Gumout.  It got better but not as good as the other two carbs.

[Medyo Bastos]

Great stuff for sure! I once forgot a set of carbs separated in 4 cans of the stuff for well over a week. I was worried about what I’d find after I remembered that I left them in there! They came out remarkable! No boo#$%*! I didn’t realize it was outlawed, too bad.


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[Maltboy]

Maybe that's what I'll do.  I'm going fishing this weekend anyway.  Perfect timing.

[Mr. Mike]

I’ve used a standard torch tip cleaning set. Canned carb cleaner to wash, and canned air to blow them out. Those tiny holes on the sides of the emulsion tubes need to be cleaned thoroughly and blown out with air. Hold them up to a light to see through each hole sides and center.

[seanbarney41]

In my experience, Gunk brand carb dip buckets work significantly better than Berrymans.

[pjlogue]

A lot of these old carbs have had a multitude of insults done to them over the years by PO's.  The carb body air passages need to be mechanically cleaned with a soft (copper) wire while flushing them repeatedly.  The low speed jets are particularly troublesome.  The tiny hole in the jet can be damaged easily with a drill or wire.  They are easily clogged with dirt, gum deposits and even fuel stabilizer which can powder up if dried on.  Check each low speed jet by holding up to a light.  You should see light through it.  Ultrasonic cleaning is the safest way to clean these jets. 

You can get new jets here:  https://jetsrus.com/a_jets_by_carburetor_type/a_jet_kits_keihin_selection.htm 

-P.

[Maltboy]

Thanks for all the great tips. 

I'm going to get a pail of Gunk cleaner and soak them for a few days. 

I'm curious how to get a copper wire into the idle passages because of all the turns.  I'll definitely give it a try.

The idle jets are in good shape.  I inspected them under magnification and all the holes are clean and uniform.  I even swapped the jets between the good carbs and the bad ones and it made no difference.

That leaves me with only one answer::  Even slightly restricted idle passages can make enough difference to cause the cylinder to not fire at idle.  That seems to be be the case with my bike.

[pjlogue]

You can slowly tease a small copper wire into the passages.  Do it from both directions.  Be careful of the brass metering inserts on the carb intake side.  These are a specific diameter and you don't want to change the orifice on them. 

-P.

[BenelliSEI]

All good advice. I have a small ultra sonic cleaner that only handles one carb at a time. 45 minutes in there, followed by compressed air and they are spotless.

Having to split the rack ensures I replace the fuel tee “0” rings, so a tiny, cheap unit really is a great investment. My wife gave me a small basket she used for loose tea. I hang all the jets and small parts in it and the tank gets them spotless. I’ve had no more carb issues since I bought it. Best tool investment I ever made.

[Maltboy]

Thanks for all the advice folks.  This forum is the best.