Mikuni float bowl passages

[mikestp]

I have found that with some Mikuni float bowls the part throttle passages are half way blocked. They might actually pass some fuel even after you shoot them with carb cleaner. That small passage in the float bowl that supplies fuel to the part throttle port just past the idle position of the throttle plate, work from that point till about 2500 rpm. After that time, both the idle port and the part throttle ports are no longer measurably effective because at 2500 rpm your main jets come into play. What I usually do now on all my Mikuni rebuilds, is get a 1/64th size twist dril, set the depth to not drill through the bottom, and drill out the float bowl passage. In most cases I can feel the drill "thunking" into the lower chamber as it drills through the blockage. No metal shards have ever been seen, so I know I am drilling petrified gasoline. After reassembly the carb has taken on a whole new personality where throttle response is quick and tight with no apparent flat spots. At lease not while on the centre stand. At that point I can take the bike out and test for main jet adjustments.

I have never seen a message regarding the drilling of thses ports in any forums. But after reading sooo much stuff in these forums with people searching for reasons the carbs are not working..... I have found just about 100% success rates after I drill out the passages.

I hope this note might help a few of you...

Mike...

mikestp@yahoo.com

 

[Steve]

Can you please post a picture of this "float bowl passage"? :-k

There are several "passages" in the float bowl area, just wondering which one you are drilling.

.

 

[chuck hahn]

No way in hellll would you find me drilling on any carb passages. Thats what carb dip and proper carb cleaning is all about.

 

[mikestp]

No way in hellll would you find me drilling on any carb passages. Thats what carb dip and proper carb cleaning is all about.

First off... I am speaking of the float bowl not the actual carb body.

Secondly you are assuming I have not tried to clean with Carb Cleaner (Berrymans if you wish to know),or Gun Wash, Ultrasonic, wire probes.... To no avail. As I told you in the previous message, I have done this now many many times and each time I look for any signs of metal to show me I have possibly drilled into casting. And no times have I ever seen any. Do what you wish, but I am not a newbie. 40 year veteran on aircraft maint. and semi retired now. But you can continue to try and try and try to resolve off idle problems...

Good luck with that....

 

[mikestp]

Can you please post a picture of this "float bowl passage"? :-k

There are several "passages" in the float bowl area, just wondering which one you are drilling.

.

There is only one passage in the float bowl on 1985-1999 Mikunis that I am aware of. A fuel pickup tube fits into it. That tube goes to the part throttle ports of the carb. I have not seen any other passage in a float bowl.

 

[chuck hahn]

Thats the starter circuit ( choke) tube. And i was assuming nothing, but based on your description one can never tell what a newbie is talking about. And if you soak the bowls also, all you need to do is flush the hole the pick up tube goes in with air and carb spray and use a thin wire to poke the hole at the bottom open anyway. Sounds like your over complicating trying to reinvent the wheel to me,,,and its an unneeded thing.

And if you wanna be real anal about the hole, a simple Qtip stick cleans it out just fine also.....and doesnt do any potential damage like a drill bit can.

 

[Nessism]

Yup, it's a choke circuit pickup passage. The idle circuit is fed by the pilot system. Fuel comes in the main jet and then branches over to pilot side. The factory service manual for the GS850 has a nice section detailing the various carb circuits, including diagrams showing how the passages connect inside the carb body.

 

[btbarb]

Yup, it's a choke circuit pickup passage. The idle circuit is fed by the pilot system. Fuel comes in the main jet and then branches over to pilot side. The factory service manual for the GS850 has a nice section detailing the various carb circuits, including diagrams showing how the passages connect inside the carb body.

So would drilling the passage in question be the equivalent of having your choke slightly engaged?

 

[chuck hahn]

no. The only way the choke can be engaged is if the plungers are manually lifted or the rubber ends are cracked and hard and letting fuel seep past. Dont friggin drill the hole out!!!! Useless and totally irresponsible drivle. Soak the bowls, use a Qtip stick to scrub the holes sides, poke the bottom hole from the bowl side, then flush with spray and air. The bottom of the bowl isnt real thick and if you slip your fuucked!!!

 

[londonboards]

I asked the same question in my thread: http://www.thegsresources.com/_forum/showthread.php?p=2013506#poststop

and in my blog: Carb Float Bowl Question

Mine were completely blocked and I wasn't sure if they were supposed to be:

Nessism came to my rescue telling me to hammer away and make them work.

Greetings

 

[chuck hahn]

Fuel comes into the well via the hole at the bottom of the hole the pick up tube sticks into,,then its sucked up into the carb body and thru the starter circuit via the choke plungers. T

 

[btbarb]

So would drilling the passage in question be the equivalent of having your choke slightly engaged?

I re-read and see mikestp was not changing the hole just clearing it.

 

[chuck hahn]

yes...but its kinda wrong to suggest that the gunk needs to be drilled out when it in fact doesnt!! thats my entire point and I have a problem with what i consider hack advice. Simple dipping and rinsing WILL clear the tubes of any buildup.

 

[Fjbj40]

You know what? What he is saying is to use a drill bit and twist it by your fingers, this will not remove any metal unless you are not thinking. It is tried and true simple practice used in many applications. Sometimes that hole is plugged solid, no Q-tip will remove the material no matter how hard you try, grab an UNDERSIZED drill bit and voila, cleared in about 5 seconds. Then you can use the Q-tip to remove the rest because you made an opening in that rock hard ****.

His advise is a valid bit of info. But as always it is your choice to take someones tried and proved ways of doing a job differantly than what you do.

It is not hack advise, that is your opinion!

 

[Agemax]

You know what? What he is saying is to use a drill bit and twist it by your fingers, this will not remove any metal unless you are not thinking. It is tried and true simple practice used in many applications. Sometimes that hole is plugged solid, no Q-tip will remove the material no matter how hard you try, grab an UNDERSIZED drill bit and voila, cleared in about 5 seconds. Then you can use the Q-tip to remove the rest because you made an opening in that rock hard ****.

His advise is a valid bit of info. But as always it is your choice to take someones tried and proved ways of doing a job differantly than what you do.

It is not hack advise, that is your opinion!

Totally agree. i have a set of micro drill bits, 0.3mm up to 1.6mm and quite often use them in a small precision hand held "chuck" to clean crud from jets and tiny orifices! Done carefully it clears them quickly with no harm, and as Fjbj40 says, it then makes cleaning and flushing the hole properly a much quicker and easier chore.

i cant see how you manage to fit a Q tip in the tiny hole in the bottom of the choke pick up tube.........

 

[chuck hahn]

You dont fit a Qtip into the small hole at the bottom and I never said so Agemax!!! You clean the big main hole with the Qtip..STICK part ( not the cotton end on because it wont fit ). And you use a thin wire to poke the tiny hole at the bottom. I have a wooden handle wire brushn that then wires are plucked from for poking holes and the jets. They are real tiny and perfectly straight..great poking wires.

And yes, it is my OPINION that its hack advice. If the hole is that plugged just how would you remove the bowl..seems it would be sealed around the pick up tube also..right?? If the bowl is off then the layer is minimal and the dip will VERY VERY EASILY dissolve it.

 

[Guest]

I agree about the proper use of jet drills, but his description of setting the depth and "thunking" indicates using a power tool. I had been warned their use was strictly by hand and to be careful at that.

 

[tkent02]

Totally agree. i have a set of micro drill bits, 0.3mm up to 1.6mm and quite often use them in a small precision hand held "chuck" to clean crud from jets and tiny orifices! Done carefully it clears them quickly with no harm,

A pin vise is what it's called here. Useful tool. Don't need them on carbs much but I suppose if it was blocked up hard enough?.. But if the easy to reach choke feed hole is that bad, just think what the other passages where a drill bit won't reach are like. It's going to need to soak in the dip or be ultrasounded for quite a while.

The OP never mentioned turning the bit by hand, he sounded like he was using the power tool.

 

[mikestp]

A pin vise is what it's called here. Useful tool. Don't need them on carbs much but I suppose if it was blocked up hard enough?.. But if the easy to reach choke feed hole is that bad, just think what the other passages where a drill bit won't reach are like. It's going to need to soak in the dip or be ultrasounded for quite a while.

The OP never mentioned turning the bit by hand, he sounded like he was using the power tool.

I have used a small power drill to clear the more petrified crap out. The passage is not a jet. It is not acting as a meter. It is simply a hole drilled to allow gasoline from the bottom of the float bowl to get to the brass pickup. That brass pickup IS metered. I suggest cleaning it but the holes in any of the brass parts of a carb might be metering holes and to drill those out would likely damage the jet.... unless you use small bore "jet drills".

For the passage in question a 1/64th size twist drill is smaller that the diameter of the passage at the bottom where it meets a cross drilled passage at a 45 degree angle. I also recommend using the same twist drill from the bottom to ensure it meets the vertical passage.

I most often use a twist drill with my fingers to bore out the crud. But when I have really hard crystallized concrete in that passage, a power attachment at low speed works when even poking, prodding, and cursing wont work. Make sure the drill is in the chuck to the right depth so as to not drill through the float bowl (although if that is what you do, then I think fixing bikes is not for you.) Some of these float bowls out there are soooo badly crudded up soaking in Berrymans for a week did nothing.

I have a lovely ultrasonic tank (2 gallons) that I bought from a US Government lab for 50 bucks... Even it does not get that tight passage cleared. I have used gun wash as a carb cleaner, with generally good result, but it wont ungunk that crud. I have always looked for shards of metal in the debris that is drilled out and I have never seen any. Be careful, drill slowly, measure the length from your chuck tip if you use a power attachment, and go slow.

In general THINK before you do any task such as this. I would have thought that did not need stating, but I will refrain from giving advise if this is what the results are. Silly people squabbling over their own misunderstanding.

I will continue to repair Mikuni carb float bowls in this manner. In my opinion it is the only method that gives me great results. I see so many bikes that stall on a quick twist of the throttle and this passage is the likely culprit.

All the bikes I have done have miraculously lost the temperamental part throttle response, and run as new again. I assume of course that the rest of the carb is not cross threaded, the jets are clear, the carb passages are clear, and clean. The floats levels are set and the needle and seat are sealing.

If you fail to clear the passage so that it flows (not just seeping when you spray carb cleaner down the vertical passage.) Try the drill. you will be pleased with the results.

Mike...

 

[chuck hahn]

Same with all the ones I do WITHOUT a drill...apples and oranges. And the only thing that "meters" it is the tiny hole in the end of the brass tube. There are 4 bigger holes at the top right where the tube goes into the carb body..those dump the excess fuel back into the bowls. And they allow a little air to be mixed as they are sipping fuel initially.