Carb leak questions

[Gilfi]

Hi all

I have a bit of a strange problem with my carbs leaking. They are Mikuni flatslides 27A 02 from a Suzuki (VM29 – GSXR750 ´85 - ´87)

 

As you can see in the pic, they seem to slant backwards. I mounted them and ran the motor (just a couple of times) and after a few seconds after stopping the motor a stream of petrol comes out of the bottom of the carb (overflow?) closest to the petrol tap (#4 carb). I'm not exactly sure from where as I only ran the bike a couple of times and when it happened I was busy scrambling for a rag to put in there.

It's also not a huge amount of fuel, I would estimate 40-80ml. It just free flows out in a stream until finished, no dripping etc.

Some things to keep in mind,

I pulled the carbs apart (not completely) to clean them. New float bowl seals and rubber pipes between carbs.

I have a Pingel fuel tap. As you know there is no vacuum to stop fuel free flowing.

 

I contacted Topham Mikuni (Germany) and Stephen thinks that it might be that a needle valve is not seating correctly (needle, seat or o-ring are worn, dirty etc). He also mentioned that the angle of the carbs shouldn't be a factor.

For my logical thinking, the angle of the carbs would matter as the leak could come from an overfflow caused by the floats not sitting correctly. I also thought it might be an overflow draining the fuel in the line from the tank to carbs caused by the Pingel tap but would this happen? Would it not keep flowing?

Just wondering if the brain collective here have any other ideas.

[TonyGee]

if its just leaking out of one carb then i would guess a float needle, if your tap is on the left of your bike then its number one carb, (from left to right) 

[Gixer1460]

As you say, carbs are from a GSXR and their inlet rubbers canted the carbs over a bit so that the bowls were roughly level with road. A leaky / non sealing float valve is the likely cause for the leak as said. As an experiment, try turning all the GSX rubbers 180 degrees as this may get carb angle a bit nearer where they should be?

[Gilfi]

The other problem is that I don't know exactly where the fuel is coming from.

I was going to test them last night by filling them up again and seeing if I can force the leak somehow. As I rocked them back and forth there wasn't enough clicking sound coming from the float bowls so I thought I'd take them apart. 2 of the floats and needles are completely gummed up but I wasn't really surprised as they have been sitting for about four and a half years.

Time to find an ultraschall bath and give them a good clean.

[Gilfi]

Hi guys

So, just an update. I got my ultrasonic bath and disassembled the carbs a bit. Dropped them in for about a half hour total.

Didn't really clean them as I had expected according to all the info I've read and vids I've watched. They seem to be pretty much in the same condition as when they went in, i.e. not really clean. They just turned a dark grey colour.

At this point I have to say I'm not too sure about this process as it's a first for me. Maybe I'm not using enough cleaner, temp too low (was about 55-60°C) or not enough time. Any suggestions are welcome. Was just a bit disappointed as I have seen a lot of pics where this has been done with carbs that are a lot dirtier than mine and they come out nice and clean.

 

 

 

 

[Isleoman]

Look I'm no expert on GSXR carbs but I've done enough manual cleaning of other Mikuni carbs to ask questions?

Have you pushed appropriate sized wire (guitar string) through all the removable jets?  Have you used the same wire to clean out various pathways inside the carbs?  I don't think you can just rely on Ultrasonic cleaning, in fact I wouldn't do the ultrasonic cleaning until after using the wire. 

They typically sell float needle  carb rebuild kits for less than $20. Should look like this sorta kinda  Those round O-rings are important, yours are probably brittle and no longer sealing, but I agree with previous responses that the needles seating are the current issue.   It's not about the external appearance it's the internals that matter.  

Float height is adjustable via the little tabs. In you situation you need to find out what the proper orientation of the carbs is for the factory specified float height and then translate that measurement to the orientation of the carbs when installed on your bike. I would think that the float bowl rim should be just about horizontal for the factory setting to be accurate.  In the picture in your first post it doesn't look like your float bowls would hold enough gas to support the engine's full operating range

Again I'm just a punter.  

Edited February 11, 2022 by Isleoman

[Gilfi]

I'm not too bothered about how it looks, was just surprised that it didn't come out with a cleaner silver colour but actually turned a darker grey.

After another run in the ultrasonic bath (40 minutes with a whole lot more cleaning product) I had to scrub them a bit with a toothbrush to get some stuff off that I thought would be done by the ultrasonic bath. A clean with some Never Dull and then with little carb cleaning brushes should make it a little better.

Just wondering that instead of all this BS that I should just invest in a set of RS 36 carbs.

[Jaydee]

3 hours ago, Gilfi said:

was just surprised that it didn't come out with a cleaner silver colour but actually turned a darker grey

The carbs you have are made of a magnesium alloy. Under a microscope the surface is nothing but creators and holes. Tiny little pours that fill with crap. Your carbs are clean but are stained, like a tattoo. And the only way to get the stains off is to remove a layer, a micron or two off the surface, i.e. bead, grit soda blasting. There's no point fizzing them for hours. The tank has done its job. Now let it rest before you burn out a transducer or burn a hole in the bottom of the tank

If your carbs are going dark, you've either boiled the absolute bollocks out of them. Carbs go black any temp over 65-70(ish)C left in for too long. Or the solution you are using is unsuitable and is reacting with carbs. Or a combo of overheating and wrong solution. You know a solution is wrong when the next day your carbs will be covered in white oxide after they dry.

Oh, and a tip. Looking at your flatsides in the tank photo. They are practically fully assembled, bar the top covers and bowls. At the very least, you really should be removing the back plates (that fit to the air box manifolds). Remove the 4 security screws, lift off plates and gaskets. The amount of shit to be found in those cavities is amazing. And you really should be pulling out the accelerator pump nozzles too as you'll never clean any of the galleries behind them. 

[Jaydee]

6 hours ago, Isleoman said:

Have you pushed appropriate sized wire (guitar string) through all the removable jets?  Have you used the same wire to clean out various pathways inside the carbs? 

No wire should be used anywhere near carbs. It can distort or damage jets, get stuck in galleries. Every manual that mentions cleaning carbs states it.

Beware of those keyrings of assorted wire and brushes being sold on eblag as carb tools. They are really for cleaning torches for gas welding.