Topic: Mikuni carb flooding problem

[dkruitz]

Hi all:
I thought I'd  try my "other" message board even though this is for a Suzuki as I'm hoping we might have some Mikuni experts here who can help.  I do have a CB500 on post on here
http://www.elginoutboard.com/jonsbike/

(assumptions: #1 carb is the left side carb when seated on the bike)

My brother-in-law and I are trying to get his project '82 GS450GA (semi-automatic shaft drive) going and are still stuck on the carbs. We found they had flowed into the crankcase and have drained the oil now. We're pulled the carbs and have one that we can't keep overflowing - it comes out the hole to the right of the inlet on the left carb.  (Photo below)  The thing that gets us is when we pull the bowls and emulate the floats rising it stops the flow of gas, even if we don't have the springs on the needles fully compressed.

We found the carbs to be missing pilot jet plugs aka passage plugs and that's resolved now, but no luck with the flooding.
We are assuming a PO messed with these quite possibly....

Photo Description:
The vent on the bottom of the #1 carb goes up and through the side and out the short vent hose that goes into the matching barb on the #2 carb. Next to that, is a brass barb with a hose on it, that we've never been able to figure out where it attaches to (still don't).

Now, here's the problem the #2 carb where the #1 vent hose attaches to on the side is plugged - or actually, never drilled through into the vent hole that the brass barb and hose use to vent the #2 fuel bowl! So in effect the gas coming into the #1 bowl creates an air pocket in the top of the bowl (because it cannot vent) and new gas coming in forces the smallish amount of gas up and out the needle jet to flood out the hole. Chances are there's not enough fuel for the floats to rise and shut off the needle/seat. Fuel pressure from the #2 bowl is also contributing to this as it has a higher fuel level and is transfered via the little gas line joining the two bowls to keep the fuel level even.

So we think, all we need to do is either drill out the pot metal barb on the #2 carb to match the #1 vent, or do we just run a longish vent hose out into the air for the #1 vent hose like the #2 vent hose? Whatcha think?

Here's a link to the full-sized image:  http://www.elginoutboard.com/carbs.jpg

[Bodi]

I would drill the vent hole. The question I have is - this is an '82 engine... how has it run for 26 years as is without the overflow/flooding problem?? And secondly why doesn't the fuel just go out the bowl overflows rather than into the oil - you don't show the bowls but as far as I've seen Mikuni uses oveflows similar in function to the Keihin ones...?
My point, if I actually have one, is that drilling the missed vent hole is a good idea but I doubt if that's causing your overflow. If it was, the first owner some 25 years ago would have been back to the dealer to get it corrected. Something else that's been done by the PO or is just normal decay is to blame, I suspect.

[dkruitz]

Hi Bodi:
Now that you mention it, I'm not aware of any overflow hoses, other than the one in the air box.  Could the two brass barbs that face each other on each bowl be the overflows?  When we got the bike, they were joined together - I had assumed ithey were plumbed together to keep the fuel level constant between the bowls?

[754]

Well....

If they were joined together & are overflows..

then.......................

Any excess gas would ether run out the back or your carb or into the motor!! whatever is downhill...

 

[CBJoe]

Silly question.... but I think that the GS's had those metal drum type floats.   Have you shaken the floats to make sure that there isnt any fluid in them.  I worked on an overflow problem on a friends 81 GS450LX and found that one side of the floats had fluid (gas) inside the float.  All that I can figure is that there was a micro crack in one of the welds and when heated up over the years it allowed fluid in, but when cooled the crack would seal up the fluid inside.

It's now hanging as a lamp cord pull in the garage to remind me 

I'll go check the extra set of GS carbs i have an see if that passage is sealed or open.

Cheers.. .Joe

[dkruitz]

Hi Joe - these appear to be the solid type floats.  Keep in mind these might be oddball carbs, they are for a Suzukimatic, and have an accelerator pump on them too.

[Bodi]

An overflow barb generally attaches to a brass tube sticking up inside the bowl, somewhere above the correct fuel level. When fuel level gets too high, gas drains out through tube. Easy to blow in the barb and see where it goes. If blocked/joined, any overflow while parked will end up in the engine sump. A very slow leak may not cause running problems when the engine uses more fuel than leaks... but will dilute your oil in time.