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Message started by metri on 10/04/08 at 15:32:10

Title: Mikuni carb flooding
Post by metri on 10/04/08 at 15:32:10

Hi all,

I'm having a bit of a problem with my Mikuni Carb. The one Lancer sells. I shouldn't say the carb is the problem, as I'm not sure of that, but it is where the symptom is manifesting...

I was getting major pouring of gas out of the overflow tube while driving. I needed to get where I was going so I corked it. Gas found it's way to come out of the breather lines too... I got where I was going and parked. I've had some gas come out of the overflow line before, but it wasn't consistent so I ignored it (mistake I'm now trying to correct). I went ahead and pulled the carb off the bike and cleaned it thoroughly. For the first few miles no gas out the overflow, but soon after that, I get some when driving, but none at idle. I'm trying to figure out what the problem could be.

I have a few issues with my bike that may be contributing. The vacuum petcock doesn't work, so it's always on. As I understand carbs, the needle valve controls gas flow to the carb, so the petcock is more of a backup. That being said I do clamp my gas line when parked to avoid possible trouble.

Questions; On a fairly new carb how likely is the needle valve to be the problem?
Could float level be an issue? and if so what should the level be set to.
Can the petcock cause this? (I don't think so)
What tests can you sugest to track down the problem.

Thanks everyone,

Brian

Title: Re: Mikuni carb flooding
Post by LANCER on 10/04/08 at 16:57:45

Your petcock is the problem. When they start messing up it is not uncommon for it to allow fuel to flow down the vacuum line which will cause the over flow.
I have sold over 300 Performance carb kits in the past few years and there have been a few folks who have thought that the needle valve was bad; I did send them a replacement just so they could verify it for themselves but that did not solve the problem. It has always gone back to the petcock.

Running in the "prime" position can help temporarily in come cases, but you need to either replace the petcock of do the mod that makes it a straight on/off valve. The info for the mod are in the tech section on this site.

Title: Re: Mikuni carb flooding
Post by metri on 10/05/08 at 05:56:31

Thanks Lancer,

I will try that. I will try it on prime (while waiting for a replacement petcock). If all else fails I may buy a new needle valve, but I didn't think that was the problem as they should last many many years... Thanks

Brian

Title: Re: Mikuni carb flooding
Post by LANCER on 10/05/08 at 06:02:42

I would not buy another needle valve for a new carb. I have never found a bad one on a new carb, and those little suckers cost about $25.

Title: Re: Mikuni carb flooding
Post by metri on 10/07/08 at 16:09:28

It was the petcock. Putting it on prime fixed the problem, confirming it is the petcock. It makes perfect sense. The diaphragm has a whole in it, so the petcock is always on, but when the carb draws air, it is supposed to create a vacuum, but it can't because the diaphragm is broken... and what's on the other side of the diaphragm.... gas. So it sucks extra gas through the vacuum line and floods the carb. Easy fix. Replace petcock or convert it to manual. Thanks yet again Lancer.

Brian

Title: Re: Mikuni carb flooding
Post by LANCER on 10/07/08 at 19:46:46

2028393F244D0 wrote:

My pleasure :)