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 Post subject: Mikuni 38 carb question
PostPosted: Sat Sep 01, 2018 9:30 pm 
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paso grand pooh-bah
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Joined: Wed Jan 19, 2005 12:00 am
Posts: 702
Location: CNY, US
model: 750 Paso
year: 1988
This begins by me having a strange, intermittent running issue. Basically it felt like the engine just wasn't getting enough gas - it would idle fine, but the engine would not spin up quickly at all. From a stoplight, it took lots of gas and clutch feathering. The engine did not like a load. Once it was moving, it was lethargic, but every once in a while I'd get a 1-second reminder of how it should be. Like "blah blah blah WHEE blah blah"

So I drained the gas (thinking water had accumulated), replaced the fuel lines, and has the carb cleaned. Upon putting it back together, it appears one carb is leaking gas profusely from a small hole on the front side (under the bowl?) I'll try to attach a pic. There's a screw associated with the hole; I wonder if it wasn't something the shop left open? Does that screw close the hole, and should I tighten it? I know carbs work by science and dark magick - mostly dark magick - and that's about as much as I dare mess with it.

Is this an overflow? Am I getting too much gas to the carb? I did have to replace the Y in the fuel line that splits between the carbs and the tank. I couldn't find a Y fitting, and opted for a T fitting. (The fuel goes straight through the top of the T to the carbs)

I know diagnosing stuff via the internet is a little crazy, but I'm hoping this is just one of those "well DUH" things. ;)

{Looks like I'll have to edit the picture before I can upload it}

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1988 Paso 750 #753965
1997 Monster 750


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 Post subject: Re: Mikuni 38 carb question
PostPosted: Sun Sep 02, 2018 5:49 am 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1633
Location: Newzealand
model: 906 Paso
year: 1990
Hard to say without seeing the pix but it sounds like that hole and screw is the carb bowl drain screw and it shouldn't leak ( there's your "duh " :D ) and yes you should tighten it just enough to seat it . As for the Y thing , a T will do but there needs to be some restriction in the return line to the tank . Some peeps use a small carb jet but just a bit of an extra long return line will restrict flow enough to do the job my return line ( on the 906 ) is about 600mm of 1/4" fuel line. Do check the float valves for leakage but if it is the float valves it more likely that there is a bit of crap jamming the valve open ...... if indeed there is actually a float issue .... sounds like its not getting enough juice and if you have just had a tutu with the fuel supply side of things it would pay to check that its getting enough pressure to the carbs , but not to much ..... hence the return line which stops the carb from seeing full pump pressure. Another fix is to dump the electric pump and go for a diaphragm type pump off a monster which runs off manifold vacuum and wont over feed the carbs, so you wont want the return line with that. Just as a thought are you sure that it running on both pots when you put it under a bit of load ( might idle ok with no load and then drop one pot when you start to let clutch out, and they can sound almost ok when running on one pot to the unfamiliar ear..........just no go power :)
Hamish

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 Post subject: Re: Mikuni 38 carb question
PostPosted: Sun Sep 02, 2018 1:00 pm 
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paso grand pooh-bah
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Joined: Wed Jan 19, 2005 12:00 am
Posts: 702
Location: CNY, US
model: 750 Paso
year: 1988
Thanks, Hamish.

I can actually check a bunch of those boxes you mentioned: I do have a carb jet in the return line to the tank, and I have eliminated the stock electric pump in favor of a vacuum pump. So I'll see about closing up that weep hole and see if that solves my issue!

I like simple solutions - it's just hard to believe that'd be the trick sometimes! I'll keep you posted!

Thanks again!
Jake

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1988 Paso 750 #753965
1997 Monster 750


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 Post subject: Re: Mikuni 38 carb question
PostPosted: Sun Sep 02, 2018 1:41 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5252
Location: southern Germany
model: 750 Paso
year: 1987
On the bottom of the float chamber there are two "holes". In the center one there is an adjustment screw. Don't touch that. The short pipe with the screw on its side is the float chamber drain. The rear/higher connection between the two carb bodies is the overflow.


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