SuzukiSavage.com - Carb Leak Diagnosis

vroom1776

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Carb Leak Diagnosis
04/09/07 at 11:13:53

Hey all,

My 38 mm Mikuni VM rounslide carb keep spitting gas out the overflow tubes. This is NOT the stock carb...

Also, I stuck the stock carb (40 mm Mikuni BS40) back on, runs fine. This implies that it is not an issue with the petcock.

So, step

1) Make sure the float needle valve closes
2) Make sure the floats themselves do not leak (they float)
3) Make sure the float arm is not getting stuck.
4) Check float height.

It usually spits gas when I come to a stop. Ever since I got the thing on there, I've more or less had to run it on "PRI" at the petc0ck in order to keep it from stalling... THAT part may be related to the idle set screw & spring. The vacuum line seems to work properly, though. If I set it to "PRI" and disconnect the vacuum line, it dies (no vac to pull gas into engine). If I put my thumb over the vacuum port {edit ->} on the carb, and restart, it will run, so the vac's okay.

Am I missing anything?

Also, I am very unsure how to check the float height.

Thanks gang!

V

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Savage_Greg

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Re: Carb Leak Diagnosis
Reply #1 - 04/09/07 at 11:34:36

My simple rule of "thumbs" is to adjust the float to where the float arm is parallel with the flat edge of the bowl when you hold the carb upside down. Just bend the tab and eyeball it...

Also, try blowing into the fuel inlet while moving the float up and down. If the float needle valve is working right, you'll know.

That's what I do when I don't know the measurement, don't have a manual, get lazy, or don't give a crap Tongue

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vroom1776 Serious Thumper Offline Posts: 2110 Gender:	Re: Carb Leak Diagnosis Reply #2 - 04/09/07 at 11:37:09 my plan to test the float needle was to fill the fuel line with gas, gently push up on the float arm (i.e. seat the needle valve), and see if gas leaks... float height... okay, that's about where it was last time I checked (months ago). Ug, now I remember posting on this... Thanks greg.
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Savage_Rob Serious Thumper Alliance Member Offline Mechanically Inclined Amateur Posts: 6972 Texas (Dallas area) Gender:	Re: Carb Leak Diagnosis Reply #3 - 04/09/07 at 11:37:32 vroom1776 wrote on 12/31/69 at 16:00:04: If I set it to "PRI" and disconnect the vacuum line, it dies That seems strange to me. PRI should not require vacuum at all.
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Savage_Greg Serious Thumper Alliance Member Offline Don't go around your ass to reach your elbow... Posts: 7844 SW Washington State Gender:	Re: Carb Leak Diagnosis Reply #4 - 04/09/07 at 11:48:59 Savage_Rob wrote on 12/31/69 at 16:00:04: That seems strange to me. PRI should not require vacuum at all. He didn't mention if he plugged the hole in the carb intake when he disconnected the vacuum hose. It could have gone lean and died then... Just a guess...
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vroom1776 Serious Thumper Offline Posts: 2110 Gender:	Re: Carb Leak Diagnosis Reply #5 - 04/09/07 at 12:09:38 sorry, me not being clear enough. edited above. response was going to be: "PRI" doesn't require vacuum to deliver fuel, but the carb still needs an internal vacuum to pull up fuel from the float bowl spray it into the engine. leaving the vac port on the carb open to air will kill the bike (no fuel delivery).
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Savage_Rob Serious Thumper Alliance Member Offline Mechanically Inclined Amateur Posts: 6972 Texas (Dallas area) Gender:	Re: Carb Leak Diagnosis Reply #6 - 04/09/07 at 13:38:23 I can see that.
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Savage_Greg Serious Thumper Alliance Member Offline Don't go around your ass to reach your elbow... Posts: 7844 SW Washington State Gender:	Re: Carb Leak Diagnosis Reply #7 - 04/10/07 at 05:27:58 But you can run the bike with the vacuum hose off, and the hole plugged, and the Petc0ck on PRI. That we know. I can't remember the original problem now...
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vroom1776 Serious Thumper Offline Posts: 2110 Gender:	Re: Carb Leak Diagnosis Reply #8 - 04/10/07 at 07:49:10 sorry just trying to be clear... maybe for nubes? anyway, everythiung seems to be in good shape, but one of the floats seems to occasionally stick... no, for my own sanity.
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Savage_Greg Serious Thumper Alliance Member Offline Don't go around your ass to reach your elbow... Posts: 7844 SW Washington State Gender:	Re: Carb Leak Diagnosis Reply #9 - 04/10/07 at 08:48:18 vroom1776 wrote on 12/31/69 at 16:00:04: sorry just trying to be clear... maybe for nubes? anyway, everythiung seems to be in good shape, but one of the floats seems to occasionally stick... no, for my own sanity. How do one of the floats get stuck? Mine are connected and move together. This forum helps your sanity?
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vroom1776

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Re: Carb Leak Diagnosis
Reply #10 - 04/10/07 at 09:40:17

this aint the stock carb, and the floats are independent. They are hollow (of course) plastic disks with brass pins that push on the float arm. They slide up and down on vertical brass rods with stops at the ends. They are free to rotate around the pins, until they hit the sides of the float bowl.

I had the bowl about 3/4 filled with carb cleaner, so that the floats were not all the way up against the stops. pushing one down should push the other up. occasionally, one would bind a little and not move. I cleaned the heck out of the bowl and pins, gonna stick it back in tonight, see if it was just gummed up. Also, the floats did not appear to leak.

Also, a 250 W heat lamp is my friend. The stock carb-engine rubber flange needed some heat lamp love to get the carb back in there.

diagram:

my floats are round.

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Savage_Greg

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Re: Carb Leak Diagnosis
Reply #11 - 04/10/07 at 09:46:37

vroom1776 wrote on 12/31/69 at 16:00:04:

this aint the stock carb, and the floats are independent. They are hollow (of course) plastic disks with brass pins that push on the float arm. They slide up and down on vertical brass rods with stops at the ends. They are free to rotate around the pins, until they hit the sides of the float bowl.

I had the bowl about 3/4 filled with carb cleaner, so that the floats were not all the way up against the stops. pushing one down should push the other up. occasionally, one would bind a little and not move. I cleaned the heck out of the bowl and pins, gonna stick it back in tonight, see if it was just gummed up. Also, the floats did not appear to leak.

Also, a 250 W heat lamp is my friend. The stock carb-engine rubber flange needed some heat lamp love to get the carb back in there.

diagram:
http://www.motorcyclecarbs.com/images/mikuni/carbs/explview/vm20_44explview.gif

my floats are round.

What is the purpose of independent floats?

Geez, how to take an overly complicated carb and make it even better Tongue

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Savage_Rob Serious Thumper Alliance Member Offline Mechanically Inclined Amateur Posts: 6972 Texas (Dallas area) Gender:	Re: Carb Leak Diagnosis Reply #12 - 04/10/07 at 09:59:54 That carb is very similar to my Amal Mk2, though the Amal uses a single float.
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vroom1776

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Re: Carb Leak Diagnosis
Reply #13 - 04/10/07 at 10:03:34

Savage_Greg wrote on 12/31/69 at 16:00:04:

What is the purpose of independent floats?

Geez, how to take an overly complicated carb and make it even better Tongue

hmmm... that's a good question. I don't know. Actually, I like this carb... when it's running well. VERY responsive. First gear is pretty ridiculous. Also, the throttle attachmetn is way better (easier to get on & off) than the VM28s that came with my RD350...

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Savage_Rob Serious Thumper Alliance Member Offline Mechanically Inclined Amateur Posts: 6972 Texas (Dallas area) Gender:	Re: Carb Leak Diagnosis Reply #14 - 04/10/07 at 10:06:09 They seem to be yoked together by item 23. That looks like they function as a single item.
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