Mikuni Venting Issue
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Outstanding HDF Member
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From: Philly Suburbs
Mikuni Venting Issue
My Mikuni HSR42 has started dropping some fuel out of the vent line. It's very consistent in its behavior... It's completely dry when the engine is running. It drips about two or three tablespoons of fuel at a fairly quick pace after the engine is shut off (provided it ran long enough to be fully warm). It doesn't start right away. After shutting the bike off and having it on the side stand it takes about three minutes before it starts. Then a steady drip of fuel begins out of the vent house which lasts for a minute or two, then it's done. It seems to be related to heat-induced pressure, as it only happens if I close the petcock as soon as I shut off the engine. If I leave the petcock open, no drip. I can close it ten minutes later and nothing drips. It seems like engine heat is causing fuel in the bowl to expand. If it can't expand up the fuel line (due to a closed petcock), it finds the vent line. I first noticed this about a week before recent dyno tuning, during which the needle was replaced. The drip behaves exactly the same since tuning as it did before, leading me to think it isn't needle-related, nor related to anything done during tuning. Float too high? Maybe it's related to the fact that every ride I've taken in the last month has been in 98 degree weather? I've also noticed a reduction in fuel economy lately, but that may be due to a larger main jet recently...
Other than the float level, is there anything else I should be looking at?
Other than the float level, is there anything else I should be looking at?
Extreme HDF Member
Joined: Jun 2014
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From: NW Ohio
I'd order a new float and check/adjust the level while replacing. They're cheap and you gotta open it up either way. Just be careful removing the pin. If that's too risky, just adjust the level.
That may also force a minor tune adjustment. Often, a lower fuel level in the bowl will result in a leaner mixture.
That may also force a minor tune adjustment. Often, a lower fuel level in the bowl will result in a leaner mixture.
Tourer
Joined: Aug 2018
Posts: 337
Likes: 214
From: California
I have the same issue. I stop it from happening by leaving the petcock open when I park it or by closing the petcock on the last block be for parking in the garage. Checked and adjusted the float but it still happens.
Cruiser
Joined: May 2018
Posts: 193
Likes: 51
From: Cumming, GA
I know you said the needle was replaced, but if the adjusting the float doesn't work, you might need to double check the needle isn't at fault. Once the fuel hits the vent line, gravity will siphon everything out up to the petcock. I'd lean towards the float first.
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Outstanding HDF Member
Joined: Mar 2015
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From: Philly Suburbs
Thank everyone. I took Chris's advice and just ordered a new float from Jets R Us, as well as a float pin. I'll check out the needle as well. I'll measure the existing float adjustment before taking it out, and will probably try the new part with the same adjustment initially (unless something seems instantly screwy - I don't really want to lean out my mixture if I don't have to)... Maybe it's set to the right height but is hanging up somewhere...
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Road Captain
Joined: Aug 2013
Posts: 515
Likes: 191
From: Sugar Hill, Ga
It might be the temp. I filled my tank to the rim in 104F and minutes later I had gas running out the vent. Gas expands much more dramatically than water.
So, if you float bowel is full with gas that was much cooler in the tank and is allowed to warm up to ambient, it could dribble out. Now if it keeps dribbling, you need to check your peacock. It should stay shut while there is no vacuum but if you are losing more than a bowel full, I'd look there also.
But as others have said, replacing a float is pretty trivial.
So, if you float bowel is full with gas that was much cooler in the tank and is allowed to warm up to ambient, it could dribble out. Now if it keeps dribbling, you need to check your peacock. It should stay shut while there is no vacuum but if you are losing more than a bowel full, I'd look there also.
But as others have said, replacing a float is pretty trivial.
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Thread Starter
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Outstanding HDF Member
Joined: Mar 2015
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From: Philly Suburbs
But it is true that I've only noticed this issue while it's been really hot... I'll check on the effect of cooler weather... If it cools down in the next decade...
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Outstanding HDF Member
Joined: Mar 2015
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From: Philly Suburbs
Well... I replaced the float and the pin, but the issue is unchanged. I just ordered a new needle valve. If that doesn't fix it, I'll try float height at 19 or 20mm... I measured the old float before removing it, 18mm (which is what's called for). The new float right out of the box was also 18mm. The manual does say +/- 2mm...