Author Topic: Fuel in the airbox? What does this mean  (Read 1006 times)

Offline BikerBoy

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Fuel in the airbox? What does this mean
« on: May 07, 2019, 12:57:36 AM »
Hey, so Ive been working on my 2008 gs500f, converting it to naked, rejetted the carbs to 20-65-142.5, new spark plugs, new air filter, new headlight and such. After I rebuilt the carbs/rejetted, my bike started right away on the first crank last week! I thought it was perfect.

However I went to ride an hour ago and it wouldnt catch unless I rev the throttle a touch, and even then its very finnicky. I cant go anywhere and risk killing my battery or calling a towtruck when its like this. a user told me today that I had the lines on the bottom of the airbox mismatched (bottom one is for drain hose, one slightly above connects to engine) so I took it apart to access these lines better. When I took the hose off the bottom of the airbox (the one going to the left-side of the engine) then TONS of gasoline flowed out of the airbox.

For what its worth the vacuum lines seemed to work when I rode it a few days ago, even though I forgot where the black charcoal canister thing goes (ive been told you can remove it, so it shouldnt affect anything). All the vacuum lines "go somewhere", the only one not connecting anywhere is the lone canister line. (picture of vacuum lines from other thread:

So, what caused my airbox to get flooded? I assume thats a major part of why my bike is having trouble starting now! Fuel is going to the carbs, fuel was going there when the bike was running in ON. Should I plug the vacuum lines with JB weld?

The engine turns over and such, battery is being trickle charged again.

Offline max

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Re: Fuel in the airbox? What does this mean
« Reply #1 on: May 07, 2019, 01:11:09 PM »
Thought I'd move this into your thread rather than this taking over jimbo98's:

That black cannister is your vacuum pump and should connect somewhere on the right hand side of the carb. I'm pretty sure the highlighted hose below is the one that connects to the vacuum pump, and shows it connecting to the carb.

The breather cover is circled below too, and the hose from it should connect to the top of your airbox.

I'm pointing to the 2nd air unit in one of the images too (don't worry, it was all clean when I finished with it!). The thick hose from this connects to the airbox directly above the drain hose.


I had fuel in the airbox before too - it means that your carbs are overflowing and should be sorted as otherwise you're likely getting fuel in your oil too. Did you check your float levels when you rebuilt the carbs? And was 'rebuilding' just new jets or all o-rings too?
« Last Edit: May 07, 2019, 01:34:30 PM by max »

Offline grader

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Re: Fuel in the airbox? What does this mean
« Reply #2 on: May 07, 2019, 07:20:53 PM »
dont know about your line problem but never plug a vacuum line with jb weld or any other putty, use a small bolt or screw that fits tight, so it wont be sucked into the carbs or engine.
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Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #3 on: May 07, 2019, 11:21:03 PM »
Thought I'd move this into your thread rather than this taking over jimbo98's:

That black cannister is your vacuum pump and should connect somewhere on the right hand side of the carb. I'm pretty sure the highlighted hose below is the one that connects to the vacuum pump, and shows it connecting to the carb.

The breather cover is circled below too, and the hose from it should connect to the top of your airbox.

I'm pointing to the 2nd air unit in one of the images too (don't worry, it was all clean when I finished with it!). The thick hose from this connects to the airbox directly above the drain hose.


I had fuel in the airbox before too - it means that your carbs are overflowing and should be sorted as otherwise you're likely getting fuel in your oil too. Did you check your float levels when you rebuilt the carbs? And was 'rebuilding' just new jets or all o-rings too?
Thanks for the detailed advice. I already have a tube connecting to the right hand side of the carbs, everythings a closed circuit but the cannister. I went up and down looking for a T section it might go to but found nothing.

I did just get the bike to run first start though, after I drained the carbs somewhat, took out my airbox and drained it, and fixed the hoses that were plugged into the bottom of the airbox (they were switched up, drain hose was slighty above the hose connecting to left side of engine).

WHAT I THINK I DID WAS leave the PRIME petcock setting on, and this worsened my issues, causing it to flood even more. I have the breather cover connecting to the top of the airbox like you said.

When I rebuilt the carbs, I did not check the float levels, because as some have advised on here, float levels are almost always fine how they are, and its best not to mess with something that isnt causing trouble. Also its difficult to measure it accurately. I should probably do a ghetto/hillbilly float check by using those clear tubes and seeing if its above or below the gasket. I rebuilt it with new jets and new o-rings on the jet outside where you screw it in, didnt put new floats in. Cleaned it up a little too. My bike is only 11 years old now. Im just gonna leave the cannister tube dangling, my vacuum lines seem to work fine without it for RES and ON. Thanks again  :cheers:
« Last Edit: May 07, 2019, 11:22:03 PM by BikerBoy »

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #4 on: May 07, 2019, 11:35:26 PM »
Ive heard I should change the oil too, in case gas got in there. I only changed the oil last year and barely put any miles on it since then though. I didnt smell any gas when I opened up the oil dipstick area though.I should probably play it safe and change it, but really dont want to  :flipoff: lol

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Re: Fuel in the airbox? What does this mean
« Reply #5 on: May 08, 2019, 05:41:27 AM »
Leaving the bike on Prime can certainly cause fuel to get where it shouldn't (unless everything in your carbs is absolutely perfect, from what I understand).

I learned that the "hard way" within the last year or two :technical: and had to change my oil. The smell of gas wasn't really strong but the oil level was higher than what it should have been and I had fuel leaking out of the airbox drain hose.

Any time this kind of thing comes up on the fb group, I strongly try to steer people away from ever leaving their bike on Prime. Even the owner's manual advises against it.
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Offline max

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Re: Fuel in the airbox? What does this mean
« Reply #6 on: May 08, 2019, 01:11:17 PM »
Sounds like my previous issue too, where the fuel flowing (either bike on in on/res, or bike on OR off in prime) would result in fuel overflowing into the airbox.

Getting into the carbs is the only hard part of checking the float levels, then you can easily use a caliper or ruler. The clear tube method will give you an even easier initial check too.

The floats can be an issue if the needle isn't seating properly (either degraded rubber tip of the float needle or incorrect float height), or if the seat o-ring has failed. This video demonstrates the point well: https://www.youtube.com/watch?v=nAe_5YwWoV0

I think the manual suggests changing your oil as part of a yearly service anyway, but it never hurts to keep it fresh!

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #7 on: May 12, 2019, 04:49:48 AM »
I took my biike for a ride today, it was a bit of a pain to start. The choke does literally nothing, it used to take it to 4k RPM's before rejetting/air filter upgrade last month, but now I have to give it a little throttle to get it to idle, but dies a couple minutes later if I dont give throttle. Once it gets going it didnt have problems dying on me though.

Its important to note, I have a small bit of fuel leaking out of the airbox still.

My problems are like this guys ( http://gstwins.com/gsboard/index.php?topic=69380.0 ) except I dont have to give it throttle during warm-up the whole time, everything else is similar. some people said its probably a tight exhaust valve. Ive never done a valve clearance check on this bike before. Im curios if people think this sounds like a valve clearance issue or more likely an issue with my carbs or something.

Any ideas or suggestions on what might be wrong? This seems to have happened pretty quickly, but my bike used to run very hot, like 4k RPMS at idle because the PO didnt clean the carbs, just messed with the air mixture on the bottom of the carb, and Ive heard bikes that run hotter have their valves tighten more.
« Last Edit: May 12, 2019, 05:14:58 AM by BikerBoy »

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #8 on: May 15, 2019, 08:57:53 PM »
Hey, I think I found my issue. Remember that black charcoal canister? Well it plugs into the vacuum line, specifically the one going to the petcock. I believe leaving it open meant fuel was constantly going to the carb, as the petccock vacuum was open. See this picture:



Pretty sure that explains my flooding/running problems. The thing is so far down I couldnt see where it connected once I had it in. Good lesson to connect the vacuum lines before putting the throttle cables on. Ill let you know how it runs after I get it buttoned up today

Offline Kookas

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Re: Fuel in the airbox? What does this mean
« Reply #9 on: May 15, 2019, 10:23:48 PM »
Hmm, strange. I would've thought it would be the other way around - an open (no vacuum) petcock line causing the petcock to not open in ON at all, only in PRI.

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #10 on: May 16, 2019, 02:44:29 AM »
I heard on another https://www.southbayriders.com/forums/threads/104436/ site from a mechanic thats supposedly worked on many GS's, that the petcock vacuum is designed to prevent fuel from running when the engine is off. It can cause lean running, rich running, or flooding. im not sure yet if it will fix my problem but it seems like the most obvious solution I have right now. Would make sense since I didnt have these problems befoire I rejetted and had that vacuum line plugged to the canister.

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #11 on: May 16, 2019, 04:48:30 AM »
OK after fixing vacuum line, I started it up, still idles too low when the chokes on. At least it runs, and I dont see any fuel leaking from airbox for now. So thats the good news.

First time I started it it sounded like a gunshot, I take it this is a backfire. I hear this can be a problem from re-jetting. Perhaps my jets are too big (20-65-142.5) instead of stock jets, I didnt have this problem before I rejetted. Should I go back to the stock jets/air filter?

Heres a youtube vid.  https://youtu.be/GwLRYnDlbPA
0:05 I start with choke on, no throttle. Dies.
0:12 Start it with choke on and a little gas. Runs low RPMs for about 2 minutes then dies. Does it sound like its only firing one cylinder some of the time?
2:15 Starting it with choke off, no throttle. Dies.

At this point I feel like taking the carb or the bike to a mechanic. I could re-jet once more but Im not sure if it would do anything.

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #12 on: May 16, 2019, 09:05:04 AM »
One reader from another thread http://gstwins.com/gsboard/index.php?topic=60591.0 says to check the fuel pickup for choke on the float. Might that do the trick?

Offline Kookas

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Re: Fuel in the airbox? What does this mean
« Reply #13 on: May 16, 2019, 12:21:35 PM »
I heard on another https://www.southbayriders.com/forums/threads/104436/ site from a mechanic thats supposedly worked on many GS's, that the petcock vacuum is designed to prevent fuel from running when the engine is off. It can cause lean running, rich running, or flooding. im not sure yet if it will fix my problem but it seems like the most obvious solution I have right now. Would make sense since I didnt have these problems befoire I rejetted and had that vacuum line plugged to the canister.

There's no vacuum when the engine is off - I think you've misunderstood the mechanic. The use of vacuum is to stop fuel from running when the engine is off, but the vacuum itself is what opens the taps once the engine is on. It's the springs in the petcock that close it again once switched off.

Offline herennow

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Re: Fuel in the airbox? What does this mean
« Reply #14 on: May 17, 2019, 10:36:33 AM »
Your bike is too rich, the mid jet is actually a choke jet, its where the gas gets sucked in when you enrichen the mix through the lever.  There is a manual of a quad that uses the same carb that uses the correct name for this.  It makes no difference to normal running, just screws up how your choke works. Go back to 60 and leave it like that. Trust me on this.

Once that is done you will find a big dead spot around 6 to 7 k RPM under hard acceleration, that is because your main is too rich. Many say this bike is jetted too lean from the factory. I am not one of those. I've seen these 6k RPM blues on several GS500s with only one size larger jet. Maybe its a bit lean elsewhere in the range but I cant live with a dead spot....

Offline roxxer69

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Re: Fuel in the airbox? What does this mean
« Reply #15 on: May 17, 2019, 01:38:47 PM »
I took my biike for a ride today, it was a bit of a pain to start. The choke does literally nothing, it used to take it to 4k RPM's before rejetting/air filter upgrade last month, but now I have to give it a little throttle to get it to idle, but dies a couple minutes later if I dont give throttle. Once it gets going it didnt have problems dying on me though.

Its important to note, I have a small bit of fuel leaking out of the airbox still.

My problems are like this guys ( http://gstwins.com/gsboard/index.php?topic=69380.0 ) except I dont have to give it throttle during warm-up the whole time, everything else is similar. some people said its probably a tight exhaust valve. Ive never done a valve clearance check on this bike before. Im curios if people think this sounds like a valve clearance issue or more likely an issue with my carbs or something.

Any ideas or suggestions on what might be wrong? This seems to have happened pretty quickly, but my bike used to run very hot, like 4k RPMS at idle because the PO didnt clean the carbs, just messed with the air mixture on the bottom of the carb, and Ive heard bikes that run hotter have their valves tighten more.

I just fixed a somewhat similar issue with my 2001. My symptoms were gas leak from the fuel/air mixture screws, oil smelt of gas (I think I left it on Prime but I am not 100% on this), and the choke did absolutely nothing, I had to hold the throttle to get it warmed up.

I cleaned the carbs thoroughly (making sure not to expose rubber to carb cleaner) and replaced all major o-rings. I've been keeping an eye on my oil level to see if it rises (due to incorrect float levels) and everything seems to be working fine. (I did not have gas in the air box)

Not sure if this information helps, but a good, very thorough carb cleaning and o-ring / seal inspection may help.

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #16 on: May 17, 2019, 11:33:12 PM »
Thanks for the replies. I switched my jets back to stock, as well as the air filter. Put everything back together and it started up ok. I took it for a ride and it was barely giving me any power and was firing funny so I went home and let it idle. Weirdly, when I took it off choke (upwards) it soars to 5000 RPM, but put it on choke, and its at 1000 RPM.  :dunno_black: This is the exact opposite of how it should be.

I need to adjust the master idle mixture on the bottom of the carb since I rejetted but didnt warm it up enough when I rode it to do that, since its running so weirdly. Also I still had some fuel leaking from the airbox sadly.

I think one of the problems was I didnt screw in the air mixture screws on the 2 carbs enough. I read they should be lightly seated here, and I thought maybe when I heard the spring squeaking then that was lightly seated. But when I rejetted again I screwed them in more and I guess lightly seated needed to be tighter than it was before. So those were probably too rich. But what do you guys think of the choke issue? It shouldnt be reversed, and run much lower RPMs on choke, super high RPMS without choke, could this be a master idle screw adjustment problem?

Offline BikerBoy

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Re: Fuel in the airbox? What does this mean
« Reply #17 on: May 18, 2019, 12:28:25 AM »
Anyone think this could be a petcock issue? Heres a video of it running after rejetting to stock, with the weird choke issue. It was slightly warmed up. I didnt do any carb syncing since Im not sure if Im done tinkering with carbs. Thanks

https://youtu.be/AnikkJJ-tE4

:10 starting it with choke on
:42 switching it to choke off (upwards), idles at 5000 rpm
:57 switches back to choke on, dies down somewhat
1:30 choke off, RPMs go to 5000