Bike stalls after 5 minutes

[longranger44]

Try loosenig a drain screw just to be sure it will open then close it as normal. AS SOON as the bike dies open that drain screw and see if the carbs are out of gas. If yes find out why there isnt fuel flow in the ON and PRI positions. If its a leverless petcock put a piece of vac line on the small nipple and a piece of fuel line into a catch container and suck on the vacuum line to see if the petcock flows fuel and that its a strong stream
And when you were in te carbs did you replace the rubber plugs over the pilot jets? Doesnt take much fuel seepage around them to cause trouble.Also be sure no choke plungers are hanging open

Ok will check the drain plugs. The petcock is the original one with a lever and 3 positions - reserve, on, and prime. I had it rebuild by a local repair shop a month or so ago.
Yes I replaced the rubber plugs. Funny, the old ones were so petrified they'd fall right out yet it ran fine with them like that for months.
Will check the plungers also and report back.

 

[longranger44]

All good. So didn't touch the pilot screws? Did you replace the seats or just the needles? Did you re-check the float levels after fitting the new needle valves? There's no guarantee that the Keyster part is the same length or has the same spring tension on the plunger pin as the part it replaced. Bowl level also affects mixture. If it were me, I'd look at the plugs, its a tell-tale whether is getting too much fuel or not enough when it stalls.

Right I didn't touch the pilot screws. I thought the problem was the float valves so with the pilot screws I didn't want to fix it if it wasn't broken. Also they're easy to get at with the carbs installed in the bike so I figured I could always do that later.
I replaced the needles and seats. I checked the float levels with just a caliper but didn't do the fuel level clear tube test.
I'll pull a bowl drain plug and pull spark plugs and report back with my findings.

 

[chuck hahn]

AHHH you said the magic word...rebuilt. Rebuilding petcocks have a very low sucess rate.

 

[Rich82GS750TZ]

I'm getting the idea that just certain things in the carbs were taken apart and cleaned, some o-rings replaced, some other parts replaced, rather than a full strip, dip, clean, and re-assemble. Am I wrong?

 

[rphillips]

Post #14 said pet cock was flowing fine but still leaking a little when "ON and "RES". sounds like maybe didn't clean or polish the seat the plunger seals against. He said when it starts to stall, blipping the throttle will make it run another 15 to 20 sec then start to stall again then blipping the throttle again will get another 15 or 20 sec. Can keep it running by blipping every time it starts to stall. That should tell something, I just don't know what.

 

[dgates1]

I would be suspect of the petcock, diaphragm messed up or something (especially with it being "rebuilt" and you said it was leaking). Maybe getting slow fuel leak down the vacuum side. Seems to be flooding it out too rich.

I think Rob nailed it in Post # 2

 

[rphillips]

All things possible. He said just rebuilt the pet cock, the kits I've seen & used include a new diaphragm (not say'in a new one can't be faulty). Also thinking completely flooding 1 cyl. on a 4 cyl. GS, through the vacuum line, shouldn't kill the eng.

 

[longranger44]

All things possible. He said just rebuilt the pet cock, the kits I've seen & used include a new diaphragm (not say'in a new one can't be faulty). Also thinking completely flooding 1 cyl. on a 4 cyl. GS, through the vacuum line, shouldn't kill the eng.

Yes I'd tend to agree as I've accidentally left a single plug wire unplugged before and it still ran, suprisingly well actually lol.
I'm suspicious of the petcock also but I'd like to know how exactly it could be malfunctioning that could be causing this problem

 

[longranger44]

I'm getting the idea that just certain things in the carbs were taken apart and cleaned, some o-rings replaced, some other parts replaced, rather than a full strip, dip, clean, and re-assemble. Am I wrong?

You're correct that's what I did

 

[longranger44]

Try loosenig a drain screw just to be sure it will open then close it as normal. AS SOON as the bike dies open that drain screw and see if the carbs are out of gas. If yes find out why there isnt fuel flow in the ON and PRI positions. If its a leverless petcock put a piece of vac line on the small nipple and a piece of fuel line into a catch container and suck on the vacuum line to see if the petcock flows fuel and that its a strong stream
And when you were in te carbs did you replace the rubber plugs over the pilot jets? Doesnt take much fuel seepage around them to cause trouble.Also be sure no choke plungers are hanging open

Ok so I pulled the drain plugs for cyl 1 and 4 right as it died. Both seemed to have plenty of gas in them.

 

[longranger44]

All good. So didn't touch the pilot screws? Did you replace the seats or just the needles? Did you re-check the float levels after fitting the new needle valves? There's no guarantee that the Keyster part is the same length or has the same spring tension on the plunger pin as the part it replaced. Bowl level also affects mixture. If it were me, I'd look at the plugs, its a tell-tale whether is getting too much fuel or not enough when it stalls.

The pilot screws are easily accessible with the carbs installed in the bike just by taking the tank off so I figured I could mess with those later if necessary. Do you think that'd be a good thing to just do?

 

[longranger44]

So I think I've ruled out the petcock/tank being the issue. I fashioned an auxiliary "tank" from a plastic bottle (just a bottle with a vent and a nipple) and disconnected the fuel line from the motorcycle tank while the engine was hot, right after it had stalled out. With the new "tank" connected the bike behaved identically to when it was connected to the actual tank.
Am I correct in thinking this rules out the petcock/tank and that this must be a carburetor issue? If so, what would cause it to slowly stall out only after it gets hot, but can be made to keep running just by blipping the throttle every 10 or so seconds?​

 

[KiwiAlfa156]

So I think I've ruled out the petcock/tank being the issue. I fashioned an auxiliary "tank" from a plastic bottle (just a bottle with a vent and a nipple) and disconnected the fuel line from the motorcycle tank while the engine was hot, right after it had stalled out. With the new "tank" connected the bike behaved identically to when it was connected to the actual tank.
Am I correct in thinking this rules out the petcock/tank and that this must be a carburetor issue? If so, what would cause it to slowly stall out only after it gets hot, but can be made to keep running just by blipping the throttle every 10 or so seconds??

Fault finding is always a process of elimination, and you've taken the petcock out of the equation, so.... your down to just the carbs. Just checking that you have blocked off the vacuum hose? I'm assuming that you have, you just didn't mention it.

I had a similar problem with the bike idling and then slowing and stalling while sitting at the traffic lights (I had rejetted the bike for a single pod on the back of the front airbox/plenum chamber and the pilot mixture was the last piece of the puzzle). The fuel screws were set too rich. leaning them out fixed the problem and drastically improved roll-on carburation from a closed throttle when cornering.

I found the following video very helpful. I've used various methods to set the fuel screws and 'blip and clean return to idle' method is now my go to.

https://www.youtube.com/watch?v=q9A2TL9RvwQ

 

[longranger44]

Fault finding is always a process of elimination, and you've taken the petcock out of the equation, so.... your down to just the carbs. Just checking that you have blocked off the vacuum hose? I'm assuming that you have, you just didn't mention it.

I had a similar problem with the bike idling and then slowing and stalling while sitting at the traffic lights (I had rejetted the bike for a single pod on the back of the front airbox/plenum chamber and the pilot mixture was the last piece of the puzzle). The fuel screws were set too rich. leaning them out fixed the problem and drastically improved roll-on carburation from a closed throttle when cornering.

I found the following video very helpful. I've used various methods to set the fuel screws and 'blip and clean return to idle' method is now my go to.

https://www.youtube.com/watch?v=q9A2TL9RvwQ

Thanks for the reply, I've kept mine with the stock airboxes and didn't change any settings on the carb jetting. All that is past my pay grade, eventually I might go to pods but for now I just want it to run in its factory configuration.

 

[longranger44]

Well good news, it would appear that the bike is fixed! I was doing some research and a guy on a Yamaha forum mentioned how sometimes gas will leak past the rings and into the crankcase from leaky carbs, and will run fine at idle, then once the engine gets hot that gas in the crankcase will vaporize and go past the rings the other way and cause it to run rich and stall. I had checked the sight glass level a while back and it didn't seem to have gone up any, but yesterday I noticed when I pulled the filler cap I could smell gas bigtime. So I drained it and it was definitely diluted. Not nearly as thick as normal. So I changed the oil and filter and voila! No more stalling. I let it idle for 20 minutes with no issues then rode it up to the corner station and filled up. No issues at all even when stopped at lights. Time will tell whether it's truly fixed or not but I'm pretty optimistic because of how consistent the stalling issue was, I'm going to go on a longer ride tonight and see how it does. I'm going to change the oil again just to be thorough and make sure it's all flushed out. Hopefully I didn't cause any damage from 20+ minutes of idling with gas diluted oil.

Anyway thanks to everyone who replied and helped me rule out other possible causes, you guys are great!

 

[rphillips]

Hoping that's got your running problem fixed. If gas is getting past the rings, into the oil, you've got a pet cock problem. That's exactly what that vacuum pet cock is supposed to keep from happening.... Just say'in, I've had gas in the crankcase twice, but did not cause anything near what you've described.

 

[longranger44]

Hoping that's got your running problem fixed. If gas is getting past the rings, into the oil, you've got a pet cock problem. That's exactly what that vacuum pet cock is supposed to keep from happening.... Just say'in, I've had gas in the crankcase twice, but did not cause anything near what you've described.

Well the gas that got into the crankcase was from before I had the petcock rebuilt and before I did the new float valves and orings. As far as I can tell I'm not getting any gas leakage since I did the float valves. With properly functioning float valves the bowl shouldn't overflow even if the petcock is slowly seeping gas, correct? Or am I wrong?
You mentioned that the guy who did the petcock maybe didn't polish where the plunger seats against, is that something I could do myself? I'm not exactly sure what part you're referring to inside the petcock.
​

 

[rphillips]

Correct, you got to have at least 1 needle and seat leaking "and" a leaking pet cock to get enough gas into oil to even notice. The reason for the (newer than 1977 auto petcocks) was how hard it was to keep all four needle and seats sealing properly and folks not remembering to turn fuel off after every ride. For me polishing the seat in the pet cock (with Q-tip or pencil eraser) and slightly stretching the spring for just a bit more pressure has worked good...I'm not recommending anything, just telling what experiences are.

 

[longranger44]

Correct, you got to have at least 1 needle and seat leaking "and" a leaking pet cock to get enough gas into oil to even notice. The reason for the (newer than 1977 auto petcocks) was how hard it was to keep all four needle and seats sealing properly and folks not remembering to turn fuel off after every ride. For me polishing the seat in the pet cock (with Q-tip or pencil eraser) and slightly stretching the spring for just a bit more pressure has worked good...I'm not recommending anything, just telling what experiences are.

Gotcha, I also heard that it was to stop fuel from flowing in case of an accident, kind of like an inertia switch in a car idea. Sounds like I'll have to pull out the petcock and try to get it to seal better by polishing like you said.

I've never owned anything with a vacuum operated petcock before, just on/off ones. Just how finicky are they even when properly polished and sealed? Even if they are working well are they prone to start leaking randomly after a year or two? Has anyone added a second simple on/off inline valve in the line downstream of the petcock? Obviously I'd have to remember to turn it on before riding but I guess I'm old school and like the idea of being able to turn off gas flow mechanically just in case the float valves start to get tired and the petcock decides to leak. Any reason not to do this besides it looking a bit redneck? :nevreness:

 

[Rijko]

i have installed an electronic fuel valve on several bikes ao my Kawa GTR1000, which has no overflow tubes and is known for hydrolock.
Around 15$ off Aliexpress, hooked it up to a switched 12v ... Bob's your uncle.
They are not very big and my buddy mounted one on his GT750 as precaution.
Won't protect you if the petcock leaks into the vacuum hose but adds extra safety.