Bike stalls after 5 minutes

[longranger44]

So I put brand new Keyster float valves in my 82 gs750t, a rebuilt petcock, as well as an o ring kit courtesy of Nessism (great kit, great price and super fast shipping I might add), and the carb leaking into the air box and onto the ground problem is gone. The bike starts fine and idles normally when cold, but after about 5 minutes or so the idle slowly starts to fall until the bike stalls out completely. I'm a bit perplexed as it never did this before, any thoughts on what could be causing this? Thanks in advance!

 

[Rob S.]

I'll guess petcock.

 

[tom203]

Yes, just get a new petcock and rule it out as a problem….

https://www.ebay.com/itm/2016463896...RSR0K5ISZu&var=&widget_ver=artemis&media=COPY

 

[Rich82GS750TZ]

Before you do that, Check the gas tank cap vent. After the bike dies, remove the gas cap. If you hear a wooshing sound, like a vacuum release, your cap is not venting properly and not allowing the petcock to flow fuel properly after running for a few minutes. Or just run the bike w/out the gas cap in place. See if if still dies. Cap can be carefully disassembled and cleaned, but the only way I've found to replace a worn seal is to buy a cheap cap for your bike on ebay and swap out the ring seal.

You don't mention using choke it all. If starting the bike on choke, revs should go a bit high. incrementally reduce choke over a few minutes till the bike idles. Idle speed (the knob under the carbs) should be set to about 1100 rpm when the bike is warm.

 

[rphillips]

Quickest simplest thing, put petcock on PRI. and see if things change. On PRI. the vacuum and working parts in the petcock are bypassed.

 

[Rich82GS750TZ]

But even w/ petcock on Prime, if the cap vent is clogged, gas won't flow properly.

 

[rphillips]

All good to check on, but thinking cap vent will take way longer to create vacuum in tank. Think how little the volume of gas in tank will change when idling for 5 min... No doubt got to check everything.

 

[Rich82GS750TZ]

I don't disagree with your thinking at all, rphillips. I just go to the least expensive things first and rule them out.

 

[rphillips]

I Like your thinking, cheapest, easiest things first... It's amazing how often you won't need to dig into the bigger stuff.

 

[longranger44]

Before you do that, Check the gas tank cap vent. After the bike dies, remove the gas cap. If you hear a wooshing sound, like a vacuum release, your cap is not venting properly and not allowing the petcock to flow fuel properly after running for a few minutes. Or just run the bike w/out the gas cap in place. See if if still dies. Cap can be carefully disassembled and cleaned, but the only way I've found to replace a worn seal is to buy a cheap cap for your bike on ebay and swap out the ring seal.

You don't mention using choke it all. If starting the bike on choke, revs should go a bit high. incrementally reduce choke over a few minutes till the bike idles. Idle speed (the knob under the carbs) should be set to about 1100 rpm when the bike is warm.

I hadn't even thought of the gas cap. I tried again today pulled the cap off as it was beginning to die, unfortunately it made no difference. Good thought though it never even occurred to me.

 

[longranger44]

Ok so I started the bike again today and here is the sequence of events-
On full choke it fires immediately and immediately wants to Rev to the moon on full choke, turning it down to about half choke brings the idle speed down and the choke can be turned off completely within 10 seconds or so. It then idles around 1100 for a few minutes but gradually climbs to 1500 around the 5 minute mark and stays there. Then around the 8 minute mark it gradually begins to slow until it stalls out completely.
Once it's stalled it can be restarted immediately with a bit of throttle and it will idle for 20 seconds or so before it stalls out again. Also, if you blip the throttle as it's trying to stall, it goes back to idling normally for 10-15 seconds before stalling out. This can continually be done to keep it running which I thought might be notable. I took a video of this in case it means anything to y'all. Let me know what you think and thanks for the help so far!
​

 

[Rob S.]

(Intake) air leak?

 

[tom203]

”Ok so I started the bike again today and here is the sequence of events-“

The sequence is odd. Being able to turn off choke after ten seconds and hold idle is odd for a cold motor . To me this suggests that engine is getting a extra fuel dose from somewhere ….but this eventually dwindles and bike stalls until blipping the throttle restores this extra fuel dose..somehow .
make sure your petcock works on PR…test it and reconnect to carb fuel rail….disconnect and plug vacuum line to carbs.

 

[longranger44]

”Ok so I started the bike again today and here is the sequence of events-“

The sequence is odd. Being able to turn off choke after ten seconds and hold idle is odd for a cold motor . To me this suggests that engine is getting a extra fuel dose from somewhere ….but this eventually dwindles and bike stalls until blipping the throttle restores this extra fuel dose..somehow .
make sure your petcock works on PR…test it and reconnect to carb fuel rail….disconnect and plug vacuum line to carbs.

Ok I disconnected the fuel line and With the petcock on prime it flows fuel freely, with it on "on" it shuts fuel off but not 100%, it will slowly drip like one drip a minute. Same with reserve.
I tried turning it to prime while it was running and there was no change in the stalling behavior. I'm not sure what you men's by plugging the vacuum lines?

 

[chuck hahn]

Be sure the fuel line isnt kinking when you set the tank down after hooking up the lines. Check the vacuum line going to petcock also.may be leaking vacuum.

 

[KiwiAlfa156]

”Ok so I started the bike again today and here is the sequence of events-“

The sequence is odd. Being able to turn off choke after ten seconds and hold idle is odd for a cold motor . To me this suggests that engine is getting a extra fuel dose from somewhere ….but this eventually dwindles and bike stalls until blipping the throttle restores this extra fuel dose..somehow .
make sure your petcock works on PR…test it and reconnect to carb fuel rail….disconnect and plug vacuum line to carbs.

Your on the right track, but I think the clue is that you can turn the choke off after 10 seconds, which to me says the pilot mixture is rich.

The best way of thinking about the the throttle is that it controls the AIR flow. NOT fuel flow, especially with CV carbs where there is no direct mechanical connection between the throttle and anything metering fuel delivery. Opening the throttle increases the amount of air the engine can ingest. Correct jetting adds the right amount of fuel to create the best AFR for that volume of air.

So opening the throttle on a fully warm engine initially ads more air to the rich pilot AFR and it picks up revs.

Rich mixture on a cold engine is required to start and warm up because the 'cold' head causes a lot of the fuel (which is a soup of many hydro-carbons with different volatilities) to remain liquid and therefore harder to ignite. So adding more fuel via the choke/enrichener increases the ignitability, as the amount of the lightest most ignitable volatile compound presented to the spark plug is increased. In a warm engine all the fuel is evaporated into vapor and easily ignited and burned.

When the engine is warm the rich mixture burns too slowly to sustain the idle.
My guess is that the pilot screws (fuel screws) are too far out (rich). Letting it die and then pulling the plugs will give you a heads up whether its rich or lean.

The pilot/fuel screws has maximum affect on the mixture on a closed throttle (idling), as its metering almost ALL the fuel into the engine, as you open the throttle the butterfly moves to uncover the by-pass orifices which draws more mixture from the pilot jet/pilot air jet, so the the proportion of the total pilot mixture trimmed by the fuel screw decreases rapidly as the throttle is cracked open.

If the plugs are sooty, progressively screw the fuel screws in a little by the same amount across all 4 screws and see if there is improvement.

 

[longranger44]

Be sure the fuel line isnt kinking when you set the tank down after hooking up the lines. Check the vacuum line going to petcock also.may be leaking vacuum.

Ok checked them both, both seem fine

 

[longranger44]

Your on the right track, but I think the clue is that you can turn the choke off after 10 seconds, which to me says the pilot mixture is rich.

The best way of thinking about the the throttle is that it controls the AIR flow. NOT fuel flow, especially with CV carbs where there is no direct mechanical connection between the throttle and anything metering fuel delivery. Opening the throttle increases the amount of air the engine can ingest. Correct jetting adds the right amount of fuel to create the best AFR for that volume of air.

So opening the throttle on a fully warm engine initially ads more air to the rich pilot AFR and it picks up revs.

Rich mixture on a cold engine is required to start and warm up because the 'cold' head causes a lot of the fuel (which is a soup of many hydro-carbons with different volatilities) to remain liquid and therefore harder to ignite. So adding more fuel via the choke/enrichener increases the ignitability, as the amount of the lightest most ignitable volatile compound presented to the spark plug is increased. In a warm engine all the fuel is evaporated into vapor and easily ignited and burned.

My guess is that the pilot screws (fuel screws) are too far out (rich). Letting it die and then pulling the plugs will give you a heads up whether its rich or lean.

The pilot/fuel screws has maximum affect on the mixture on a closed throttle (idling), as its metering almost ALL the fuel into the engine, as you open the throttle the butterfly moves to uncover the by-pass orifices which draws more mixture from the pilot jet/pilot air jet, so the the proportion of the total pilot mixture trimmed by the fuel screw decreases rapidly as the throttle is cracked open.

If the plugs are sooty, progressively screw the fuel screws in a little by the same amount across all 4 screws and see if there is improvement.

Well I didn't touch the pilot jets when I went into the carbs and it wasn't doing this before, I just did the needle valves, fuel tube, and vent tube o rings.

 

[chuck hahn]

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

 

[KiwiAlfa156]

Well I didn't touch the pilot jets when I went into the carbs and it wasn't doing this before, I just did the needle valves, fuel tube, and vent tube o rings.

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.