Bike stalls after 5 minutes

[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.

[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.