82 GS300L - Right-Side Drop Outs, Bogs, Stalls

[Gorminrider]

When you describe the rough test for the petcock with residual bowl contents, is this meant to test the vaccuum, i.e. that fuel is successfully being pulled down into the float bowls?

Pretty much. With full carburetor bowls, on a warmed up bike, disconnect the fuel line from the petcock, start and run the bike and see if and how the fuel runs into a jar. Obviously a spare piece of fuel line or a funnel is wanted to make it easier.

I think this is closer to reality than just sucking on the petcock's vacuum line.

more ...I have one spare petcock that audibly clicks as the diaphragm pops the plunger in and out of the valve...it needs a fair amount of suction too. I don't know how it'll work on a bike-haven't tried it yet but it's an example of variations possible.
Another factor is the spring against the diaphragm that counters the vacuum pressure. If it's too weak, a full tank of fuel has enough pressure to actually push the plunger out. .... When you finally add in the o-ring on the plunger's fit in the valve hole, a simple thing gets complicated.

[LAB3]

An awful lot of phantom issues suddenly went away when I put a crowbar in my wallet and swapped out my old petcock for a new one.

[philmotion]

So here are some things I tried today:

1. Petcock Test: as you described (with a longer fuel line into a clear glass container) and fuel flowed instantly when the bike was started and vacuum pump activated. Seems to be healthy.

1. Air Filter: Cut out by hand type. I changed this earlier this summer. The manual says to dip it in oil and carefully wring it out. I notify that it still had some oil in it and some visible residue in the airbox. I ran into without the filter and it seemed hold idle better and longer albeit hunting for it from time to time. So I cut a new filter, did NOT dip in oil and put it in the bike and the bike seemed to keep a steadier in neutral, better than yesterday.

2. Pilot Screws: Once I got a good steady idle on the bike, I brought the RPMs down as low as I could and tried to bring these screws in, then back these screws out until I got most consistent idle again on each side and then brought the idle adjust screw back up. I wanted to be sure this was in a good spot because I fiddled with it with no rhyme or reason the other day desperately hunting for a solution. Not sure I am doing this 100% correctly but I think I am close enough though correct me please if I am not.


OBSERVATIONS:

The first 10 minutes of the ride were almost flawless, better than the last weeks. As I rode on, it got worse. In the high rpms through higher gears I have no issues. When downshifting in particular today and pulling the clutch in, the bikes wants to die. As this progressed, and I came home, even the bike in the driveway in neutral was behaving erratically with 10 seconds of bog and 10 seconds of high rpms.

Any thoughts?

Pretty much. With full carburetor bowls, on a warmed up bike, disconnect the fuel line from the petcock, start and run the bike and see if and how the fuel runs into a jar. Obviously a spare piece of fuel line or a funnel is wanted to make it easier.

I think this is closer to reality than just sucking on the petcock's vacuum line.

more ...I have one spare petcock that audibly clicks as the diaphragm pops the plunger in and out of the valve...it needs a fair amount of suction too. I don't know how it'll work on a bike-haven't tried it yet but it's an example of variations possible.
Another factor is the spring against the diaphragm that counters the vacuum pressure. If it's too weak, a full tank of fuel has enough pressure to actually push the plunger out. .... When you finally add in the o-ring on the plunger's fit in the valve hole, a simple thing gets complicated.

[Gorminrider]

But just to lay the petcock to rest,

As this progressed, and I came home, even the bike in the driveway in neutral was behaving erratically with 10 seconds of bog and 10 seconds of high rpms.

this would've been the time to test it. As well as anything else that is easy... the surging idle seems like a symptom you can get your teeth into...I think a " lean-running engine when warm" is hard to "settle" with the throttles' idle setting and at the idle-mix screws so you might move on to checking for airleaks at the intakes....

[philmotion]

Thanks again. I will move onto that next and give that petcock another test after a good ride if it comes to it again.

But just to lay the petcock to rest, this would've been the time to test it. As well as anything else that is easy... the surging idle seems like a symptom you can get your teeth into...I think a " lean-running engine when warm" is hard to "settle" with the throttles' idle setting and at the idle-mix screws so you might move on to checking for airleaks at the intakes....

[philmotion]

Hi again,

A little update here:

AIR LEAKS: Looking to airleaks at the intakes I did see that my boots from the airbox to the carb were not seated as nicely as they could be so I adjusted that. The bike responded beautifully on one day (a warm day) and went back to the same behaviour the next (cold day). When performing a spray test with WD-40 and Carb Cleaner I did not notice any major change in idle.

PETCOCK TEST: I did let it warm up a good deal and tried the petcock test after a short ride but nothing struck me as not performing as it should. I tried a clear vacuum hose on the vac line and it visibly also appears to be doing its job. I also tried putting a long fuel line into a clear bucket and manually sucking on the vacuum line in "reserve" and "ride" and it is also operating seemingly as it should; free flowing fuel, a clear stop to how much I can draw back on the line with fuel flowing freely and stopping as it should.

Any other ideas?

Much appreciated.

But just to lay the petcock to rest, this would've been the time to test it. As well as anything else that is easy... the surging idle seems like a symptom you can get your teeth into...I think a " lean-running engine when warm" is hard to "settle" with the throttles' idle setting and at the idle-mix screws so you might move on to checking for airleaks at the intakes....

[roeme]

So it's an intermittent issue, and not readily reproducible.
Last time I had one of these, it turned out to be a wonky electrical connection.
Second to last time, first obstructed and then missing petcock screen inside tank (debris of unknown origin).

[fbody_mike]

The only thing I can really offer at this point is to let it act up, and let the motor stall instead of reviving it with throttle. Once that happens, quickly pull the plugs and check to see if they are carbon or fuel fouled. I'd expect to see them fuel fouled if blipping the throttle revives the engine. I think you got a tough gremlin on your hands.

[philmotion]

Thanks Mike. Tough gremlin indeed. I will keep chipping away. First real day of snow today up in Ottawa, Canada so the pressure of the last rides is long gone...will keep chipping away over the winter. Thanks for all the tips.

The only thing I can really offer at this point is to let it act up, and let the motor stall instead of reviving it with throttle. Once that happens, quickly pull the plugs and check to see if they are carbon or fuel fouled. I'd expect to see them fuel fouled if blipping the throttle revives the engine. I think you got a tough gremlin on your hands.

[Gorminrider]

But the fun is in guessing, gremlin or not. It should be a comfort that it's almost always something simple, on these simple engines.
But I like to practise my manualese: so
blah: The motorcycle has two basic "circuits" with jets for each. Low speed and high speed. There is a "shifting" from one to the other circuit as the throttle is opened and revs increase... Simply, tweaking mixture screws has virtually no effect on high speed running and very little at medium speed.. you can see that by its small size and position close to the nearly-closed throttle plate, where vacuum is very high when the plate is closed... But the low speed jet is on the other side of the throttleplate...it's a kind of "background" to the varying high speed jet

It still sounds like a variable fuel supply at lower speeds and it sounds to me that one cylinder might still be dropping out per your original post

Since your trouble seems specific to low-medium rpm, the low-speed circuit is a suspect. But, since your idle-mix screws DO have a noticeable effect, the passages and screw are probably good. But the lowspeed jet, (with its tiny holes) as a strong suspect....I know you cleaned the carb but even "cleaned" means various things. It may even loosen deposits and these return as problems...

By the way, the idle-jets have rubber plugs over the jet-well on my bike. Does yours?

More fuel: Float needle not sealing and fuel level too high? (say, high speed consumes the extra fuel, low speed does not and bike balks) There'd be smoke at exhaust too, when running...and it'd create a puddle of gas under the bike or in the sump! (smell the oil) when tap is left on Prime .

To set mind at ease? fuel-level test: if you have a piece of clear "pvc-similar" tube, warm it til it stretches creating a "neck", let it cool and cut it where it matches the bowl drain hole diameter. Screw the cooled tube into the hole and hold the other end of the tube above the bowl. Turn fuel tap to prime. Fuel level should be steady very near the gasket joining bowl to carb....the angle of the bike will be reflected so don't puzzle that level changes as sighting level relative to the gasket line..

.Fuel level: Floats can get hung up on a gasket or..?, float needle sticking , needle spring inoperable(the float needles actually have a tiny spring on the pin that contacts float- that would affect bowl's fuel level and also render the system more sensitive to bumps and vibration).

A loose battery connection can come very close to your symptoms when the bike is running! but if the bike is starting consistently, running well at high speed, it's less likely.
Ignition: the bike has a mechanical ignition advance. If it is balky and not returning to low rpm position smoothly, the timing will be too far advanced at low rpm Check it is working properly, unimpeded by a stray wire or rust or etc. A strobe light would check this when bike is running badly.

[LAB3]

It's an 82, the ignition advance is done in the ignitor box. Since winter is upon you my attention would go toward a thorough inspection of the electrical system. Make sure all your connectors are pulled apart and cleaned as well as your grounds. Grab some wire and run a common ground between points instead of relying on the frame go do the job.

I'll also agree that it's something very simple, finding the simple issue is the frustrating part!

[Gorminrider]

It's an 82, the ignition advance is done in the ignitor box.

Oh shoot! Sorry-My mistake!

[fbody_mike]

I once had some float needles that would not flood, but they wouldn't fully shut off in certain situations, like 20 minutes into a ride at constant speed of say 60 mph. I would have to speed up or rev the heck out of it to keep it from dying. Couldn't see anything wrong with the float needles, other than they must have been worn to the seat in such a way that when the rotated they would leak. Drove me nuts for a while because I could not find the problem, ended up changing the seats and needles and the problem went away. So yeah, it is hard when it is intermittent and not extreme to cause a stall, just have to keep after it. Come to think about it those needles could have been sticking shut and cause the same problem.

[philmotion]

So I took a spin yesterday, a rare occurrence in this late in November in Ottawa. Bike ran pretty well over all without having touched it. When I last looked at it I put a clear vacuum tube on just out of curiosity. I noticed that some condensation develops in the tube, more at low rpms as it accumulates and less at higher rpms as it dissipates, presumably getting sucked into the carb. If so,

1) Could this condensation, or in other words these water droplets be finding their way into the float bowls and cause the erractic behaviour?

2) Would this pull us back to a petcock issue?


Thank you.

P

But the fun is in guessing, gremlin or not. It should be a comfort that it's almost always something simple, on these simple engines.
But I like to practise my manualese: so
blah: The motorcycle has two basic "circuits" with jets for each. Low speed and high speed. There is a "shifting" from one to the other circuit as the throttle is opened and revs increase... Simply, tweaking mixture screws has virtually no effect on high speed running and very little at medium speed.. you can see that by its small size and position close to the nearly-closed throttle plate, where vacuum is very high when the plate is closed... But the low speed jet is on the other side of the throttleplate...it's a kind of "background" to the varying high speed jet

It still sounds like a variable fuel supply at lower speeds and it sounds to me that one cylinder might still be dropping out per your original post

Since your trouble seems specific to low-medium rpm, the low-speed circuit is a suspect. But, since your idle-mix screws DO have a noticeable effect, the passages and screw are probably good. But the lowspeed jet, (with its tiny holes) as a strong suspect....I know you cleaned the carb but even "cleaned" means various things. It may even loosen deposits and these return as problems...

By the way, the idle-jets have rubber plugs over the jet-well on my bike. Does yours?

More fuel: Float needle not sealing and fuel level too high? (say, high speed consumes the extra fuel, low speed does not and bike balks) There'd be smoke at exhaust too, when running...and it'd create a puddle of gas under the bike or in the sump! (smell the oil) when tap is left on Prime .

To set mind at ease? fuel-level test: if you have a piece of clear "pvc-similar" tube, warm it til it stretches creating a "neck", let it cool and cut it where it matches the bowl drain hole diameter. Screw the cooled tube into the hole and hold the other end of the tube above the bowl. Turn fuel tap to prime. Fuel level should be steady very near the gasket joining bowl to carb....the angle of the bike will be reflected so don't puzzle that level changes as sighting level relative to the gasket line..

.Fuel level: Floats can get hung up on a gasket or..?, float needle sticking , needle spring inoperable(the float needles actually have a tiny spring on the pin that contacts float- that would affect bowl's fuel level and also render the system more sensitive to bumps and vibration).

A loose battery connection can come very close to your symptoms when the bike is running! but if the bike is starting consistently, running well at high speed, it's less likely.
Ignition: the bike has a mechanical ignition advance. If it is balky and not returning to low rpm position smoothly, the timing will be too far advanced at low rpm Check it is working properly, unimpeded by a stray wire or rust or etc. A strobe light would check this when bike is running badly.

[Grimly]

Condensation in a vacuum line is just the moisture vapour present in the air being dropped out of suspension because of the vacuum.
In other words, it's normally part of your engine's daily diet and nothing to worry about - indeed, your engine runs a bit better when there's a bit of humidity.
Only start to worry if it condenses out in the tank and settles along the bottom, rusting the seam out.