Bike died last night

[ferda]

So the unthinkable happened last night. On my way home last while in heavy stop and go traffic my bike started to bog down. The only way I made it to my destination was on full choke. The return trip was a nightmare. I could not accelerate at all, I could only make my way slowly via full choke and by letting the bike and gearing keep me at speed. However, that didn't last long. My bike instead of screaming at 5+k on full choke, it barely stayed alive. It finally sputtering to death.
Any ideas as to where I can start?
I have new plugs, good 94 octane fuel. Carbs were pulled and cleaned in the beginning of the season. I find the bike bogging on acceleration when it is cold but it comes to life after a few rounds around the block.

[mr72]

I would think bulk fuel delivery first. Try running it on PRI .. vacuum line coming loose or leaking bad at the petcock could cause this.

[Watcher]

I would think bulk fuel delivery first. Try running it on PRI .. vacuum line coming loose or leaking bad at the petcock could cause this.

If you have an in-line fuel filter, check (remove) it.

[ferda]

I would think bulk fuel delivery first. Try running it on PRI .. vacuum line coming loose or leaking bad at the petcock could cause this.

Tried starting the bike up in PRI and still no luck.

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[ferda]

I would think bulk fuel delivery first. Try running it on PRI .. vacuum line coming loose or leaking bad at the petcock could cause this.

If you have an in-line fuel filter, check (remove) it.

I don't have an inline filter, just the normal in tank one.

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[herennow]

Running better when warmed up points to too lean. Id also guess fuel starvation. How do your plugs look? How does your tank look inside, clean? Dirt could be getting into carb and blocks main jet that sits down in bottom of float bowl, but this is rare. Does petcock filter look clean?
Also, When last were valves checked?

[mr72]

More likely a vacuum leak, slides not coming up, etc.

[ferda]

More likely a vacuum leak, slides not coming up, etc.

How can I test this theory better? What might be the possible fixes?

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[Watcher]

More likely a vacuum leak, slides not coming up, etc.

How can I test this theory better?

You can see them.  Just take off the air-box and watch the slides.

[herennow]

Slides not coming up would mess with revving up the engine.  However, is probably not causing your starting problem as the bike should start and idle even with problem diaphragms. The idea of a vacuum leak on the intake manifold might cause revving problems but would need to be a serious leak to prevent the bike from starting.

Can you describe what exactly happens when you try to start it?  (throttle, choke, etc)

What have you done to diagnose so far? Have you had a good look over the bike to see if anything is amiss, carb boots are tight etc?

For something to happen so suddenly something major must have happened, hose or hose clamp fallen off, something sucked into the air filter, etc.
How do your plugs look?
How does your tank look inside, clean?
Does petcock filter look clean?
When last were valves checked? Tested for manifold leaks (hint WD40) but the bike has to be running for this.

[ferda]

So I finally had the time to look at the bike closely. I do have spark, however it will not fire at all. Did a compression test, left cylinder was 105, right was 85. Pulled the tank off, however the fuel in the lines didn't seem to be pulled into my engine. The carbs also seem to be a bit dry. I am thinking that the frame rail petcock might be blocked?

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[mr72]

Compression numbers are very low. That might even be enough to cause low vacuum and keep the petcock from opening all the time. Try running it on prime, see if that helps. And plan on a top end rebuild ASAP if you intend to rely on it.

[herennow]

Just to make sure,  did you do the compression test with the throttle open?
Low compression could be due to rings or valves, a little oil can identify which is which.

[ferda]

Just to make sure,  did you do the compression test with the throttle open?
Low compression could be due to rings or valves, a little oil can identify which is which.

No it was done with the throttle closed off. Adding a touch of oil didn't increase the values very much.

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[ferda]

Compression numbers are very low. That might even be enough to cause low vacuum and keep the petcock from opening all the time. Try running it on prime, see if that helps. And plan on a top end rebuild ASAP if you intend to rely on it.

Putting it on pri doesnt seem to make a difference.

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[herennow]

Ideally a compression test should be done warm and with throttle wide open.

[mr72]

Re-running the test with the engine warm and throttle open won't increase the number from 85 psi up to something more reasonable like 130+ psi, IMHO. Can't hurt to try just to be sure. Actually if you run the bike until it dies when hot and then do the compression test, you might find the number to be lower. As the engine heats up, everything expands including clearances that are causing the low compression. Especially if it is valves causing the low compression, it will be much worse when hot, which is one possible explanation of why the engine is dying when hot. The aluminum head expands much more than the steel valve so the valve-to-seat clearance will go up.

When my bike had nearly identical compression numbers, it would run for like 15 minutes and then refuse to run under about 7K rpm. If you let it cool down completely it would run again for 15 minutes. But it would ordinarily die while idling at a stop light or while cruising at low rpms and I'd have to wait hours for it to cool down to get it running again. A top end rebuild was the first step in making this bike reliable.

[herennow]

Josh, that's exactly right, the compression must be tested warm to see what's happening in real life operational circumstances.
I've checked compression with both open and closed throttle and the difference can almost be a hundred percent.
Note one should limit your number of rotations. If you crank the engine enough you can get almost any compression you want.

[mr72]

I've checked compression with both open and closed throttle and the difference can almost be a hundred percent.

I have not done the same so I will take your word for it. However, I would guess that it also depends on how much the intake leaks air with a closed throttle. Most old GS500s have a ton of leaks and have the throttle propped open enough by the "idle speed screw" to allow enough air to be drawn in at 100 rpm to perform a compression test... I mean enough air gets in with the throttle "closed" to allow the engine to run at 1000+ rpm.

BTW I had doubts about my compression numbers until the mechanic showed me another test with my head on the bench sitting vertically with intake ports facing down, valvetrain in but cams removed. All the valves were as closed as they can get under spring tension. He poured gasoline (...I would have used water, but anyway) into the exhaust ports and it literally poured out of the closed valve on the side that had 85psi compression. It just plain leaked out the valve on the side with 110 psi. The valves were visibly bent so one edge didn't sit flush against the seat. Amazing the engine would run at all like this.

[herennow]

I know what you are thinking MORE TOOLS dammit!!!

But...

It's amazing how much you can learn about your engine from a vacuum gauge (besides just balancing carbs)

I use two cheapish vacuum gauges with clamps to constrict the hoses and tune out the cyclic RPM variation which causes needles to shoot up and down. Will post photos later.

these guides explain all the types of problems you can diagnose.

http://www.secondchancegarage.com/public/186.cfm

https://www.autoequipment.com.au/vacuumgauge/vac1.htm

Things to note :

You must find a way to constrict hoses to eliminate most needle flicker (but not too much or response is too slow.)

On bikes the values you see are generally much less than cars - all these guides are for cars so dont worry if you see less vacuum mine was a bit under 20 KPA or 6 inches mercury)