misfiring cylinder, vacuum issue. sound familiar to anyone?

hey everyone, my bike's been doing some weird junk the past couple weeks. it's been running great all summer. I tried to get home from work after a bad rain storm and it sputtered almost the whole way. it wouldn't rev above 2500. it was seriusly like an engine computer or governor was holding it back. i noticed then that cylinder 3 (that's what i call the inner-right cylinder. maybe it's generally accepted as #2...i dunno) was hot hardly at all. I pulled the spark plug and it hardly affected how the engine ran. it WAS sparking though. the next day it cranked up fine though! it sounded smooth, and revved up all the way with no probs. I did research on the forums here and tried swapping the plugs for 2 and 3. cyl 3 is still the one that runs cold.
Logical conclusion: fuel issue. i'm thinking i should take the carb apart, but it's such an ominous task: I've never done it before, i have no garage, and my college semester starts on monday (ugh..)

so get this. after letting it warm up, i got me a couple blocks, sputtered for a few seconds, and quit abruptly. i cranked it, choked, it, lightly gassed it, nothing. turned the petcock to prime (gas tank isn't empty, so it's not that im' out of gas) and after a few more cranks it fired up and seemed to run normally--just a little short of its usual power.

so that's how it's been for the past few days. The vacuum line to make the petcock work is connected to carb 3, and carb 3 coincidentally isn't working right. no vacuum to the petcock?

what are the possible implications?

it's probably time to replace the carb air boots--they're hard and starting to get cracks.

could this all be due to a vacuum issue caused by cracked air boots?

 

25+ year old coils and plug wires seldom like getting wet.

Odds are you lost a hole due to misfire when stuff was wet. Now that it's dried out your problem could be a spark plug that got fuel fouled when it wasn't firing.

 

Re: misfiring cylinder, vacuum issue. sound familiar to any

thx for the reply,

the coil wasn't wet, and it's been rained on the whole summer. I pulled the #3 plug and it looked normal. it was starting to get covered in a thin layer of black, but that was all. i'll switch that plug to another cylinder after work tonight to see if the problem follows it. that still doesn't explain the reasoning why the petcock stopped working in the "on" position. any ideas on that?

 

Sounds like your vacuum line is collapsing when you rev the bike....

 

Re: misfiring cylinder, vacuum issue. sound familiar to any

any ideas on how to remedy that?

 

Re: misfiring cylinder, vacuum issue. sound familiar to any

The coils don't actually have be soaking wet, as if someone poured a bucket of water in them. The extremely high humidity when it is raining is enough to cause a short if that is what is happening.

If your number 3 cylinder wasn't firing, it would stand to reason that you wouldn't be getting enough vacuum to properly run the petcock, would it not? And you might actually have a catch 22 here. The vacuum line may be worn and cracked, preventing a good vacuum for that cylinder to properly fire, and keeping the petcock from working properly. If it is collapsing as was already suggested, it would probably cause the same problem. Inspect the vacuum line by pulling it off and looking it over for cracks. Put your finger over one end, and blow in the other. If it is leaking, you will hear and feel it, as the hose won't be holding the air pressure. That would indicate you need to change that piece of hose.

You can also try placing the hose on one of the other adapters. It doesn't matter which one it goes on. If the problem follows the hose, you know what to do.

Before you go condemning the boots, look at them from the inside. Many times, boots look like hell on the outside, but if they haven't dried and cracked all the way through to the inside, there is no reason to mess with them. You might put some RTV on them, but I wouldn't automatically go replacing them.

 

Test the Petcock
Hook-up a new piece of vacuum line to it.
Place the Vacuum Hose to your lips and draw a vacuum.
See if the flow from the Petcock is controlled by observing the flow through a clear hose attached to the Petcock outlet.

If you taste Fuel ... The Vacuum Membrane in the Petcock failed.
Replace or refurbish the Petcock.