My shadow is possessed.

1983 VT-500c with ~85Kmi.
The darn thing comes up with new symptoms before I get the old ones
solved.
Was running mostly ok. Let it sit for two weeks.
Now it starts fine, but won't run above idle.
Give it a little throttle and it just dies.
Doesn't sound like it's running on one cylinder.
Sounds like they both just quit.
Let it idle for 5 minutes and it runs fine.
I pulled the carbs apart and they look pristine.
no obvious plugged jets, diaphragms are pristine.
I examined the boots carefully for potential air leaks.
Compression is 90 in each cylinder when cold.
Not as high as I'd like, but it does have 85K miles on it.
I think this means I don't have a bent valve or valves adjusted too
tight...I think...

I've run three tanks of gas thru it, so it's probably not bad gas.
Spark plugs were too white for my taste, so I richened her up a little.

I have no idea where to look next.
Suggestions, other than a rag in the gas tank and a match?
mike
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I'm dying to know the solution to this, as I have a friend with very
similar complaints about his four-cylinder bike.

 

Thanks, it's an electronic ignition. Over the last few months I've
changed the whole ignition system except for the trigger coils inside
the engine.
mike

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Okay, I'll try ...

Is there crap in the carb bowls? Shut off the fuel and drain the bowls
into a tuna can to check for sand, rust, and other stuff that could plug
the idle jets. If you find crap in the bowls, you need to improve your
fuel filtration (inline paper filter, maybe), and you should probably
replace your idle jets. The orifices in those jets are remarkably small
(something like 1/75") and easily plugged.

 

Like I said in the original post, I just disassembled the carbs.
The things that trouble me are:
1) it came on all at once.
2) both carbs seem to be affected
3) how can a plugged idle jet, ignoring the fact that I've just cleaned
'em, cause an absolutely repeatable symptom that only lasts for the
first five minutes while it's warming up?

And these idle jets are huge. The tiny jets are in the CB125 that has a
rusty gas tank. I'm very familar with the symptoms of plugged idle
jets...at least in a Makuni carb. ;-(

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