Another Newbie here-- bought a 97 GS500 with only 6k miles,
and arranged through the seller and his favorite shop to have a tune up and carbs cleaned
(it had been sitting six months). It ran great until tonite. (The service was 3 weeks ago, but I messed up my back so I've only ridden it about twenty miles, but I kept it started every day.) Also, I drained the tank of the old gasoline (but not the float bowls), and filled it up with new gas, back when I got it from the shop. Well, today was the first long day of riding. It rode like a dream, with twice the power that my old '95 GS had. Then tonite
it started to have an erratic misfire at idle,
with a "pfft" or popping  sound, which increased the longer I rode.
Only at idle.

I searched all the past posts about float levels, carbs, choke, etc.
I now realize that the fuel petcock has been left on PRI for the last 3 weeks
(an honest mistake by the shop); can we confirm that you should NOT
leave it on PRI or bad things will happen? Is there gas in my oil now?


Also, I replaced the handlebars yesterday with straighter ones, and
had only a minor issue with the throttle cable sticking. But I read back in the
archives about the choke cable-- if it's binding, it will choke one carb
but not the other. That must have affected things, no?

Do either of these 2 things I did sound like they would cause a sudden uneven idle, after three or fours of good riding?  I hate to go stomping back to the shop; originally I figured they left something undone, or
there's more gunk in the carbs. But I am trying to figure out if I am the culprit.  Tomorrow I will check all boots and hose connections
and plugs anyway..  But right out of the shop it rode beautifully.



PS the shop actually did the carb cleaning twice...  the head mechanic
didn't like the way the other mechanic had done it, so he tore it down
and did it over again while I went to get fast food. I think that's a good sign.. maybe?


cheers,

steve

My last 2 bikes were a 75 Honda and a 73 BMW. I was SO looking forward
to being very lazy with carbs and ignition systems and just RIDE. Oh, well.

That noise at idle is probably cam shaft end play...... completely normal.

your idle miss fire could be caused by a few things,  the first thing I'd check is the compression to see if you have bad rings from the bike sitting, then I'd check the carbs to see if they have been synched, then worry about spark plugs and figure out how they are runnning.  nevermind, I take that back check the plugs first.

You might well have gas in your oil, so check that right away; the diluted oil will not lubricate and protect your engine properly, so don't run it again until you've ruled that out, or dealt with it properly.

Don't overlook the sparkplug wires; are the sparkplug caps screwed into them tightly? Check the sparkplugs themselves; are they tight? Are the caps seated tightly? Are both exhaust headers bolted into the cylinder head tightly? Even a little loose is no good...

Since the motorcycle worked properly when you picked it up from the shop, odds are good that something else has happened in the interim. You might want to drain the fuel tank, remove and overhaul the petcock, flush the tank to get the sediment and goo out of the crap pockets...

You've got some basic troubleshooting to do, but if you're logical and methodical about it, you should be able to track this down fairly quickly, without a lot of fuss. Does the misfire at idle actually go away at higher revs, or does the engine noise hide the problem?

Looks like you'll have to roll up your sleeves if you want to diagnose this correctly; keep us posted on your progress, OK?

This morning I started it up, warmed it up with half-choke, and
it took about five or seven minutes, but..  pop! misfire, followed by a
temporary drop in idle. Then, 24 seconds later, pop (not loud, more like an old outboard boat motor), and a drop in idle. Then one ten seconds later. Then a whole minute before one more.

Now, the oil smells like oil--  how does one detect gas in it? would it
be an overpowering smell? It seems fine. It seems brand new.
If there were a whole lot of gas in the oil would it affect the ignition
of the spark plugs? I guess I will drain it out today anyway.

The plugs (NGK, of course) seem clean, thoough if they are gapped it's a big gap (don't know the gap size). I've only heard bad things about
NGK plugs here; should I get rid of them today?

What's confusing to me is that this acts like something I had before
with a very old bike. The plug wires were original, and were grounding out
on the frame. Intermittent spark, or lack of spark. I figured it out one night, in the dark. But the idle on that bike was generally horrible.

But this idles beautifully for a minute, then,  "pop!"  It's not sounding like it's coming out of the tailpipe, but right around the cylinders.

Could've sworn that by '97 they would have went with fuel injection.
Can I convert to it? I am so sick of carbs! I an going on a No-Carb Diet!javascript:emoticon(':lol:')

So today I will do everything that I paid the shop $250 to do, all over
again. Yay!

Yep, watching in the dark makes it much easier to see a "spark leak."
 
Probably the first thing I'd try is replacing the plugs (gap to 0.08-0.09mm).
Then, I'd try checking float level with U-tube method.  It's possible that one side is too rich and fouling-out (but I don't think it should pop as it fouls).  An over-rich mixture is fine after a cold start, and fine as you ride around, but when the engine is hot and you try to idle, it can foul.

Maybe when the idle has dropped-down, it is running on one cylinder.  Try waiting for the idle to drop down, then unplug one of the plug wires (be careful about not getting shocked).  Done on one side of the bike, there would be no change; on the other, the engine would just quit.  That would help narrow the problem, and you can swap plug/wire/coil from one side to the other to see if the problem follows.

tonite I ran it (started instantly, warmed up, etc) and the pop/drop
thing started again after about 5 minutes, so I covered the headlight
so I could look for spark leaks in the dark. None.

I pulled one wire and the engine lowered, and stopped misfiring.
It just ran lower and a little choppy. So I put the wire back on and
went over to pull  the other one. Same thing-- the engine lowered
its rpms, a little rough, but NO misfire. Both sides reacted exactly the same with the wire pulled--  lower rpm but no misfire.

Only misfires when both plugs are hooked up.

I replaced the NGK's today with Champions. Didn't help.
The old  NGK's were standard, not the hot type. Also they were
both clean but baked a little white on top.
But both plugs exactly the same. No black, no soot. White.

Now, the pop/miss is at idle AND at gradual low acceleration. But it hasn't wanted to die out.  Yet.

Next step will be dropping the float bowls and looking for crud in them.
And somehow checking the float levels. I am no good at that.

All of this in the last 24 hours. Ran well in the 3 weeks leading up to this
(after the carb service)

sr

From what you've told us, the engine is (typically) lean, though I don't know if the work you've had done should have corrected that or not. Perform the same tests with the choke ON halfway, and then ON full, and let us know if things improve, get worse, or if the symptoms disappear altogether.

My guess is you've got a lean condition, and I suspect an air leak at the carb intake boots. One way to diagnose it is to run the engine, and spray starter fluid at the intake boots. Spray the end that attaches to the carbs, then spray the end that bolts to the cylinder head. Check both sides (L & R), and all around the intakes.  If the engine starts to run better, you've found your leak. If there is no air leak, the starter fluid won't have any effect on the engine.

Make sure the carbs are mounted tightly, and that the air filter is installed properly. If it has slipped out of position, you'll get a lean condition.

Also, be SURE the exhaust header bolts are TIGHT, and that the header itself doesn't have a crack or pinhole in it. Just something to do for the peace of mind while you try to resolve this.

Well, good luck with this; keep us posted, OK?

I think you hit the nail on the head.

I super-tightened the carb boots this morning,  and went off for a ride.
It took almost the whole day before it misfired once.

The carb cleaner spray affected it minimally, but it did affect it.
So it was the carbuerator  boots, as you said.

(I had checked these boots before, but now I see that one clamp was
crooked. )

I now need to take the tank off and finish tightening the screws, the ones which are not accessible from the sides. There's a few screws which I think could use a re-torque-ing.

The shop had done this carb clean twice; the second time was in a hurry because I was there to pick up the bike.  So it all makes sense now.


Now that I've pulled out all my hair...  


I owe you one--  how about some drink coasters, made out of hair?javascript:emoticon(':lol:')

sr