I've posted on this before and bugged Lancer today, but this issue is driving me crazy and making my bike unreliable.  I did the half-spacer mod in the winter or spring, and it worked out well until the weather got more warm and humid in Houston (more meaning like a steam room 24x7).  Now, if I start up cold when it's dark, for example, or encounter rain, the bike won't run worth anything or idle.  It will stall requiring open throttle starts and constant revving while at a stop.  Various changes like a main jet, air mixture screw, and Dyna Muffler install have not changed or eliminated the problem.  I tried a stretched carb slide spring for a while (sticky slide?), and that helped but cut back too much on throttle response/top end.  It's not necessarily rain related, but it seems to happen after sitting and starting in the morning or at night.  The only thing that improves it is a long, stumbling on-the-road warm-up.  Then, it usually runs like $$$ all day, unless rain is encountered.  Pulling the choke normally makes it worse.  Maybe that's part of my problem.  The bike has 4000 miles.  Plug problems are not intermittent right?  I'm about ready to put that spacer back in (gasp, you can't do that!!!!).  What about the half-spacer and this crazy carb icing?  Thanks in advance for any opinions!

T Mack 1 - FSO wrote on 07/14/08 at 09:26:54:


Excellent question.  I will look at it tonight and ascertain "tanness".  I didn't think a bike with that few miles could have a spark plug issue, but clearly I have an issue.

Thank you, sir.  I have a 150 main right now.  I had no issues last summer (all stock), but the issue did occur with my stock main jet (and spacer mod).  I will look at the plug tonight and try putting that spacer back in.

Serowbot wrote on 07/14/08 at 10:31:44:

Thanks.  Every thing you have mentioned has been verified.  The carb is very clean.  The vacuum hose going to the petcock as been changed a couple times.  Are you saying that if it was blocked there would be too much vacuum available to the slide, keeping the needle up?  Remember that this problem often goes away with warmer or drier conditions.

Quote:


Actually, carb and wing icing are two completely different things.

Well, except the ice part.  

Carb icing occurs due to water vapor in the air cooling while being sucked through the carb (Boyle's law) and solidifying as ice on the walls of the venturi or on the butterfly valve (or equivalent - in this case it would be a slide valve, I guess) - the key is that the ice gums up the works and causes a reduction of air to the engine (i.e. richer mixture) and thereby a reduction in power.

Note that carb ice (as was previously stated) is a function of temperature, but also of altitude and most importantly, humidity. I'm sorry to say that we lose people here in Florida every year due to that. ("It's 70, it can't be carb ice...") I think I read somewhere (Continental or Marvel manual - one of the two) that you can get carb ice as warm as 90 degrees F given the right humidity.

In wing icing, on the other hand, the concern is that formation of ice or rime on the surface of the wing disrupts the flow of air over the leading edge by turbulating the laminar boundary layer. This leads to a lower C_Lmax and resultant lower critical AOA or higher speed stall or both..... uh...... It makes the plane lift less, m'kay? WAYYY off topic.

Anyway, the way cars and airplanes solve carb ice is by use of a manual (typical in airplanes) or automatic (typical in cars - called a diverter valve) valve that typically sucks some inlet air past the hot exhaust headers to prewarm it or mixes in a little exhaust gas (how and what depends on your setup). Either way, there's less O₂ to work with (both because it's warmer=less dense, and because it may actually be exhaust) so it's actually a lot less efficent, but it's so much warmer that it eliminates the ice.

How do you keep this from happening in a M/C carb without such a system? Well, you could theoretically rig something like what an airplane has (a manual carb ice diverter) by running some of the inlet air....over the exhaust would be easiest, I think. and using a valve with a plunger...but practically, leaning it out seems a much better idea to me.

I'm finishing this thread out hoping it will help somebody else out.  It turned out after all that diagnosis and messing around that the problem was the petcock.  When I first got the bike, when it had 1700 miles, I had a problem with open-throttle stalls.  Replacing the petcock vacuum line cured that.  Little did I know that the petcock would act up again later in a different way.  This time, it had been not opening properly at lower idle speeds until it was good and hot.  Had I just thought to try PRI weeks ago when this dumb behavior started, I could have cured it.  I was so sure that rich running was causing the problem that I refused to think of another cause  .  It turns out I was running out of gas unless I operated the bike at high speed/open throttle conditions.  My last 2-3 rides, I rode on PRI, and everything was fine.  Yesterday I converted the petcock to manual operation, so I should be set for a while.  I did a seat lift also.  I think I'm happy for now.