Help me Obi Wan Kenobi, your my only hope

[racerxnico]

Throwing myself at the feet of the collective wisdom that is the GS Resources in the hope of enlightenment.

I have a bike that had sat for several years. I got it cleaned up and was pleasantly surprised with the ease of start up (I had cleaned carbs, changed fluids, checked valves ect.) and it ran great. A bit loud as the muffler had some rust holes along the bottom, but otherwise no problems. I ran it around like that for a couple months last season.

Over the winter I sourced some universal mufflers to replace the rusted out ones. It doesn't start quite as easily now and it almost sounds like the starter is chasing the engine for a bit till it fires. It runs ok for a while, but then it "seems" to get hot and starts to sort of chuff and back fire (more like a burp than a real loud backfire though) and will die unless kept at higher revs. The heat thing seems to be confirmed by the fact that I took it for a test drive yesterday (it was warmer out here in Seattle than the last time I rode it) and this behavior happened more quickly than on the cooler days.

Is this an issue of new mufflers not giving enough back pressure and breathing too freely causing a lean condition? Does anybody have any ideas about what else it could be?

If it is too lean, and I have to rejet, I imagine I'll have to change pilot jets too so as not to stall at idle?

thanks

[chef1366]

Lean conditions do create heat issues. Be careful not to burn your bike up. Only your spark plugs will tell the tale. Tan is good, white is lean. If it is lean you can try to richen the pilot circuit with your idle mixture screws. If this doesn't work you might have to go up on the pilot jet. Your new pipes might warrant rejetting. Only the plugs will tell.

[Jethro]

Take it from me, a lean condition can melt the head and pistons, don't run it lean...

If the exhasut has been changed you may need to re-jet. More flow out of the engine means more flow into the engine. All circuits probably need to be re-jetted. As said, check your plugs.

[racerxnico]

So, is it the general consensus that this is the most likely answer? Is there anything else that it might be that should be ruled out first?

thanks

[bwringer]

You got it. It's running way lean and you'll need to rejet ASAP. Don't ride it until you do. There's nothing worse than overheating an air cooled engine. I have some lovely burned exhaust valves around here somewhere that would absolutely turn your stomach.

While you have the carbs off, you might as well check for intake leaks and replace the intake boots if needed and definitely replace the intake o-rings (if any -- which bike is this?). Also, make sure the airbox is sealed correctly.

You could also consider placing a restrictor in the exhaust to bring backpressure back into line. You'll have to experiment a bit. These are sold for cruisers under the euphemism "torque enhancers" or somesuch, or it should be easy enough to make your own. Just make sure you find a way to make two identical ones.

[racerxnico]

Thanks,
I was wondering about trying to restrict the exhaust and wondering if that would work. I'm going to try to get ahold of a compression tester to make sure I haven't already done any damage.

As for which bike, I have a confession. It isn't either of my Suzukis. It is a little Yamaha XS400. After I got it running last fall I started making cafe racer mods to it. Over the winter I had to look for quite a while to find a pair of mufflers that would work with the rear sets I fitted (they were from a 2005 Ninja).

It was so much fun to scoot around town with last fall I couldn't wait to get it out this summer. I'm pretty happy with the way she's looking and really hoping I can get her dialed in and running right again.

thanks again,

jason

[chef1366]

Boo Hiss Darth Vader:-)

[racerxnico]

I had a thought the other night. It seem like most lean problems that I have heard about are in the primary carb circuit and can be solved with bigger jets. I never really see people with idle circuit problems.

Since my problem doesn't "seem" as prevalent at higher RPM's, I wonder if my problem could be simply a clogged pilot jet creating a lean condition at idle? It also seems like rejetting isn't usually necessary unless both the air box and exhaust are changed.

I can't figure any real way to test this theory though so I think I'll just have to pull the carbs and see if a cleaning fixes things.

I know the XS isn't a GS, but I have found that the knowledge base on this site tends to have a little more depth than some of the others I've come across.

thanks
jason