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High idle rpm, popping in exhaust

  • Thread starter Thread starter Bkan
  • Start date Start date
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Bkan

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Hey all, having trouble with my gs425L. It idles around 3000 rpm, and when I screw the air screw on the carb almost all the way in, it'll go down to 2000 but will start making popping noises in one muffler. I also noticed that there's an exhaust leak where the two mufflers connect underneath the bike. There's quite a bit of white smoke.

The idle mix is completely disengaged. I thought it might be a vacuum leak but I can't determine where it could be. I sprayed WD-40 around likely suspects like the boots and hose sections, but didn't notice any changes in rpm. Thoughts?
 
Popping on the overrun is often caused by a lean condition, where the improperly burnt fuel (doesn't burn correctly as it's too lean) comes through to the exhaust and explodes in there. It can also be valves not closing correctly and needing adjustment or re-seating. It can also (more usually on 4 cylinders) be a carb out of synch:


As Tom says, what have you done to it recently? Or are you recommissioning it after a long idle period?

Lean can be exhaust leaks, intake leaks, vacuum hose leaking, no air filter, blocked pilot jets and so on.

White smoke however, is often an indication of oil being burned. That's another issue but one that might go away once the engine is warmed up and the rings have had a chance to bed in.
 
The bike is actually a restore project. I'm not sure how long it's been sitting but I finally got it to run. I lapped all the valves, got new shims that have the correct clearance, replaced all the gaskets, fully cleaned the carbs. Maybe I should have replaced the rings as well since I have noticed the oil level is going down, so I must be burning oil. I'm pretty sure the lapping went well tho, since the compression on one cylinder went from 80 to 115 psi.

I'll try to replace all the hose and see if that gets me anywhere. There was also a piece of hose that I'm not sure goes where. It was rather long, about 2 feet. The carb hose and fuel tank and airbox hoses are all in, does anyone know where this could go to?
 
I have all those hoses in, where the heck did this one come from...
 
Try your wd40 spray trick on two other places.
- right were the boot mates up to the head. Maybe the orings there are old and shrunk and hard.
- right were the choke/enricher rod goes into the carb. Is oring there.

I had similar high idle (as engine got warm) and hanging idle.
Replaced the intake book oring and it got some better, but it keept getting worse, untill I broke down carbs and replaced ALL the orings in the Cycle ORing kit this time.
 
So it's definitely the piston rings causing all the smoke. Put a little oil in the cylinder before the compression test and it shot to 155psi from 120. I think the air leak is actually from my tachometer cable. When removing it, some of the plastic came loose, but when I pushed it in, idle went way down. Still pulses, but hopefully after replacing cable and rings, it'll be all set.
 
Putting oil down the cylinders will have the same effect on any bike - until it gets burnt up. The test you made and the conclusion you have reached are not valid at this time in your investigations.

You need to run the bike for several 100 miles to see if the piston rings sort themselves out and are the cause of the smoke. In my experience, it's more likely the smoke is caused by valve guides or valve stem seals and the oil is seeping into the cylinders from above.

Leave that aside for now and continue to try and get the bike running correctly with the throttle responding and the idle behaving.

Greetings
 
Hmm but I just replaced the Valve seals. I thought they might be leaking, but I read that usually you'll get oil on the side of the spark plug, while for piston rings you get oil on the tip or over the whole center? Which is what mine look like. The amount of oil burnt is also extremely fast. Testing it on the centerstand for 10-15 minutes makes the oil level go from high to low.

Right now I have it idling around 1000 but it dips down sometimes and dies. Other times it randomly revs up to 2-3k. Pushing the tacho cable in seems to help, but not sure.
 
Pushing the tacho cable in won't make any difference to the actual engine speed. What is will do, if it's not properly inserted in the first place, is cause the tacho cable to slip and then the tacho will register inaccurately.

The question is; is it the tacho that revs up and down (according to how you push it in) or it is the actual engine revving up and down?

Can you post a video of this happening?

Greetings
 

Here's a video of it running. Not really sure how, but it sat at normal idle speed. Usually it'll sit there, and then ramp up and down. Maybe since today was cold and rainy? Its 50f outside and when I usually try it its around 85.

The reason it stays revved is because I unscrewed the throttle from the handle bar. Normally it returns back to close quickly on it's own. But as you can see, a lot of smoke, especially on the right side.
 
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