Stiff Diaphragms/Weak springs??

I don't have a hanging idle but on my Seca 650 if I ride on the freeway for maybe 5 minutes and then pull off at the first light my rpms hang at around 1500 before slowly making their way down to 1100rpm. If I rev up the bike to like 3000rpm the rpms immediately come down but hang at 1500 for a few seconds before making it back to 1100. It does not sound like a typical vacuum leak and I've checked that. I've read that sometimes stiff diaphragms or weak springs can sometimes cause this. Is there a way to soften the rubber on the diaphragms and/or strengthen the springs?

 

That could be the culprit, but it's also caused by buildup in the slide bores. Polishing the slide bores (not the slides) fixes that. If the diaphragms are hardened, then only real remedy is to replace them with new. The springs won't be an issue, and only need to be replaced if they were damaged during a carb rebuild. The only other thing would be a blocked/ restricted air-jet. All work can be done by removing the top hat of the carbs.

Before you go that far though, make sure that the throttle cable is lubricated, routed properly, and that there's no crud built up on the carbs. Check that the throttle tube moves smoothly as well. I coat the inside with graphite just to make sure that it runs smoothly on the bar.

 

Thanks K-moe. I've checked the throttle cable and tube. All work pretty well. I'll check the slides again. What is graphite?

 

How did you set the idle mixture screws , did you use a color tune?

I would be tempted to richen it slight by adding a 1/2 turn CCW to all four and see if that helps

 

It's a mineral that's used for many things, including pencil "lead" and as a dry lubricant for locks, among many other things.