Topic: CB500 erratic idle RPM

[dudah]

i'm having some idle issues with my 72 cb500f. when i start it to warm up in the morning, it starts right away, and jumps to about 2200 RPM. i usually put it in first gear and let out the clutch a little bit and the idle drops down to about 1100 and stays there.

once i start riding, with the clutch is disengaged, the RPM's jump to a little more than they were before disengaging the clutch, no more than 4100 RPM, then settle around 2200. when i come to a stop light and have it in neutral it idles around 2200 rpm. again, once i put it in first gear and let out the clutch a little bit it will settle down to around 1100 RPM but the problem resumes once i start riding again.

i had some vacuum leaks earlier that i fixed. had a torn carb boot and patched it with electrical tape and really cranked down the clamp. sprayed a lot of WD40 around the boots and it seems tight, can't get the engine to jump when spraying WD40 all over the boots.

thought maybe the idle jets were clogged so i ran a bottle of seafoam and that helped only a little bit, before it would idle past 4k. thinking maybe the idle jets are still clogged and i should run seafood without riding it around, just idling in my driveway. air fuel screws are about 1 turn out.

[flybox1]

adjust your idle set screw?

[dudah]

forgot to mention that. i set it to the point where when i put it in first and i can get it to idle around 1100, otherwise i don't mess with it. loosening it when it's idling high has no effect.

[flybox1]

barring any remaining vacuum leaks, and assumed properly cleaned, bench sync'd and vacuum sync'd carbs (seafoam will not clean your carbs.  you must manually do this yourself), do your slides completely close when your idle set screw is turned all the way out?

[TwoTired]

The idle knob is set for 1050 RPM +/- 50 RPM, AFTER the engine is brought to full operating temperature.  See Honda shop manual.

If return to idle is erratic after warm up, then check vacuum sync, free cable movement, and proper slack in the cables.

[dudah]

thanks for the responses! when i bought the bike, i don't remember this being an issue. this is my first bike, and i did a clean and rebuild on the carbs. they were super clean, but needed new gaskets so figured i'd do the whole thing. one thing i forgot to do was run guitar strings through the idle jets. i did a bench sync and a local shop did the vacuum sync.

adjusting the idle screw doesn't really do much. when it's idling high i can screw it all the way out and it has no effect. i read that the head gasket seal may cause a vacuum leak - shot WD40 all over there and couldn't get the RPM's to jump. same with all the other seals around the carbs.

i've started to use the choke to help warm the bike up in the morning. it starts fine without choke, and idles around 2500 RPM. if i tweak the choke - about 85% closed, i can get the idle to go down to 1000 RPM and stay there. i'm doing the same thing at stop lights now.

i recently set the points and the bike seems to be running fine, but haven't got a timing light yet to check. sometimes when it's cold and i'm in 1st it sputters for just a second - could this also be causing the problem?

something worth mentioning is that it was missing the airbox when i bought the bike. i made a makeshift one out of duct tape that seems to work fine. the top of the filter is open while the front (facing the rear tire) is a little covered by a piece of plastic i'm using to press it against the air boot box. if this isn't making a good seal could that be throwing it off?

[TwoTired]

adjusting the idle screw doesn't really do much. when it's idling high i can screw it all the way out and it has no effect.

Almost certainly the individual slide adjusters were tweaked/skewed enough to prevent full closure of the slides.  And this is why the idle knob can't dominate the idle speed or the proper low position of the slide.  You'll have to now start over with the carb sync, starting first with making sure a slide can reach full closure and declare it the master carb.  Then adjust the others to the master's vacuum and never change the master slide position again.

i've started to use the choke to help warm the bike up in the morning. it starts fine without choke, and idles around 2500 RPM. if i tweak the choke - about 85% closed, i can get the idle to go down to 1000 RPM and stay there. i'm doing the same thing at stop lights now.

You'll soon begin having problems with the plug fouling with soot, with such frequent use of choke.

i recently set the points and the bike seems to be running fine, but haven't got a timing light yet to check. sometimes when it's cold and i'm in 1st it sputters for just a second - could this also be causing the problem?

Who knows?  It can sputter from any number of causes, including fouled spark plugs, and bodged point gaps and timing.

something worth mentioning is that it was missing the airbox when i bought the bike. i made a makeshift one out of duct tape that seems to work fine. the top of the filter is open while the front (facing the rear tire) is a little covered by a piece of plastic i'm using to press it against the air boot box. if this isn't making a good seal could that be throwing it off?

Of course screwing up the air box arrangement can make the engine run unpredictably.  But, you already know this.