Topic: how to clean timing advancer?

[manguy]

my 1981 cb650 keeps having a high idle after I rev the engine past 3000. it will stay at 3000 untill i bring it down by slowly letting out the clutch. I contribute this to a bad timing advancer because there are no leaks in my intake, and the carbs were just cleaned. I looked at the advancer and it is a little rusty.

is there anything i can do to clean it? or do I have to buy something?

[scunny]

the cam pulls off the base, then a little tidy up with some steel wool or scotchbrite pad. don't think that's got anything to do with a hanging idle tho.

[Gordon]

A sticky advancer can cause hanging revs, but so can unbalanced carbs.  Did you synchronize the carbs after the cleaning?  What was your method for determining the absence of intake leaks? 

[manguy]

i just bench synced the carbs based on the butterflies. and i sprayed carb cleaner all around the intake boots and there wasn't any difference in idle. i have yet to do a vacuum sync. could that be the culprit? I wouldn't know where to attach vacuum lines anyway. The only place i see is after the carb before the intake boot. which would defeat the purpose of testing the vacuum on the carb.

[scunny]

butterflies are the choke, it's the sliders that are synced. don't know if you have CV or PD carbs but they both have ports for syncing on the motor side of carbs.

[manguy]

cv carbs. you dont have to make sure the butterflies on the motor side are synced too?

[scunny]

humble apologies, forgot the CV's had two sets(I blame age)the front set control the vacuum.

[Gordon]

i have yet to do a vacuum sync. could that be the culprit? I wouldn't know where to attach vacuum lines anyway. The only place i see is after the carb before the intake boot. which would defeat the purpose of testing the vacuum on the carb.

Yes, unbalanced carbs, as in not vacuum synchronized, is one of the common causes of hanging revs.  The screws plugging the vacuum ports before the intake boots are where you attach the vacuum guages.  You're not measuring the vacuum on the carb, you're measuring the vacuum created by the piston on the intake stroke which, among other things, is determined by the position of the throttle valve. 

Before doing a vacuum synch make sure all other tune-up procedures have been done first. 

[manguy]

oh interesting!!! im used to aircooled vw's and you measure the airflow through the carb inorder to sync them. I guess the only air that gets into the cylinder is air that goes through the carb.... so its the same thing! you're genius!!! lol
i haven't really done a tune up on the bike like i should. valves and timing. the guy said it was good to go. it wouldn't hurt though. im sure there is a resto thread i can find that'll tell me yeah?

is the flag looking screw the air mixture screw? syncing is done via the grub screw that controls each butterfly on the throttle?

[Gordon]

I was also first introduced to the concept of carb synchronizing on an old VW.  I had a '74 1800cc dual carb bus, and I still have the Uni-syn I used to synch the carbs.  It's basically the same concept on a sohc4, but much more sensitive to slight differences between carbs and you absolutely need four gauges so you can see the reading on all carbs at the same time. 

All other tune-up items will affect the cylinder vacuum, so everything else needs to be done before performing the synch.  Maybe the PO did a good tune-up, maybe he didn't.  It's up to you whether or not you want to spend your time doing a carb synch based on an assumption that may be incorrect.  It doesn't take that much time to at least check the settings. 

I've never done a carb synch on a 650 before, so I can't help you with specifics on your bike.

For step-by-step procedures and technical specifications you can find free manuals for download at the top of the sohc4 tech forum page.