Topic: help me get '71 cb500 running right

[uwjberg]

ok - i'm in the middle of my '71 cb500 resto. i cleaned the carbs and installed rebuild kits (stock jetting). new airfilter, stock airbox. new plugs, oil, etc. when i start the bike, it will run on choke for a bit, then surge to around 3k rpm. i can back the choke off, but then the rpms drop, and it has trouble running. when it's fully warm and running with no choke, the idle is very high - 3k rpm or so, even though i have the idle adjustment screw backed all the way off. it also surges a bit up and down while idling.

today i did the wd-40 thing, and sprayed it on the carb boots while it was running to see if that was it. didn't really seem to make a difference. still idling very high, still surging.

any ideas on what i should check first? my inclination was the carb boots, but the wd-40 should have indicated that, right? i'd hate to drop $100 on new boots if i don't have to. any other ideas?

thanks in advance...

[DanEarl]

Did you check the float level when you put the carbs back in?  I believe it's supposed to be 7/8". 

[TwoTired]

Are certain the Slow/Idle/pilot jets were cleared during your carb cleaning.

Did you feed the cleaned carbs dirty fuel?

[uwjberg]

yes, all clean fuel to the carbs.

if i recall, the floats were all fine. i'll have to pull them and double check. while i'm in there, i'll double check the slow jets, but i gave them all a really good cleaning and they looked fine when i installed them.

just for my info, what does float level govern (i know the fuel level in the bowl, but what effect does it have when floats are too low/high)? also - if the slow jets were plugged, wouldn't it be running at a lower rpm (less fuel)? i'm a bit of a carb newbie, so excuse the ignorance.

[number13]

Did you synchronize the carbs?

[1timduke]

Did you replace the fuel rubber?   Could have started to deteriorate over the years letting in little bits of gunk.

-Tim

[TwoTired]

The throttle valves are supplying fuel above 1/4 throttle, and are mostly closed off when the throttle is at idle position.  This is where the Slow/Idle/pilot jets dominate the supply of fuel to the engine.  Choke creates abnormally high carb throat vacuum, which can cause more fuel to be drawn from the mostly closed throttle valves.

The jet system(s) feeds fuel in the the carb throats where the vacuum draws from the fuel in the bowls.  The farther the fuel from the carb throat, the harder it is to get fuel to rise that far.  If you take each example of carb throat vacuum during run operations, lower fuel levels create leaner conditions, and higher ones cause richer conditions.  The float adjustments determine fuel level in the carb bowls.

Cheers,

[uwjberg]

i bench sync'd the carbs using A 1/8" drill bit as suggested here on the site. all new fuel rubber.

looks like i'll be pulling the carbs to check the jets again, and to re-check the float levels.

[TwoTired]

A bench sync does not replace a vacuum sync.

[uwjberg]

indeed - but it should get it close, correct? close enough that i would think that it wouldn't cause the high idle and surging? i know i will need to vacuum sync the carbs - and i will certainly do so -  but i wanted to at least get it running ok and make sure there were no issues before doing it. i was under the impression that the vacuum sync was pretty much the last step in tuning, as you have to do it while the bike is running, and at a constant speed - am i wrong?

[TwoTired]

One of the problems with the drill bit method is that while you can adjust all the carbs close to even, the adjusters can be position so that the slides may not fully close, even with the idle knob backed out.

If you have biased all the individual slide adjusters too far in one direction, it would explain your high "idle", particularly if you can't make it die by backing out the idle knob.

Doesn't really explain the surging, though.  So, you may have multiple problems, that a vacuum sync won't cure.

Cheers,

[1timduke]

When you bench synced, did you put the 1/8 bit on the intake side of the throttle slide?   If you put it on the other side then you have far too large of an opening I believe.   

Ask me how I found out

-Tim