Motorcycle Repair: engine noise at idle and ignition timing/carbs, clutch basket, air leaks

Question
I am restoring a 1977 cb750k. The motor starts and runs but makes rattling noises at low idle and up to 1800 rpm. The noise is completely unheard at 2000+ rpm.  I have adjusted the intake and exhaust valves, gapped the points and set the ignition timing several times.  I believe those settings are correct.  I've cleaned the carbs inside and out and believe them to be in good condition. The bike will idle reasonably well at 1300 to 1400 rpm but falters below that range.  Have not yet syn'ed the carbs.  This bike has not been in service for about 7 years prior to a month ago.  I know nothing about the previous owner nor the way the bike was treated other than it was in rough shape comesticly.  Thanks for any suggestions!  Roger

Answer
Roger, You left out the indicated mileage on the bike and you don't say where the rattling noise is coming from! So some guessing will be involved here.
1. Factory idle setting is 1,000 Plus/minus 100. If it won't idle smoothly below the settings that you indicate, then the possibilities are:

Uneven compression, carbs out of synch, idle mixture screws not set correctly, ignition timing is not accurately set (check running for best results), spark advancer is stuck/sticking or advancer shaft is bent, air leaks at intake manifolds.

My guess at your concerns about rattling sounds are probably related to the primary chain/tensioner. When these engines get a lot of miles on them, the primary chains stretch out and the tensioner rollers get hard, so the tension is uneven at best. The noise is aggravated if the engine is not running properly.... see above.

Do a compression check first. If you have a low/dead cylinder, then the bike will always rattle in the lower end. If the carbs are out of synch you will have a rattle. If the ignition timing is uneven between 1-4 and 2-3 cylinder pairs, then the lower end will rattle. If the bike has a LOT of miles on it, the clutch basket may also be contributing to the rattle sounds, as they get loose on the shaft eventually.

Check also: www.sohc4.us for more info...
Bill Silver