Street Motorcycles and Parts Discussion, Questions and answers that pertain to street motorcycles and parts can be found in this forum.

1972 CB 750 K2 Wont run smooth, Tried everything, HELP!

Old 06-07-2010, 04:01 PM
  #1  
Junior Member
Thread Starter
 
Join Date: Jun 2010
Posts: 1
Default 1972 CB 750 K2 Wont run smooth, Tried everything, HELP!

Hi

Last year i bought a 1972 cb750 K2, as far as i remember the bike ran smooth and great last year, i didnt really ride it much, or hard at all, so im not sure if this problem came during the winter storage, or if it has been there from before i bought it.

Problem is, when idling it idles pretty nice but u can hear a little "pinging" from number 3 and 4 exhuasts. drive it gently and it runs smooth and has great power. Running it hard, like over 6000rpms, it starts stalling, sputtering, shooting from the exhuasts, on all 4 i think. let go of the throttle a lil, it revs up a bit more, but still stuttering and sputtering and wont run clean at all on high revs.

Its driving me crazy, the engine is in good shape, except for this.

So far i have;

Cleaned the carbs, changed the sparkplugs, the condesators, point breakers, the timing advancer, i even tried a diffrent igniton plate with points, condensators and everything on. also changed the coils, adjusted the mixture screws. one odd thing is, i can take out all of the mixture screws and the bike runs just the same, thats pretty weird isnt it?

The last parts makes me wonder if i have a airleak in the inlet tubes between the carbs and the head, but i tried spraying them with start gas, and the engine doesnt react to that, but i guess the leak could be so small it will only be a diffrence at higher revs?

I also gone over the electric net to see if any earths/grounds or anything has come lose, but so far i havent found anything, anyone know of a ground/earth point in a smart location i should check?

Has anyone any suggestions on whats wrong or maybe even recognize the symptomes?

any help is greatly appriciated.

Thanks in advance

Tom
K1X2K2 is offline  
Old 06-15-2010, 02:22 AM
  #2  
Junior Member
 
Join Date: Jun 2010
Location: Australia
Posts: 3
Default

Had the same problem with my 1979 CB 750, it turned out to be a small vacuum leak in the number 3 manifold. It wasn't picked up with start gas, how I found it was to let the motor warm up and put brake fulid on the manifold and small bubbles appearred telling where the leak was. It took me awhile to find it. Also it could be the inlet tubes between the carbs, old age may cause them to suck in under revs, restricting vaccum, or incorrect rubber will cause the same problem. Hope this helps.
Shagga is offline  
Old 04-07-2011, 04:26 PM
  #3  
Junior Member
 
Join Date: Apr 2011
Location: Plymouth Ma.
Posts: 9
Default

That is good advise. Did you put new spark plug caps on when you did the plugs? I have found that when I do a tune up if I adjust all the valves as well as the cam chain tensioner correctly it quiets down the motor and it runs substantially smoother. One other thing you should do if you want it to run perfectly, get yourself a manometer if you don't already have one and sync your carbs. Good luck!
snaglehawk is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
antanipoith3246194
Coffee Break
3
07-04-2013 10:29 AM
JerseyRider
Street Motorcycles and Parts
0
12-24-2012 10:02 PM
nate
Introduce Yourself
1
10-13-2006 06:16 PM
imanewbie
Performance Tips and Shop Talk
8
07-01-2005 02:30 AM
PHAT-BOY
Street Motorcycles and Parts
2
03-11-2003 06:31 PM


Thread Tools
Search this Thread
Quick Reply: 1972 CB 750 K2 Wont run smooth, Tried everything, HELP!


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.