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

Old 06-07-2010, 04:01 PM
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Default 1972 CB 750 K2 Wont run smooth, Tried everything, HELP!


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

K1X2K2 is offline  
Old 06-15-2010, 02:22 AM
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Join Date: Jun 2010
Location: Australia
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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
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Join Date: Apr 2011
Location: Plymouth Ma.
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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  
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