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

[k1x2K2]

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

[Re-run]

Hi Tom.  Sounds like you have done the throw money at it so we don't need to cover that!

Put your idle mix air screws back in and lightly seat them and back out about 1.75 turns or so.
Double check your points gap. I think minimum is .012, any less than that and the points may not open right at higher rpm.
 Now get yourself a timing light, a cheap $12 model from harbor freight will do just fine. Get the bike warm and then pull of the points plate and hook up the light. Start the engine and let it idle. Now check your timing.
Adjust it so it is spot on. It helps if your garage is not brightly lit. Now rev to about 3000rpm and hold it there. Check timing again. If your ignition is not fully advanced, then you need to look at your advance mechanism.

After you have verified your timing, we will try something else.

With the bike idling, spray the carb boot to the engine and air box with wd-40. Listen for rpm changes. If you have changes, then you have air leaks. You need to tighten up the clamps, or get new ones if they wont get any tighter.

After that, try a plug chop. Get the plugs clean and take it too the road. Get that sucker up in the rpm range and then hit the kill switch and pull in the clutch. Stop and check the plugs. See what color they are.

Let me know. Also, what air filter do you have?

[k1x2K2]

Hi

thanks for ur answer.

I have checked the timing advancer, i have actually changed it too and the entire timing setup from a bike that ran perfect. i have also double checked the point gaps.

I will try the timinglight as just a way to totally eliminate any fault in the timing, even tho im pretty sure already the timing is set on its marks. i will double check it that way to see if the advance is doing what its supposed to do.

i already sprayed the boots checking for airleaks, i cant hear any changes in the rpms, but i suspect its a very tiny leak since its noticeable only on higher revs and then maybe i can have a leak even tho i cant hear it when idling/reving the engine?

Stock airfilter and box, i even taken the filer and the buttom of the box out, and it makes absolutely no diffrence on how it runs, thats weird isnt it? i also taken all 4 mixture screws completely out of the carbs and even that doesnt make a diffrence. i had many bikes, but never one that runs without the air/mixture screws. maybe this is normal on these bikes tho?

I have done a plug chop several times, if i run it below 6k where it runs pretty good, the plugs are a bit black, but not too bad, running above 6k the plugs are pretty white, so it seems they dont get fuel.

The clamps around the intake boots i cant tighten anymore, so i will have to get new clamps, but im thinking about getting new intake boots too just to make sure its not leaking anywhere.

Also another strange thing, when i come above 6k, i tried to yank the choke a lil just to see if it gets better, but just a tiny movement on the choke and the bike totally dies, its not like it sputters or growls or anything, it just dies, weird or what?

Thanks again for ur suggestions, any more thoughts on my problem is greatly appriciated.

Tom

[Re-run]

The plug chop tells a lot.
 Your bike will probably run in the idle range without the screws but it will make things leaner than normal.
But back to the plugs. They should be about a caramel brown or so.
For now, leave the idle mix alone.
Check out the mains. On the bottom of the jets is a number. I think for your bike it should be 105 or 110 but I am not sure. If you have not downloaded the manual here, do so and check it out to be sure.
Unscrew the mains and check out the emulsion holes to make sure they are all clear.
Open up the tops and raise the needle by lowering the clips. Try the second from the bottom slot.

Also, did you install any new carb kits? Do you know if the previous owner did? I ask as some of the kits are poor in quality. Keyster is kinda lousy, jets are ok but their needles do not work well. K&L is kinda the same way. The honda kits are the best.

Also, what spark plugs do you have? You should have D8EA.

[k1x2K2]

Hi

Thanks again for ur answers.

I switched coils again today, got new clamps for the intake boots, got them tight and nice, adjusted the ignition again with a timing light and also checked the advanced timing, everything seems perfect.

I also checked the valve clearance and its all good.

Now the bike runs a lot better, it doesnt missfire at all, and runs great above 6k, the strange thing now is that it got no pull from the buttom rpms and up to like 4-5k.

I have also tried to change the mainjets, it was 110 in them, but i put in 115, didnt seem to make any diffrence at all, or maybe that caused me to lose the buttom pull? i never remember how that is, does 115 give more air and less gas, or just more has? or is 110 actually giving it more fuel than 115? maybe i should try go back to 110 again now just to be sure.

I dont know if they have bought any rep kits for the carbs.

My next step now will be cleaning another set of carbs, new intake boots, change the jets back to 110 and see if it will finally be good. i feel like i got a step closer today, getting ridd of the missfiring issue.

Thanks again for all ur help, and keep throwing out ur thoughts :)


Tom