Topic: 1977 cb750k not running on all 4 / misfiring

[Xander^]

having some loud knocking sounds when i start up my bike.. pipes were getting pretty warm except for number 3, it was slightly less warm than the rest.

i've installed new spark plugs
timing was set
carbs rebuilt and looked over twice

could it be my coils? they are still the stock ones..

heres a quick vid.

[flybox1]

are all plugs sparking?
what do the insulator tips of the spark plugs look like?
are all carb bowls filled with the same level of fuel? (clear tube test)
Who did your carbs and are you sure the pilot circuits are all supplying fuel?

[Robbo]

Really tough to diagnose an 8 second video clip.


Sent from my iPhone using Tapatalk

[Xander^]

Really tough to diagnose an 8 second video clip.


Sent from my iPhone using Tapatalk

here is a longer vid from the first initial start 6 months ago.. still sounds the same.
since then ive done those 3 things ^ i posted in the first post.

[Xander^]

are all plugs sparking?
what do the insulator tips of the spark plugs look like?
are all carb bowls filled with the same level of fuel? (clear tube test)
Who did your carbs and are you sure the pilot circuits are all supplying fuel?

when i installed new plugs yesterday, the old ones (which were only used for 5-6 start ups) were not looking bad at all. ill try to get a picture.. one of them wasnt are dark as the others tho..

insulator tips looks brand new.. ill have to go check since ysterdays start up.

ill have to check the bowl levels, but 6 months ago i was getting a leak in number 2 carb, since rebuilding them no leaks.. had someone experienced with cbs do my carbs.

[Xander^]

Here is what the old spark plugs looked like. Old as in used for 5 start ups or so..
The other two look like the darker one from this picture

[scottly]

That motor sounds like somethings being hammered on!!?? Do you have any oil pressure??
Oh, and the black sooty spark plugs are carbon fouled= too much fuel/too little air.

[chicagobob]

In addition to the plugs, I'd check the #3 carb.  Is it getting fuel? (open the drain and see if any flows out.  If not, the float could be stuck).  Is the float at the correct level?  Any clogged jets?

[Yamahawk]

It sure sounds like a valve needs set, like the clearance is too loose on one cylinder. Also, I noticed that you seem to be running a 4 into 1 header, without any type of can on it, or baffle in it. This isn't a good idea, as you need some back pressure in your engine. Get a slip on can and install it.
I would go through the valve setting procedure, and see what you get. That tapping noise could be a bearing, but most likely is in your valve train. And, as Scottly said, Check Your Oil Pressure!!! If you don't have any oil getting to the bearings, that could destroy the engine.
Charlie

[Xander^]

In addition to the plugs, I'd check the #3 carb.  Is it getting fuel? (open the drain and see if any flows out.  If not, the float could be stuck).  Is the float at the correct level?  Any clogged jets?

i can check that, nothing should be clogged, rebuilt the carbs and went thorugh it all twice. level of float could be the only thing off but even then shouldnt be. if float was stuck id get overflow from the bowl right?? that hasnt happened either.

[TwoTired]

In addition to the plugs, I'd check the #3 carb.  Is it getting fuel? (open the drain and see if any flows out.  If not, the float could be stuck).  Is the float at the correct level?  Any clogged jets?

i can check that, nothing should be clogged, rebuilt the carbs and went thorugh it all twice. level of float could be the only thing off but even then shouldnt be. if float was stuck id get overflow from the bowl right?? that hasnt happened either.

Heh, if you flip a light switch twice, it will still be off.

Anyway, do the same thing wrong twice and it will still be wrong.  Seems most rebuild reports on this forum take three times minimum to get it right, and often more.   The slow jet are pressed in and if not yanked and cleaned, can cause issues.

A stuck float can be stuck in either open or closed position.  Only one of these positions creates an overflow.  And depending on bike position may overflow into the carb throat rather than a plugged overflow tube.

Cheers,