Topic: 78' CB750K8 Carb tuning

[zeus87]

well, i just rebuilt my carbs...Keyster kits.  All is good, fired right up runs strong.  Now for tunning?  It hesitates bad when whacking the throttle from an idle, i deffinetly have some hot and warm pipes.  i dont have a sync gauge for now, but would like to get the pilot screws dialed in.  Also i put a K&N stock filter in, that is it the rest is bone stock.  #3 cylinder has some backfiring going on, but a 1/4 turn out cleared that up.  Any tips or suggestions?  Also i have amazing throttle respone when the choke is up, but only breifly as the RPMs naturally rise with it up.  I started out with the pilot screws out 1 1/2.  thanks in advance!!  also let me know if a video would help out. -J

[TwoTired]

You can't properly adjust the pilot screws until the engine draws equally from each carb.  Carb sync first. And, get all pipes evenly hot.  Pilot fuel jets clear?  Did you pull them for cleaning?

Throttle "whack" issues are most likely accelerator pump related.  Check valves - stuck, tiny squirters clogged, diaphragm torn, passageways clogged, etc.

Cheers,

[zeus87]

alright, i will get the sync done.  Another thing i noticed, number 4 exhaust is twice as hot as 1-4, but the plug comes out black.  i tried backing out the pilot screw, and i had no change in exhaust temp.

[Cvillechopper]

Like TT said, got to get a synch done before anything else.  Assuming all of the passages are clear the synch should get you to the point that you can start working on adjusting the pilots.  You should start with them all set equally after the synch and adjust from there.  Also, was the bike running well before rebuilding the carbs or is this the start of the project?  Just wondering if you've gotten all of the other adjustments done already that should precede carb work such as cam chain tensioner, tappet, and points adjustments.

Good luck

[zeus87]

project, wasnt running great.  Yes i already adjusted everything else.  I noticed that pilot screw on the 4th carb (cyl 4) doesnt seat as far as the others.  1 1/2 turns out shows alot more threads than the others.  hmmm..

[lrutt]

I'd recommend going through them again and check the low speeds. You have have dislodged the oring or small washer in the #4 pilot and that is why it's out farther.

Also, There are 2 K8's I know of that the accelerator pumps were not functional from the factory. The small diaphram in the #2 carb float boal did not have a hole punched in it for gas to fill the accel pump chamber. I know mine didn't. I made the appropriate hole and then had pumps and what a difference that made. I'd suggest you pull the air box and work your throttle to test the accel pump circuit in all 4 carbs. This could also be your problem.

Plus you know you can pull the pilot jets out very carefully with pliers and make sure you get those passages clean. Lots of writeups on that if you search on K8 carbs.

[TwoTired]

project, wasnt running great.  Yes i already adjusted everything else.  I noticed that pilot screw on the 4th carb (cyl 4) doesnt seat as far as the others.  1 1/2 turns out shows alot more threads than the others.  hmmm..

The pilot screws on the 77-78 models were mixture screws ,that adjust the total flow from the pilot circuit which a fuel air mixture in itself.  The screw behaves as a fuel  screw, turning it out allows for a richer mixture.  The pilot screw tips have a very shallow tapper and can get wedged into the carb body if installed too tight. Unscrewing them, then breaks off the tip and totally blocks the the pilot fuel flow.   A blocked pilot circuit will make the cylinder burn very lean and hot. Given what you say about seating compared to the others, I'd say a close examination of the carb body pilot screw assembly is in order.

Cheers,