Topic: Changed throttle cable and...

[mlinder]

Now everything went to hell.
69 K0 750, with the separate cables to each carb.
Sputters and dies at idle, actually, sputters from a couple of cylinders below 5k rpm. Feels great at 5k.
Bike was running REALLY well before I changed cables...
I'll do a carb synch today, they may just be so out of whack from changing cables that it wont run, but you'd think the stock idle screw at stock settings would make it ok.... Anyway, as I said, carb synch today, but any other ideas welcome.

[Johnie]

I hear ya'...I have a 1970 CB750KO built in 1/70.  I bought it in July with 17,000 miles and it had not been run since 1990.  Unbelievably rust free.  Took me 3 weeks to clean the carbs and take care of the gas tank, but it fired right up.  I also had to replace a broke top end throttle cable.  Then you have to get those 4 bottom ends exactly the same length through adjustment.  For now I just keep it running with the throttle and will do the rest later.  I did the sync with my Morgan Carbtune and could not get all carbs synced perfect.  But I mainly just wanted to see where it was at in the sync area.  I know the previous owner had the points set wrong and now I have to check his timing and then I will fine tune with the sync last.  Good luck and tell me how it goes/

[Johnie]

So tell me how you raised the idle on your 69.  My large screws on the side of the carbs do not see to do much at all.  I did see a change in the vacumm when I had the sync on it and I turned those side screws.  But like I said, I could not get all synced to the same number.  And I could not get the idle high enough to keep it running.

[mlinder]

OK, so I'm dumb.
I had swapped 1 and 3 carb piston. Meaning they were in backwards. "Hey, why doesn't this thing run right?"
Duh
(expert, my ass..)


Johnie, didnt do synch today, but...

Remember that both the idle screws and the cable adjustments on the top of the carb bodies on these 4 cable carbs will stop the piston from decending all the way. You need to loosen the cables until they allow the pistons to drop to the idle screws, and adjust cables from there.
Also, if the mix on the air screws is too high or too low, the idel adjust screws may or may not do much of anything.

[Johnie]

mlinder...so how can you tell that you had 1 and 3 carb pistons switched?  Are they different sizes or something? 
Thanks

[mlinder]

No.

The pistons in 1 & 2 are opposite of 3 & 4.

The side that takes in air, from the airbox/pods/stacks/whatever has diagonal 'cut', to let air pass in more smoothly. The other side is 'straight', no cut.
With the pin guide one one side and the cutout for idle screw on the other, if you swap any from 1 or 2 to 3 and 4, the diagonal cut will be on the wrong side, presenting a crappy, flat edge for air to try to come around.

[736cc]

I did that backwards slide thing a while back, too. Duh.