Topic: CB750 carb synchronizing and tuning.

[efyou]

Aloha;

Kei hin carbs and pods on a chopper (Yeah, I'm getting frustrated but not enough for $600 Mikuni solution yet) .  Had 115 mains and experienced typical loss of power at 3/4 to WOT.  Changed to 120 mains and retained 40 slow jets.  Followed Clymer manual for synchornizing carbs and can only get 5" of vacuum.  Manual says adjust to 8". With 5" vacuum, and idle adjusted, needles position is at or below the 1 thread minimum recommended by Clymer.   Bike is still running rich.

I went up in jet size to get rid of power loss and it worked tip-top.  Pulls like a train going uphill WOT.  Now running rich (little bit of smoke at idle and revving, black sooty plugs after riding).  I'm not sure where the needle position is set (bought carbs rebuilt as "stock") but am assuming that I need to raise clip to drop needles further into seat to lean out?  Am I on the right track or lost going down some dark siding?

Mahalo

[mystic_1]

You won't get the full 8 inches of vaccuum without the stock airbox.  Regarding sync, the most important thing is that all four carbs are drawing the same vacuum.

Needle clip position doesn't affect 3/4 to WOT very much, that's mainly for tuning the midrange.  Here's a handy chart that someone posted a while back:



mystic_1

[efyou]

Understand about the synch (that's good news).  Although there's lotsa playing around with dampening guages to get them not to flutter "more than 1 graduation".

How do I lean out mixture?  Maybe I screwed up idle adjustment (before bike was fully warmed to op temp.)?  I currently have idle jet backed out 1 turn per Clymer, and never readjusted from carbs (just main idle screw.)  Is idle jet where I lean out mixture at idle after or as part of synch/tuning? 

I'm preparing to start the process all over again, and am hoping I've moved down the learning curve a bit.

Mahalo.

[Duke McDukiedook]

Ah yes, the fun of tuning pods. Good luck.
You're going to have to experiment until you get the right combination.
There is no cut and dry formula to get them to work properly if you ever do.
There are tons of threads on the subject if you run a search, might help you get the winning combo.

[efyou]

Am I at least right about the idle adjustment?  i.e.: readjusting from the initial 1 turn out?

Mahalo

[BobbyR]

If you are synching the carbs what vacum they are pulling is not importtant. What is important is that they are all pulling the same.

If you are mixture adjusting that is a different matter.

[efyou]

Aloha;

I thought I was doing both.  I think that Clymer synch procedure (don't have the book with me) says to adjust idle first before synching.

So the way it stands now is even though I can't get 8" vacuum with pods, I can synch.  I think that I still need to adjust mixture, because I have rough idle.  Seems rich; a little black smoke when at or when I rap throttle from idle. 

[Duke McDukiedook]

Synching properly will give you a smooth idle if you performed a tuneup before hand.