Topic: high idle 78 cb750 pd42b carbs after jetting and other stuff

[mAd mOrdigan]

Hey guys I'll be brutally honest, i really dont know all the intricacies of carbs and completely understand how they work. Originally the bike had pods, 4-4 exhaust, the previous owner apparently kept the jets the same, 110 main, 35 slow, dont remember the fuel mixture screw settings.. it was pretty obvious when the bike would fall flat on its face under acceleration especially at high RPM's

So.. after lots of reading here. I put in 120 mains, 42 slow jets, have my fuel mixture screws 1 turn out, pretty sure i setup the floats right at (off the top of my head) 15mm, the accel pump definitely works as i see fuel squirt into all 4 carbs. I did all this, left the pods on, put on a motogp werks 4-1, checked valve tappet clearances, adjusted the cam chain tensioner, and checked the timing although didn't adjust it as it was pretty much on point. I fire it up first try but it idles at like 4K! choke or no choke. I turn (what im pretty sure is) the idle screw over by carbs 3 and 4 and after a lot of turns either left or right, the idle just goes up about 500rpms. Am i missing something here?

[750K]

Did you do a proper sync and is your fast idle cam set properly?

[harisuluv]

I can usually solve this with an adjustment of the mixture crews.  If you have PD style carbs and you are running rich as you seem to be turn them IN to adjust.  Perhaps a quarter turn at a time.  They are hard to reach.

If you are not getting much adjustment from the idle knob then it is also possible it is set incorrectly relative to the syncing of your slides.  However if you didn't fiddle with that or have the rack apart it is probably the former.

You don't need to understand everything if you can follow instructions. 

[flybox1]

Leave your mixture screws at 1 turn, and the rest of the bike as it is now.  You're close.
Your slides are unable to bottom out because they were bench syncd too high, whick keeps your idle high and not effected by idle set screw changes.
You can verify this by removing a pod, and looking in.  There should be little to no slide cutout showing when your idle set screw is out all the way, and your choke knob is pressed in.
Remove your carbs and re bench sync them to #2...

[mAd mOrdigan]

thanks for the advice! on my way to work I wondered if maybe I set the throttle cable too tight which seems like the equivalent of troubleshooting a computer that's not plugged in. But hard to imagine i set it so tight that it would idle at 4k! I'll check the slides tomorrow I do remember them each having the slightest gap but didn't see if all four were at the same height. I didn't really fiddle with anything else outside or on top of the carb except to open it up and spray some carb cleaner in there. I'll check it all out tomorrow and report back. I have my morgan carbtune waiting on the shelf, kind of excited to get this all sorted and try it out.

[lucky]

I can usually solve this with an adjustment of the mixture crews.  If you have PD style carbs and you are running rich as you seem to be turn them IN to adjust.  Perhaps a quarter turn at a time.  They are hard to reach.

If you are not getting much adjustment from the idle knob then it is also possible it is set incorrectly relative to the syncing of your slides.  However if you didn't fiddle with that or have the rack apart it is probably the former.

You don't need to understand everything if you can follow instructions. 

IF you did not inspect the mixture screw O rings then all bets are off.
The carbs need to be off to do that easily. Make sure the O rings are not mangled.

Otherwise turn the mixture screws all in lightly until they stop and back them all out 1 full turn.
#2 is the hard one to get to.
But if you have the right screwdriver it helps.
You can go to the supermarket and in the Duracell battery display one of the packages includes a small screwdriver with special handle and that is perfect for adjusting the mixture screws of the 1978 CB750 carbs.

Put a mark on it with a magic marker so you know
how many turns out you are setting the screw .