Topic: 1978 CB750K carb changes

[Doggie]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

[conwaylale]

Read through that. It has everything you need to know about carb mods.
http://forums.sohc4.net/index.php?topic=5410.0

[flybox1]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

the K8's are lean from the factory.  An aftermarket pipe will typically make you leaner...
Accurate plug chops, starting with new plugs and a WOT chop, will let you know.
Jetting mods in my signature for a pipe slightly more open than stock might help...

[AintNoEasyWay]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

I've got a CB750 K7 with 4 into 1. I'm looking for an answer on this as well, as I imagine we would need similar jetting. I've just finished rebuilding my carbs with standard specs on everything, but waiting on a couple more parts before I try to start my bike.

[Doggie]

I'm rebuilding mine too. (3) times in fact. Mine is hard starting. All jets and passageways are clear, new accelerator pump, new needle and seats, floats set, idle screws set at 1 1/2 turns out, and it still doesn't want to start. Had to crank up the idle to make it start and then slowly drop it down. I'm ready to send it out.
                                Doug

[750K]

I'm rebuilding mine too. (3) times in fact. Mine is hard starting. All jets and passageways are clear, new accelerator pump, new needle and seats, floats set, idle screws set at 1 1/2 turns out, and it still doesn't want to start. Had to crank up the idle to make it start and then slowly drop it down. I'm ready to send it out.
                                Doug

Make sure your fast idle cam is set up properly, if its not raising the pistons enough cold starts are really tough. Your manual will tell you how to adjust it properly.

[brewsky]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

If you have a stock airbox and filter, and a MAC 4/1 with stock baffle, you may not need any changes, as it is plenty restrictive.

If you open up the airbox, use a K & N, or open up the baffle, have a different brand exhaust etc., you probably will.

Here are my results, yours may vary:

[Doggie]

Thanks for all the help....Doug

[flybox1]

I'm rebuilding mine too. (3) times in fact. Mine is hard starting. All jets and passageways are clear, new accelerator pump, new needle and seats, floats set, idle screws set at 1 1/2 turns out, and it still doesn't want to start. Had to crank up the idle to make it start and then slowly drop it down. I'm ready to send it out.
                                Doug

^^^then your fast idle cam is not working properly.
when you pull your choke knob, the cam, right between #2 & #3 carbs,  will raise the slides slightly, then give her a shot or two of gas with a throttle twist (accel pump shoots gas into the carb throat) and hit the starter or kick it over.  should fire easily.  Fast idle cam (+ the accel pump) properly working is tits!

[TwoTired]

the K8's are lean from the factory. 

Nit pick, perhaps, but I don't believe that true. 
They were peak tuned to match the induction and exhaust parameters of production units.  The PD's were considerably less sloppy than earlier carbs which made those earlier carbs "over rich" in nearly every operational range.

An aftermarket pipe will typically make you leaner...

Quite true, if you leave the PD's in stock tuning.
If you mess with perfect balance motorcycle tuning, things go awry.

[lucky]

If you have the stock airbox and stock air filter all you should need is a slight mixture adjustment -richer.

[Doggie]

And you mean richer, as going in on the idle screw. I was 1 1/2 turns out, now 1 1/4 turns?

                                      Doug

[750K]

Air/fuel screw, not idle screw. Pd's have one on each carb body, idle screw doesn't adjust fuel mixture.

[TwoTired]

And you mean richer, as going in on the idle screw. I was 1 1/2 turns out, now 1 1/4 turns?

                                      Doug

The idle screw sets the idle RPM.

The mixture screw (four of them) on the PD carbs adjusts mixture.  Inward leaner, outward richer.

[mrbreeze]

I'm rebuilding mine too. (3) times in fact. Mine is hard starting. All jets and passageways are clear, new accelerator pump, new needle and seats, floats set, idle screws set at 1 1/2 turns out, and it still doesn't want to start. Had to crank up the idle to make it start and then slowly drop it down. I'm ready to send it out.
                                Doug

If it a problem with the fast idle adjustment thats one thing but you didn't mention if the carbs have been vac. synched or the state of tune (valves,cam chain,timing,dwell,etc.)You can have the cleanest carbs on the planet but it won't make a sh!t if it isn't tuned & synched.

[Black 750K8]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

If you have a stock airbox and filter, and a MAC 4/1 with stock baffle, you may not need any changes, as it is plenty restrictive.

If you open up the airbox, use a K & N, or open up the baffle, have a different brand exhaust etc., you probably will.

Here are my results, yours may vary:

brewsky
 Do you have a pick of just where you placed the four 1" holes in your airbox? Would be a big help.
 Thanks in advance.

[brewsky]

I have been reading about carb changes (jets & needle). My 1978 CB750K has a stock air filter but after market 4 into 1 exhaust. Are there any changes to be made?

                                  Thanks, Doug

If you have a stock airbox and filter, and a MAC 4/1 with stock baffle, you may not need any changes, as it is plenty restrictive.

If you open up the airbox, use a K & N, or open up the baffle, have a different brand exhaust etc., you probably will.

Here are my results, yours may vary:

brewsky
 Do you have a pick of just where you placed the four 1" holes in your airbox? Would be a big help.
 Thanks in advance.

See reply #30 in this thread...
http://forums.sohc4.net/index.php?topic=129596.25

[Black 750K8]

And you mean richer, as going in on the idle screw. I was 1 1/2 turns out, now 1 1/4 turns?

                                      Doug

The idle screw sets the idle RPM.

The mixture screw (four of them) on the PD carbs adjusts mixture.  Inward leaner, outward richer.

TT:  Very good point a lot of people don't understand that the mixture screws on this (1978 750K) bike works different they are on the other side of the carb. So in or out on the screw is in reverse compared to the non PD carbs.

[Black 750K8]

I'm rebuilding mine too. (3) times in fact. Mine is hard starting. All jets and passageways are clear, new accelerator pump, new needle and seats, floats set, idle screws set at 1 1/2 turns out, and it still doesn't want to start. Had to crank up the idle to make it start and then slowly drop it down. I'm ready to send it out.
                                Doug

^^^then your fast idle cam is not working properly.
when you pull your choke knob, the cam, right between #2 & #3 carbs,  will raise the slides slightly, then give her a shot or two of gas with a throttle twist (accel pump shoots gas into the carb throat) and hit the starter or kick it over.  should fire easily.  Fast idle cam (+ the accel pump) properly working is tits!

Fly
 I'm in the learning stage on these carbs. I think I remember my uncle (bollingball) talking with you about this fast idle cam. But maybe it was someone else. He is not riding any longer and has a hard time getting out in the shop. But he gave me his bike.