!978 CB750 build

[Piute]

                                                             

 looks good

[Lucky 1]

Thanks Plute!!

i am already lining up my next project.

[shinyribs]

Yeah,rain sucks.It kept me off the bike this weekend,but gave me plenty of time to sort a few issues. I found one thing I was not real happy with.We have some rough roads around here and a few times the rear of the bike has hit pretty hard.I never thought it hit hard enough to bottom out,but I saw a witness mark on the bottom of my seat that says otherwise.Not cool.Gotta make a bump-stop of some sort I suppose.Glad you were able to ride some at least!
 Next project? One just finished and already on another?...cool. That how you do it

[Ghost-Flame]

Beautiful Lucky, just beautiful! I hope mine turns out half as good

[Lucky 1]

Well I like those exhausts. I would retain those if possible.

[Ghost-Flame]

Lucky 1 wrote

Well I like those exhausts. I would retain those if possible.

Really, They are just the stock exhausts as far as I can tell. I haven't decided yet. But, I was thinking of 4 into 1. to clean up the busy-ness of the bottom look of the bike.

[Ghost-Flame]

Lucky,

Who did your seat and rear fender work? Did you do it yuorself or someone else do it? I like it a lot.

[Lucky 1]

Ebay-Viet Nam. Very expensive.

I would not do it again.

[shinyribs]

You bought Vietnam off of eBay?! Boy,I bet that was expensive! Whatcha gonna do with all that land?

[Joey Stevenson]

Could you let me know what coils you purchased?  I have a 1975 CB750k.

Thansk.
Joey

[Lucky 1]

Dynatech coils. Dynatech ignition.

[cdaiscool]

Make rubber I bet. :D

[Dirty]

Looks good Luck1. You used a 78 tank and carbs correct? Is there two fuel lines from petcock to carbs? I think my 78 has aftermarket petcock on it because only one barb on it and a hose with a screw in it from carbs.

Also I was wondering if the contact breaker assembly is removed when installing a Dyna electronic Ignition? Had a malfunction loading bike on trailer ...ended up with a smashed cover and a bent breaker assy.!!!
Thanks, Dirty

[Lucky 1]

Yes I have 1978 carbs.
I have a 1978 tank and 1978 petcock. Just one fuel line.
Your petcock could be a 1978A Automatic.


Your lucky you did not break the engine case when you
damaged that breaker cover and breaker.

Anyway the breaker plate is very easy to get, and many parts places sell
The points and condensers with the plate. With the Dyna ignition you would not need the
Points plate.

[Lucky 1]

I have taken it out for two short rides of only about 1/4 mile.
Still working on the jetting.

One more week should do it.

I started with velocity stacks.
4 into 2 into 1 exhaust with internal metal baffle.
#45 idle jet.
#120 main jet.
.56 shims on the needle.

NOW
Stock air box.
Stock filter.
4 into 2 into 1 exhaust with internal baffle.
#42 idle jet.
#120 main jet
No shims on the needle.

Going to:
Stock air box.
Stock filter.
4 into 2 into 1 exhaust with internal baffle.
#40 idle jet.
#120 main jet
No shims on the needle.

If the midrange is thin then I will add
one.020 shim under the needle.

A #38 idle jet will NOT work with this exhaust.
So all that is left is the #40 idle jet. ( always the last one !)  LOL..lol

I am finding out that if the idle smells rich - it IS too
 rich, and needs a smaller idle jet.
Mid range is a separate problem.

Some other people are working on this same problem with the 1978 carbs
on the (other CB750)forum.
They are using dyno tests and Dynajet A/F ratio real time on the bike.
Getting valuable data.

The stock air box limits the intake area to 3.7 inches x 1 inch.
The carb intake area combined is 10.67 sq. inches.
So this air box is limiting the air intake to about 30%!
It is like a giant choke making the mixture  richer.

[Dirty]

On my 78 there are two hoses going to carbs. One is fuel delivery 1/4 ish and other is 3/16 ish going to fuel transfer pipe on #2 carb. Any idea on that?

[Lucky 1]

That is not fuel transfer tube. It is a float bowl vent tube that interconnects all of the float bowls.

[Lucky 1]

Final jetting 1978 CB750K


May 9, 2012

Final Set Up:

1978.
CB750K.
Stock air box with stock filter.
Exhaust 4 into 2 into 1 with metal internal baffle-Cycle Shack.
idle jet #40mm Edit*** went back to the #42
Main jet #120mm
Needle non adjustable -no shims.
mixture screws turned out 1-1/4 Edit*** 3/4 turn out.


Runs perfect. Road tested.

[Piute]

              Appreciate it Lucky ,
           All testing and Detailed info you gave out will be put to great use on my bike

     I was thinking that if I put A 17 tooth gear on with my stk.43 rear (630 chain)
   just to try out before I go to the 530 chain,stk..MY take off and low torx / speed no prob ,
                                     but need more for hwy 90% of the time.
                                         
             other bikes front 1=3 rear then would be around like 34 t.

[Lucky 1]

I do not think a 17 would fit even if you could get one,will it?
I would still go with the #530 chain. When you see the difference
in the chains you will not believe how much rotating weight will be removed.