Topic: 78' CB750 acceleration issues

[zeus87]

( 78' cb750k8 )got the carbs rebuilt, cleaned completely and synced.  Now it runs real strong and idles mint at 1000 (warmed up) but i have terrible throttle response without the choke half engaged.  It is bone stock except for a new box filter which was an upgrade to the K&N, i set the air screws at 1.5 out.  Any pointers?  thanks

[754]

Sounds like the accelerator pump is not squirting..

[zeus87]

hmm, i drilled it to get it back to new....how do i test it without pulling the carbs for the 5th time? haha thanks!

[754]

not sure what you mean by driled it??

I think they can be worked on w/o removing carbs... I got a set laying on the bench, but have not run them in decades..

[ekim98]

I can change jets on my 78 750k on the engine, but I run pods so I may have a little more room to work??

[zeus87]

i can make room, what is the process to make sure it works? thanks again!

[jjwaller]

i can make room, what is the process to make sure it works? thanks again!

If you pull the airbox off, you can see gas spraying through the brass tubes in the front of the carbs
when you actuate the throttle.  But then again, seeing you have the airbox off, which is the hard part about
pulling and especially reinstalling carbs, you might as well pull carbs and test them on bench. The accelerator pump is actuated by a spring when the throttle arm is twisted.  Spring might be stretched out and no longer
has enough tension to work the pump.  My carbs were like this.  Installed a new spring and it works great. Pulling the carbs is a pain, but it's a whole lot easier to see what's going on then with them installed.  Just pulled and reinstalled mine tonight to Re-jet to 115 mains.
Like I said, the worst part is getting the airbox rubbers back on the carbs properly.
Hope this helps.

[754]

I hate them airboxes so bad..

 Last one I ran was in the 70,s.

[zeus87]

alrighty, i will let you know what i find

[754]

If you have had these off 5 times, I think you may be working in too big a hurry..

[zeus87]

alright, well im getting no spray from those verticle posts, and all i hear is a slight vaccum noise from the accelerator pump.  what now?  is there a way to prime the pump?  thanks!

[TwoTired]

The Accelerator pump is at the bottom of #2 float bowl.
There are two check valve in the system, one on the inlet to the diaphragm one on the outlet. They must be free to move and seal in their seat under pressure.  They aren't really removable and are held in place by a square block. Carb cleaner to dissolve sticky bits is one way to get them clear.
Check the diagram itself for resiliency. There are two holes in the outer band that must be as open as the holes it mates to.  You can test the pump with the bowl off the carb,  Fill it with gas and actuate the pump shaft.  Wear eye protection.

After the pump is verified, then you check the path to the squirters and the squirters themselves to verify they aren't clogged up.

Cheers,

[eurban]

There is also the mechanical / actuating part of the acell pump that can have issues.  Whacking open the throttle indirectly (there is a buffer linkage) pushes the pump rod down.  I have seen the springs worn / improperly installed and or the buffer linkage rusted up to the point it doesn't move freely and or the gap between the linkage and the pump rod too large (it should be essentially zero gap)  Basically you should be able to see the rod push down when you whack open the throttle.

[zeus87]

got it!! the o ring inside the pump cover was shot, i saw bubbles coming from the outside.  replaced it and it runs GREAT.  thanks all!

[zeus87]

well well well, i finally got enough done to take it around the block.  Still having carb issues!!  it needs half choke to be real smooth from idle to about 4000(it seems fine normally, but under load it sputters) after 4000 it doesnt want to accel at all(still half choke).  If i drop the choke after 4000 it runs great 4000 and above.  Again when i removed the carbs, the air screws were out 1.5 turns, after fully cleaning and doing full rebuild kits, i added the factory style K&N filter.  On assembly i backed the air screws out 1.5 (as what they were prior to dissasembly).  What do you think is going on here?  i found that my float is a little off on carb #1 because im dripping fuel when the kickstand is down.  Im open for ideas, thanks everyone!

[zeus87]

here are some shots of the plugs, one of them touched the upper area around the threads and got some dirt on it, so i wiped them all a little (probably doesnt make for an accurate picture) but anyway, plug 1&4 were pretty white, and 2&3 were not as white as 1&4.  Maybe electrical? 

[jjwaller]

It looks like you're awfully lean.
The plugs also to me look like extremely lean.
You said you put in a K&N, Did you go up at least one jet size?
(the 78's came jetted extremely lean for EPA)
However, it sounds like you still have clogged low speed jets
if performance at sub 4-3000 needs choke.  The plugs
agree with that.
On my old 78 F bought new in 78, I installed a K&N and
drilled out the exhaust baffles (hey, I was young and stupid! )
anyway, installed 120 mains and performance went way up.
Fuel mileage went from 42 to 50 MPG. Yes with bigger jets.

[zeus87]

no change in jets, how about going 1/4 turn out? or does it look like i should go 1/2 out.  Anyway, i will give it a try.  I want to go with pods and kirker exhaust in the future, where can i purchase bigger jets?

[TwoTired]

Are you reading the clymer manual?

I ask because the 78 carbs have an Idle Mixture Screw (IMS) that Clymer may not know about.  These adjust backwards from an Air adjust screw.  Out makes IMS richer.
Also, the IMS has a very shallow taper, which, if over tightened, can damage the needle and/or seat, or wedge itself in so tight, that the tip breaks off and blocks the idle circuit output completely.

Your performance issues/ problem description still seems like a slow circuit carb issue.

Cheers,

[jjwaller]

no change in jets, how about going 1/4 turn out? or does it look like i should go 1/2 out.  Anyway, i will give it a try.  I want to go with pods and kirker exhaust in the future, where can i purchase bigger jets?

I'd go at least 1/2 to 1 turn, just to try it, but you need to re-jet at least 1 if not 2 sizes up.
CRC , among many others, has jets for our carbs. Check the FAQ for parts suppliers.  Best reference anywhere!
And as TT said:
Also, the IMS has a very shallow taper, which, if over tightened, can damage the needle and/or seat, or wedge itself in so tight, that the tip breaks off and blocks the idle circuit output completely.

Your performance issues/ problem description still seems like a slow circuit carb issue.

Be very careful of breaking off the IM needles.  Nasty.  Have an XS1100 with this problem.
Check your low speed circuits.  There's a reason they  call it triple cleaning carbs.
Have fun

[KeithB]

Those plugs do look like you are running way lean!
I added low restriction exhaust and air filter pods.
Drilled out the main jets with a 1/32 drill bit which is about .031.
Runs great!

[zeus87]

now what exactly is the SLOW CIRCUIT end of carbs?  ha thanks!

[754]

I think this is the time, where you should pick up a manual. Then read, reread, and then read again  till you somewhat understand the carb section..pretend there will be a test..

Actually there already was a test, If you had your carbs off 5 times, it looks like you did not pass.

And now you are asking, what is the slow circuit...??

[TwoTired]

now what exactly is the SLOW CIRCUIT end of carbs?  ha thanks!

The slow circuit in each of your carbs consists of:
the slow jet (pressed into the carb body),
The slow jet emulsion tube (attached to the slow jet),
the slow air jet (at the entrance to the carb throat),
the Idle Mixture Screw (just in front of the float bowl),
the delivery hole in the carb throat, (not too far from the pointy end of the IMS,
and the passageways drilled into the carb body to allow fuel and air movement between the aforementioned components.

Can you guarantee all of the above are clean and free to operate as intended?

[zeus87]

well i got the plugs looking good, but sure didnt fix my dragging throttle response.  2.75 turns out on the air screws, and plugs look to be much happier.  I understand the carbs, but just never herd the term slow circuit (relax im brand new with this).  I have a hard time believing it is the carbs due to cleaning them spotless twice, so what do you think about timing?  Again i have awesome throttle response with 1/2 choke but without it hesitates and sputters a little.  Without choke it runs great above 4000, but with choke it sputters at high rpms.  Let me know,  if you say carbs, i will go to them.  (p.s. i posted about checking ignition timing with a gun, as i really dont know how and i dont want to burn out anything)  Thanks guys..