Topic: Question for the PD42 carb guys ('77-'78)

[Stoli]

The reason you have a hesitation when gibing it quick throttle on this 78 is because the slide needle needs to be one step richer.
Otherwise the clip on the needle needs to be moved down from the top one notch.

I have it shimmed 1mm already. Do you think I need another washer or two?

Current config:

APE Pods
Mac 4-1 header w/ Cone engineering coreless cone/reverse cone
Slow: 42
Main: 135
Needles: 1mm shim (2 M2.5 washers honed to .0195" each)
Screws: 1 turn out

[flybox1]

just one shim on each. sound about right.... go for it.

[MOONDOGNYC]

Stoli, any word yet

[Stoli]

Stoli, any word yet

I decided that the hesitation is normal (my son's stock '77F does the same thing). Mind you, I really have to try to make it happen - overly aggressive throttle snap at speeds over 45mph. It's not a movement that you would normally make. It kind of makes sense that if you were cruising along and instantaneously opened the air passage to full throttle that the fuel will take a moment or two to catch up. The accel pump compensates at the lower throttle range but I am already over 1/3 throttle when I can cause the hesitation. Since the bike is running so well, I decided not to monkey with the current config.

I also solved my starting woes by finding a sweet spot at about 1/2 choke. I put it at full choke to start and then slowly move it down to 1/2 choke immediately after it starts and it will warm up there at 1500-2000 rpm.

[flybox1]

do you give it 2 full turns of the throttle before pulling the choke knob to 'prime' the carb throats with fuel?

[K3Owner]

Stoli,

Glorious bike.

I'll add a little here. The carbs on these machines are direct slides. Grabbing a handful at low RPM in high gear will produce a delayed response, period. The drop in vacuum cannot be accommodated by the carb design that quickly. This was solved for us ham-fisted types when the CV carb was introduced. I could do this all day on my Bandit 1200 and it had no accelerator pump. The slides will rise gradually based upon the vacuum level making transitions very smooth. Still, we messed with the springs to alter (slow) the rate when we went to pods and exhausts so CV carbs have their own challenges.

Roll on the throttle and find that max rate for your machine.

[Dave Voss]

The carbs on these machines are direct slides. Grabbing a handful at low RPM in high gear will produce a delayed response, period. The drop in vacuum cannot be accommodated by the carb design that quickly.

This is well stated, and probably not stated often enough about these older motorcycles, which really do prefer to be ridden 'briskly', as opposed to 'abruptly'.

[flybox1]

^ this is true if you are cracking it open from any throttle position above 1/4 turn, but then you are not taking advantage of the accel pump as it was intended.
If you are at 1/4 or more throttle, cruising along at 5k in any gear, let the throttle go, just for a second (so the throttle can reset, and the slides can drop.  this also allows the diaphragm to raise, filling the accel pump bowl) then hammer it, and hold on to your butt.  I get no delayed response or lag because of my accel pump when doing this.

[lucky]

250 miles ago

Edit:  I cleaned them myself after they sat with gas in them for 20 years and I could have missed something.

If you did not take the idle jets out and actually look through them to see daylight then you will have to do it.
If they are clogged with 20 year old varnish you will need a .010 thousanths electric guitar string to clean out the .013 thousandths hole. Then carb clean and compressed air to finish.

A paper clip will just ruin the soft brass jet.

[Stoli]

do you give it 2 full turns of the throttle before pulling the choke knob to 'prime' the carb throats with fuel?

If the bike has been sitting for a a few days (which has not been often) I do this but sometimes forget and it doesn't seem to make much difference. The bike always fires right up to 2000+ rpm at full choke when it is cold then starts rolling back within a second or two.

Flybox - I'm going to try your suggestion next time I have a chance. Makes perfect sense to bring the accel pump into the equation and will serve to validate what K3 said.

K3 - you said that much better than I could.

Lucky - I replaced those jets when I put the pods on. The old ones were very gunked up in addition to being too small.

[TwoTired]

The carbs on these machines are direct slides. Grabbing a handful at low RPM in high gear will produce a delayed response, period. The drop in vacuum cannot be accommodated by the carb design that quickly.

This is well stated, and probably not stated often enough about these older motorcycles, which really do prefer to be ridden 'briskly', as opposed to 'abruptly'.

Accelerator pumps were added to the 77-78 Cb750 models as well as later Cb650 models, to allow rapid twists of throttle at low RPM and still get brisk, even abrupt, pickup.

They ain't all the same fellas, even if they do have direct mechanical slides.  If your machine has no accelerator pump and direct control slides, then about 1/2 of snap throttle twist should still produce reliable and brisk pickup.  If not, somethin' needs fixin'.

The stock exhaust back pressure was needed for stock carb jetting to make all operating ranges user friendly.  Changes beget changes. NOTB

[Dave Voss]

Accelerator pumps were added to the 77-78 Cb750 models as well as later Cb650 models, to allow rapid twists of throttle at low RPM and still get brisk, even abrupt, pickup.

They ain't all the same fellas, even if they do have direct mechanical slides.

Of course you are right, my point was that these motorcycles are no longer new, but have 30~40 years of time and use on them, no matter how well preserved or rebuilt, and so an expectation that they will respond instantly is perhaps a bit optimistic.  If we ask for 9/10th's performance, we'll all be happy, it's when we ask for 10/10th's performance that we start to complain.

[Stoli]

If your machine has no accelerator pump and direct control slides, then about 1/2 of snap throttle twist should still produce reliable and brisk pickup.  If not, somethin' needs fixin'.

Really?  This sounds like a good topic for a poll. I'd like some feedback from guys who are riding SOHC CB750s. Next time you are out on the road, find a place where you are going 40-45 in 4th gear (that should be about 1/3 throttle or so). Then, as fast as humanly possible, snap the throttle to full. What happens? Please include any details regarding deviation from the stock configuration (carb type, intake & exhaust).

- My '78K with pods & a 4-1 exhaust (stock carb with 135 mains and needles shimmed 1mm) hesitates for about 2 seconds as if to say (what the f&ck is it that you want me to do?) then slowly responds into a stable acceleration.

- Tried it on a '77F completely stock and it does the exact same thing as above. 

I guess both need fixin' ?

[flybox1]

^ done this    almost daily
definitely not the feel of FI, but my K sure doesnt have to think twice about accelerating 

[MOONDOGNYC]

Ok...just finished putting on K&N pods, 120 mains and 42 slow.
Runs and idles better than I could have ever imagined.  DONE !

[lucky]

I would prefer to talk thousandths of an inch not mm's.
One mm=.039 thousandths., so ONE shim sitting down in the pocket in the bottom of the slide is correct.

Two shims would be way too much. That would be almost .080 thousandths.
 That would be over 1/16 inch=.062 thousandths

Good that you went with the #120 main jet.

[lucky]

I will say this again . You do not have to accept poor throttle response!

If you were going to race that motorcycle and could afford a tuner,
Would you accept poor throttle response. NO

[Stoli]

^ done this    almost daily

Interesting. To me, it is not something I normally do and I would consider myself an aggressive street driver. When I take a fraction of a second to roll the throttle from 1/3 to 3/4+ (which I often do) the bike responds well, but when I try to make the motion instantaneous both bikes will hesitate for a bit. The comments from K3Owner and Dave Voss would appear to be spot on to me:

"The carbs on these machines are direct slides. Grabbing a handful at low RPM in high gear will produce a delayed response, period. The drop in vacuum cannot be accommodated by the carb design that quickly."

"This is well stated, and probably not stated often enough about these older motorcycles, which really do prefer to be ridden 'briskly', as opposed to 'abruptly'."

[TwoTired]

I guess both need fixin' ?

Yep

[briancwynn]

Bringing this thread back to life.  What was the outcome of all the Carb adjustments? Did you live with it or find a perm fix?

[freezerburn]

Well, late to the party sheesh! First off thanks for all the info here for us to get things running right again.
Alright here are my jettings and settings and basic description of the bikes mods in hopes of contributing too:)

1978 cb750k with 4 into 4 straight pipes
K&N 2+2 pods (bird catchers really)
Unmodified engine as far as I know
Sea level on up to about 4000 feet

#125 mains
#40 slow jets
Mixture screws 1 1/2 turns out

Fired right up and ran perfectly everywhere in the ranges
I did lower the idle once. Once assembled and started it was dead nuts a perfect running bike.. and the stumble off of idle was history. Couldn’t be happier and I would suggest the above jetting etc for a good base line for anyone with a similar mod setup. Be cool, freezerburn