CB77 Carburetor Tuning

[nander]

Hello All,

This is my first post! I need a little assistance tuning my 1962 CB77. I just recently got it running although I've owned it for (far too) many years. I have read the posts on this forum at length, but I still have some carburetor tuning problems/questions...

Having followed the Ed Moore tuning recipe, I seem to only be able to get a state of tuning where either a). the bike idles reliably, but will initially stumble off the line and in the transition to second gear (idle too rich?) - OR - b). an inconsistent idle, but better throttle response and off the line acceleration (idle too lean?).

Also, on a couple of occasions I have set the carbs in such a way that the exhaust delivers a popping sound upon deceleration under load ("pop", "pop", "pop" as I go down the road). I interpret this as some form of backfiring also due to a rich throttle setting. Is this correct? Or is this a normal experience this with CB77s?

Finally, when the bike is warmed up, I sometimes encounter a problem where the the idle will hang at 2000-4000 rpm when I blip the throttle. It will continue to hold that idle even in gear. If it gets above 4000k I kill it with the ignition switch. Starts back up at a lower idle. I know this has come up in other posts but, I can't seem to find any description of a remedy. Could it be an intermittent hangup in the cable, or am I fiddling with the bike so long in one place it is getting overheated and the idle gets erratic?

Spark plugs are blackish brown, but neither sooty nor fouled.

My only adjustments so far have been with the idle and air screws. However, here are the variables I've considered playing with:

1. Trying hotter D6HAs instead of the D8HAs I am currently running.
2. Changing the position of the needle clip -- from the middle to the second highest setting -- to lean it out when I begin to roll on the throttle in order to improve response.

Also, for what it's worth, I am running Emgo pod air filters and repro mufflers with no baffles. Do I need to consider rejetting for those "modifications"?

Any suggestions or advice would be greatly appreciated...

nander

[Spargett]

I had exactly this problem for the most part before I started the restoration. I didn't get a chance to troubleshoot it in any extent, but I did see that the bike had been running very rich upon disassembly of the engine.

So I have no surefire solution based on experience for you right now, but I can say that an overly rich mixture seems to be happening (as you suspect).

As far as the RPMs floating, I'd check to make sure the nuts on your carbs aren't too tight. If so, it can bend the body which tends to pinch the slides a bit higher up. I've been told to hand tighten them, then just a dash more with a wrench. Fixed a sticky throttle problem I was having.

I'm curious as well to hear what some of the other more knowledgeable members have to say.

[LOUD MOUSE]

# CB72/77 - Remove the air intake tubes and put middle finger in left carb and thumb in the right carb.
CL72/77 - Remove the tool can and put middle finger in left carb and thumb in the right carb.
# Move throttle handle and if both slides don't move at "exactly the same time" adjust the screws on the top of the carbs so they do. Be sure to have at least 1/8 inch slack in the throttle handle.


# Remove tools and turn slide screws in a couple of turns.
# Choke lever to 3/4 closed.
# Turn gas lever on. WAIT!
# Engine in "NEUTRAL"
# Key "ON".
# CB72/77 - Push starter button.
CL72/77 - Kick start the motor.
# Engine will fire and run somewhat.
# Run until you can OPEN the choke all the way.
# Adjust slide screws to even cylinder speeds.
# When engine is hot adjust slide screws so engine idles at around 800 RPM.
# Adjust air screws. 1 1/4 turn max here. Usually open a small amount to smooth idle. Never more than 1 1/2 turns open from bottom total. May need to open slide screws after this to lower RPM.
# Install the air tubes and go ride.

Do the above again. You have one slide higher than the other and your air screws are open or closed to much/not equal. .............lm

Hello All,

This is my first post! I need a little assistance tuning my 1962 CB77. I just recently got it running although I've owned it for (far too) many years. I have read the posts on this forum at length, but I still have some carburetor tuning problems/questions...

Having followed the Ed Moore tuning recipe, I seem to only be able to get a state of tuning where either a). the bike idles reliably, but will initially stumble off the line and in the transition to second gear (idle too rich?) - OR - b). an inconsistent idle, but better throttle response and off the line acceleration (idle too lean?).

Also, on a couple of occasions I have set the carbs in such a way that the exhaust delivers a popping sound upon deceleration under load ("pop", "pop", "pop" as I go down the road). I interpret this as some form of backfiring also due to a rich throttle setting. Is this correct? Or is this a normal experience this with CB77s?

Finally, when the bike is warmed up, I sometimes encounter a problem where the the idle will hang at 2000-4000 rpm when I blip the throttle. It will continue to hold that idle even in gear. If it gets above 4000k I kill it with the ignition switch. Starts back up at a lower idle. I know this has come up in other posts but, I can't seem to find any description of a remedy. Could it be an intermittent hangup in the cable, or am I fiddling with the bike so long in one place it is getting overheated and the idle gets erratic?

Spark plugs are blackish brown, but neither sooty nor fouled.

My only adjustments so far have been with the idle and air screws. However, here are the variables I've considered playing with:

1. Trying hotter D6HAs instead of the D8HAs I am currently running.
2. Changing the position of the needle clip -- from the middle to the second highest setting -- to lean it out when I begin to roll on the throttle in order to improve response.

Also, for what it's worth, I am running Emgo pod air filters and repro mufflers with no baffles. Do I need to consider rejetting for those "modifications"?

Any suggestions or advice would be greatly appreciated...

nander

[nander]

Thanks for the suggestions gentlemen.

Spargett - I had initially thought I might have a vacuum leak, so I have the carbs on just a little more than finger tight.

On another note, I read in one of your posts that you had your bike serviced at Charlie's Place. I just got mine back from him. He got it running, which was no small feat considering the years I'd spent trying. But, the bike had the pop-pop-pop on deceleration and hesitation when accelerating from a stop - so i started messing with it.

LM - This morning I readjusted the slides, although there wasn't much of a one before, I can neither hear nor feel (with thumb and forefinger) a difference in their movement now.

Following your suggestion about unequal air screw balance between carbs, I changed out one of the air screws. It looks like I had an original air screw in the left carb and one of the slightly longer keyster air screws in the right. The bike idled and accelerated much more happily after swapping for a matched keyster air screw.

However, after riding around for 15 minutes or so (and taking it to 60 in 3rd gear, up hill, with a couple of grand left to go on the tach!), I found the bike idled less consistently and would intermittently hang at 2000 rpm -- and other times die when I came to a stop.

Anyway, I think I still have some air screw and throttle cable adjustment to do, I probably don't have enough slack in the cable as you suggest. This afternoon, I will again re-sync the slides with a little more slack to see if I can really fine tune this thing.

Here's a question: how hot is the bike when you are tuning it? As the bike gets hotter does it require a leaner mixture? I think I am tuning it to the point where it runs well when it is warm, and then when it is hot it runs a little rich.


Thanks again,
nander

[LOUD MOUSE]

QUESTION FIRST: are you trying to turn each screw out/in the same/equal number of turnes?
If the carbs are adjusted correctly the idle at running temp will be smooth and not surge or high rev.
All these 2 carb engines will need to get to that "running temp" and will snap and pop untill it does.
I get my engine running (after it has been tuned to running temp) and keep it reeving up/down for a minute or so and then I go for a ride and by the time I have to stop the engine temp is there and the idle is as adjusted. I'm of the opinion you have one slide a little higher than the other and it is causing your eratic idle problem. .................................lm

Thanks for the suggestions gentlemen.

Spargett - I had initially thought I might have a vacuum leak, so I have the carbs on just a little more than finger tight.

On another note, I read in one of your posts that you had your bike serviced at Charlie's Place. I just got mine back from him. He got it running, which was no small feat considering the years I'd spent trying. But, the bike had the pop-pop-pop on deceleration and hesitation when accelerating from a stop - so i started messing with it.

LM - This morning I readjusted the slides, although there wasn't much of a one before, I can neither hear nor feel (with thumb and forefinger) a difference in their movement now.

Following your suggestion about unequal air screw balance between carbs, I changed out one of the air screws. It looks like I had an original air screw in the left carb and one of the slightly longer keyster air screws in the right. The bike idled and accelerated much more happily after swapping for a matched keyster air screw.

However, after riding around for 15 minutes or so (and taking it to 60 in 3rd gear, up hill, with a couple of grand left to go on the tach!), I found the bike idled less consistently and would intermittently hang at 2000 rpm -- and other times die when I came to a stop.

Anyway, I think I still have some air screw and throttle cable adjustment to do, I probably don't have enough slack in the cable as you suggest. This afternoon, I will again re-sync the slides with a little more slack to see if I can really fine tune this thing.

Here's a question: how hot is the bike when you are tuning it? As the bike gets hotter does it require a leaner mixture? I think I am tuning it to the point where it runs well when it is warm, and then when it is hot it runs a little rich.


Thanks again,
nander

[nander]

I have been making equal adjustments to both sides, i.e. if i turn the right air screw in 1/8, then i do the same for the left.

LM - good diagnosis, I think you were right about the unequal pull on the throttle cable, I spent some time this morning re-syncing them and making sure the bolts were tight and the rubber boots were snug, and I didn't have any idle surge problems when I took her out for a spin.

Also, just as I had found with the air screws, there was a slight difference in the length, taper, placement of threads etc. between the right and left idle screws. So, I dug out a pair that look to be properly matched and that seems to have helped.

I took the bike out today and it idled pretty reliably (save for an intermittent stoppage of fuel flow to the left carb). Although I am still searching for the sweet spot that gives me the best take off in 1st gear, I think smooth performance will be attainable.

On that note, one last question: how much sensitivity do you experience in the air screw adjustment? Do you find yourself making 1/16th turn adjustments to get the perfect state of tune, or should that not really be necessary?

Cheers,
nander

[LOUD MOUSE]

Thanks for the answer for the way you adjust the screws.
Lets go to start-up. Open the slow speed screws apx. 1 1/4 turn.
After ya have made the slides lift together just turn the SLIDE screw a turn or two out from bottom.
Start the engine and go Left/Right and turn the screws in OR out until ya have the engine idling smoothly.
The slides screws will seldom adjust the same number of turns out.
It's a matter of Left/Right screw In/Out until ya have it running smooth.
After that ya can turn the slow speed screws In/Out (no more that 1/2 turn more out) until the POP-POP is gone. ...............lm

I have been making equal adjustments to both sides, i.e. if i turn the right air screw in 1/8, then i do the same for the left.

LM - good diagnosis, I think you were right about the unequal pull on the throttle cable, I spent some time this morning re-syncing them and making sure the bolts were tight and the rubber boots were snug, and I didn't have any idle surge problems when I took her out for a spin.

Also, just as I had found with the air screws, there was a slight difference in the length, taper, placement of threads etc. between the right and left idle screws. So, I dug out a pair that look to be properly matched and that seems to have helped.

I took the bike out today and it idled pretty reliably (save for an intermittent stoppage of fuel flow to the left carb). Although I am still searching for the sweet spot that gives me the best take off in 1st gear, I think smooth performance will be attainable.

On that note, one last question: how much sensitivity do you experience in the air screw adjustment? Do you find yourself making 1/16th turn adjustments to get the perfect state of tune, or should that not really be necessary?

Cheers,
nander