CB77 Carburetor Tuning

[Spargett]

Excellent information guys. All the details are really helpful.

[leonvjames]

From a long time since I have done the carbs on my CB77 my method was,
Warm up engine to normal operating temperature.
Set both mixture screws about 1.5 turns out.
Wind both idle stop screws out clear of the slides.
Check both carbs are mechanically together with fingers on them etc. refit aircleaners
Pull off 1 plug lead, say left one and screw in right carb idle screw a couple of turns
Start bike, runs on one cylinder ok. and adjust right carb for best slowest idle using mixture and idle screws
Stop engine, refit left plug lead, remove right plug lead and wind in idle screw to suit
Start engine and adjust left carb for best slow idle as above.
Stop engine and refit right plug lead and screw idle screws out an equal amount, 1 turn
start engine and then adjust idle screws equally until correct idle speed is obtained, about 1200rpm
Hope it all makes sense but it always worked for me.
Leon, long time owner, bought bike new in 1965, still gets occasional use.

[LOUD MOUSE]

I (along with others) watched a VJMC club member do your adjustments at a meet in Kentucky to a CB350 and (not to my surprise) when he started it with both cylinders firing it revved to "off scale". If ya say so. ............lm

From a long time since I have done the carbs on my CB77 my method was,
Warm up engine to normal operating temperature.
Set both mixture screws about 1.5 turns out.
Wind both idle stop screws out clear of the slides.
Check both carbs are mechanically together with fingers on them etc. refit aircleaners
Pull off 1 plug lead, say left one and screw in right carb idle screw a couple of turns
Start bike, runs on one cylinder ok. and adjust right carb for best slowest idle using mixture and idle screws
Stop engine, refit left plug lead, remove right plug lead and wind in idle screw to suit
Start engine and adjust left carb for best slow idle as above.
Stop engine and refit right plug lead and screw idle screws out an equal amount, 1 turn
start engine and then adjust idle screws equally until correct idle speed is obtained, about 1200rpm
Hope it all makes sense but it always worked for me.
Leon, long time owner, bought bike new in 1965, still gets occasional use.

[nander]

Leon,

In my recent mad search of the internet for carburetor tuning advice, I have read on other forums about similar methods of tuning two cylinder engines by initially isolating each cylinder as you describe.

I was about to try the one cylinder at a time approach a couple of weeks ago, but I shied away from doing so when I came across a warning on some forum that running with one spark plug wire disconnected on a motor timed by an electronic ignition that uses a hall effect sensor, will damage the sensor. I no longer have points on my bike. I am running the electronic ignition (got it here from Michael Stoic), and since as I recall it uses a Hall effect sensor, I am a little wary of running the bike with one spark lead disconnected.

Anyone know if this is a genuine concern?

nander

[leonvjames]

nander,
it might be possible with the electronic ignition to run the disconnected plug lead into another spare plug that was earthed on the motor somewhere. That would show a normal load to the electronics but wait till you get some more replies from someone who knows.
Leon.

[britman]

From a long time since I have done the carbs on my CB77 my method was,
Warm up engine to normal operating temperature.
Set both mixture screws about 1.5 turns out.
Wind both idle stop screws out clear of the slides.
Check both carbs are mechanically together with fingers on them etc. refit aircleaners
Pull off 1 plug lead, say left one and screw in right carb idle screw a couple of turns
Start bike, runs on one cylinder ok. and adjust right carb for best slowest idle using mixture and idle screws
Stop engine, refit left plug lead, remove right plug lead and wind in idle screw to suit
Start engine and adjust left carb for best slow idle as above.
Stop engine and refit right plug lead and screw idle screws out an equal amount, 1 turn
start engine and then adjust idle screws equally until correct idle speed is obtained, about 1200rpm
Hope it all makes sense but it always worked for me.
Leon, long time owner, bought bike new in 1965, still gets occasional use.

Leon,

Just FYI. This is the same method described in most period publications for adjusting carbs on old British twins.

[nander]

Here's an update to my carburetor tuning saga.

Accomplishments to date:
1. No more idle surge - cable adjustment and proper cable routing fixed that.
2. Rich idle condition cured (well sort of, read below)
3. Air screw adjustments fall within in the expected 1-1/2 turn range.
4. Off the line acceleration is improved.

Because I couldn't seem to cure my over rich idle condition, I decided I should eliminate the oft mentioned possibility of worn needle jets. My bike does have 20k miles on it.

I got a set of NOS needles and needle jets from Fastfred. During the needle jet swap I compared new and old and the bore of the old needle jets were noticeably larger than the new.

Put the carbs back on the bike, startup was now impossible without choke. During warmup I heard the pops, snaps, and spitback I've heard described, but never experienced firsthand. Pulled the plugs after the first ride - they were unfouled and the porcelain was light tan!

Case closed.

Haha...wouldn't it be nice if that was the end to the story?

Problem now is that I have lost midrange (and above) power. The bike sputters around 4000k and I either have to reduce throttle or drop a gear. I am dealing with an odd trade-off: before replacing the needles and needle jets, the motor hesitated when accelerating from a standstill, but ran like a top and had power all the way through the range above 4000k. Now, the bike accelerates from a standstill, but stumbles and refuses to cooperate above 4000k. What gives?

After my most recent ride, the left cylinder plug looked clean, right looked fouled. Now this is just a hunch (hard to tell when you're riding), but I feel like my problem may be just with the right cylinder not pulling it's weight.

Here are the variables I am considering are contributing to my situation. Perhaps someone can enlighten me with their first hand experience...

1. Fuel Starvation?
Although I've checked the floats (and I'm gonna check em again), is an out of adjustment float a likely culprit? In this scenario I imagine the right carb is incorrectly metering fuel (hence the fouled right plug).

2. Bad Coil?
Recently, on several occasions at startup, the motor wouldn't fire consistently on the right cylinder. After clipping a 1/4 inch of the end of the plug cable and tightening things up, I was able to cure the problem. I think it was a temporary problem resulting from me loosening connections by pulling the plug wires so frequently (after every ride). However, might I be dealing with a coil that's on the fritz?

3. Spark Advance?
Because the power problem appears to kick in around 4k rpms and above, I started to think my timing might be off. I have the electronic ignition. I set it up years ago. I am reviewing the instructions right now and digging around in the garage for the dynamic timing light...

4. Needle Clip position?
I'm at the middle position right now. I was thinking of dropping it a couple of notches and seeing if a richer mix cleared things up.

5. Main jet size
I am running no baffles in stock pipes and some aftermarket airfilters that I would imagine are a little more free-breathing than the stock ones. Could these minor mods be leaning out the motor enough that I need a larger main jet?

Who knew this could be so tough?
No matter.
I will win in the end damn it!