Topic: 72 CB750 stumble off idle when hot

[KeithB]

Finally got a chance to get the bike out on the streets for a test. Couldn't really do a proper highway plug chop but overall, the plugs look good.
Pulls great with no "flat" spots but is a bit hard to get going off idle once warmed up. Need to get the revs up and slip the clutch. Doesn't do this when cold.
Double checked timing and it is dead on and advances correctly. Also tried some air screw adjustments and it seemed to get slightly better when turned in. Almost closed.
Plugs show light beige with 5 minute idle test.
From what I have read in the carb FAQ, it sounds like I might need to up the pilot jet from the stock 40 to smooth out idle to partial throttle transition.
Stock airbox and filter 120 mains Dunstall 4-2 exhaust.
Anything else I should look for before I dig in?

[ekpent]

 Are those stock Kehin brass bits in your carbs or aftermarket ? Kehin parts are marked with a stylized 'K'.

[KeithB]

I am going to pull one to check.

[KeithB]

Kehin jets confirmed.

[flybox1]

Adjust jets last.   Lets be sure its not something simple.
New air filter recently? Type? Paper or oiled (K&N)
My thought is if its new and flows air better than before, it might be leaning you out just enough to make it stumble.

[markreimer]

I have the identical issue on my 74 750K. I found a small tear in one aftermarket carb boot, which I just replaced. But even when they were new it would have a momentary stumble when starting from a stop. I'm running 40 slow jets and 105 mains (stock for a later '74). I could reduce the stumble a fair bit by leaning the mixture (turn the  adjustment screw OUT to allow more air) but then I'd be popping a lot on deceleration. I could tune the popping out but then the stumble came back... not sure what to do next.


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[KeithB]

It was pods but now is stock paper filter in stock air box.
Didn't ride it long enough to get hot with the pods.
Fair amount of back fire on decel as well.
All carb boots good and tight, no tears

[edwardmorris]

Popping on decel can be fixed by adjusting slide height. Were the carbs synced?

[flybox1]

Popping on decel can be fixed by adjusting slide height. Were the carbs synced?

..and correcting idle A/F mixture.

[evinrude7]

how is slide height adjusted?

[flybox1]

how is slide height adjusted?

With a vacuum sync of the carbs

[KeithB]

Installed 42 pilot jets and it definitely helps the cold start and idle.
Too cold to road test so have to wait for better weather.

[markreimer]

Please report back when you can. On my 750, I find the stumble usually goes away, or is at least reduced, when I lean the idle circuit using the air screws. I'm running the stock 40 pilot jets, airbox with K&N filter (same results with stock paper filter).

If you went to a richer jet, it makes sense that it would start easier, but I'd expect the stumble to get worse, not better.

[flybox1]

A stumble above idle can occur with a lean or rich condition.
Adjust your IMS based on A/F needs (plug chops or gas analyzer)

[KeithB]

Please report back when you can. On my 750, I find the stumble usually goes away, or is at least reduced, when I lean the idle circuit using the air screws. I'm running the stock 40 pilot jets, airbox with K&N filter (same results with stock paper filter).

If you went to a richer jet, it makes sense that it would start easier, but I'd expect the stumble to get worse, not better.

Looks like it will be a while before the weather warms up again

[markreimer]

Yeah for me too... I just re-jetted for a new exhaust and all of a sudden the temp plunged. It was +5C that day. today it's -18C. It's going to be at least two weeks till I can take the bike out now.

[KeithB]

Installed the 42s but no real difference.
Backed the idle air screws out 3 full turns and things cleared up. Stumble was a rich mixture problem.
However, it started to rain before I could get back out for full throttle test.
I'm considering installing 38s and that should get the air screws back to the range they should be in.

[markreimer]

Sounds like you're honing in on it. I may have to try the same thing now


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[flybox1]

38's will most likely require your air screws to be near seated as they make you too lean.
A stock airbox and a 4:2 shouldnt require much more than air screw adjustments for the idle circuit.
Go back to your 40's (if they are not Keihin brand, this could be causing the problem) and 1 turn out of the air screws.
Plug chop again, with new plugs, and make air screw adjustments as needed.

[KeithB]

Go back to your 40's and 1 turn out of the air screws
A stock airbox and a 4:2 shouldnt require much more than air screw adjustments for the idle circuit..

I agree. However, the bike has it's own opinion.

I started with 40s  was 2.5 turns out  and still had problems even though the plugs looked "good" with an idle test. 1 turn out didn't work.
 I could try the 40s again but probably will end up with the screws turned out more than they really should be.
It's easy to try the 38s and if they don't work, you will be the second to know 

[flybox1]

Would love to see the 'results' of your idle plug chop.
Post a picture when you can, and be sure to list the essentials used for the test...Time, jet (size and brand), air screw setting, etc...
...and post a picture of your intake setup/carbs/engine/headers.
Thanks

[KeithB]

Restored to 40s and left air screws out 3 turns. Seems OK is basic testing but need to get out on the road.
A bit chilly right now but can do idle test.

[markreimer]

For what it's worth, I've noticed on my 750K that what runs well at idle often doesn't run well 'on the street'. And what also runs well initially on the street, sometimes doesn't run well once the engine is up to full operating temp. Which is pretty frustrating for tuning purposes!

Just to confirm.... your air screws are on the airbox-side of the carb, right? I can't remember which years were different, but some carbs have them on the opposite side, which means turning the screws out makes it richer. I'm pretty sure yours will be on the airbox side, meaning 3 turns out is leaning it out pretty much to (if not past) the the max adjustment range of the screw.

[HondaMan]

Several things to look at:

Air screws: 7-8 to 1-1/8 turn out on the 657 series carbs.
At 1.5 turns out they are fully as rich as they will possibly go, at 3/4 turn out they are as lean as they will get and still run.
If you notice a further difference in performance beyond these ranges, something else (like jet size or float levels) is not right. Float levels should be 25-26mm for smoothest performance, 24mm with perfect float valves and hotter plugs will raise the low-end torque a little bit.

When the low-end is very rich, as yours now is, it will stumble at the point where the slide is open too far for the engine speed at that mixture. Another thing on the K0-K2 that makes this happen is: the spark advancer springs are too soft (or even loose) and/or the shim washers underneath the points cam are gone, letting the cam wobble at low speed and then straighten up at about 1800 RPM. This sudden timing shift (backward) as it straightens up causes the stumble.

There should be NO MORE than 0.008" worst case clearance between the face of the advancer cam and the back of the big nut.

A third thing: if the timing of the spark (especially on the 2-3 points) jitters back-and-forth under a timing light, the long bolt under the spark advancer is bent. This can be straightened, but it got that way from pulling the big nut CCW with the sparkplugs installed. This should be no more than 0.002" out-of-round, or the spark advancer tilts, causing the cam to bind at low speeds, then unbind above about 2k RPM, causing a stumble, too.

[markreimer]

Several things to look at:

Air screws: 7-8 to 1-1/8 turn out on the 657 series carbs.
At 1.5 turns out they are fully as rich as they will possibly go, at 3/4 turn out they are as lean as they will get and still run.
If you notice a further difference in performance beyond these ranges, something else (like jet size or float levels) is not right. Float levels should be 25-26mm for smoothest performance, 24mm with perfect float valves and hotter plugs will raise the low-end torque a little bit.

When the low-end is very rich, as yours now is, it will stumble at the point where the slide is open too far for the engine speed at that mixture. Another thing on the K0-K2 that makes this happen is: the spark advancer springs are too soft (or even loose) and/or the shim washers underneath the points cam are gone, letting the cam wobble at low speed and then straighten up at about 1800 RPM. This sudden timing shift (backward) as it straightens up causes the stumble.

There should be NO MORE than 0.008" worst case clearance between the face of the advancer cam and the back of the big nut.

A third thing: if the timing of the spark (especially on the 2-3 points) jitters back-and-forth under a timing light, the long bolt under the spark advancer is bent. This can be straightened, but it got that way from pulling the big nut CCW with the sparkplugs installed. This should be no more than 0.002" out-of-round, or the spark advancer tilts, causing the cam to bind at low speeds, then unbind above about 2k RPM, causing a stumble, too.

Hondaman, I thought turning the screws out let more air in, leaning the mixture. You're saying it richens the mixture. I have the 657B carbs off a '74k and the screws are on the airbox side of the carb. Prettt sure opening them up leans the mixture?


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