Topic: 77 750k Stumbles.

[cl4yd0h]

I have a pretty good stumble and lack of power off the line. Its takes a second to catch back up with itself, then screams.
Its only off the line from a stop or slow speeds. If you want a good jump on someone you have to really take the revs up there or else it falls on its face for just a second. Plug chop yields good results with a nice tan color on all four.

I disassembled/cleaned one way circuits and replaced diaphragm on accel pump (works awesome now!) which really helped the stumble but it is still there.

These carbs have been apart 10+ times now, just checking and verifying my work.. everything is CLEAN and functioning. Floats at 14.5mm

Spraying cleaner around manifolds doesnt change idle.
I should also mention I am running velocity stacks, with 120 main and 35
It performs REALLY well in the higher revs but down low its gutless.

If anyone has any experience with this that would be great!

Thank you!

[toytuff]

Did you try backing out the air/fuel screws? The book states 1 1/2 turns but I had to set mine 2 -2 1/2 turns out.

tt

[Johnie]

Did you sync the carbs after all the work? Just checking...

[flybox1]

...but what does your 5 minute idle plug chop tell you?  LEAN, im sure of it.

Stacks + 35 pilots doesnt sound right.  40's or 42's is in the right ballpark. 
I bet you are really lean at idle, and 1.5 turns out wont do it. yes, 2.5 turns is closer, but nearing the range where small adjustments wont make a big enough difference.
IIWM, id go right up to 40's and drop the needle clip one position(richer)(if its stock setting right now)
idle mixture screws can stay at 1.5 turns out.
if you go to 42's, idle screws will be 1 turn out. needle down 1 position(richer)

[cl4yd0h]

Carbs were synced with traditional vacuum gauges.

I have found 2 1/2 turns runs best, but still not completely right. I wouldn't feel comfortable going any father out, there isnt a whole lot left after 2 1/2 out. I may have to up the idle jets.

My mixture screws are going to be somewhat different as I used hardware store washers for them, I was missing two and they are about twice as thick.

Plug chop at idle shows light tan.

Also PD carbs don't have an adjustable needle, I have read threads on this forum about people shimming them, worth a try?

[lucky]

I have a pretty good stumble and lack of power off the line. Its takes a second to catch back up with itself, then screams.
Its only off the line from a stop or slow speeds. If you want a good jump on someone you have to really take the revs up there or else it falls on its face for just a second. Plug chop yields good results with a nice tan color on all four.

I disassembled/cleaned one way circuits and replaced diaphragm on accel pump (works awesome now!) which really helped the stumble but it is still there.

These carbs have been apart 10+ times now, just checking and verifying my work.. everything is CLEAN and functioning. Floats at 14.5mm

Spraying cleaner around manifolds doesnt change idle.
I should also mention I am running velocity stacks, with 120 main and 35
It performs REALLY well in the higher revs but down low its gutless.

If anyone has any experience with this that would be great!

Thank you!

I see the problem now.

#120 mains and 35 idle jets.  #35 was stock idle jet.
That will never work with velocity stacks.
For that set up you need:

#120 mains.
#42 idle jets.
Slide needle clip moved down one notch from stock.
Mixture screws open one full turn.

[lucky]

I have a pretty good stumble and lack of power off the line. Its takes a second to catch back up with itself, then screams.
Its only off the line from a stop or slow speeds. If you want a good jump on someone you have to really take the revs up there or else it falls on its face for just a second. Plug chop yields good results with a nice tan color on all four.

I disassembled/cleaned one way circuits and replaced diaphragm on accel pump (works awesome now!) which really helped the stumble but it is still there.

These carbs have been apart 10+ times now, just checking and verifying my work.. everything is CLEAN and functioning. Floats at 14.5mm

Spraying cleaner around manifolds doesnt change idle.
I should also mention I am running velocity stacks, with 120 main and 35
It performs REALLY well in the higher revs but down low its gutless.

If anyone has any experience with this that would be great!

Thank you!

I see the problem now.

#120 mains and 35 idle jets.  #35 was stock idle jet.
That will never work with velocity stacks.
For that set up you need:

#120 mains.
#42 idle jets.
Slide needle clip moved down .
2nd notch down from the top.
Mixture screws open one full turn.

[cl4yd0h]

Thanks guys!
I ordered a set of 40s before I saw your reply lucky..
I will test them out and maybe have to get 42s..

I will keep updated and let you guys know what works for me.

[cl4yd0h]

what appear to be helicoil threads came out of two of carb bodys for bowl screws..

Gonna have to insert new coils or tap them into something else..

[lucky]

Thanks guys!
I ordered a set of 40s before I saw your reply lucky..
I will test them out and maybe have to get 42s..

I will keep updated and let you guys know what works for me.

#40's will not work. Don't waste all that time installing them.
Sell them to a CB750 owner that has an earlier model.

[lucky]

what appear to be helicoil threads came out of two of carb bodys for bowl screws..

Gonna have to insert new coils or tap them into something else..

Those were original threads probably.
Just tap to next SAE size this time instead of metric, if there is enough material.
Do it by hand. Do not use an electric drill!

[78whiteorbs]

I was under the impression stacks were for wot performance- not bottom end accelorating. If you Rev it up pretty high and dump the clutch does it still hesitate?  Not trying to start anything in your thread but was always under the impression stacks on a street bike was a fashion over function thing.  At the drag strip its all about wot and to have that aggressive look on the street is very cool looking but doesn't play well with stoplights and traffic cruzing.
again as always just was my understanding and could be wrong but the airbox fixes lots of hanging idles and flat spots and hesitations. Hope I didn't kick the hornets nest here fellas. I bet the stacks draw lots of looks and complements but on the open road not to big of an audience. Least were I like to ride.Now bike nights will draw a crowd.

[cl4yd0h]

you are probably on to something white orbs..

The bike came with pods and no original air box..

I REALLY enjoy the look of the stacks.. I knew it was going to be a headache from the get go but hopefully I can get it dialed in. I am also a pretty aggressive rider, cruising in the high end most of the time..

I will screw with jetting and see if I can get them perform well enough for leaving stop lights at low speeds and if not I might have to toss them..

Thanks lucky, I ordered a set of 42s as well. I will try both and see what happens..

Also, the two places the threads came out were the strait through passages, so I am lucky enough to put a screw and a nut through without risking damaging or cracking something with a tap..

I just got a 1971 Suzuki Titan 500 that I stripped down the to frame and started cleaning today.. A lot going on in my garage!

[Black 750K8]

I was under the impression stacks were for wot performance- not bottom end accelorating. If you Rev it up pretty high and dump the clutch does it still hesitate?  Not trying to start anything in your thread but was always under the impression stacks on a street bike was a fashion over function thing.  At the drag strip its all about wot and to have that aggressive look on the street is very cool looking but doesn't play well with stoplights and traffic cruzing.
again as always just was my understanding and could be wrong but the airbox fixes lots of hanging idles and flat spots and hesitations. Hope I didn't kick the hornets nest here fellas. I bet the stacks draw lots of looks and complements but on the open road not to big of an audience. Least were I like to ride.Now bike nights will draw a crowd.

Could not have said it better. All show and no go. That is in town stop and go then jump on the interstate.

[lucky]

Velocity stacks are used in all kinds of motorcycles.
Drag racing ,road racing, and even dirt track with foam filters.
Racers would not want them if they did not perform perfectly.

The velocity stacks WILL work and not hesitate IF...you use the #42 idle jets. on the 77-1978 CB750 and change that needle setting.

I have already done all of the work and testing and you get the benefit for free.
#40 idle jets will still cause hesitation when giving quick throttle.
Been there. Not just a theory.

The #35 stock jet has a hole .013 thousandths.
The #42 idle jet is .016 thousandths.


Very small increase. But it is still needed.

[lucky]

Did you try backing out the air/fuel screws? The book states 1 1/2 turns but I had to set mine 2 -2 1/2 turns out.

tt

That is because it is so lean.