Guru needed: Vacuum/air cutoff issue?

[RJ CB650]

Symptoms: 1982 CBX
1. Stuttering power up to 3500rpm, near unrideably low power.
2. Idles and starts nicely
3. On centerstand, difficulty revving up (compared to a normal running bike), occasional firecracker sounding "Pop" when letting off (leaned right out?)
4. Choke: With a 1000 rpm set idle approx, putting choke on revs it to about 3k. Further supporting way too much air and not enough fuel
5. That darn air cut off valve. Looks fine when I cleaned the carbs. On a whim, I took it out, revs and runs fine when on stand. Not taken on road yet.

Okay, to me, it looks like a serious vacuum leak in the carb assembly somewhere so getting too much air after the throttles somehow. Here is things I know for certain. The boots for the cylinder head are in great condition. I cleaned and rebuilt the carbs, but didn't do a full shaft disassembly. I blew out all the passages (Well, pretty sure I didnt miss any) and they all flowed nicely. Note, this is an issue the previous owner told me about. Took it to a professional shop to be set up and tuned, then the low speed was way off afterwards.

So all cleaned and rebuilt. Have done CV carbs multiple times in the past without issues, but all without this air cut-off. Did I miss something? Sound like something familiar?

Now one thing I need to check out. Anybody got pics on the 82 underside? Doing research, I found this vid:

https://www.youtube.com/watch?v=tTcHmS3OS_w

At 1:26, it shows different than mine. Vid shows the press in style with black cap. For myself, i have the lowspeed. The highspeed of course is normal and brain is shorted on what goes into the third post. However nothing was out of the ordinary to me there so I am assuming the 82 is just like my 82 650 and it did rebuild similar....

Anywhoo. So summation:

Jetting is to factory jet size.
Mix screws I had at two turns out.
Original airbox/etc.
Bad power, pretty sure lean. I had left my plug socket at folks house working on my other bike so need to pull plugs for inspection.
Floats set to correct height
Seems good idle, and zero issues starting, instant with button.

Require info:
Sources of vacuum leaks?
Passage that can prevent low speed fuel delivery which is countered by the choke or air shutoff being closed?
Other ideas or experience?

Thanks for any help. This is really getting to me. Bike is almost ready to get back on the road if only I could solve this issue and I am right stumped. None of my previous CV carbs had to deal with this cut-off, so I am lacking experience in how they would connect, or if it is the source of the vac leak itself? There has to be some connections somewhere between the carbs...

[daves79x]

Put your air cut-off valve back in, after making sure there are no pinholes in it. Biggie here is the special small o-ring for the air passage. Is it there? It has one flat side that goes toward the body. Make sure your spring is there also. It will not cause you any problems.

Sounds like you are still too lean in the low range. 2 1/2 turns out on the pilot screws would help some, but there could be other things wrong. Did you verify without a doubt that each slide/dome combo fit properly? Was each slide free to move in the dome? This is often overlooked and slides can stick (just one or two will give you your symptoms, especially the popping). The tower and the receptacle in the dome need carefully cleaned of varnish with a Q-Tip. You also could have a bent tower on a slide or a ding in a dome.

But you also might have some low-speed circuits partially plugged. Did you remove the #35 idle jets? They screw in in your '82 carbs and are easy to remove. That needs done to insure they're clean.

Finally, is the accelerator pump working properly? Did you test it out on the bench before installing the carbs back on the bike?

Lots of pitfalls here but I don't think you have an air leak. Test for sure around the rubbers with ether.

I'd also question whether you are running on all 6 at idle. These things seem to idle pretty good on 5 cylinders if you're not familiar with them. Check by feeling all 6 pipes at idle after start-up.

Dave

[RJ CB650]

Does not appear to be any leaks in the diaphragm. I will be getting a new one regardless since this one seems to have a small crease, but no holes and has proper o-ring.

All the slides move nice and smooth and top end was cleaned.

Accelerator pump, didn't liquid test it. Diaphragm was good and moved. But since the issue happens at sustained throttle, I am nut sure this is it.

Both jets are new ones, alas not original Keihin. Somebody, probably during the tuning had rejetted both jets. Think a 50 in the low speed and a 110 in the high if my memory is correct. Had been rebuilt in the past. I had to replace a bunch of seals so I just got ye "Okay" K&L kit but I plan on getting the entire bank rebuilt in future so will be good for a year or two.

I will double check the all six running here this evening and give an extra turn on the mix screws to see how it responds, then do the ol' leak test. Oh, and in regards to the previous tuning, it had low speed power fine, but the idle was rough according to previous owner. And of course, a plug check required first.

[daves79x]

Let's back up. Are you running the stock airbox? Are the carbs VB64? If so, there is only one main jet and it should be a 105. The idle jet in the '82 VB64 carbs screws out and they should be clearly marked as #35. That's all the jets that are in there. So if you have the stock airbox and are running 110 mains and #50 idle jets, then you are WAY too rich. If you are running pods, then you're way too lean.

Clarify what you have and I'll get you close from there.

Dave

[RJ CB650]

Lol, okay, time to restart.

I knew somebody had monkeyed around in the carbs, and bike sat for a year so cleanout by default.

Bike is stock airbox assembly, but a K&N filter by previous owner (maybe that is cutting vacuum a bit) and is there any screens etc that has been removed which could cause problems?

Previous owner had bike dyno tuned from a shop which all the guys in his area dealt with.

With seals going, damaged mix screw, etc. I ended up needing an aftermarket rebuild kit to tide me over until I do a full teardown/rebuild later.

I pulled out of it a 50 and a 110

Put in the 35 and 105. Screw style slow jet.

Choke stabilizes and really ramps up the rpm, so I am thinking it is extremely lean. Will pull plugs and inspect soon, plus tweak mix screws. Did reading and CBX is low end supposed to be 2-2.5 turns out it appears?

Also to add, on my test run when I noticed the stutter and no power being bad, once I got upper end, it smoothed out. So is definitely something not working in low speed circuit. Removal of the air cutoff smooths it out further pushing that it is extremely lean since my readings is that the cutoff closing on these carb richen it up significantly so would have similar result to the choke on. After the initial around the block test ride, I had did garage testing so choke probably is just smoothing it out by making proper mix.

[CopperCollar]

You have some fine advise coming from Dave. My 80 is dong the very same as your bike.

I pulled the carbs and found a Dynojet STG III kit in it, figured that was part of the issue. Put it back together after cleaning and new seals, proper jets and needles and she ran great then started the crappy midrange power with popping. I figured a piece of something got into the float bowl and sucked into a low speed circuit. That wasn't the problem.

My 80 has a K&N filter with a stock airbox and a Supertrapp 6-1 exhaust.

I found a slide that was sticking in the cap, didn't catch it on reassembly and I always check that. Once I was certain I had all the slides working properly I put the carbs in and she is running nice again.

Things I have learned as this is the first set of VB62 carbs I have done:
1) Slides are flat bottom as compared to the VB60 which are cut.
2) Slides have a BLACK cap vs the WHITE one used in the VB60. This is the plug that goes in the top of the slide. This seems to allow more travel of the slide up into the cap.
3) The needle is a 55a vs the O5F ? used in the VB60 carbs and #2 could explain why the needle is different for the VB62 carbs.
4) Balance the carbs, this seems to help a lot of problems on idle that we experience and really does quiet down the clutch noise.

Mike

[daves79x]

So we're back to stock, so I'm pretty sure your problem is plugged low-speed circuitry. Mike confirmed what the popping could be from, and you can remove all six domes/slides from a late model while they're on the bike. I'd carefully check that. Whoever worked on the carbs before likely didn't have a clue as to how to check that the low-speed circuitry was clean.

Dave

[RJ CB650]

Plus partially me then. Must have missed something in the circuits when blowing them clean or something got into it. I was able to spray through cleanly on assembly. So I will give it a bit more mix and see how she responds. If still bad, will pull them back out for re-inspection. Not looking forward to that, but if it must be done, have a weekend coming up and sunday weather is supposed to be terrible here. Myself as part of a motorcycle group had a bike show organized for this weekend, but weathergod may be shining on me by delaying the show so I can fix the issue ^.^

[RJ CB650]

Update: Good advice on checking the cylinders for running. I just did a warm up. 1 and 3 are stone cold, never even caught it cause any of my other bikes would never run down even one! I have spark at the wires, will be pulling plugs in few minutes to inspect.

So if a blocked passage. That would shut em down? I didn't check if those two warmed up under choke. Will try it in a second. Now that I noticed that, I noticed that while both left and right exhausts are smooth, there is clearly a pulse difference. In the mean time, if plug be firing good, what else could cause it? I guess if low speeds are 100% blocked they might not run at all on these carbs?

Darn this "learn something new every day"

[daves79x]

With real estate, it's location, location, location. With CBXs it's the carbs, carbs and carbs.

Dave

[RJ CB650]

With real estate, it's location, location, location. With CBXs it's the carbs, carbs and carbs.

Dave

In this case it wasn't the carbs actually. But something insanely simple that I never would have thought of if you hadn't mentioned about running on five cylinders. Since no longer a carb issue, I posted it up in daily discussion. Also, had to give Kudos to you!

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