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TOPIC: Mikuni VM 33 Carbs

Mikuni VM 33 Carbs 26 Feb 2016 07:17 #712568

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I've been asked about the VM 33's by a member and one last year. First off I wouldn't pay more than $300 in good condition. They're obsolete. I'm including the data on them as I go. These carbs have been a PITA not because they won't work, it's because the PO was a moron. More on that later. Once I got most of the mistakes corrected they have been great carbs. When I was getting Bossie ready for the road I talked to Mike at Z1E. He told me they were good carbs in the day but they had problems dialing them in. Too rich or too lean. Kinda like the VM 29's of the time. Both are smooth bores. Most parts are still available for them being they fit other types. The cost of new parts vs a modern carb set should be factored in as well.
Last year a member asked how much I would charge to set up his 33's. He was having leakage issues and running problems. I told him I would go through the carbs and give him my opinion before spending any money. I didn't hear from him for a few months then he asked again. I offered to set them, put them on my bike, dial them in but he would have to re-sync them and possibly rejet them for his engine. He told me he spent a lot of money buying them on eBay and didn't want to spend much more. I understand that, I'm there too. I need two slides but to set them up like the factory I'll need four and start all over. They're running good now and rather than spend another $2-300 on them, I might just buy some other ones.
I'm hoping others with experience will add to this thread. Here's some specs and data.
Steve







BTW I'm at 2.0 TV, Needle set at #2, #70 pilots and 130 mains. Plugs still look lean.
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Mikuni VM 33 Carbs 26 Feb 2016 07:59 #712573

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Case in point. Was just sent this link. I wouldn't pay more than $250 for them. Mine has a duel fuel tap where these have one. There is a vacuum tap on #3 carb but I don't see one on these. Spacing seems odd too. Jetting seems more in line with what's called for the Kaw set up but with the cracks and unknowns, :whistle:
I'm going to resync my carbs soon so I will get some pics and a clip.
I'd pass on offering more.
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 08:00 #712574

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From reading various threads about tuning 33's, I believe one issue is insufficient signal strength at lower throttle positions on stock and modestly over-bored engines, resulting in problematic tuning difficulties as compared with smaller more readily tuned carbs.

it's reportedly more difficult to achieve a strong steady idle, and more difficult to achieve a strong steady pull-away from idle, which I suspect is due to reduced signal resulting from the larger bore.

At WOT and more open throttle positions, signal strength would be expected to become less of an issue.

In short, the 33's may be "too much carb" for stock and modestly over-bored engines.

It's not uncommon to hear from folks who regret having installed 33's and wish they had opted for 29's or smaller mechanical slide carbs.

My Simple-Simon understanding of "signal strength" is suction power produced through the carb throat during the engine's intake stroke.

Good Fortune! :)
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Mikuni VM 33 Carbs 26 Feb 2016 08:20 #712577

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Regarding VM33's, would Patton's noted concerns be more or less an issue for a stock 1100 motor with 84 GPZ head/cams?
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Mikuni VM 33 Carbs 26 Feb 2016 08:24 #712578

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Something else, There's no taps for vacuum sync. They would have to be added on. Mine were poorly done. I think I'd be better off with some CV style carbs being I don't race and my riding style. I go from S/L to 4800' at least once a month.
I knew I should have kept those Suzy carbs. Dang it. :angry:
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 08:26 #712579

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daveo wrote: Regarding VM33's, would Patton's noted concerns be more or less an issue for a stock 1100 motor with 84 GPZ head/cams?


Honestly IDK. Higher compression right?
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 08:35 #712581

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swest wrote: . . . no taps for vacuum sync. They would have to be added on. . . .


29's fit into stock Z1 carb holders that have vacuum nipples used for syncing.

Do 33's require larger than stock carb holders? :unsure:

Found this helpful thread:
www.vintagebikeforum.org/index.php?topic=2486.msg15817#msg15817

Good Fortune! :)
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Last edit: by Patton.

Mikuni VM 33 Carbs 26 Feb 2016 08:47 #712583

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Yes, I have later style holders. No taps. Idle is good albeit my jetting is different than what's called for by the graph from jetsRus.
2.0 slides not 1.5. Turns out 1.5 is richer than 2.0. I had it bass akwards. :blush:
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 08:51 #712584

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swest wrote:

daveo wrote: Regarding VM33's, would Patton's noted concerns be more or less an issue for a stock 1100 motor with 84 GPZ head/cams?


Honestly IDK. Higher compression right?
Steve


Stock pistons...with increase of flow through head.
Does increased mixture volume passing through the cylinder affect compression?
If any, what impact does that have when considering carburetor size?
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Last edit: by daveo.

Mikuni VM 33 Carbs 26 Feb 2016 08:53 #712585

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Patton wrote: . . . Found this helpful thread:
www.vintagebikeforum.org/index.php?topic=2486.msg15817#msg15817 . . . .


For ready reference, here's a post by wiredgeorge in the referenced thread:
Re: More Mikuni VM-33 smoothbores
« Reply #1 on: August 30, 2014, 06:51:48 AM »
Sorry Thomas, didn't notice your post. The problem with the 33s is the large venturi produces very slow air speed and this requires them to be used on a higher compression and larger displacement bike capable of producing more air speed through the venturi. Smaller bikes, such as a stock KZ900 don't much like the 33s because while they will idle fine, if you quickly twist the throttle, there is a bog. The pilot air jet is either a 1.0 or 1.2. Dropping the pilot air jet to .6 helps the issue. Problem is still there only not as severe. The other main issue with the 33s is no sync. They can be fairly accurately bench sync'd if you have the skills. The holes on the venturi edge? One is a pilot air jet which as i said will be about 1.0 or 1.2. Then the other air jeet is the main air jet which augments airflow when the throttle is shut (main circuit is always working). The other opening is set back farther in the venturi and supplies air to the enrichener circuit. It isn't really a jet but just an open passage about 4mm across. The other castings that look like openings are for show I guess. As far as pilot jets, Sudco's suggestions are semi-insane as I recall... 17.5s are fine but the air screws should be almost closed. A lot of the setting depends on the slide cutaway number. If they are 1.5s, then about 1/4 turn out.

Good Fortune! :)
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Mikuni VM 33 Carbs 26 Feb 2016 10:12 #712587

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Patton wrote:

Patton wrote: . . . Found this helpful thread:
www.vintagebikeforum.org/index.php?topic=2486.msg15817#msg15817 . . . .


For ready reference, here's a post by wiredgeorge in the referenced thread:
Re: More Mikuni VM-33 smoothbores
« Reply #1 on: August 30, 2014, 06:51:48 AM »
Sorry Thomas, didn't notice your post. The problem with the 33s is the large venturi produces very slow air speed and this requires them to be used on a higher compression and larger displacement bike capable of producing more air speed through the venturi. Smaller bikes, such as a stock KZ900 don't much like the 33s because while they will idle fine, if you quickly twist the throttle, there is a bog. The pilot air jet is either a 1.0 or 1.2. Dropping the pilot air jet to .6 helps the issue. Problem is still there only not as severe. The other main issue with the 33s is no sync. They can be fairly accurately bench sync'd if you have the skills. The holes on the venturi edge? One is a pilot air jet which as i said will be about 1.0 or 1.2. Then the other air jeet is the main air jet which augments airflow when the throttle is shut (main circuit is always working). The other opening is set back farther in the venturi and supplies air to the enrichener circuit. It isn't really a jet but just an open passage about 4mm across. The other castings that look like openings are for show I guess. As far as pilot jets, Sudco's suggestions are semi-insane as I recall... 17.5s are fine but the air screws should be almost closed. A lot of the setting depends on the slide cutaway number. If they are 1.5s, then about 1/4 turn out.

Good Fortune! :)


Can't vacuum synchronize them? Hmm...
Bench synch :huh: :blink:
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Mikuni VM 33 Carbs 26 Feb 2016 10:29 #712591

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I've read that the slides leak air with the tops off so syncing them gives bad results right Patton? And the 29s are the same right? With 1.5 slides is a 1/4 turn out on the air screw recommended on the 29s too..... sorry Steve need to ask.
1978 KZ650 b-2
700cc Wiseco kit 10 to 1.
1980 KZ750 cam, ape springs, stock clutch/ Barnett springs.
Vance and Hines Header w/ comp baffle and Ape pods, Dyna S and green coils, copper wires.
29MM smooth bores W/ 17.5 pilots, 0-6s and 117.5 main
16/42 gearing X ring chain and alum rear JT sprocket....

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Mikuni VM 33 Carbs 26 Feb 2016 12:37 #712606

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They don't leak any worse than the 26's. Syncing them is no problem and I can and will do it. Mikuni calls for 1/2-3/4 turn. The air screws have a hole in them. Mine were soldered shut. I'm going to buy two new ones to replace the ones Snake munged up.
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 15:22 #712625

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I run them on my elr some times and no problems bu tits bored and head work. crh

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Mikuni VM 33 Carbs 26 Feb 2016 18:58 #712664

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swest wrote: They don't leak any worse than the 26's. Syncing them is no problem and I can and will do it. Mikuni calls for 1/2-3/4 turn. The air screws have a hole in them. Mine were soldered shut. I'm going to buy two new ones to replace the ones Snake munged up.
Steve


Please briefly explain the synching process for VM33's. I have a Carbtune four-column unit, which I rather enjoy using on my BS34's.
Is the process as simple with Smoothbores or RS Flatslide models?
:lol:
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Mikuni VM 33 Carbs 26 Feb 2016 19:07 #712665

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Mine have vacuum ports though poorly done. :dry: You take the tops off and sync them by the lock nut and screw. #3 is fixed, that's the lead carb. I will get the levels as even as possible, blip the throttle adjust them again until they won't change. They're easier to sync than my 28's were. I changed the needle setting so I didn't bother syncing them until I knew I would leave them that way. I'll be doing it soon and will record the process.
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 19:18 #712668

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swest wrote: Mine have vacuum ports though poorly done. :dry: You take the tops off and sync them by the lock nut and screw. #3 is fixed, that's the lead carb. I will get the levels as even as possible, blip the throttle adjust them again until they won't change. They're easier to sync than my 28's were. I changed the needle setting so I didn't bother syncing them until I knew I would leave them that way. I'll be doing it soon and will record the process.
Steve


Poorly done vacuum ports...? On the carbs from the factory, right?
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Mikuni VM 33 Carbs 26 Feb 2016 19:20 #712669

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No, the PO Snake AKA Shark was a moron. :angry: They don't come with ports from the factory. :dry:
Steve
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Mikuni VM 33 Carbs 26 Feb 2016 19:43 #712673

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swest wrote: No, the PO Snake AKA Shark was a moron. :angry: They don't come with ports from the factory. :dry:
Steve


Really, no factory supplied vacuum ports to attach synch hoses to, ok...what? :ohmy:
You got to be kidding. :dry: :angry: :evil:
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Mikuni VM 33 Carbs 26 Feb 2016 19:45 #712674

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I'll take a pic in the morning. You'll laugh. :huh:
Steve
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