VM33 smootbores

Who runs 33s? I have found that most sets I see have large mains and pilots. Usually around a #130 main and #30 pilot. I have a couple questions...

These carbs tend to bog when you are in a higher gear at a lower rpm and crack the throttle all the way open. Doesn't seem to matter what gear. Anyone experienced that issue and figured out how to fix it... Example... 40 mph constant speed in 3rd gear and then open throttle fully. BOGGGGGG... if you let the throttle off just a tad, the carbs will then pull like a freight train. Seems to do this with most sets of 33s. I have used mains from 120 to 150 and pilots from 17.5 to 30 and all combos of jet needle clip positions and still see this issue on most sets.

Does anyone use a vacuum actuated petcock and use the vacuum fitting on carb 2 (I think... or is it 3?) That fitting runs to a hole in the venturi that is mostly always covered by the slide as it is half way up the side of the bore where the slide rides. This can't work geat for a vacuum petcock.

I seem to have best luck using a smaller pilot jet (around a 20) with the air screws almost closed to get a smooth idle. Air speed is pretty fast due to the smallish cut out (1.5) so this works well at idle. Main jets for pods and 4 into 1 on a stock KZ900 seem to be around a #120.

Those carbs will pull your arms out of your sockets and really do outperform 29s (or BS34/CV34s) by a mile even on a stock 900. Anyone use them on smaller bikes? 650/750?

Sorry George, I have a set of 33s but not mounted yet. What RPMs are you speaking off, roughly, when you say "lower".

You're saying 33s out perform 29s on a stock motor? That's interesting to me because I was going to trade the 33s for some 29s with a friend, and my motor is stock - for now - save for a pipe and pods. Could you elaborate a little?

ive got a set on one of my 1170s that were put on back in early 90s,they worked good on the bigger motors but ive always had better luck with the 29s on smaller motors yself.most people dont know what they are when they see em!

My carb test bike has pipes/pods (one of them does as it is set up that way usually). It is a stock 903 with good and consistent compression. With a set of 26s it runs nice. With a set of 28s it runs nice... maybe more kick at the top with early Z1 carbs; later pumpers make little difference over 26s. 29s are an improvement over early z1 carbs when you hit 7K rpm. 34s are more consistant through the entire rpm range than are the 29s but at the top, maybe the 29s by a nose. The 33s will pull to redline insanely in 1st with no bog or slog, then in gears 2-5, if you shift over 4K rpm, the bike will pull your arms off... I would guess maybe 5 or more horsepower over the 29s and DEFINITELY more than BS34s.

The ONLY issue when tuning is that if you are riding along enjoying the scenery... you are say in 3rd or 4th gear at 3000-3500 rpm and suddenly twist the throttle FULLY, the bike bogs... If you feather the throttle even slightly, it moves cleanly through the bog spot and RIPS to redline. If you were at the strip you would NEVER notice this bog... only riding like a geezer... I messed around with FOUR sets of those carbs the other day and they all tuned the same. BOG when you open the throttle all the way at between 3K and 4K rpm.

George, that's the very same story all our customers seem to face with the 33's. Lack of throttle response in the lower RPM range.

Could this be the slide cutaway is too aggressive on the VM33's? It sounds similar to the issue of running VM29's with the 2.0mm slides on a 900 Z1.

What cutaway is on the 33's you're playing with?

I love my 33s.Way easier to jet than 29s.
With the 1075 motor it would bog with a
low RPM full throttle yank but the 1198
motor just pulls.It puts up a little stutter
at 3000 rpm in 5th but that is realy doggin' it.
I new a guy with a 900 running 28s off a Honda
750 with the 1/4 cable and except for having to
sinc them more,he really like how they performed.

George i find the exact same problem with mine (see sig below)i have found 22.5 pilots work best with air screw out 1 turn.i find if i go out more the idle is too high and can't lower the slide stop any more.with 117's i have the air screws out 1/2 turn .i have not nailed the mains yet but they like to be between 132 and 140.the slide cut is a 2.the bog seems a little better with the needle at clip#3 but it is a little rich there.i have a bog from 3500 to 4500 rpm.i also have the floats level set higher(21.5mm instead of 24).
I just got a new set cams with lesss duration in an effort to help this ,but it now sounds like this is not going to work.what have u found with rs34's?

Jeff, The 33s all come with 1.5 slide cut outs. That speeds up airflow through there nicely and they idle great. I did experiment with pilots going up from #17.5 pilots which needed to have the airscrews closed all the way to idle nicely... tried 20s, 22.5, and then went to 30s... idled nicely each time... air screws like to be mostly closed for every setting with some additional air using the larger ones but not more than a turn out. Those carbs are a rush! I get addicted to cracking the throttle. As far as the main jet, a #120 is plenty for pulling redline on the stock 900. To test for the right main, when it starts feeling like it is running out of gas, pull up the choke (you get used to taking one hand off the bar when you are pulling towards redline I guess) and if the pull gets strong again momentarily, go up in jet size... if it gets worse, you went too big.

Those big long 6 series jet needles are not fussy at all. Couldn't feel much difference in the middle three clip positions. I also tried some 5 series jet needles and the bike ran almost exactly the same on 5J9s... they are over a 1/4" shorter... go figure. I also experimented with the throttle stop to limit how high the slide would ride. It had no effect on the flat spot but did limit acceleration with the throttle wide open, as you would expect. If I wanted to hot rod a bike, these are the way to go. You just have to learn to not get into that flat spot. I also think that a better petcock might help some too. I use 1/4" fuel line and a Drag Specialties petcock. I think these carbs might need more gas on full throttle. Last, I noticed that it didn't make the least difference to cap the #4 fuel inlet and just feed gas into the #1 fuel inlet only. The fuel pipe going through these things is huge... much bigger than a 29 pipe and the pipe on them runs from 1 to 4 so I think the 2nd fuel inlet wouldn't be needed unless you had a real high horsepower bike.

Top end power will increase with larger carbs though, but the low end power will suffer. Fit a too large carb (or carbs) and the venturi drop is less for the same amount of air flow and you get a flat spot since the lack of drop in air pressure will not allow enough fuel to be drawn into the carb throat when you open the throttle. If you open the throttle gently you might get away with it, but it seldom can be tuned away. But most people who install large carbs are not the type to twist the throttle grip slowly.

Ivar... what you say is likely correct. This is what has been in the back of my mind since all of them behave pretty much the same and this particular issue isn't affected by jetting.

George,

Have you experimented with the air bleeds?

From what I've seen from your posts, I'm sure you have already but I feel that most people forget to work with air bleeds.

Mick

What do you mean by an "air bleed"? The holes in the sides of the main jet holders (emulsion tubes) on this carb don't have air bleed holes as some do but the pilot jets do have air bleed holes. The only way I know that the term "air bleed" is used is the holes in the edge of a tube that draws gas so as to allow the gas to be pulled and atomized. I am guessing you are referring to something else. What part/component are you referring to?