please help diagnose. (pilot jets/vm29's)

i'd like to run this you guys on here, and see if the collective experience can help with my diagnosis.

the engine: basically, 1015 engine, stock head, stock cams, 4/1 pipe, with baffle. brand new carb to head boots.

the carbs: vm29's, UNI pod filters(oiled), 115 main jets, stock needle jets, presumed stock needle settings, 17.5 pilot jets. sync assumed to be serviceable. air screws out 1/4 turn(...)

symptoms: engine hot, will idle. go to transition from just idling to 1/8th throttle to leave light. engine bogs and stalls if i do not rev past 1/4 throttle, and on to jet/jet needle. basically, whenever i come right off of idle, i get a serious bog, then it cleans out, and it revs to redline(or when i unwind it). adjusting of air screw makes little to no difference in idle quality. neither speeds up, nor slows down. however, if i run this out to 1 and 1/2 turns out, i will get some backfiring in the exhaust.

my best guess is that i went too small on the pilot jet, especially since it clears up after 1/4 throttle is exceeded. steady-state cruising is poor as well. i have been working around it by cruising in a lower gear to keep my rpm's above part throttle.

thanks for looking everyone.

--r

Post edited by: solomrus, at: 2006/03/03 14:14

Mikuni, in their infinite wisdom, decided to make the pilot circuits of the VM29 very tuneable. I am not sure anyone ever won a race using the idle circuit and it just makes the things fussy.

First, if adjusting the airscrews makes no difference, the reason it makes no difference is that when you open them up, it adds air. The problem you are having is that your idle mixture sounds kind of lean. The #17.5 pilots SHOULD be OK with your set up but they are not as evidenced by the bog off idle. Go to a #20 pilot. The reason I can't say what is going on is that you didn't mention your slide cut out. Mikuni used several slide cut outs... the ones initially intended for a Kaw 900/1000 were a bit smaller and required smaller pilot jets and the ones for a Suzuki were quite large and required a bigger pilot jet. You may have a Suzuki style cut out. Anyway, try some larger pilot jets. You might want to pick up both #20s and #22.5s. One of them will be right. If memory serves, #22.5s are what are used normally in the Suzuki jetting mandated by the larger cut outs...

Once you have the larger pilot jets installed, readjust your air mixture screws. If the pilot jet is correctly sized, those things are ultra-sensitive due to the holes in the tip. If I owned smoothbores, I would put JB Weld in those holes so the air screws would behave in a civilized manner but I can't afford smoothbores! bwhahahaha Besides, the BS34s I like to run will run circles around the smoothies for a fraction of the cost and are a darn site easier to tune with more linear power delivery and better economy! (sorry for the lecture - I know you probably paid a fortune for those VM29s!) hehe

george, thanks for chiming in.

no one had #20 pilot jets in town, closest that i could find were 22.5's.

right now, i'm on 22.5 pilots, and at 1 and 7/8ths turns out from lightly seated. idle seems pretty solid at 1200rpm, maybe a little less. throttle response is pretty good. pulls until the engine is just making noise.

any way that i can measure those cutouts with the slides in? any detrimental effects of using the larger cutouts on the kaw engine?

depends on what you consider a fortune. maybe one day i will try a set of the bs34's, especially since they are so inexpensive. then, i can sell the 29's for what i gave for them, and enjoy these on the street. 8^)

the beauty of old bikes. gotta love it.

--r

two words for you,oh wait thats 7,8 ah crap!anyway BOB BROWN!

give me a chance to make it work on my own man!

bob is the last resort.

it's running pretty good after riding it around for about an hour this afternoon. ride for a bit, tweek the air jet, ride around some more, tweek it a little more.

ok, so 1 turn isn't making a difference, what if i go two full turns? wow! now that is more like it. ok, out a little more, backfiring and popping. DOH! wrong way! back in a little more past 2. just a little shy of 2 is about as good as i have it so far. there is a tiny bit more to be had.

gonna get max to loan me his manometer, and sync them up next week. check needle positions, and the cutout, cuz it sure sounds like it's big.

lovin the bike tho. not nimble, not fast, but quick enough for me, and i will always prefer burnouts to wheelies. 8^)

--r

how about a burnout followed by a wheelie combination? :whistle: :evil: told ya i dont do carbonators:whistle:

ok, nice rolling burnout, with a small power wheelie at the end of it.

yeah man, i remember. you are the anal-retentive pr*ck. 8^) that is why i have the sprocket cover on my work bench to weld over the shift pattern... :evil:

gonna try to ride to b-fast in the morning. the cold is ok, hope it's dry. 8^)

--r

i bought some new carbides for my die grinder today $18 each they must be self sharpening or something!:whistle: im gonna work a few hours in the morning then im gonna try and go to ashland to work on frame,i should have some more aluminum for you to weld next week!im gonna see about building that tailpiece tomorrow if i make it to ashland.

The slides are usually stamped on the bottoms with the cut out size. A small mirror will often do the trick.
The STD VM29 (Sudco suggested) throttle valves for the CB 750 and KZ 650-1000 are 1.5 cut outs. The Suzuki 750-1000 cut outs are 2.0.
The STD primary jets for the Suz and Kaw are 25s. The Honda Primary Jets are 17.5. Suggested main jets are 115 for the Honda and Suz and are 120 for the Kaw applications. The air jet is .9 on all and all have 5DL31-3 jet needles and .6 needle jets.
Hope this helps,
Fish

Post edited by: The Fish, at: 2006/03/04 08:50

The recommendations I make are made through personal experience. I didn't use Sudco's recommended sizing chart as I have personally tested their recommended jetting and have not had it work well for me. The pilot circuit on the VM29 is VERY similar to the pilot circuit on various VM26 assemblies. The slide cut out is the critical tuning feature that dictates which pilot jets must be used to get the bike to idle properly. The cut out is the contour of the slide where the larger open area (most contour) means a higher cut out number. The cut out, allows more air and essentially combines air volume at idle with the air jet on the carburetor body. More air means more gas is needed to keep the air/fuel mixture in the proper relationship. VM26 cut outs have varied from 1.5 to 2.5 and I have quite a bit of experience jetting the pilot circuit with all variations. The Sudco recommendsations are too rich for most applications and require running the air mixture screws all the way shut. SOME air can still get by through the holes in the screw tips so the bike will idle. I have found their #25 pilot jet suggestion virtually impossible to get a bike idling properly. Last year, I had about 25 sets of VM29 carbs go through my shop with all variations of slide cuts out. In general, the Sudco pilot jet recommendations are on the rich side and their main jet recommendations on the lean side as most folks running smoothbores have engine modifications that will dictate for more gas. Their recommendations are for stock bikes I think and note they don't mention whether the recommendation is for a bike with pods/pipes or OEM set up. The very rich pilot settings are DEFINITELY too rich for a stock airbox... you could never make the bike idle properly.

I recommend 20.5 / 120 for a 1000 and 17.5 / 117.5 for a 900. I have tuned 10 bikes like this and it always works. The needle is set in the center postion also. air scew is about 1 turn out.

I forgot to mention that the tuning above is for individual filters and a header with no baffle.