29 smoothbore vent lines and idle screw

Have some 29 smoothbore on my stock 78 kz1000
Has 4 into 1 header and dyna S

Running open velocity stacks also
Needle in the middle
115 main
17.5 pilot
Everything else is to the book specs for these carbs so no need to see them posted again
Here are my questions
I get the best response at 1/2 to 3/4 turns on the air screw
Decided to close air screw completely and it made no difference
Bike idled and had good response?????????????
There is mention of vent lines on the 29s........ never ran them
Some say you need them
Seriously what is the science behind it ,why????? How long of lines and why does that make a difference ?????
Would like to hear first hand accounts from smoothbore owners ,not more cut and paste specs

The vent tubes should routed up and over the air box. In this case there isn't one. It's so bugs and crap doesn't get in the bowls. My 33's are smooth bores too. The air screws have no effect after 3/4 turn.
Steve

All 4 float bowls are vented through the two vent nipples. Usually there are two hoses (one hose to each vent nipple), both of which hoses exit to open air.

Absent the hoses, with the nipples exposed :ohmy: , it's easier for water or other foreign matter to enter the carb through the open exposed nipples, or to just clog the nipple openings (which disables the venting requirement).

My KZX900 vent hoses are run in the stock routing positions to exit underneath the seat.



The vent hoses could just as well be routed to underneath the bike and exit to open air.

Length of the vent hoses doesn't really matter, provided they aren't kinked or obstructed.

When turning the pilot air screw has no effect on rpm, there's likely something wrong inside the carb, probably an imperfect pilot circuit.

Good Fortune!

I found a gnat in one of my float bowls. I have the tubes now.
Steve

The vent info makes sense,i read previous threads that without they ran like crap
Steve you mention after 3/4 turns there is no change on your 33s
Do you mean 3/4 to closed it runs fine and above 3/4 it starts to lean out and run worst?

richard wrote: Have some 29 smoothbore on my stock 78 kz1000
....Running open velocity stacks also
........

Is this a track-only bike or do you use it on the street? If you use it on the street there's an awful lot of road grit, etc. that over time will be sucked into the cylinders, and that will shorten the engine life. If it's track only that's less of an issue since you won't be running thousands of miles. Ed

My 33's were munged up by the Po. I bought 2 new screws and 4 O rings from jetsrus.com/ I should have got all 4. Mik says they should operate at 1/2- 3/4 turn. Mine have a hole in them. Not like the other VM carbs. 1&4 are responsive in that range but 2&3 I have to go out to 1 1.4- 1/2. I've gone 1-2 steps down on the pilots to see if the screws would work better. They didn't Just got leaner.
www.mikuni.com/pdf/vmmanual.pdf
Here's my thread on them.
kzrider.com/forum/3-carburetor/601369-mikuni-vm-33-carbs
Steve

As shown in the Sudco Tuning Manual 4th Edition, the VM33 smoothbore carb main jet's access to air intake is metered by a BS30/97 air jet, which should be clear, clean and open, undamaged, unclogged and unblocked. If in doubt about 100% serviceability of the air jet, replacements are inexpensive and readily available. Efforts to clean/clear the jet by reaming it out will likely prove less than satisfactory, as I believe the orifice is designed for precise metering of incoming air flow into the main jet.



www.z1enterprises.com/product/SU002-154

I don't know why the Z1E listing mentions applicability only with reference to 29mm smoothies, and fails to mention its applicability to 33 mm smoothies.
The Sudco manual shows BS30-97 air jets on both the diagrams and parts listings for VM29 and for VM33 .

EDITING MEMO -- See following posts which indicate that the BS30-97 Air Jet is for metering incoming air intake provided to the main jet -- not to the pilot jet.

Good Fortune!

Patton, that's for the main circuit. The pilot is on the right.
Steve
jetsrus.com/a_jets_by_carburetor_type/je...ni_BS_30-97_air.html
jetsrus.com/carburetor/carb_mikuni_VM33_...oded_view_parts.html

swest wrote: . . . that's for the main circuit. . . .

Thanks, Steve!

As I understand what you're saying is that the BS30/97 Air Jet meters air intake for the main jet and not for the "pilot" jet.
My post #726741 has been edited accordingly.

MAIN JET (air intake governed by BS30/97)


PILOT JET (air intake NOT governed by a BS30/97)


It's been a while since I've sprayed carb cleaner into the small holes entering at the air box side of the 29 smoothies to see where it comes out.

It must be a different hole -- not the BS30/97 hole -- that goes to and furnishes incoming air to the pilot jet.



Good Fortune!

Yup. That's the one thing I haven't checked out. The PO may have munged it up too. Z1E is half the price of jetsRus. That's helpful.
Thanks
Steve

Richard is talking about the vent line, item #11 in the factory drawing. There is a vent for each bank of two carbs to allow atmospheric pressure be in the bowl so that fuel can be drawn up and into the jets. If this vent line did not exist, fuel would only pull until there was a vacuum in the bowl. As an example, have you ever drawn through a straw in a juice box and found it started to collapse, same idea.
Good luck Rich, it rides like a rocket!