29mm smoothbore setup?

what pilots,jets snd needle setting are you guys using on the 29mm smoothbores?

My 26mm carbs were spot on with 17.5 pilots and 117.5 mains on my 1000 engine but way lean on my 1075cc w/stock cams. so I went ahead and bought a set of 29mm smoothbores w/15.5 slides. they were also very lean so i then started increasing the pilot and jet sizes.

went from 17.5 pilots to 20 then to 25.
the mains went from 120,122,130,135 then 140.

Was still too lean so then brought the needles up one notch and it is now pretty close.

It seems like a lot to me. I triple checked the float settings and for vacuum leaks and the manifold are well sealed.

got to admit - this is one screaming ride
I havent checked my gas mileage yet but would think it will prolly suck.

Welcome back Jim.

I triple checked that......

Float level correct
carb passages were clear
no vacuum leaks
manifolds tight and sealed

the bike was surging when cruising at a steady speed.
bringing the choke lever up a little would smooth it out.

I went up in small incements to the point where i am now.

pilots = 25
mains = 140
needle = 4th notch

it is real close now.
I think i might need to go the next size up on the pilot to get it finially right.

Those jets do seem way big - it seems to point to a different issue.

You may want to make sure your fuel flow is adequate. With the 29mm carbs you only have the single fuel feed - some of the petcocks do not flow enough fuel at full throttle. I ran into this problem back in the 1970's - I plugged one of the 2 Z1 petcock fuel spigots and just could not get enough fuel through the stock petcock.

Also, check the vent in the gas cap - if that is partially plugged it'll constrain the fuel flow.

The float height should be 23mm - is that what you'd got them set to?

Do you know what size float needle valve seats are in the carbs? Usually it's 2.5, but if you are running a smaller opening, that may be part of the issue.

float level - 23mm
I am using the single line pingle petcock.
I replaced the Gas cap last year with a new one.

It makes no differance if I shut off the bike - let it sit a few min's then go again when warmed up. It shows no signs of being a fuel supply problem.

the surging was happining with no real fuel demand on the engine.

start and warm up bike.
shut off
put on jacket, helmet and gloves.
restart bike and just trying to ride thru the subdivision at 20-30mph it would surge. apply choke just a little and it would smooth out.

I would have thought that 22.5 pilots and 120/125 mains would have been fine.

jtmcculloch wrote:

I would have thought that 22.5 pilots and 120/125 mains would have been fine.

Me too...

side note - I have less than 50 miles on the 1075cc upgrade.

no idea what size float needle valve seats are in the carbs.
I may have ordered them from you when I got the pilots and mains. I would have to check my records.

jtmcculloch wrote:

...float level - 23mm...
the surging was happining with no real fuel demand on the engine... ust trying to ride thru the subdivision at 20-30mph it would surge. apply choke just a little and it would smooth out....

As earlier suggested, I would suspect over-lean mixture in the pilot circuit, perhaps due to one of the followinmg:

(1) Pilot passage not being ultra-clean, clear and unobstructed -- as known, the pilot jet picks up raw fuel from the float bowl, where air joins with the fuel creating the fuel/air mixture which travels from the top of the pilot jet on through the pilot passage and out the pilot outlet (a tiny orifice in bottom of carb bore), from where the mixture is sucked through the intake valve into the combustion chamber. The pilot passage from top of pilot jet to and through the pilot outlet should be clear and ultra-clean. I would keep blasting away with carb cleaner through the pilot passage until seeing cleaner freely emerging from the tiny orifice pilot outlet.

(2) Float bowl fuel level too low. The service level actually visible by using the clear plastic tube test (as opposed to relying solely on the upside down bench-top measuring method, which is okay for a preliminary setting but may or may not result in the true actual correct float bowl fuel level). Smoothbores require fitting the clear plastic tube to the large bottom drain plug hole. While there may be better methods, I use a tapered rubber grommet pressed into the drain hole with the clear plastic tube fitted inside the grommet hole. Would perform this test while carbs remain mounted in place on the bike with bike held level. The float levels may already be just fine, thereby casting more suspicion toward a problem in the pilot passage.

For purposes of illustration, here's a pilot circuit cut-a-way which might be helpful:



Imo the pilot circuit is more important to lower rpm operation than it is generally given credit for. For instance, at lower road speeds on a large four in top gear, the throttle slide is just barely cracked open, where incoming fuel mixture is still being influenced by the pilot circuit (before the pilot circuit is gradually phased out by larger throttle openings at higher speeds as the mixture becomes more and more influenced by the tapered jet needle.)

The history as reported seems to show more mixture problems being experienced at the lower end (pilot circuit) rather than at the higher end (main jet, jet needle, clip position, etc.) And of course, even an otherwise perfect carb can produce the wrong fuel mixture with an improper float level.

Just my 2 cents.

Good Luck!

jtmcculloch
I have 120 main, 20 pilots, needle set in the middle slot
on a stock 1000 with baffled pipe+pods. air sq. 1 turn out
On a 1327 drag bike with 29s
Same setup only 130 main jet. ran good but under carbed.

check your plugs... gray, not black.
Rule of thumb. changing the needle clip is like changing
two main jet sizes.

the float needle valve seats should be 2.5 in most applications. If they've been replaced with smaller ones, you could be constraining the fuel flow into the carbs. Also, check the filters over the float needle valve seats - if clogged they'll constrain fuel flow.

Have you checked if the rubber carburettor holders have no lekkage between the holders and the cylinder head!!. Check the screws overthere!!
I run 29mm pumpers with a standard Z1 airbox, K&N airfilter, 17,5 pilots, 120 mains, 3e clip jetneedle, 1015cc
Spark plugs are oke
Willem/Holland

the manifolds are the first thing i checked as that is a common place for a vacuum leak.


Jeff, thanks for the extra effort in your post.

Next step will be to check the float level with a tube as opposed to measuring the floats.