29 smootbores

When driving my z900 with 29 smoothbores at a constant speed of about 130-150 km the engine starts to stutter as if there is not enough petrol in the carbs. 120 mains, 117 pilot, pingel petcock and standard airbox with K&N filter.
Could it be that there is at that speed a vacuum in the petroltank? Otherwise the carbs run really great and are completely rebuild, the very thin gasgets under the jetblocks are also new. Any suggestions?
Willem
The Netherlands

There are vents on the 2 and 4 carburetors up on the side and to the center of the carburetor. Are these free and clear? If so, try riding without ANY hose or anything on the vents and see if that makes a difference. Are you using a fuel filter? If so, is there a directional arrow on the filter and is it oriented in the correct direction (direction of gas flow)? Have you tried popping open your gas cap when the bike starts stuttering? Possibly a vacuum lock caused by poor venting... Have you checked your mechanical advance to ensure it pivots freely at about 3000 rpm... you are supposed to clean/lube it occassionally.

The vents on this set are both clear and i am not using a extra fuel filter, but yes, the next time whem i am riding i planned to open the gas cap for possible vacuum lock. To check the mechanical advance is a very good idea (never checked it because it always worked) Thanks Willem

If not already done, would assure no crud/grit hovering suspended in fuel and collecting onto petcock pick-up tubes inside tank or clogging petcock screen. May need to flush out tank, and clean pick-up tube screens. This fuel starvation problem may occur while bike is being ridden.

Insufficient flow through petcock allows floatbowls to get dry and "run out of gas." Sit there for a few moments (fuel slowly drizzles down into and fills the bowls) and then engine cranks right back up as if nothing was wrong, only to repeat the problem shortly thereafter by again "running out of gas."

Imo, an accessory in-line filter should always be used, particularly with smoothbores having no floatbowl overflows and therefore especially succeptible to obstructed float needle/seat interface allowing excess fuel to escape up into carb bores and into crankcase.

Would also discontinue using the "S" shaped hose between airbox and crankcase breather, and route another breather hose around swingarm to underneath bike. Avoids possibility of excess fuel (which has risen into carb throat and flowed into airbox) from draining through the S shaped hose down into the crankcase.

An example is shown below.

Good Luck!

[Click on image to enlarge view]

Well, i have 2 completely rebuild smoothboresets, 1 with an accelerator pump. Both sets run like hell but have the same problem and that is strange becuase my 28mm Z1R set has not. The pingel petcock is new, has a nice filter and is special for large diameter tubes. Tubes are also new. So my first thought was that it could be a vacuum block in the petrol tank or (WG idea) the advancer. hmm back to my garage :blush:

Easy enough to test for continuous uninterrupted volume of fuel flow by discharging fuel from petcock line into a separate container, and keep watching to determine whether fuel keeps freely pouring on through into the container.

While doing this, can also check for proper functioning of the fuel tank cap vent.

If fuel keeps freely flowing, when thereafter reconnected to carbs, fuel would logically keep pouring on into the floatbowls.

As known, possible clogged cap vent and/or impurities clogging the petcock pick-up tubes may cause fuel starvation symptoms regardless of perfect petcocks and perfect carbs (which depend on a vented clean fuel source).

Have also seen it happen on new bikes with freshly re-painted tanks where some paint fragments got inside the tank and migrated down to huddle around the pick-up tubes. Sucked up tight against the pick-up tubes, especially under acceleration or higher speeds where more fuel supply is needed to maintain required levels in the float bowls.

Feels just like "running out of gas" because the float bowls are indeed "running out of gas".

Good Luck!

Could someone please post pictures of this carb set up the proper way with inline filters from the petcock to the carb. Thanks ...Russ

Have never experianced w/ 29's, but once w/ 26's could out-run the float valves w prolonged wfo. went from 2.0 to 2.5's & cleared prob. G

My bike is doing the exact same thing. I put an inline filter in and it sounds and feels like it is starving for gas. When I ran it without a filter it ran correct. However without the filter float bowl #1 stuck twice causing gas to trickle out of the pod so I went to the inline filter. The stock petcock angle makes it tough to utilize the reserve side and get a clean flow to the carbs. I am sure many of you have addressed the issue. I would love to see a picture of the best set up...Russ

Russ Jackson wrote:

... ran it without a filter it ran correct. However without the filter float bowl #1 stuck twice causing gas to trickle out of the pod...

Red Flag Alert -- where gas trickled out of the pod, it may well have also flowed the opposite direction into the combustion chamber and down around the pistons/rings into the combustion chamber.

If not already done, would sniff-test oil for any fuel odor and examine condition of oil for any fuel dilution.

Good Luck!

gane wrote:

Have never experianced w/ 29's, but once w/ 26's could out-run the float valves w prolonged wfo. went from 2.0 to 2.5's & cleared prob. G

went from 2.0 to 2.5's -- what do these sizes refer to?

Patton, If you look at the seat of your float valve there will be a stamped #, similar to jet sizes. it refers to the inlet dia.G. PS, as a worthless asside.I've encountered in the past, (in racing) situations where exotic fuels have eaten chunks of fuel line leaving them attop seats, and restricting flow. sometimes keeping needles from seating, sometimes not. Most current (vitron tipped) assys have screens attop inlet, many earlier did not.a good thing to check if you go there. G