VM29 Jetting Trouble

Thanks wiredgeorge for the excellent real-world info.

Don't know which slide cutaways or jet needles or air filtration trippivot is using.

Do know my Yoshi Stage 2 Z-1 with VM29's has smooth stable idle, pulls well from idle without stumble, and performs consistently well throughout the powerband. Am using K&N filter in stock airbox and believe (not positive) sizes as shown on Z1E site, with jet needle clips in middle position, standard VM26/288 pilot air screws, o-rings and springs, and 17.5 pilot jets.

Began pilot screw adjustment at 1.5 turns out. Don't remember the final setting, but does influence idle.

Thinking chetriply has similar setup, and throught maybe having similar problem due to clogged air jet. Hope he's not suffering from information overload.

Here's the BS30/97 air jet I had overlooked in earlier cleaning efforts.



Post edited by: Patton, at: 2007/05/19 22:01

Post edited by: Patton, at: 2007/05/19 22:05

Patton, what is the size of the mainjet in your configuration?
Willem

willemZ900 wrote:

Patton, what is the size of the mainjet in your configuration? Willem

Mine have the smaller size 105. But would urge reading plugs after hi-way speed throttle-chop to assure best application for your individual bike.

Mainjet size is especially critical at wide open throttle. Too lean can quickly burn holes in pistons.

Believe interaction of tapered jet needle in needle jet dominates fuel control at various throttle openings (slide positions) until the tapered portion reaches top or near top position so as to basically leave nothing but orifice in the main jet governing fuel supply at top of needle jet (where slide cutaway size probably becomes irrevelant).

And with regard to any and all of this about best setup for VM29's, I heavily value and readily defer to the broad real-world experience and vastly superior knowledge of wiredgeorge. He knows what works for many. I only know what finally worked for me.

Patton, Most folks who run 29s also use pods and an aftermarket exhaust. I generally use #17.5 pilots and #120 mains... If a stock airbox is used, you can use #15 pilots and #115 mains generally. One comment you made, I take some disagreement with... the comment about burning a piston if the main jets are too small... my experience is that the bike just runs out of gas when on full throttle. I suspect that if the the bike is running out of gas and falling on its face, you could ride along with the throttle wide open long enough to hole a piston. It might be possible but I tune so many carbs that when they hit "out of gas due to tiny main jet" problem, I just turn around back to the shop and get some larger mains. If I am in an application that I am unsure of as far as whether the mains are too big or too little, I apply the choke to see if it helps or makes the problem worse. I have holed pistons in the past by having air leaks... like from cracked carb holders so it is possible to hole a piston from a lean condition but I would be surprised if it happened by using too small main jets. For instance, some of the Suzuki jetting is over-lean for their VM series VM26s... Some of them use #90 main jets and I can't pull 6500 rpm with that jetting on my test bikes using a stock airbox. If you hold the throttle open, it just burbles out at 6500 rpm no matter how long you hold the throttle open... Slip some #102.5s in and the bike will pull to redline (#105s are even better). I can't see someone holding the throttle open for long enough to do engine damage... have you ever had experience with this happening or hear of anyone have holed pistons from too-small mains? Just curious...

Hey all.

Finally gonna have time to stick the old girl back together. One question for Wiredgeorge; Where can a man obtain the correct return spring for the 29's?

Any help would be appreciated.

There are two return springs out there for the VM carbs that have a single, center return spring... One is on the VM24 and the other on the VM26/VM28/VM29. The latter are larger diameter wire... heavier gauge. If you put a VM24 spring on a 29, the throttle actually feels nice and light BUT the lighter spring won't return the slides immediately... the idle takes awhile to settle. I have had this happen several times making me think MAYBE Mikuni stuck the wrong spring on but someone may have just mixed the spring up when rebuilding or something. Not sure if this part is still available. You could check with Z1Enterprises.com or find a junk set of 26 or 28 carbs.

wiredgeorge wrote:

Patton, Most folks who run 29s also use pods and an aftermarket exhaust. I generally use #17.5 pilots and #120 mains... If a stock airbox is used, you can use #15 pilots and #115 mains generally. One comment you made, I take some disagreement with... the comment about burning a piston if the main jets are too small... my experience is that the bike just runs out of gas when on full throttle. I suspect that if the the bike is running out of gas and falling on its face, you could ride along with the throttle wide open long enough to hole a piston. It might be possible but I tune so many carbs that when they hit "out of gas due to tiny main jet" problem, I just turn around back to the shop and get some larger mains. If I am in an application that I am unsure of as far as whether the mains are too big or too little, I apply the choke to see if it helps or makes the problem worse. I have holed pistons in the past by having air leaks... like from cracked carb holders so it is possible to hole a piston from a lean condition but I would be surprised if it happened by using too small main jets. For instance, some of the Suzuki jetting is over-lean for their VM series VM26s... Some of them use #90 main jets and I can't pull 6500 rpm with that jetting on my test bikes using a stock airbox. If you hold the throttle open, it just burbles out at 6500 rpm no matter how long you hold the throttle open... Slip some #102.5s in and the bike will pull to redline (#105s are even better). I can't see someone holding the throttle open for long enough to do engine damage... have you ever had experience with this happening or hear of anyone have holed pistons from too-small mains? Just curious...

Have been gun-shy for many years about running too lean and causing excess heat resulting in piston damage. Goes back to two-stroke days of yore, and realize the problem may be more critical in two strokes, and that pre-ignition (pinging) from improper timing is often more damaging than mixture, and that excessive compression ratio can also fry stuff pretty good.

May be misapprehending the seriousness of wfo running on too small main jets, and perhaps overcautious, but seeking to be safe than sorry. And having roasted more than one good motorcycle engine, believe excessive heat is an enemy to air cooled engines.

Being a confirmed worry-wart, here is a sampling of the stuff that concerns me (excerpts from various sources, some more authoritative than others, none authored by me) ---

Piston Diagnostic Guide:
Site for piston damage analysis with many pics of severely damaged pistons from various causes, including a hole from too lean carb setting.
Piston Dianosis Guide

From Weber Carb site:
Install a Halmeter AF30 Air Fuel Ratio Gauge to help you with tuning. It is fast and accurate and will take a lot of the guesswork out of it. This is a particularly valuable tool for cars that get track use so that you make sure you do not lean out and put a hole in a piston


From various forums:

The carburetors are delivered from the manufacture preset to run rich. This is to reduce the likely hood of burning a piston due to a lean fuel mixture.

A moderately lean main jet can cause engine seizures. A severely lean main jet can cause the engine to burn a piston (whole in top).

End gas detonation is cause by several factors of combustion: (1) excessive squish clearance, (2) lean mixture, (3) excessive compression, (4) too low octane fuel, (5) insufficient cooling, and (6) air leaks that cause a lean mixture.

If your upgraded air filter allows "more air" into the carb, then increase its main jet size (NOT 90 down to 75). This will lean the carb's mixture and in a short period of time, it will burn a hole in the piston (from running too hot).

You need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side.

Air cooled cars, turbocharged cars and cars with aluminum heads can be very sensitive to lean mixtures. If the mixture does not contain enough EXTRA gasoline to cool the combustion process, then exhaust gas temperatures may get so high that they crack or melt the exhaust valve, exhaust valve seat, the piston or the head itself!

Same theory applies to four-strokes as well; the more fuel entering the engine, the cooler the piston will be. The oil and water cooling systems are not designed to cool the piston; only the little bit of fuel that is mixed with the incoming air charge prevents your motor from seizing. ONLY after the heat has been transferred through the piston to the rings and then to the cylinder, will the cooling system get the chance to do its job.

And never forget that lean mix is the dead for all motors !!!


Even the frik'n Ebay seller:
This is the solution to your vintage racer's jetting woes. Never again will you have your Mikuni VM round-slide carb blubbering or running so lean you're burning holes in the piston


So that there's the reason I'm so darn paranoid about running too lean.

Post edited by: Patton, at: 2007/05/24 17:43