M900 jetting advice

[sur4die]

so i have a 96 M900 that i'm trying to rejet after some upgrades and i'm not having the best of luck.  without a dyno it's very difficult for me to follow the factory pro tunning instructions since i kinda need to tune the bike well enough to idle in order to take it down the street and evaluate the bike "after 10-15 minutes of hard use".  but if you read the FP instructions, fixing the idle is their last step...

so i guess i'm looking for some sort of a sanity check on my main jet selection.
I went from a stock setup to K&N pods and a set of Mark's (motocreation's) BoomTube straight pipes.  i tried both the 145 and 155 main but i can't really tell the difference on the streets near me (no real place to open it up, f-in suburbia)

is 145 definitely too small?

could people with similar setups/experience chime in with their viable configurations?

[dlearl476]

Oh man, having just finished about 40-60 hours of trial and error jetting hell school on my '81 Moto Guzzi Monza, I feel for you.  Unfortunately I can't really offer anything but moral support. 
I will offer this.  It was a bit more appropos for the Guzzi, as it has Dellorto's, but the theory is all the same, especially the part in the back about CV carbs.  I'd suggest you read it a couple of time until you understand all the sentences (not casting aspersions, it took me three or four times.  )  It will make the process easier.

http://www.thisoldtractor.com/gtbender/mg_manuals/dellorto_manual.pdf

I'd also suggesting asking the same question of whomever you bought the pods from.  I'm sure they've answered your questions countless times.

[64duc]

If it runs good :IE no popping on decel, and no coughing on accel, run it for a few miles ( a hundred or so ) and read or post pictures of your plugs. Black is rich, light brown is optimal, and white is lean.

[sur4die]

If it runs good

that seems to be my primary problem.  it won't idle/idles on the rear cylinder. on part throttle the front cylinder fires, but the rear cylinder after-fires.  it hesitates on accel and pops on decel.

and it does this basically no matter which main jet i have installed.

the carbs are clean.  i completely broke them down and dipped them.

the emulsion tubes are new.

the engine has compression. ~135-140psi on each cylinder

could it be the rubber and plastic bits in the carb? does someone make a rebuild kit with o-rings and such?

[Speeddog]

Have you synced the carbs?

[sur4die]

Have you synced the carbs?

um, no...
can you sync them without the bike being able to idle?  i have always synced with a warm idling bike.  how do you sync when you first get the thing back together? running with the enriching circuit on?

[64duc]

um, no...
can you sync them without the bike being able to idle?  i have always synced with a warm idling bike.  how do you sync when you first get the thing back together? running with the enriching circuit on?

My solution would be to turn the idle screw in untill the engine will run on it's own. Then sync the carbs, lowering the idle as necessary while doing so.

[Speeddog]

It's best to sync them at a warm idle, yes.
Having recently chased jetting on a carbie Monster (futile effort that was solved by syncing  ), I'm thinking that it may be your issue with the idle quality.

It's more difficult to sync when it's running poorly / not idling, but it can be done.

Alternative is to give the sync screw a quarter turn one direction, and see if it helps.
If it gets worse, try going the other direction a half a turn, so then you're a quarter turn the other way.

Or, you could take a stab at the pilot jets, go a step richer and see how it likes it.

[64duc]

Carb tuning is as much art as science and will definately drive you to distraction at times.

[sur4die]

sync the carbs first.  sounds like a plan to me!

i also managed to track down a cheap carb rebuild kit at https://www.carbkitscapital.com/carb_kit_list_part.php?make=Yamaha&cat=Carb_Kit

the same kit is used on the FZR, XTZ, and TDM as is used on our Ducatis.  my gaskets all have a crustiness on them and one of my o-rings has a burr.  for 20$ shipped i figure it can't hurt.

[ScottRNelson]

For a 1997 M900 with open airbox, I had excellent results with 155 main jets, needle one clip leaner than the middle position, and 4 turns out on the idle mixture screws.  Springs and pilot jets were whatever came in the Factory Pro jet kit.

[dlearl476]

For a 1997 M900 with open airbox, I had excellent results with 155 main jets, needle one clip leaner than the middle position, and 4 turns out on the idle mixture screws.  Springs and pilot jets were whatever came in the Factory Pro jet kit.

There you go. 

And back to Speedog's question,  you mention you've had the carbs off/apart?  The first step to all this is making sure there is slack on the throttle/choke cables.  If I'm following Speedog, I agree that one of your carbs is working sooner than the other.  I had the same problem with the Guzzi until I noticed the one of the throttle cables was routed incorrectly.

Carb tuning is as much art as science and will definately drive you to distraction at times.

Nonsense.  It's a black art, plain and simple.  Only pain and suffering in purgatory can free you! 

[sur4die]

i was wondering when you'd chime in Scott.     you always had good carbie advice on TOB.

i've been working on this the past 4 nights in a row and i need a break.  luckily i have first corner tickets to the F1 Canadian GP.  i'll check back in after i return from Montreal and put some MORE time into it.

[sur4die]

yeah...  so i got back to the bike FINALLY after waiting for the rebuild kits (i forgot i needed 2) to arrive.

install the new gaskets and float valves from the kits and try and fire it up so i can sync the damn thing...  still misbehaving, but on one of its smoky coughs i noticed that instead of coming out the top of the carbs there was smoke below the battery box...  now isn't that peculiar.

turns out that my fuel pump's vacuum hose had been torn off its nipple by the starter cable.    ugh...  quickly repaired that with some spare fuel/emission hose, synced it up, and what do you know! no more flames out the tail pipes!

went for a ride and it's running at least as well as it was before, if not better.   [moto]
155 main, 42.5 pilot, middle clip, and 3 turns on the idle mixture screw, which is the FactoryPro recommended setting for open intake/free flow exhaust.  plugs are nice and tan now, but i'm sure i'll tweak the clip or idle after i get a few rides in to characterize it's cold to warm transitional behavior.

[greenmonster]

155 main, 42.5 pilot, middle clip, and 3 turns on the idle mixture screw,

Soud right on w pods.