Advice on 39 or 41 mm Kehin FCR's w/ small valve 900 heads (750 heads)

[silas]

Anyone have experience w/ putting the Kehin 39 or 41mm flatslide carbs on a 900 monster w/ the smaller valve heads? Did you go w/ the 39 or 41's? Is it a worthwhile mod w/ the smaller valve heads ?

My '98 M900 came w/ the '750 heads' & valves I think. The horiz cyl head is marked 'W2' & the vertical one 'W1'. Are these the smaller 750 valves/ heads?

I've ridden sev 900ss's (91-98) w/ the 41mm FCRs' & love'em. Am I going to notice a similar performance difference w/ the smaller valve heads & the FCR's Are they set up the same from ca-cycleworks for both valve sizes on the m900 & 900ss?

My mechanic is not a big proponent of the FCR's & says I should update the heads for more hp 1st before the carbs. I just love the seat of the pants improvements the fcr's make on the 900ss's...

Thanks a lot ,

[Duck-Stew]

The 'W' heads are indeed the small cams and valves (750 parts).  Update the heads first, and then buy the carbs.

[needtorque]

The 'W' heads are indeed the small cams and valves (750 parts).  Update the heads first, and then buy the carbs.

+2 save and do the heads and carbs at the same time.

[Norm]

Check with Chris K, but I think he has gotten good results with jetting 41s for the smaller heads. 39s are also harder to sell. You can get almost as much as you paid for a set of 41s.

[krista]

IMO, the stock carbs are more restrictive than the W heads. All the pre-DS heads are choked up anyway, the W heads are just a little worse than the average V2 stamped SS head.

The jetting is the same on all FCRs: 41s or 39s; Monster or SS; 750 or 900. I've got a FCR FAQ here too. 900s get 41s and the 750s get 39s. It's about displacement and air volume, not what valves are in the heads. If you're thinking do both but want the FCRs either way, get them first and see if they are enough to make you happy.

Again IMO, no one should spend money on the insides of pre-DS engines in the name of power. Instead, they need to get a 696/1000/1100 engine and start there. We're starting to get people putting FCR41 singles on 1000DS engines. The first guy got 99hp and other than a nemesis ecu, the rest of his bike was all stock + slipons. 2nd guy is a racer and is essentially over-engineering his bike, so it's not quite done yet.

Chris

[junior varsity]

Holy hell, all this time I was thinking I had the shitty heads on my bike, and I come to find out I lucked out and got the V2 stamped heads, on a 99 M900. And an aluminum swingarm. And fully adjustable showa forks. Good googly moogly! I thought I had the bad ones, and it turns out I was mixed up.

I should still upgrade me carbs with the 41s. ASAP.

[junior varsity]

My horiz head is marked V3, the vertical is marked V2. Are the numbers just to differentiate one from another? (Wondering why mine aren't V1/V2, and are V2/V3)

[greenmonster]

My garage buddy got (to me) amazing results w  W heads w polished ports, DP cams, HC pistons, open airbox & RR exhaust and 39mm FCR:
88Hp at rear! Still hard to believe but I`ve seen the charts.

I want to try 39`s on my 900 w polished ports, HC & open breathing, wondering if velocity will be faster/higher and therefore torque better...

[brad black]

Holy hell, all this time I was thinking I had the shitty heads on my bike, and I come to find out I lucked out and got the V2 stamped heads, on a 99 M900. And an aluminum swingarm. And fully adjustable showa forks. Good googly moogly! I thought I had the bad ones, and it turns out I was mixed up.

I should still upgrade me carbs with the 41s. ASAP.

it's a Special - M900S.

[junior varsity]

Well, its certainly special, but its no "S". Had the factory crappy Boge-Sachs rear shock, standard M900 sideplates, and all the paperwork and title indicate to be a just a regular M900. Its like they went to build an S, didn't have the last few good parts, and just said "some customer is getting lucky with this bike badged standard M900"

[Duck-Stew]

My garage buddy got (to me) amazing results w  W heads w polished ports, DP cams, HC pistons, open airbox & RR exhaust and 39mm FCR:
88Hp at rear! Still hard to believe but I`ve seen the charts.

I want to try 39`s on my 900 w polished ports, HC & open breathing, wondering if velocity will be faster/higher and therefore torque better...

After all *that* work, it's hardly a fair comparison with a stock pair of 'W' heads...

BTW, the 'W' heads went to the Cagiva's after 1998.  All 900's (AFAIK) in '99 and up got the 'V' heads.

[junior varsity]

Oh, ok. I try to do all my homework (http://www.moto-one.com.au/performance/) but I never can be sure what's the deal on some things.

[greenmonster]

After all *that* work, it's hardly a fair comparison with a stock pair of 'W' heads...

I agree, just FYI what you eventually could get out of them.
Still find it hard to believe, but I`ve seen the chart.

[desmopr]

IMO, the stock carbs are more restrictive than the W heads. All the pre-DS heads are choked up anyway, the W heads are just a little worse than the average V2 stamped SS head.

The jetting is the same on all FCRs: 41s or 39s; Monster or SS; 750 or 900. I've got a FCR FAQ here too. 900s get 41s and the 750s get 39s. It's about displacement and air volume, not what valves are in the heads. If you're thinking do both but want the FCRs either way, get them first and see if they are enough to make you happy.

Again IMO, no one should spend money on the insides of pre-DS engines in the name of power. Instead, they need to get a 696/1000/1100 engine and start there. We're starting to get people putting FCR41 singles on 1000DS engines. The first guy got 99hp and other than a nemesis ecu, the rest of his bike was all stock + slipons. 2nd guy is a racer and is essentially over-engineering his bike, so it's not quite done yet.

Chris

Interesting Chris.  Can you pls elaborate.  So they swap the efi for fcrs and use a ecu for ignition??

[BumpaD_Z28]

My horiz head is marked V3, the vertical is marked V2. Are the numbers just to differentiate one from another? (Wondering why mine aren't V1/V2, and are V2/V3)

Mine are the same way V3 / V2 on my '99 M900 city

It's nice to know that we don't have the W's

~DaVe