Modified Carbs and Airbox Mods.

[koko64]

Yes, back in the day.
I think the 40 pilot is ok with the DJ 730 aggressively tapered needle and it's small root diameter. I would run a 42.5 with a FP needle or 729 DJ needle.
Yesterday I told the GLW that I have enough jetting data on the ported carbs to run different tunes for M750/900 and SS750/900 with pods, stock air boxes, open air boxes and modified air boxes. That testing is thankfully done. I put aside an hour or two each week to probably account for a weeks work, but I learned what I needed to.

Now to test against FCR's.

[koko64]

[/url]20171207_080829 by Tony Kokonis, on Flickr[/img]

Buggered why the pic so large on the transfer from Flickr. The link gives a better pic.

Found a before/after pic of the rubber stacks. It helps get the stack mouth further from the air cleaner element. Seems to have a more positive effect on 900's.

Bikeboy found little effect on a 750 on a dyno. One forum member suggested that acceleration/response would be improved even if not peak hp. I have a theory that barrel/pod filters don't negatively impact the 750 motors as badly as the 900's. I believe it's linked to the port/cylinder volume ratio mentioned by Lofgren.

[koko64]

Road tested my ported carbs against FCR41's. Of course I expected the FCR's to be better, but by how much?
The difference in response was in the 1/2 to 3/4 throttle region. Off the bottom to 1/4 throttle and low rpm transitions the ported CV carbs by their nature had the edge, but from 1/2 throttle the FCR's took over. My M900's ported heads really seem to like the bigger FCR's. Accidental 3rd gear wheelies avoiding a carelessly merging car exiting a roundabout can and will occur.
Like others here, I reckon the FCR39's would be a good upgrade on a stock or mildly modified 900. Having ridden and compared a hi comp, extensively airbox modded, light flywheel, mildly ported 750 with FCR39's against my pod filtered, light flywheeled, stock CV carb jetted 750; I have come to the opinion that the BDST38's could be made to work very well on the 750 motors at their 38mm size, or on stock 900 motors. I think the W head 900's would also be a good candidate for a ported CV carb.

[greenmonster]

 


Mine had the W heads when I bought it, ok midrange but they just choke at 140 km/h, night and day w V2.

Tony, now it`s time for the ducatigirl100 main jet mod:

http://www.ducatimonsterforum.org/index.php?topic=75075.0

http://www.ducatimonsterforum.org/index.php?topic=74413.30
"The result is a better effeciency from 1/4 to 3/4 .... is nigth and day."

[koko64]

Edited: Lofgren used to do this stuff, but he has rightly moved on to EFI. A partially educated guess has me thinking he may have extended the emulsion tube/needle jet base to the bottom of the float bowl as a main jet holder and redo the top section for more efficient atomization. I'm only guessing because he wouldn't divulge his secrets to the likes of me .
A mate has rightly pointed out that the correct relocation of the main jet is paramount to avoid fuel starvation. Reworking the needle jet for more efficient atomization and effectively adding a power jet adds to the circuits allowing an optimum main jet without an overly rich midrange. Relocating the main jet in line with the needle jet gives a direct route for fuel draw and appears to be best practice for performance carbs. Note the BDST38 takes a less direct route and I believe that this has as much to do with the slow response as anything. All these mods help reduce compromises in jetting and enhances response. This stuff has been done to HD carbs for a long time as there's nothing like a slow bike to drive performance tuning!