Ducati Monster Forum

According to the Dellorto manual, the Dellorto carbs float bowls can handle up to 40 deg downdraft angle. The carburettored belt drive long manifolds appear to be at least on a 40 deg angle to my eye. Anyone tried Dellortos on the belt drive long manifolds? I was considering 36 or 38mm Dellortos rather than the very expensive FCRs or TDMRs. TM or VM 36 or 38 carbs are relatively cheap but I would have to modify the manifolds as they do not tolerate much tilt past the horizontal. If I can just bolt on some slide carbs to the stock manifolds it would be an easy fix.

Not since I had a 69 t120

I heard that some ace tuner in Australia was going to try it.

 [popcorn]

After much eyeballing the manifolds and 888 based frame spars, I can see a relatively "easy" way to cut n shut the manifolds for horizontal carbs like TMs. It requires a friendly machine/welding shop to modify spare manifolds accordingly. I figure if you have to cut n shut the manifolds for expensive Dellortos then may as well cut a 10-20mm wedge to use cheaper horizontal TM 36 or 38's. With stock manifolds I would be looking at used TDMR40's, but that raises the question of carb size for a 750 Monster for street scrambler work. Im thinking 36-38mm slide carbs with a choke/enrichener. Any thoughts on this?
With my own manifold work, TDMR's run at about $1200-1400 used and $1700 new, Dellortos for $1200-1300ish, TM's 500-600 new with cables and throttle, forget Ducati spec FCRs with no choke and now very very expensive. What about MX style horizontal FCR37 or 39's with a choke? They are cheaper and come up used on ebay.

FCR37 horizontal carbs off RMZ 250's are common with local parts dealers.They can be had for under 200 each and come with an accelerator pump and choke. Any thoughts?

I bet you regret selling it.


That Aussie's a fool [laugh]

Quick search says 250 four strokes are ~40 HP, so I think a pair would be just fine for a 750.
Peak torques seem to be occurring at 8k-9k rpm.
Factoring that by the displacement ratio of 2/3, gives 5.3k-6k.
My 750 had it's highest torque at 5.5k.

Looks worth a shot to me.

Thanks mate. That confirms my guestimation based on 750 Pasos often running 36mm Dellortos. I remember the old adage of a flat slide carb flowing like a 2mm larger carb to an equivalent size butterfly carb.

Could have different throttle cutaway, can affect ease to get good idle.
Throttles are numbered, like 2.1 .

Thanks Gert👍

An issue with the FCR MX37 carbs is that they were oem for various models with cast bell mouths to fit specific airboxes. The FCR MX 39's I've seen have the regular bolt on velocity stack or air box adapter. The 39's cost about 50 more per piece and are easier to fit up to my pod filters. I would prefer the smaller carbs however, so I'll see what can be done with the bell mouths of the 37's before I decide.

(https://live.staticflickr.com/65535/49452522158_b777210a00_w.jpg) (https://flic.kr/p/2ikX6Wo)images (https://flic.kr/p/2ikX6Wo) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

Most of the used FCR MX 37's have these cast bell mouth/airbox adapters designed to fit specific model airboxes as oem equipment. Im trying to avoid this, so this has directed me more towards FCR MX 39's which usually have the bolt on airbox adapters/bell mouths. The 39's, although being a little more expensive, with these larger, bolt on airbox adapters allow me to use the K&N pod filters which I already have and save me modifying the cast version, which all saves money. I will also save time and money on jets and needles getting the 39's as I already have decent tuning data from tuning two sets of 39s and data from the forum (cheers Speeddog).

(https://live.staticflickr.com/65535/49453239527_44ed3705d7_m.jpg) (https://flic.kr/p/2im1MbP)images (3) (https://flic.kr/p/2im1MbP) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

This is what I can pick up for a few bucks more saving alot of hassle and money in the long run. See the bolt on stack/adapter? These are compatible with different filters I already have. The spigots are also in the ball park with my spare manifold rubbers compared to the 37's so its worth a change in plan to go with the 39's. Add in the money saved on brass and needles and it adds up quickly. I reckon the larger carbs are more likely to have adjustable slow air jet screws which are worth their weight.

So this with the 37's(https://live.staticflickr.com/65535/49453241527_6bdfa4fb3f_w.jpg) (https://flic.kr/p/2im1MMi)images (1) (https://flic.kr/p/2im1MMi) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

Vs

This with the 39's

(https://live.staticflickr.com/65535/49453007351_f96f11ef00_w.jpg) (https://flic.kr/p/2ikZAaM)images (2) (https://flic.kr/p/2ikZAaM) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

It will save me JB welding or epoxying the cast version and porting to get the shape I want for air flow and filter compatibility.

 I scored two of these FCR MX 39's after making low ebay bids to test the waters, so I'm committed now ;D
(https://live.staticflickr.com/65535/49453007351_f96f11ef00_w.jpg) (https://flic.kr/p/2ikZAaM)images (2) (https://flic.kr/p/2ikZAaM) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr
(https://live.staticflickr.com/65535/49457535996_9be183c26e_h.jpg) (https://flic.kr/p/2imoNnS)20200129_121203 (https://flic.kr/p/2imoNnS) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

Setting up for manifold mods to suit 2 x FCR MX 39's. I'll get Powerbarn to make me a 2 into 1 throttle kit and I'll start measuring for the cut 'n shut of manifolds. I have to decide which way to point the front cyl carb based on clearance to the battery box. I note that 900 manifolds are a little shorter with a nicer radius  so I may go modified 750 manifold on the Vert cyl and 900 on the Hor cyl. If I point the Hor cyl carb back to the motor it may not need a cut n shut with the angle of the front cyl being tilted upwards. I can decide which is the greater pita, relocating the battery box to behind the steering head or cut 'n shutting the front/Hor cyl manifold. The longer, straighter 750 manifold will serve well on the Vert cyl to allow the carb to clear the frame crossmember.  We'll see after real world mock up stage.

Regarding jetting. I usually use fattter than ideal slow jets for the FCR's without a choke, but won't have to make that compromise with these carbs. I can jet down for economy and smoothness and with 39's have good base line settings to try. Being able to remove the breather pods, carb breather hoses, extra carb heating oil lines and tap will really clean up access for roadside trouble shooting on this analogue bike. I'm really excited about this little project.

there are also the very good Mikuni TMR 41 from KTM and other
😉

 [thumbsup] merci :)

This will be handy.
(https://live.staticflickr.com/65535/49462568256_886cb63497_h.jpg) (https://flic.kr/p/2imQAhY)Mikuni-45-degree-rubber-sleeve (https://flic.kr/p/2imQAhY) by Tony Kokonis (https://www.flickr.com/photos/150482584@N03/), on Flickr

That's a cute biscuit, where do they come from?

Mikunioz, our local importer/distributor.
https://www.mikunioz.com/shop/mikuni-rubber-45-degree-mounting-manifold-sleeve-for-40mm-spigot/?v=26e945ed8310

I'll test it on the Hor Cyl manifold to bring it around before cut n shut'n it. Then both carbs will face forward and be sidedraft. I've enquired with Powerbarn about the 2 into 1 cables they sell with split FCR's, but if they can't help me I'll modify the local Mikuni 2 into 1 throttle to suit. The Vert manifold will be cut at its straight end and machined to take a stock donut. This should be long enough to clear the main cross member and high enough to clear the front, angled strut (from my measurements). The manifolds will no longer be similar lengths, but I'll test it first before chopping both. I dont mind the long manifolds for the "scrambler" application. I can leave the battery/ignition box alone if this works. My goal is simplicity, less plumbing, no carb freezing, flatslide response (with a choke), and ease of maintenance.

Gotta say, I'm looking forward to removing the still-air chambers, choke cable, extra hoses, vacuum lines and baffles, and the float bowl to oil cooler plumbing to really clean up the area around the carbs.

It's a tight fit with 38mm inners and may require a trim.

Since the question was resolved, I'll log the progress of the MX FCR Project in a new thread.

Thanks for everyones contributions and advice, it really helps to have a sound board to sort out ones ideas. I feel I can now proceed with a clear direction and plan.
Cheers.