Tuning a Mototrans 250cc

[NewKidOnTheBlock]

Thank you all for your replies. It would seem that I am about ready now to begin the work, once my piston arrives I shall start getting everything else in place. I will endeavour to update you on here.

[DewCatTea-Bob]

[quote= LaceyDucati ...
" He does have the green and white profile, but I wouldn't bother as it won't be suitable. "

____ I wonder WHICH Green & White cam-profile !? ...
I'd agree that the '250F1' G&W.cam would be unsuitable, (for anything other than racing on circuits which don't ever call-for reengaging 1st.gear),, however the 350Mk3.G&W version is just a step hotter than the M1/Gray-cam, and not so awfully wild.
It's really too-bad that Ducati ever allowed these two (only somewhat-similar) cam-models to become color-coded the same !


Dukaddy-DUKEs,
-Bob

[DewCatTea-Bob]

[quote= NewKidOnTheBlock ...
" see the photos on this link, they will probobaly help to answer more questions than I have with words alone: "

____ I scrolled-through about 18-pix
I'll
see what I can do to make their subject-targets more visible.

____ I've chosen the most useful pix you had, and will now soon post their subject-targets within this post (and perhaps another as well).


____ After seeing the picture of your 200/250-cyl.head and noticing it's finned-top, I then realized that I've forgotten of something which I've always been well aware of...
Those 200-type cylinder-heads rather have 'FLAT-tops' which don't have the rounded/lifted-dome that the newer head-design employs so as to create additional cam-lobe cavity-volume for the cam-followers of the rocker-arms (which may have-to work with cams that have higher lobe-tops) !
__ So another possible issue with fitment of newer n-c.cams, is possible insufficient clearance between the cam-followers & the cavity's overhead-ceiling.
However a simple solution for such issue, is to simply grind-off (& polish of-course) up to .5mm from the top-sides of the followers, (for your Scr.cam).
__ I wouldn't have thought of this as being much of a concern,, but obviously Ducati had, or-else it seems they wouldn't have bothered with replacing their nicely designed fins with the rather bare-dome atop their later head-designs.
__ But still this overlooked concern shouldn't be any issue for adapted-installation of w-c.cams,, as THEIR base-circle diameters are about 2.5mm smaller, thus compensating for taller cam-lobes.

[LaceyDucati]

Bob

The G/W he does is the mark 3/ SCR cam.

My experience is that this cam is okay for a 250 race bike, not the best but reasonable top end. Nothing special in a 350 race bike and lacking midrange for the road in either (even with an open exhaust). The other G/W is not well known in the UK, so no experience.

Nigel

[Eldert]

i posted this before in another topic but here it is again

a Mototrans 69 bore head but it looks like they modified the mould to accept a high lift camschaft or it could be a differend casting .



Eldert

[DewCatTea-Bob]

[quote= LaceyDucati ...
" The G/W he does is the mark 3/ SCR cam.
The other G/W is not well known in the UK, "

____ I've covered details about this before, so I gather that you either missed them or didn't absorb or remember the info.
__ There are slight lobe-height/lift differences between the two G&W.versions (however tolerance-differences blur the base-figures)... intake - 10mm vs. 9.85mm ; exhaust - 8.5mm vs 8.35mm,, and the greatest difference between the two versions is the 'overlap' valve-timing, of which the 250F1-version has 135-degrees (at-least 15-degrees more than the 350Mk3.cam).
__ That BOTH these camshafts got factory-installed into 350-Scrambler models, is likely the fault of Berliner -(U.S.importer), as I understand that a white-cam was LOGICALLY installed in 350Scr.models for some other countries.
As I think I've determined, it seems that it was late-1968 when the G&W switchover from the 350Mk3 to the 250F1 version occurred,, SO if your country got 350-Scramblers with such non-scrambler/G&W.camshafts all the years since w-c.models became produced for 1967, then you also ought-to have examples of both G&W.versions !
So I hope you have a collection to check-out & see if you happen to have both versions.
In the past, Eldert has (separately) posted the specs of both example-types he's measured, so I gather that HIS country has gotten both G&W.versions as well.


Hopeful-Cheers,
-Bob

[DewCatTea-Bob]

[quote= Eldert ...
" a Mototrans 69 bore head but it looks like they modified the mould to accept a high lift camschaft or it could be a differend casting . "

____ Until I saw NKOTB's picture of his Mototrans 250-head, I had assumed that his example was the same as Eldert's posted head-casting.
Eldert's square bolt-pattern 250-head must be a newer casting, as it has added casting-space for optional machining of the wider rectangular bolt-pattern as well.
__ Also, it seems to appear that Eldert's cyl.head may have the standard inlet-mouth dia.size as well.
Eldert, can you confirm the dia.size of it's in.port,, and also, tell us what size the stock valves were in your Mototrans 250-head ?
__ I'm still sort-of surprised that the older Mototrans-head has an intake-port that's well under 25mm.
However I've been aware of (non-Ducati practiced) theory for matching-up wild-cam/valve-timing along-with rather narrow intake-porting. ...
Back in the '70s, MotoGuzzi (who's cyl.heads had overly large intake-ports), had discovered that reducing the diameter-size of their in.ports, actually INCREASED max.HP at top-RPM (rather than reduce power, as would be common-sensibly expected).
The related theory is that at higher RPMs, there's then no time for engines to take-in a 'big-gulp' of atmospheric-air,, so-thus BETTER to take advantage of the rather full RAM-effect (with an optimally-narrow in.port), so as to stuff the cylinder to a fuller degree (than otherwise might be attained with an in.port that's too-wide).
__ So since the Mototrans 250-head has such an extra-small intake-port that's expected to work together with a camshaft which has such extra-WILD valve-timing, that seemingly odd combination seems to be TOO-much of a peculiar circumstance to have come-together without any consideration of the probable outcome !
So it seems rather likely, (now that I've thought about it), that Mototrans had quite possibly 'tuned' their chosen port-diameter & camshaft/valve-timing combination to reach a certain compromise which Mototrans had deemed to be best overall, (combined with whatever exhaust-system was supposed to-be employed along-with).
__ Am I giving Mototrans too much credit ? _ I once would've thought-so,, but the overly peculiar extremes of cam & port, would seem to indicate NOT !
What does anyone-else think ?


Dukaddy-DUKEs,
DCT-Bob

[LaceyDucati]

Bob
All the Green and white cams here in the UK are the same as far as I have seen, the ones with the lower lift.

I had seen the previous imformation, but as I haven't come across this other G/W in the last 25 years of my involvement in Ducati singles, there seemed little point "absorbing" the imformation! If I need it I will use the search

Personally my Brain is fully absorbed trying to run a business and what seems like the Worlds Ducati single requirements and problems via the wonders of email. Can't really complain as it is good to be busy

Good job this site is here otherwise the situation may be worse!

Regards Nigel

[Eldert]

Eldert, can you confirm the dia.size of it's in.port,, and also, tell us what size the stock valves were in your Mototrans 250-head ?

Hi Bob

the intakeport is a little over 27 mm . there are no valves in the head to measure

Eldert

[DewCatTea-Bob]

[quote= Eldert ...
" the intakeport is a little over 27 mm . there are no valves in the head to measure

____ THANKS Eldert !
__ I had thought perhaps that you would've also had the valves & cam, etc which came with that 69mm/small-bore 250-head.
I'd assume that it's valves were likely std.type 36 & 33mm items.



____ Since Eldert didn't mention anything about the std.type intake-port appearing as possibly being non-factory looking,, and-also, since the fact that his presented small-bore/square-pattern 250-head is a newer casting than the older/200-type example with the finned-top,,
then either Mototrans later abandoned their practice of small-ports with wild-cams, or-else the newer small-bore 250-head is from a DIFFERENT long-stroke 250-model, (which probably came with a different camshaft also).
In either case, mystery abounds with these small-bore/long-stroke 250-engines !
__ I sure wish Ducati had also built all the strange motor-models that Mototrans did, cuz then I'd have still more interesting stuff to know about and not be so mysterious to me.
Too-bad that there's not an English-speaking counterpart to myself who I could deluge with questions about Mototrans Duke-models !
____ Anyhow, I'm now thinking that rather than going-through all the work of porting-out the older finned-top cyl.head,, it would be preferable to keep it as stock, and also acquire another small-bore 250-head like Eldert's example. _ As such an example ought-to be found cheaply enough in Spain, (if Eldert is unwilling to part with his), and-thus could possibly save it's worth in porting-work.


Dukaddy-DUKEs,
-Bob