Tuning a Mototrans 250cc

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NewKidOnTheBlock
Posts: 29
Joined: Tue May 13, 2014 1:22 am
Location: Oxfordshire, UK

Tuning a Mototrans 250cc

Postby NewKidOnTheBlock » Wed May 14, 2014 4:47 pm

First, hello to the forum! A small introduction but I shan't bore you all: I've got a (currently) 160cc narrowcase single, which started life as a humble Monza Jr but now is under a cafe racer guise. I have joined the forum after many many google searches have lead me back here and I see a wealth of knowledge. I have gone head first in with this project and am looking to make this bike truly mine, but you must forgive me if I ask some basic questions as this is my first engine build.

I have bought a Spanish 250cc mototrans engine with the 4 speed 'box, it's currently stripped and cleaned. I've been planning the rebuild for some time and am just getting the details straight before I start the proper work. This bike is to be used on the road but I'm looking for more power, but this being a road bike without sacrificing too much rideability. I'm doing as much of this work as I can myself except leaving the porting, reboring and crank rebuild to the pros.

My plan in general is new bearings, gaskets, seals throughout. Reduce weight in the original clutch hub through machining radial holes then balance it. Rebore the cylinder oversize and fit an oversize piston (Just to clean up the bore, not aiming to increase displacement as the cylinder wall is so thin anyway). Porting the head and fitment of oversize valves. A slightly more aggressive cam. Rebuild the crank (using the original con-rod if it's serviceable). Replace the oil pump with a high flow unit. Possibly reduce flywheel weight. Remove the generator as I'm using a total loss system already. And basically replace any damaged parts along the way.

The questions which are holding me back at the moment are:

Does anyone with previous experience building one of these engines have a recommendation for valve sizes to fit, it's originally 33mm inlet/28mm exhaust.

Does anyone have a recommendation of a cam to fit? Ideally I don't want to break the bank, the prices of new manufactured parts are really pushing my budget, I see some companies can regrind profiles, any experience with this? Or alternatively does anyone have one, perhaps a scrambler cam they'd part with?

Also, does anyone know what material the original valve seats were made from? Should I be running leaded fuel? And/or does anyone have any ways to identify the two? I can't fit it on the hardness tester we have at work due to clearance and even then it wouldn't be sitting on a flat surface.

Finally, what oil do people recommend running these engines on?

Sorry that my first post has been such a monster, if anyone can answer even one of my questions it would be appreciated.

Cheers!

Bevel bob
Posts: 1042
Joined: Thu Apr 15, 2010 8:01 am
Location: Bromley Kent UK.

Re: Tuning a Mototrans 250cc

Postby Bevel bob » Wed May 14, 2014 5:26 pm

There are better qualified guys on here than me ,but here's my view, Don't remove the valve guide boss unless you want to be fitting bigger guides every few thousand miles. Don't lighten the flywheel if you need a tickover.Fit valves from a Dianna and don't exceed 27mm on the carb if you want all round power.10 to 1 compression is plenty, A Mark3,Mach1 or Dianna cam will work well.Fit some sort of electronic Ignition and a free flowing exhaust.Valve seats don't seem to suffer unduly from todays fuel, pump delivers more oil than the motor can drain when cold ,so a bigger pump? ,As the oil is going to be chucked every 1K use cheap and cheerfull chatsworth 40 or equivalent .

DewCatTea-Bob
Posts: 2897
Joined: Sun Nov 01, 2009 10:53 am
Location: Near SE side of Lake Michigan

Re: Tuning a Mototrans 250cc

Postby DewCatTea-Bob » Wed May 14, 2014 11:08 pm

NewKidOnTheBlock wrote:I've got a (currently) 160cc narrowcase single,
I have gone head first in with this project and am looking to make this bike truly mine, but you must forgive me if I ask some basic questions as this is my first engine build.
I have bought a Spanish 250cc mototrans engine with the 4 speed 'box,
I've been planning the rebuild for some time and am just getting the details straight before I start the proper work. This bike is to be used on the road but I'm looking for more power,
Rebore the cylinder oversize and fit an oversize piston
Porting the head and fitment of oversize valves. A slightly more aggressive cam. Remove the generator as I'm using a total loss system already.
Does anyone with previous experience building one of these engines have a recommendation for valve sizes to fit, it's originally 33mm inlet/28mm exhaust.
Does anyone have a recommendation of a cam to fit?
if anyone can answer even one of my questions it would be appreciated.

____ I've condensed your post down-to just that which is most relevant to what I-myself am most apt to address with you.
__ First-thing needing to get CLEARED-up is, are your questions concerning your "160" or your 'Spanish 250 4-speed' motor !?
Are you intending to replace the MonzaJr's motor with the 250-Mototrans version ?
What is the bore-size of the particular engine which you're intending to use, (61 or 69mm) ?
____ I-myself am one of few people who don't care much to ASSUME what the chosen post-wording of others are actually meaning to convey, as doing-so can quite possibly lead to mass-confusion (sometime down-the-line). _ So it's appreciated (by me, at-least) when everyone uses rather redundant wording so as to make no mistakes concerning exactly what one really-means to convey (without having to assume anything).
__ I suppose that it's possible that Bevel-bob's reasoning-sense somehow assumed correctly, and gave you relevantly related info that's actually on-track with that which you're really meaning to seek-out. _ But if you really don't have a standard-type -(74 x 57.8mm) 250-engine, (as B.bob seems to have somehow gathered),, then I'm afraid that some of that which has been posted by Bevel-bob, is actually irrelevant to you, (since his suggested valves & camshafts won't fit 160/200-type cyl.heads).
__ Once you get us clearly straightened-out on what-all is actually what, then we can better help answer your questions appropriately.
And BTW,, all 160-motors are n-c.versions, (so no-need to ever note that yours is-not a w-c.model),, however it may've paid to bother mentioning that your 250-Mototrans motor is-not a wide-case version.



Hopeful-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

NewKidOnTheBlock
Posts: 29
Joined: Tue May 13, 2014 1:22 am
Location: Oxfordshire, UK

Re: Tuning a Mototrans 250cc

Postby NewKidOnTheBlock » Thu May 15, 2014 5:51 am

My apologies for any ambiguity in my post. The current engine fitted in the frame is the 160cc, this is not the engine I am rebuilding. The engine I am rebuilding (to swap into the frame once complete) is a mototrans 250cc with a 69mm bore.

Cheers

Bevel bob
Posts: 1042
Joined: Thu Apr 15, 2010 8:01 am
Location: Bromley Kent UK.

Re: Tuning a Mototrans 250cc

Postby Bevel bob » Thu May 15, 2014 8:38 am

The words Tuning a Mototrans 250 were a fair clue , But yes i do have some psychic powers,(but i can't spell!).

DewCatTea-Bob
Posts: 2897
Joined: Sun Nov 01, 2009 10:53 am
Location: Near SE side of Lake Michigan

Re: Tuning a Mototrans 250cc

Postby DewCatTea-Bob » Thu May 15, 2014 9:33 am

[quote= NewKidOnTheBlock ...
" The engine I am rebuilding (to swap into the frame once complete) is a mototrans 250cc with a 69mm bore. "

____ Adapting a 250-motor into a 160-frame is the kind of project that I always used to very-much like ! _ (And this is pretty-much the same as what I had once considered doing once I completed the 300-motor I always wanted to complete.)
__ What had kept me from jumping to conclusion as to exactly what you were referring-to,, is that you mentioned a "28mm" valve, which seemed too small for any 250 ! _ So I was left wondering which cyl.head you actually meant.
__ I don't know very-much about all the details of 69x66mm 250-motors,, but I do understand that they employ 200-type cyl.heads, and I know that camshafts from larger cc.capacity engines are too-long to fit within the smaller-engines (without modification to the bearing-support/cap) ! _ So obtaining a hotter camshaft for your project, is an even tougher problem than you may've been aware of.
Bevel-bob mentioned valves from a "Dianna" - (I'm left to assume that he likely meant a 'Mark-III'), but those would be the same size as that of a standard 250-Monza (which is 36 & 33mm).
Anyhow, larger valves would probably hurt more than help performance, unless you also enlarge the inner-diameter of the valve-seats.
Are you sure your 250's valves are only 33 & 28mm sizes ?
__ What is the diameter of your 250-head's inlet-mouth (of the intake-port), 25mm ?
__ Hopefully someone like Eldert (who has Spanish 4-speed 250 motors in their country), can help guide you to the performance-parts you desire. _ But I'm afraid your hopes for such will be dashed.


Hopeful-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

Eldert
Posts: 771
Joined: Sun Nov 01, 2009 3:23 pm
Location: Hazerswoude Rijndijk Netherlands

Re: Tuning a Mototrans 250cc

Postby Eldert » Thu May 15, 2014 11:54 am

i did hop up a 69 bore engine once . i put a 39 mm intake and a 34 mm exhaust valve in it . i used valves with a 7 mm valve stem

the camshaft i just made shorter in the lathe . i never found out how it run tho .

the owner put a external oilfilter on it and the camshaft and rockerarms got ruined due to oil starvation .

i know Nigel did a 69 bore engine for one of his customers to run it in the Manx GP

Eldert
Last edited by Eldert on Thu May 15, 2014 6:57 pm, edited 1 time in total.

NewKidOnTheBlock
Posts: 29
Joined: Tue May 13, 2014 1:22 am
Location: Oxfordshire, UK

Re: Tuning a Mototrans 250cc

Postby NewKidOnTheBlock » Thu May 15, 2014 5:44 pm

Thank you all for your replies. I've spoken to Nigel and he's suggested shortening a regular camshaft, this is something I am happy to do. I cannot confirm the valve sizes right now as I don't have them to hand but I will double check. Bob - I do intend to open out the valve seat inner diameter also along with the porting work. I believe from memory that the intake mouth is approximately 22-23mm diameter.

Cheers!

DewCatTea-Bob
Posts: 2897
Joined: Sun Nov 01, 2009 10:53 am
Location: Near SE side of Lake Michigan

Re: Tuning a Mototrans 250cc

Postby DewCatTea-Bob » Thu May 15, 2014 8:48 pm

[quote= NewKidOnTheBlock ...
" I've spoken to Nigel and he's suggested shortening a regular camshaft, "

____ Normal 250 camshafts are (as I recall) 6mm longer, but I think they only need to be shortened near 5mm to fit.
I've never tried to cut one down in a lathe because I had assumed the shafts to be too hardened to do-so without crazy damage happening to the std.cutting-bits,, but perhaps with diamond-tips or along-with grinding-stones, the job could then be easier done with lathing.
__ If you could acquire a Red/Mark-III camshaft, I don't think you'd find it too wild for your taste, (but don't let anyone talk you into a Green&White-cam).
If interested, I may consider swapping you a White-cam for your stock-cam,, but since the valve-timing of your stock-cam is unknown to me, I have doubts that you'd feel any performance difference between the two.
Perhaps Eldert is aware of the state-of-tune of the particular camshaft that's stock for the 69x66-engine.



" I believe from memory that the intake mouth is approximately 22-23mm diameter. "

____ That would seem about right for a 160-head, but I think your 200/250-head ought-to be more like 24mm.
I wouldn't recommend increasing the in.port-size much more than 28mm, how big were you thinking of ?
__ As for valves, I think you ought-to settle for 37 & 33mm sizes.
What does the crown-shape of your intended piston look like ?
____ How is it that you've chosen a 69x66-250 motor for your 160-project, anyway ?
It seems kind-of a waste (to me) to bother with modifying such a long-stroke 250-engine (that's already better tuned for more advantageous low-end power) and merely end-up with a motor that will be lucky to obtain the same amount of peak-power as that of a stock n-c.74x57.8-250 Scrambler-motor.
So I wonder if there's some reason I'm ignorant of for why you haven't instead preferred to choose a much more common short-stroke 250-motor for your project ?


Hopeful-Cheers,
-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob

Jon Pegler
Posts: 459
Joined: Sun May 16, 2010 6:19 pm

Re: Tuning a Mototrans 250cc

Postby Jon Pegler » Thu May 15, 2014 10:30 pm

Mototrans camshafts varied with different models and years.
The most common camshaft fitted into the 250 De Luxe had the following events-
Inlet opens 45 BTDC
Inlet closes 85 ABDC
Exhaust opens 65 BBDC
Exhaust closes 45 ATDC

The 24 Horas motor had the following events-
Inlet opens 55 BTDC
Inlet closes 90 ABDC
Exhaust opens 85 BBDC
Exhaust closes 55 ATDC
The 24 Horas cam was also fitted into a few n/c 250 Road models so may appear in the US occasionally.
How many of these original cams are left after so many years is hard to say.
Some of the Spanish cams were not too clever to begin with.
It's not easy tuning a n/c Mototrans machine because so much is different from the Italian machines.
Also, the world of Mototrans parts can be a bit of a minefield, the Road models probably being the most incomprehensible.

Jon


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