Bring back the midrange power.

[Bevel bob]

I'm looking to de-tune my 250 Mark3 , A cam change is on the wish list and i'm thinking of getting the inlet welded to a "D" shape to speed up airflow and re-instate support for the inlet guide. My question is has anyone done this with Devcon or similar resin product? , Is this reliable enough to be considered?, I read of possible issues after a number of heat cycles. Is it practical to sleeve a 29d back to 27mm ?.

[DewCatTea-Bob]

[quote= Bevel bob ...
" I'm looking to de-tune my 250 Mark3 ,
i'm thinking of getting the inlet welded to a "D" shape to speed up airflow and re-instate support for the inlet guide.
Is it practical to sleeve a 29d back to 27mm ?. "

____ I think you're looking-down the wrong road, as it would be a certain shame to try screwing-around with a stock Mark-3 cyl.head that's still in good original working-order !
__ Rather, I think you should consider trying-out swapping your current head for a Monza-cyl.head (with it's 26.5mm in.port & mild cam).
If we were neighbors, I'd certainly let you try-out one of my spares to discover how well you'd actually like it.
Have you been keeping watch on eBay for any such listings ?


Hopeful-Cheers,
-Bob

[Eldert]

Bob there is no substitute for cubic inches . stick in a Sebring crank , barrel with piston and bevelshaft and make it a 350 .

Eldert

[Nick]

If you do a search for high velocity intake port or something similar there are a couple of good sites that go into some detail about the D-shaped ports. One of the guys uses JB Weld, after properly preparing the surface.

[Bevel bob]

Hi all, My 250 was race ported and cammed by the previous owners, gave me a magic holiday on the Isle of Man where it was really happy revving its head off on empty roads. However for the rest of the year in small twisty lanes and traffic its a waste of time and wearing.Also concerned that the inlet guide is probably going to get loose again. I dont need a motor that makes extra power at 10000 rpm at the expense of good manners at 4 to 9K, I'm aiming to reduce the cam duration and increase gas velocity at lower revs to improve the carburation. Would prefer to keep the Mach1/ Mark 3 look though.Trying to do it on the cheap as i have other classics to support on a pension!.

[DewCatTea-Bob]

[quote= Bevel bob ...
" My 250
gave me a magic holiday on the Isle of Man where it was really happy revving its head off on empty roads. However for the rest of the year in small twisty lanes and traffic its a waste of time and wearing.
I dont need a motor that makes extra power at 10000 rpm at the expense of good manners at 4 to 9K, "

____ Welcome to the real-world, as you've realized what the real-deal is !
Remove & save the hot-head for those rare racing days, and substitute a Monza or Scrambler cyl.head for the rest of the (more sane) time of your riding-life !
__ How much do you expect to spend on the de-tuning of the present cyl.head ?
You could possibly buy a complete Monza/Scrambler-head for about 200-U$.



" Trying to do it on the cheap as i have other classics to support on a pension!"

____ If you're getting too old,, then I'm afraid you have-to face-the-wall & realize that you need to part-ways with your life's-stuff and sell-off any non-Ducati/non-riders, so as to then become better enabled to support your current life's-fun for the present. - (So sorry to say.)


Hopeful-Cheers,
-Bob

[double diamond]

Bob,
Not going to be able to sleeve a SSI 29 to 27mm. But bare 27mm bodies show up from time to time and all the other parts will fit up. Just make sure you get a 27D and not a 27A. I would think some racer type would be receptive to trading your Mach 1 head for a more pedestrian version. What you probably want is a scrambler head/cam/rockers but likely scarce in the UK. Matt

[tobydmv]

I have a stock monza head, intake, and carb. I also have a mikuni and intake for a scrambler. If you are interested in a trade i'm sure i'm not the only one who'd oblige you.

You could just put the monza intake and ub24 carb on there and see how it reacts.

[Bevel bob]

I had not thought of having a spare set-up to swap over, replacing the racing stuff for I O M trips. I'm not looking to downgrade as far as a Monza or 24mm carb,I would be very happy with Dianna or Mach1 state of tune,with a more stable inlet guide support. Its possible that a cam change or follower reshape and a little epoxy work would do enough to make the bike more flexible.

[DewCatTea-Bob]

[quote= Bevel bob ...
" I'm not looking to downgrade as far as a Monza "

____ There's something to be said about narrower ports with their resulting increased flow-rate & ram-effect,, and thusly with it's 3mm reduced in.port-size, the Monza-head more-so enjoys that advantageous effect.


" or 24mm carb, "

____ While for likewise operation, the smaller carb enjoys better throttle-response,, unfortunately, of-course it's downside is a max.full-throttle setting that's only equivalent to near 2/3rds-throttle of a 29mm.carb !



" I would be very happy with Dianna "

____ What's a "Dianna" ? ... A 'Diana 250' has the same top-end as a Monza ! _ So I assume you must actually mean a 'Mark III' cyl.head,, which (just-like a Scrambler), has a 28.5mm in.port.


" or Mach1 state of tune, "

____ While your 'mind-set' may think it would be "very happy", your seat-of-the-pants perception would perceive no noticeable difference, and-thus there would be virtually no notable reward for the effort of having made such a very minimal*change, (* depending on which G&W.cam you currently employ).



" Its possible that a cam change "

____ Yes, a considerably milder cam, (perhaps a Scr.cam) would certainly provide you with a notable power-increase (below top-RPM) to a significant degree greater than the resulted-loss at top engine-speed,, thus an overall power-improvement !



" or follower reshape "

____ That type of modification is often overlooked as an alternative to cam alterations.
Sometimes done to end-up with valve-timing that's wilder than that of a Scr.cam, but-yet not so extensive as that of a Mach-I/Gray-cam,, the radius of the cam-followers is increased to cause a Scr.cam to preform hotter (more like a Mark-III/Red-cam without being quite so wild).
__ However in your case, you'd actually instead require that your rocker-arm's follower-radius rather be reduced ! _ Which means you'd prefer that the std.radius be altered from 32mm down-to perhaps just 24mm or-so.
While that change would-not directly affect max.lift, (although would lower average valve-lift),, it would indeed significantly reduce the valve-timing duration and-so result with near similar performance as expected by a Mark-III or Mach-I cam (with std.rockers).
Although it's hard-saying which is easiest to get accomplished, (reduce the follower-radius or replace the camshaft with a milder version).



" and a little epoxy work would do enough "

____ I really wouldn't recommend doing such as that to screw-around with the in.port shape,, as the involved troubles with doing-so, is really not worth the insignificant possible gain of which it alone would go unnoticed anyhow.



____ I think the bottom-line here is that you need to realize that your realization (that your racing-type top-end is-not very pleasurable to take advantage-of as a daily-rider), actually requires a rather major-change in order to make it all worth-while. _ Cuz the road which you currently seem to have your sites set on, is going to be a considerable amount of work for next to no noticeable gain in the direction you really wish to go.
__ Have you ever actually raced* your souped-up Mach-I against a stock Monza (* from street-light to street-light, or on a tight-twisty back-road) ? _ No I don't think you ever have (or-else you'd show more respect for the Monza), so I'm sure you'd be surprised ! _ As it's not so much of a "downgrade" as you think (top-end wise, [as the difference in compression-ratio is rather where near half of the power-difference is at]) !


Hopeful-Cheers,
-Bob