2 ques, shifter box and modern carb

[DewCatTea-Bob]

[quote= Timmy250 ...
" pic shows where the solder laid, "

____ The head's chamber was too dark to see well enough in your pic, so I've brightened it up, (as seen below).



" lots tighter at the flat perimeter edge, so that's not the correct place to measure. "

____ Well, while it may indeed not be the correct place to measure for checking the squish-band clearance,, it however must be the MOST-important piston-to-head clearance, to be concerned with... As THAT location would be the foremost area to be concerned about piston to head contact ! - Right ?
This is an issue which those with std.bore pistons don't need to be concerned with.
__ If your overlapping ledges were set at .030" clearance, then I'd say you could decrease it by 10 to 20-thousandths, (providing your con.rod is staying good).
How have you been setting your head-clearance ? _ Just with paper cyl.base-gaskets ?
If so, how many pieces did you have stacked-together under your cyl.jug-base ?



" valves are 37 and 33. "

____ That's consistent with older Monza-heads. _ But I suppose someone could've installed a Monza intake-valve (thinking that doing-so would increase a GT-head's performance).



" pic shows biggest reading I could get on intake port. "

____ Have you tried using that same caliper to measure the diameter of a penny (to confirm that it's calibration is correct) ?



" Diameter of hemi part of head is 74 mm. "

____ You must mean that the diameter of the head's ENTIRE combustion-chamber is "74 mm",, cuz the spherical-area of the hemi-dome is only within the upper surface-area, beginning directly above the upper-border of the squish-band...
See the white-with-green line (that I've added to your cyl.head-pic), which indicates the location & width-limit of the 'squish-band'.


____ BTW, I've now fully edited my previous-post (cuz it's original wording wasn't so clear as it ought-to have been).

Dukaddy-DUKEs,
-Bob

[Timmy250]

Bob, I tried a 0.5 mm aluminum shim, too much, so just two gaskets. Measured with head off, with depth gauge, get .012, with solder get .010, so if .004 is minimum safe, think it's good. New crank pin and piston/pin, old rod, hope it all holds.

Squish band at .030.

Tim

[Timmy250]

Bob, I now understand better what you mean by squish band vs pure hemi, pictures are so much better for me to get it. I think your edit changed your stated minimum clearance in "flat" areas of head and piston from .004 to .005-.010, or did I re-read too hurriedly? Still ok, at .010.

Here is pic of new carb installed, haven't tested yet, waiting for new clutch basket bearings to arrive. Thanks for helping me think through this.

Tim

PS, noticed prominently displayed in background is a hammer, my primary tool of choice.

[DewCatTea-Bob]

[quote= Timmy250 ...
" I now understand better what you mean by squish band vs pure hemi, "

____ If the upper area of the combustion-chamber was left uninterrupted by the squish-band, then it's 'hemispherical' shape would be left totally-pure. _ However on the other-hand,, if the entire chamber's shape were rather-instead only fashioned same as that of just the squish-band's shape itself, then the entire chamber would be 'cone' shaped.



" I think your edit changed your stated minimum clearance in "flat" areas of head and piston from .004 to .005-.010,

____ You're right. _ Ya-see I couldn't leave it the way it was, as I came to realize (when I later read what I had originally posted), that the minimum-figure was rather only intended for just cast-pistons (with over 8% silicon-content). _ And since we were only dealing with forged-pistons in this case of yours, it became pertinently appropriate for me to correctify the posted figure. _ Cuz a near silicon-free piston (like the MC.piston more obviously is), will expand somewhat faster (than the rest of the cylinder), and therefore requires extra clearance-space (until the rest of the top-end fully heats-up as well).
Otherwise without the extra-clearance,, such an overboard engine-cylinder which sounds (relatively) quiet when cold or fully heated, may possibly make a teenking-sound at some point during warm-up (as the over-sized piston's top-edge then [temporarily] comes into slight contact with the edge of the head's combustion-chamber).



" Still ok, at .010. "

____ Yes,, in any case, you ought-to be good there. _ Especially since more modern pistons (like your new-one is claimed to be), are supposed to have adopted the updated-belief that increased silicon-content is rather desirable.
__ Now with that minimal (piston to head) clearance, your comp.ratio ought-to be as high as possible with a 350-piston, (about 7.4:1).



" Here is pic of new carb installed, "

____ I expect it ought-to perform as well as it looks, (as it's obviously not a want-to-be Mikuni double).
__ I see that it looks like it's manifold-flange fits nicely onto the mounting-studs & against the head's port-mouth... Were it's mounting-holes spaced well for easily slipping-over the studs ? _ Does it's mounting-flange include a built-in O-ring seal (like Amal-carbs have) ?
Did the port-holes (of, carb; isolator-rubber; & head-port) all line-up fairly well for you ?


Hopeful-Cheers,
-Bob

[Timmy250]

My understanding is that it is a Keihin made carb, modified by a racing engineering firm to fit Brit bikes to replace Amal, licensed by Keihin for re-sale. Something like that, built is of high production quality.

Because it was made to fit Brit bikes, 2" on center studs, I had to remove a small amount of material to get the Ducati studs to fit without interference. It does have an o-ring, and the match-up was great, I chamferred the rubber insulator slightly to match carb's 26 mm to the intake port's 25 mm. It's a straight shot to the valve, concentric and no sharp edges. Had to shorten the throttle cable outer sheathing about 3/4" to get the throttle to fit properly, but used the original stuff that went fine, throttle works great. Wondering how I will operate the enrichment "knob" for starting, given that it's spring loaded and has to be held up by hand to work. I'll figure something out.

Tim

[Timmy250]

Update, pic of final install of new carb, per Bob request.

Right now has 105 main jet, 35 pilot, am still working on correct slide and needle to get rid of too rich condition idle to 1/4 throttle, will report back on best final set up.

Tim

[DewCatTea-Bob]

[quote= Timmy250 ...
" pic of final install of new carb, "

____ Thanks, that's a pretty-nice pic with a fine view-shot at the mounted carb !
It seems that either I had missed seeing or forgot that you previously had such a picture already posted within THIS same thread (above). _ But at-least we can now see that your carb's installation doesn't seem to cause any fitment-issues with the fuel-tank's installed mounting-position. _ However what-about the activation-action of the cold-start setup, is it manageable well enough ?
__ Your chosen carb appears to-be a perfect fit with a nice & clean installation ! _ So I'd now suggest this "modern carb" before an old Amal or most Mikuni-models for replacing the stock Dellorto-models with side-bowls, (for DUKEs that are to be ridden rather than merely looked-at) !



" am still working on correct slide and needle to get rid of too rich condition idle to 1/4 throttle, "

____ If your carb was pre-jetted for most any 250cc 4-stroke single-cylinder, then it's jet-settings ought not be far-off enough for you to tell !
RATHER than the slide-# (or especially the size-# of the needle-valve), for controlling the air/fuel-mixture under 1/4th.throttle,, most carbs have an adjustable idle-circuit 'screw' to effect the setting of the mixture for within that range ! _ And since the idle-circuit can possibly effect the slide's control of the mixture up near 1/3rd.throttle, I'm fairly certain that adjustment of your idle-circuit is ALL that you really need to be concerned with, (and least-of-all, the needle-valve's size).
Have you not yet found an idle-mix.screw & tried to correctly adjust your low-speed fuel-mix, by that means alone ?
If you have already done so, then I'd figure that you'd need a leaner pilot-jet.


Hopeful-Cheers,
-Bob

[Timmy250]

Hi, thought I'd update. Finished restoring the Ducati awhile ago, but got busy in the meantime restoring a couple others, including the Puch also in the photo below. These two bikes share vintage, number of wheels, displacement and color, but not personalities!

Anyway, the Duc runs great now, but definitely had to fiddle with the modern carb to get it to work, and much differently jetted than it came as supposed to work with Brit twins - see discussion above. The main problem to solve was transition from idle to part throttle. But, runs great.

However, the motor is very noisy mechanically. I've never heard another one run in person, although listening to youtube videos, they do sound pretty rattley, but hard to really know. Is that others' experience, sounds like piston contacting the head, but have assured myself it's not, added a shim to ensure not, clearance as measured should be adequate, and no damage evident. Anyway, is mechanically noisy motor the norm, like more than a Honda from the same era, you know? Thanks,

Tim

IMG_20150515_171022.jpg

[Nick]

Well, with all those bevel gears whirring around in there, there is a bit of noise. Too, for someone accustomed to liquid-cooled engines (sound-insulated by water!) the Duc might sound a bit noisy. My Duc always seems to run better when I wear earplugs....

[DewCatTea-Bob]

[quote= Timmy250 ..
" thought I'd update. "

____ It's always nice to have a fellow-member bothering to provide us with an update !



" the Duc runs great now, but definitely had to fiddle with the modern carb to get it to work, and much differently jetted than it came as supposed to work with Brit twins "

____ Then the seller must get some complaints from those who purchased that same carb.model for Brit.bike-engines as well !



" the motor is very noisy mechanically.
sounds like piston contacting the head,
but have assured myself it's not, "

____ While the meshing of the bevel-gears doesn't make any such audible noise, it's possible that the tower-shaft's bevel-head is-not being kept firmly held in it's normal position (thus allowing the varying valve-spring pressure to slap it around within it's place in the head-casting)...
You could remove the bevel-cover to check & see if it happens to be clanking-around* side-to-side (actually back & forth) within it's location pressed into the cyl.head, (* while at idle or during kick-over).
__ What's been known to occur,, is that the tower-shaft's hole in the head-casting can become somewhat elongated, thus allowing a shifting-movement that allows the valve-springs to bang the bevel-head back & fourth against the head-casting (and-thus possibly make a clanking-noise similar to piston-slap [but not as hard & loud]).
__ So when you have a look-see at the meshing bevel-gears,, if you then see any such loose-play movement of the tower-shaft's bevel-head being pushed-around, then that's most likely responsible for the specific top-end noise you've been hearing !
The source of the slap-clanking noise results when the std.bushing (of the bevel-head ball-bearing) is pressed-forward (against the inside/front of the head-casting's hole) while the tower-shaft is trying to turn the cam against the valve-spring pressure,, (and after that resistance-pressure is overcome), then next the releasing spring-pressure conversely tries to turn the tower-shaft, thus-then the std.bushing is pushed-rearward (against the inside/rear of the hole). _ So-thus when this back & forth movement occurs in rapid succession,, the std.bushing then gets SLAPPED back & forth against the rear & front inside-areas of the hole in the head-casting, thus resulting with the consequential clanking-noise emanating from the cyl.head !
__ Ducati had been aware of such possibility, and-so produced replacement-bushings (which replace the std.bushing that fits between the special double-row ball-bearing & the head-casting), with an over-sized OD for better obtaining a fully-tight press-fitting into so-worn holes of well-used head-castings (for the obvious purpose of preventing bushing-to-head slap) !
__ Other than the (slowly developed & eventually disconcerting) cylinder-noise,, the only notable operation consequence of continued running with the std.bushing still left operating within a worn/elongated-hole, is that the sloppy-movement of the tower-shaft's bevel-head tends to reduce the intended full-action of the cam-lobes, thus resulting with an effectively milder-cam, (although insufficiently so). _ And this milding-effect diminishes as revs increase, (as does the related clanking-noise also become reduced as well [as revs climb away from idle]). _ These two effects (allowed by an elongated-hole) both tend to dwindle-down as RPMs increase because, the valve-springs then don't get as much time to brutally 'slap' the std.bushing rearward (fully against the backside of the worn-hole) !
__ (DESMO-heads [and others of OHC.engines smaller than 175-models] don't suffer near as much from this issue concerning elongation of the head-casting hole, since those models' valve-springs are much weaker [than those employed within std.type 175~450-cyl.heads]).



" is mechanically noisy motor the norm, like more than a Honda from the same era, "

____ As far as top-end noise of new engines goes,, I'd say yes, that a Duke's valve-gear workings is somewhat slightly more audible than that of a Honda XL250, (but then such Jap.bike-engines employ the unfair advantage of rubber-dampeners wedged-between their cyl.fins, [which well curtails fin-radiated internal engine-noises] !).


Enlightening-Cheers,
DCT-Bob