Original/Stock CARBurator-Swapping of various DUKE-models

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DewCatTea-Bob
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Original/Stock CARBurator-Swapping of various DUKE-models

Postby DewCatTea-Bob » Tue Jul 13, 2010 5:49 am

____ Hey-there all good fellow-DUKErs,,
I decided to start this thread so that we'd have one on the intended subject with a topic-title which makes itself fairly-clear so as to more exactly indicate what kind of subject-matter ought be found within. _ (Hope this consideration starts a useful-trend, so that we all don't continue to make our seemingly unmarked 'Box-Of-Chocolates' any more extensive than it has already become!)

____ Anyhow, I recently received a PM which brought-up the subject of switching/swapping the various factory-original carbs (which came stock on various DUKE-models), over to DUKE-models which were never factory-equipped with same.
Since I may have more experience with such carb-swapping than most anyone else, I figure I ought to open a port to pass-on some of the related stuff I know (which may not yet have been independently learned of by others).
__ I've done so much carb-swapping between models, that it might be easier to make a list of all the possible match-ups, which I have NOT done...
At the extremes, I have never tried to put a 24mm-DeL.carb on a 450, nor a 30mm-Amal on a 200,, but otherwise, I've tried pretty-much any other match-up ya might think of.

____ To start this off, I'll bring-up what I'm recalling may be my very first unfactory-original carb-swap job.
Back in 1969, I had a 1965 250Scr. & also a 1966 350Sebring...
I then thought it peculiar that the 350 had such a smaller carb -(24mm) than the 250's -(27mm), and my primary interest at the time, was to improve the 350's power. _ So I swapped the two carbs between those two DUKE-models, and the first thing I noticed was that the 350 had become a bit harder to get started, while the 250 got notably easier !
Also, the throttle-response worsened for the 350, while the 250's improved.
On top of those changes, it seemed that the 250's performance as a 'scrambler' was actually improved more than was the 350's overall-power improved,,
all of which was unexpected.
__ While I could tell that the 350 had generally suffered a bit from the swap,, I could not really tell for certain if the top-end power of the 350 had actually been increased, as was expected. _ But I chose to leave the 27SSI-carb on the 350 anyhow, because I figured that it's mid-range & top-end power certainly had to have been increased, at least a little.
Conversely, the 250's top-end power must have been hurt at least a little, but I never really could tell for sure. _ Since in both cases at the time, the 'control' bike -(a small-valve Mk3, ridden by an equal-sized friend), was found to provide inconsistent comparison-results, seemingly due to how full the gas-tanks were, and varying wind-conditions plus even rider's choice of clothing ! - (In the real-world, it seems that The Specs do not always equate with actual-results.)
Still, there was little doubt that the 250's overall-performance had been fairly well improved for it's role as a 'scrambler' ! _ So it was forever left with that 24mm DeL.carb ! - (As I sold it in Spring of '72, and bought it again in 1980 (as a basket-case due to busted oil-pump), still with the same 24mm DeL.carb!)
And for both DUKEs overall, they both were deemed to be improved by that carb-swap, (even though the DUKE which was the UNintended-beneficiary of the swap, benefited most).
__ I never sold the Sebring ! _ (And by the following Spring, the cyl.heads of those same two DUKEs, were also swapped, (with the 250 then also getting a 80-thousandths oversize MC-piston)... But those details are another story, better placed within another thread.)
So that Sebring has gone through additional carb swapping...
It's next carb was a 28mm-Amal (from a 650-BSA), which seemed to be a better choice (overall) than the 27SSI but, it wasn't long after that when it was next replaced with a 27mm square-slide DeL.carb (from a Harly 350-Sprint), which much improved starting ! _ All three carbs were tried on the n-c 350 -(no longer fair to be called a 'Sebring', since it no longer had it's stock-head), during the same riding-season, in combo with the 250-Scrambler cyl.head.
My n-c350 was then compared to a stock Sebring (which I also still have), and both still stayed neck-&-neck until 3rd-gear was about half wound-out, at which point the larger carbed n-c350 (with 250SCR-head) would begin to pull away from the stock-Sebring.
I kept the last combo (with the Sprint carb) for the rest of that riding-season, and the following season I only switched the SCR-cyl.head for a 250Mk3 (small-valve) cyl.head. _ THEN, I had truely felt that I finally had a 350 which had reached my desired goal of a fast-350, as it was then running notably faster than any other stock 350-DUKE.
Then during the next following riding-season -(1972), my n-c350 got it's final set of engine-mods... a w-c (modified) piston, a large-valve Mk3-cyl.head, and a 29mm DeL.carb -(from my 450Mk3). _ Those changes made a very impressive increase in power ! ... Much more so over the previous-combo -(small-valve Mk3-head & 27mm carb), than that previous-combo had made over the stock-Sebring's ! _ I'll never forget about how my n-c350's power then became so strong (that after installing a new set of friction-disks), it then became very wheelie-prone whenever I grabbed some throttle (in 1st-gear with 49/15t gearing) ! - A scary-delight at the time !


Dukaddy-DUKE 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

DucBevellvr
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Joined: Sat Apr 10, 2010 3:02 pm

Re: Original/Stock CARBurator-Swapping of various DUKE-models

Postby DucBevellvr » Tue Jul 20, 2010 10:28 pm

I note this thread was posted a week ago, yet it seems funny nobody else has already asked by now what should be a very relavent question!
And that is:

Bob, what about the jetting for any of those carburator swaps??
Did you just leave them all exactly as they were, or did you make some jetting changes in any of them?

I'm particularly interested in using a 27VHB (from a 70s Harley) on my Monza.
Is there much rejetting involved in that? And if so, where would be a good place to get the correct sizes?
Would there be any other problems to consider, with this carb swapping project?

Thanks,
JeffP

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

Re: Original/Stock CARBurator-Swapping of various DUKE-models

Postby DewCatTea-Bob » Wed Jul 21, 2010 10:30 am

" Bob, what about the jetting for any of those carburator swaps??
Did you just leave them all exactly as they were, or did you make some jetting changes in any of them? "

____ I'd usually just do the carb-swap with no internal changes, and then do a 'plug-reading' (using a new spark-plug), and then usually find that no jetting change is really needed.
As a rule however, whenever moving a stock-carb from a larger cylinder to a smaller one, I'd never make any jetting changes,
although when taking a stock-carb from a smaller cylinder for use on a larger cylinder, I'd just raise the needle-valve one notch and increase the main-jet two or three sizes.


" I'm particularly interested in using a 27VHB (from a 70s Harley) on my Monza.
Is there much rejetting involved in that? "

____ A 26 or 27mm carb-model like that would be a good choice for your Monza,, and if it's from a 'Sprint', then it's stock-jetting should be fine as is, but, if it's from a 2-stroke model, you'll have to redo most of it's jetting.


" where would be a good place to get the correct sizes? "

____ These days -(since the '90s), I'm sorry, I really don't know ! _ Hopefully, somebody-else can give you a tip on that !


" Would there be any other problems to consider, with this carb swapping project? "

____ Well, unless you also swap-over the throttle & it's cable (which was used with the intended carb), you'll likely have to re-adjust your throttle-cable. _ And also, the manifold for attaching the carb to the cyl.head may be of a concern to deal with.
Please let us know, as I'm interested in this project of yours !


Fun-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


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