Starter gear spring / leaf spring ?

[Desmoto-M.3]

Bob, I don't considder my own self to be one of those "toe-jabber" types! But I think i know the type you must be referring too. They quickly jab their foot on and off against the lever while speed shifting. Right?
I think that's unnecisarry abuse, so i rarely ever do any speed shifting.
But I think i do lift my toe off the lever before i let the clutch back out, like you claim most riders do.
So I can't wait to start trying to use your sugjested method. Unfortunately that will be months from now because my Ducati is parked and set for the winter now.

About my gearing question, are you sure just 1 tooth less on the front sprocket will make very much change for what I want? The figures you give don't look like they are very much different to me, to be worth doing. Especially if my first gear would still be just as high as second gear on a stock 250 Scrambler.
Could I switch to a 16 tooth front without making fifth gear as low as my stock fourth gear?
I often have to ride through my hometown to get to one of two roads connecting to a city about 20 miles away. Then I go to a couple places within the city, and return back home. I hate the city driving but really enjoy the stretch between!
I don't think I'm TOO big for my Mark 3, but i guess i should loose 20 pounds to be better suited for it. I'm currently about 190 and 5' 9". Do you think I'm too big for for this size bike?

Thanks for all the info!
Ken

[DewCatTea-Bob]

" Do you think I'm too big for for this size bike?
About my gearing question, are you sure just 1 tooth less on the front sprocket will make very much change for what I want? The figures you give don't look like they are very much different to me, to be worth doing. Especially if my first gear would still be just as high as second gear on a stock 250 Scrambler.
Could I switch to a 16 tooth front without making fifth gear as low as my stock fourth gear? "

____ No Ken, I don't think that you're too big for it but,, without a good tail-wind, you are probably never going to be able to get your stock Mark-3 up to it's claimed 110-MPH top-speed.
So you might as well lower your gearing to make it more useful to you for your in-town riding.
____ I know what you mean about the gear-ratio figures seeming to not be very much changed. _ But actually, 1-tooth on the front-sprocket makes a fairly significant change.
While it's true that your 1st-gear will be about the same as a Scrambler's 2nd-gear, it really depends on which model that you're actually referring to...
There's the 1965 model (that was actually the last of the 'Motocross' models), which had 55/14t gearing; then there's the 1966-Scr which used 50/14t gearing; & then the wide-case SCRs, which came with (considerably higher) 45/15t gearing.
With the 40/17t gearing, your 1st-gear overall-ratio would be virtually the same as the 2nd-gear of the stock 1966-Scrambler. _ However, keep in mind that your 1st-gear would still feel a good bit lower than that because, the Mark-3's rear-wheel tire is smaller than a Scrambler's! _ That difference in rear-wheel circumference can be roughly equal to the same difference that 3-teeth on a rear-sprocket would make !
__ It seems you were wondering how much you can lower your gearing without causing top-gear to become as low as the stock 4th-gear is. ...
Well the difference between 4th & 5th is nearly 13%, and a 17t would lower the gearing only around 6%,, thus making the new top-gear just about centered between your current/stock 4th & 5th. _ While the 16t sprocket you asked about, would lower your top-gear about 12%, and bring it down to only about 1% higher than the stock 4th-gear, (6.05:1 vs. stock-4th @ 6.13:1)... So close to the stock 4th-gear, that you would not be able to tell any difference. _ (So I hope you like how things have been, while cruising in your current 4th-gear.)
The 16t would also lower your 1st-gear overall-ratio to 15.81:1, (same as if you retained the 18t and replaced the 40t rear-sprocket with a 45t!).


" I don't considder my own self to be one of those "toe-jabber" types! But I think i know the type you must be referring too. They quickly jab their foot on and off against the lever while speed shifting. Right? "

____ Yes Ken, you understand exactly what a meant about toe-jabbing at the shift-lever!
Bad style!! _ It's just luck that allows that method to get by! Nothing more!
Also, if the clutch is not dropped at the exact instant needed during an extra-hard/fast toe-jab, it could cause even gears that would've otherwise fully engaged, to bounce back out of reach (of dog engagement), and knock the engagement-fork into a false-neutral spot, (all while the rider's foot is still blurring away from off the shift-lever!).
As I've always claimed, "Toe-Jabbing" is bad-news! _ So try not to do it!


GoodCheers,
-Bob

[DewCatTea-Bob]

By captpaul :
" my last is a 1965 bronco.
I have to paint and cosmetically restore a 2556 mi (lo millage ) bronco. The parts shop manual has an exploded view of the shifter that might be adjustable.A low millage bike shouldn't need dogs machined, would they? "


____ Of course such machine work should not need to be done! _ I assumed that Eldert was just kidding about needing to do such extensive work, (or, he's never worked on the strange 'Bronco' model before.) _ Because.....
You stated that 1st-gear will stay engaged as normal, only after being forced to stay engaged for a (unspecified amount of) time.
Here's why that's the case.....
The very tip-edges of the engagement dogs are ever so slightly rounded -(un-sharpened to a point), and since the adjustable range of your shift-lever is not set far enough to engage the dogs of 1st-gear to a location that's well past those tip-edges of the dogs, the dogs are then pressured to push each other apart, (since they were not engaged far enough).
However, when you hold the shift-lever down (as you mentioned), and thus prevent those dogs from being pushed apart,, vibrations from the engine & road, along with varying pressures on the dogs (due to irregularities in throtle-setting & suspension-action), will then allow the dogs to move only in the direction of further engagement. _ And so once the dogs happen to move past their tip-edges & reach each others slant-cuts, they then pull each other towards one another (as the slash-cuts are meant to do!) and become fully engaged... Thus at that point, 1st-gear then remains engaged ! ...
So you see, IF the slash-cuts of those dogs were indeed in need of renewing, then 1st-gear would never stay engaged. _ But since you claim that, (without further force required), 1st-gear is able to remain engaged,, (that's once they've had time to work themselves further together),, THAT-fact indicates that the slash-cuts of the dogs are still (at least) good enough to do their job as they should, (once fully engaged as they should be).
__ So lets get the fork-engagement rod properly adjusted & reset.
If you don't know how, then please show us some pictures of the mechanism.

GoodWill Cheers,
Bob

[Desmoto-M.3]

Thanks Bob,
I never rode a Motocross or Scrambler older than 1966, but my Dad had both the 1966 and the widecase 1967 250 Scrambler models, and i got to ride both of them for a number of years back in the '80s and '90s.
He always said the '66 was a little quicker, but i only remember the widecase had a smoother and more pleasing ride.

After your point about wheel-size, and that a 16 tooth would make fifth gear almost the same as my current fourth, I've desided to go with your sugjested 17 tooth front sprocket, or else a 43 or 44 tooth rear sprocket.
Because on the country road i like to ride most, I often switch between fourth and fith gear, so i would miss not having a gear that's higher than my current fourth gear. And any amount that my first gear gets lowered by a gearing change which still keeps my fifth gear noticably higher than my current fourth gear, will just have to be acceptable.
Since there's no way for me to spead out the gear ratios, this new gearing i've chosen seems to be just the right amount for me!
I did some math of my own and figured that the top speed of a stock Mark-3 using the 40 / 17 gearing, would be lowered from 110mph down to 104mph.
Since mine has a DESMO head on it plus a 350 Scrambler piston, wouldn't that make my Mark 3 even faster?
Would you care to comment any on how all these changes would stand up against a stock 250 Mark 3 ?

Does anyone know where i could get my choice of sprockets?

Thanks,
Ken

[DewCatTea-Bob]

" Would you care to comment any on how all these changes would stand up against a stock 250 Mark 3 ?
Does anyone know where i could get my choice of sprockets? "


____ Yes I would Ken. As I've also installed DESMO-heads on narrow-case engines and 350-pistons in 250-motors, (but never in this same combo that you have)!
However I must say, we are getting considerably off-topic from this thread's original subject!
__ First the piston change... While the extra 2mm in bore-size will provide a 5.5% increase to 262cc, the 350SCR-piston will also lower the compression-ratio from the stock 9.8:1, to only about 7.7:1 . _ So the very slight power-increase that the extra 14CCs should have provided, is more than wiped-out by the loss of power that such a decreased compression-ratio would cause.
That particular piston change (with no other changes), likely would lower top-speed from 110 to under 100mph.
__ Next the head change... The most significant difference between a DESMO-head & a 1966 Mark3-head, is the cam! ...
The standard DESMO-cam (which is based on the 250F1-cam), is considerably wilder than the 250-Mark3's cam! _ Thus it produces slightly more power at a higher RPM !
While an otherwise stock 250Mark-3 would likely still be able to obtain 110mph with the DESMO-head, the gearing would likely have to be adjusted slightly lower, so that the new & higher RPM at which peak-power output is produced, would still occur at (or slightly higher than) 110mph. _ (As I really doubt that a 250-DESMO produces as much power as the stock Mark-3 does at the Mark-3's peak-power RPM.)
__ Overall, it's good that you plan to lower your gearing,, cuz as your Mark-3 currently is, (I assume it's carb & exhaust are still stock), I'm quite sure that a stock Mark-3 will out accelerate & out-run yours! (Providing the riders are same size!)
After you lower your gearing, your DESMO-M3 then stands a better chance to keep-up with a stock Mark-3, but, I'm afraid to say that I'd still expect the stock Mark-3 to come-out ahead.
__ On a positive note, your altered Mark-3 should be a heck of a lot easier to kick-over! (Due to the DESMO-cam's wilder valve-timing & that 350-piston.)

____ I have a great selection of sprockets which fit the counter-shaft of all the OHC-singles!


DUKE-Cheers,
Bob

[Desmoto-M.3]

Needless to say Bob, I'm really disappointed with your acessment of the outcome of all the changes done to my Mark 3.
From everything you covered, I gather the only thing working against my Mark 3's power output is the low compression do to the 350 piston. But it's been my understanding that the 350 Scrambler's piston has the highest c.r. of 9.5 , right? So how does it get so low as just 7.7 in the 250?
I thought that the 250 heads had the same sized combustion chamber and everything else, as the 350s. That is correct, right?
And, doesn't the 350 piston go all the way up to the top of the 250's cylinder, with it's crown pushed just as far into the head's chamber, as when it's in a 350 engine ?
So unless I'm wrong about at least one of these assumed facts, i don't see why the compression would get as low as you stated.
I do remember how my Dad always said the Mark 3 got easier to kick start and his 350 which the DESMO head came from got a whole lot harder to kick start, after their heads were swapped between them. So you must be right about my Mark 3 being easier to kick now as it is. And i do realise that it does kick though eazier than any of the other 250s I've tried. So i guess the head models must have something to do with the amount of compression.
What I still question is how does the 350 piston seem to get most of the blame for the reduced power output?
If you could just clear me up on this last issue, I'll be sure to stop this sidetrack of this thread. - As you've pointed out, we have wondered off topic.
And by the way, since you brought it up, my carb is still the original Delorto but the mufler seems to be like something from a old Dunstall Norton, or the like. It's not very quiet either!

So how could i get a 17 tooth sprocket from you?

Thanks a lot,
Ken

[DewCatTea-Bob]

" I gather the only thing working against my Mark 3's power output is the low compression do to the 350 piston. But it's been my understanding that the 350 Scrambler's piston has the highest c.r. of 9.5 , right? "

____ That's right, the wide-case 350's comp.ratio is 9.5:1, (and the N-C 350's is 8.5:1).


" So how does it get so low as just 7.7 in the 250?
I thought that the 250 heads had the same sized combustion chamber and everything else, as the 350s. That is correct, right?
And, doesn't the 350 piston go all the way up to the top of the 250's cylinder, with it's crown pushed just as far into the head's chamber, as when it's in a 350 engine ? "

____ As is standardly done, the "7.7:1" was a CALCULATED ratio, not necessarily the same as a 'measured' ratio.
__ While the combustion-chambers are indeed the same with the two heads, a 350-head has a 76mm-bore while the 250's is 74mm. -(That bit of machine-work is the only way to tell the difference between otherwise identical 250 & 350 heads, when they're completely bare of any other parts!)
__ While the 350-piston could indeed fit equally as far into either head,, whether or not it reaches to actually fit flush with the top of the cyl.sleeve, depends on the actual height of your cylinder. _ My calculated 7.7 was based on it doing so, therefore your actual comp.ratio is likely even a bit less than that!
The top-edge -(base of the crown) of your 350-piston is at least as high (above the piston-pin) as with any 250-piston, so your 250-cylinder/jug may have a 1mm thick spacer-shim installed under it. - Have you noticed any such shim between the base of your cylinder & the motor-case?
____ Keep in mind that while any air that's squeezed into the chamber of either head model is indeed compressed into the same restricted volume of space, (as seems to be your concern),, the AMOUNT of air that's getting squeezed is NOT THE SAME amount! _ As I attained my 7.7-results by using figures which reflect the fact that your 250-cylinder has 78cc less displacement than the 350!


" I do remember how my Dad always said the Mark 3 got easier to kick start and his 350 which the DESMO head came from got a whole lot harder to kick start, after their heads were swapped between them. So you must be right about my Mark 3 being easier to kick now as it is. And i do realise that it does kick though eazier than any of the other 250s I've tried. So i guess the head models must have something to do with the amount of compression. "

____ Actually it's not the model of the head that's the cause, it's the model of the camshaft within the head!!
You see at kicking-RPM speeds, wilder cams don't cause the valves to capture as much air (to then need to be squeezed into the chamber), as milder cams do!
So actually, it was the milder Mark-3 cam, (not the M3-head), that caused the 350 to get much harder to kick-over!


" What I still question is how does the 350 piston seem to get most of the blame for the reduced power output? "

____ Well Ken, the dome-top on the 350-piston is much lower than that of the 250Mark-3 piston! _ Therefore whatever amount of air gets trapped in the cylinder, that air doesn't get squeezed as much into the combustion-chamber (by the 350-piston)!
And as I've explained, there's up to 78-CCs less air-volume to be compressed (in this case of yours).
__ Also, more compression causes higher combustion temperatures, which in turn create considerably higher pressures which push-down on the piston!! _ And that makes your motor go lots stronger!
So you see, the amount of compression is fairly important, as it can make a big difference in power!



" So how could i get a 17 tooth sprocket from you? "

____ I guess I'll list some of what I have on eBay this next spring.



ChummyCheers,
DCT-Bob

[Eldert]

Hi Guys
i found a picture of the improved kickstarter spring design for the narrowcase on my computer and like to share it with you all . no more leaf spring but there is a coil spring onder that bridge over the gear



Eldert

[DewCatTea-Bob]

" i found a picture of the improved kickstarter spring design for the narrowcase on my computer and like to share it with you all . "

____ Thanks for posting another of your pix Eldert! _ Your posted-pix are always useful & nice to look-over! _ Keep-em coming !
__ As a side note for MikeN, he can now get a better look-see at the empty hole that he was concerned about having a missing bearing or something (where the shifting-drum fits).


Happy-Cheers,
-Bob

[miken5678]

thanks for the shot

Down home with the folks this weekend and in my fathers garage so I shall get some working time in and start on my wheels.. gonna get busy buffing.... couldnt quite fit the frame in the car to bring it down to bead blast but it will come soon enough.. will grab a couple pics of the other bikes in progress and post them up

sorry to go off topic..