Cure for an ankle buster?

[JimF]

This new-to-me 250 is a real ankle buster. Almost without fail the first time it fires off it kicks back sharply and forcefully.

This is a battery-less 250 4-speed. The points are opening at 30 degrees BTDC.

Suggestions?


Jim

[MotoMike]

Hi Jim

I looked back to try to find which 250 you had but couldn't figure it out. Don't the one's with AAU have a static timing of 5-8 degrees with 33 to 36 total at 3k rpm? What ever it is supposed to be, couldn't you dial it back to the point it doesn't kick you and see how it drives?

Mike

[Bevel bob]

Fit a 60w alternator and battery ignition ,change the AAU for one that suits and have a bike you can use. Mach1 settup . Ignition 5 to 8 degrees static.Kickbacks so feeble now this OAP with dud Knees can cope.Otherwise its polish and park it.

[DewCatTea-Bob]

By: Jim...
" This is a battery-less 250 4-speed. The points are opening at 30 degrees BTDC. "

____ I assume then that what you have is an early-scrambler/'Motocross' model which has it's equivalent-AAU locked at the full-advance setting (which I believe is supposed to be set at about 39-degrees BTDC, [not just "30"]).
I-myself have never owned one of those models that was still in running-condition, so I don't have much personal experience to pass-on but do recall being afraid of the dreaded intense kick-back by such early 250-models with no working AAU.
Whenever I did have to try to start such a Duke (other than by push-starting), in order to get them more safely kicked-over using the kick-lever, I'd roll the Duke backward in 1st-gear until up-against the compression, then shift into neutral and then give the kick-lever a real-hard full-fledged full-force kick-through, (with high-hopes that another attempt wouldn't have to be done again !). _ Otherwise, with the crankshaft left wherever it happened to come to rest (between attempts), ya could then expect a kick-back about every 4 out of 5 tries.
__ The recommended-fix for the much dreaded kickback-issue was said to be to install a kill-switch/button and learn how to hold-down the button while kicking and develop the knack of knowing just when to let-go (of the kill-button), so as to get started-up without running-into any kickback-occurrence. _ And while I have installed such kill-buttons, I-myself never bothered to fully master that touch-&-go method, (since it seemed kick-backs were still possible during the learning-process, if ya weren't patient enough to be steady & careful at it).
I believe the proper process (during the kicking-method) was to hold-down the button until comp.TDC became fully encountered, then let-off the button (just as the full-comp.TDC was occurring),, with that process run-through rather quickly with strong confidence !
Apparently if accomplished just-right, the timing of the ign.spark would be delayed a few (random)- degrees, so that it would occur with less likely-hood of severe kick-back.
The reason the process could work is because the kill-button would effectively short-out the points, (thus temporarily rendering their set timing useless), and when the button is next (properly!) released, IT instead would then allow the spark to be created,, somewhat delayed compared to the normal/set ign.timing.
A really extra-useful process for those oldest models without any working AAU !
__ Preferably, the original (locked type) points-cam ought to be replaced with an AAU-controlled points-cam setup.
Have you checked-into whatever setup your old 250-Duke has ? - (The points mounting-plate has to be removed, in order to see if there's any workable AAU behind inside there.) _ The older battery-less type ignition models (like you probably have), all came-stock with locked-in-place points-cam setups, (and if ever owned by a richer type owner, probably since got modified).
If you happen to already have a working AAU installed, then your current static-timing is set about 8-degrees too-far advanced, (assuming your stated "30 degrees BTDC" is a 'static' reading).


Hopeful-Cheers,
DCT-Bob

[JimF]

Here's what's under the points cover:

points.jpg

I don't see an AAU but them mabye (certainly) I don't know what the AAU looks like. I would have expected to see some kind of sprung plate that changes the timing under centrifigal force, but I also imagine there are many ways to implement an AAU.

Please comment!

Also it seems to be the points are opening at 30 degrees BTDC. Now I believe the laws of physics are such that I will get spark when the coil is energized and when it's de-energized (which is the case now.) But now is as good a time as any to make sure that I am not jinxed by someone before me setting the points up wrong.

Near as I can figure this was a street bike (narrowcase.) The typical giveaway is the frame loop over the rear tire.

The frame's tail has been lopped off and the loop welded on vertically to form a bumstop. I can't see any evidence of there haveing been a loop of frame over the rear tire.

There are several bits on the bike that would also suggest to me it was likley a street bike.

Bob, Re-reading your post would seem to indicate that an AAU (if present) would not be visible without removing the points, correct?

[DewCatTea-Bob]

" Here's what's under the points cover: "

____ The cam-lobe type appears to be the short-duration version, as expected for the battery-less type ignition-system, (which employs the green-label type ign.coil).


" I don't see an AAU but them mabye (certainly) I don't know what the AAU looks like. "

____ As I had mentioned, in order to see inside behind there, the points-PLATE has to be removed, (not just the points inspection-cover). - (In order to remove the plate, it's two screws [located near 1 & 6:30 o'clock] need to be taken-out.)


" I would have expected to see some kind of sprung plate that changes the timing under centrifigal force, "

____ That would indeed be expected IF your n-c motor had been a newer model, however due to it being an old/4-speed model, it ought to have a locked -(no advancer/AAU) type points-cam setup.


" Also it seems to be the points are opening at 30 degrees BTDC. "

____ You mean during STATIC checking, is that correct ? _ That needs to be confirmed, in order to realize if it's ign.timing is set too advanced or too retarded, (as it is currently not set correctly).


" Now I believe the laws of physics are such that i will get spark when the coil is energized and when it's de-energized (which is the case now.) "

____ I'm not too sure of exactly what you're really meaning to say by that wording, (since we are now not concerned with battery-powered type of ignition), however the (green-type) ign.coil is indeed energized -(providing spark), when the points open.
Your (green?) ign.coil should not make any spark when the points close & short-out/de-energize the ign.coil.
__ (Only the red-type [bat.powered] ign.coil produces it's intended ign.spark, when it's de-energized [when the points open].)
Since the green-type (peak DC-pulse powered) ign.coil is intended to be FULLY energized only when the points open, (thus creating an expanding-flux type spark),, the intended rotation-point when the points close, is thus then meant to occur when the alt.supplied AC-waveform has happened to become too weak to be able to create a collapsing-flux type spark (like the red-coil setup does).


" Near as I can figure this was a street bike (narrowcase.)
There are several bits on the bike that would also suggest to me it was likley a street bike. "

____ If your Duke's engine is stock, then it's probably the only other 4-speed 250 which used that type of ignition,, which was the (rare!) early 'Diana Mark III' !
__ I believe the Motocross-model had two 6mm holes in the chain-case (to hold the internal chain-guard), so if you don't find such evidence, then the motor must be that of a 4-speed Mark-III.
__ Does the alt.cable have 2 or 3 wire-leads ?
__ How about a bunch of pix of this very unusual Duke-find of yours ?


" Bob, Re-reading your post would seem to indicate that an AAU (if present) would not be visible without removing the points, correct? "

____ Actually, the points (as well as the condenser) can be left remaining attached to their 'plate', and the (complete)- plate will need to be pulled-out, so as to see just-what's inside behind where that plate is mounted.
(You may wish to re-read my previous-post once again, as I've since added more detailed-info into it which may be of interest.)


Hopeful-Cheers,
-Bob

[MotoMike]

If it has an AAU under the plate, can't you rotate the points cam moving the weights against the springs and it would pop back when rotating force is released? And if so, you would know that your static is way too advance if the 30 you are talking about is indeed static?>

[Bevel bob]

You need to check the range of the advance by setting up a timing disc and rotating the cam (i use a pair of pliers with a bit of lead in the jaws to protect the cam) and a screw cup fitted under the screw in the middle of the cam to jam the cam on full advance) ,once you have determined if the AAu has any advance you can experiment .But if you have a fixed unit I would think about converting it to something practical.

[JimF]

I took out the points 'bucket' and there is no AAU behind it.

The points that were in the bike looked bad, that is to say that both contact surfaces were dark with irregular coloring on both disk surfaces.

Since the bike is having trouble revving past 6K rpm and recent attempts to get the bike above 6K RPM by changing carb jets and needle position have proven ineffective, I am changing out the points for new.

Right now I am looking to find the amount of static degrees BTDC that I should be setting these non-centrificugally advanced points to.

year: Unknown
Engine # 88103
Gearbox: 5-speed
Frame: narrowcase - street version
Electrical system: 'Magneto' (AC) with no Automatic Advance Unit.

http://www.motoscrubs.com/Ducati_Electr ... ywheel.htm

Looks to me like 38 to 41 degrees. Is that correct?


Jim

[DewCatTea-Bob]

" I took out the points 'bucket' and there is no AAU behind it. "

____ You say "bucket" ? _ You must mean the 'plate' which the points & condenser mount to !?
Once that plate is removed, I've always ever seen either a suitable working AAU or one that's been welded into it's full-advance position.
Do you have a pic to show what you actually have ?


" Since the bike is having trouble revving past 6K rpm and recent attempts to get the bike above 6K RPM by changing carb jets and needle position have proven ineffective, I am changing out the points for new. "

____ That may help (insignificantly) but, I wouldn't expect any notable improvement (due to your ign.points-condition).


" Right now I am looking to find the amount of static degrees BTDC that I should be setting these non-centrificugally advanced points to.
Looks to me like 38 to 41 degrees. Is that correct? "

____ Yes.
And it's more likely that your revving-issue is due to your current advance-setting being set way-off. _ So before you simply go-ahead & set it properly, first try to learn where the advance-setting has actually been at (during your recent testing).


" year: Unknown
Engine # 88103 "

____ It's a 1965 model.


" Electrical system: 'Magneto' (AC) with no Automatic Advance Unit. "

____ I'm unclear as to why it doesn't have a working advance-unit... I thought the locked to full-advance feature was stock on OLDER ('magneto-system') models. _ Do you have any previous reason to suspect that your points-cam setup is not stock ?


Hopeful-Cheers,
-Bob