Laverda 1200 1977 Potential DMC Ignition Timing Problems....

[MOS]

Hi Lav crew

I have a 1977 Laverda 1200, that has been set up to Mirage Specs. All has been going well until it refused to run during a ride ( with my wife on board ... she forgives me for having my daughter pick her up)
I thought it was the coils. I replaced them .. nice strong spark. ... tried to start... new fuel, new plugs... backfires our of carbies
I pulled off the carbies and gave them a clean/ blow through
Tested the compression .. all OK ad 130. ( have not tested leak down)
All I can think of is that it might be something in the DMC 1 ignition or the pickups / wiring
Will try looking at timing with light. Any suggestions much appreciated !!!

 

[1200ts]

If you are getting a good spark on all three cylinders and it is spitting back through the carbs, my guess is the ignition timing has moved.

 

[Exmoor beast]

Obvious really but have you connected coils/ leads to correct cylinders?

 

[mauricejota7604]

Is that you Ash

 

[Sir Sidney Ruffdiamond]

Recently my Jota stopped on a run and started again for not so long and in the end came back in a van - with the help of Rob Bradbury we checked through the system using the test setting - me having fitted new plugs etc and checked the voltage voltage at the coils - Piet diagnosed a pickup issue and with a new cable was sorted - Rob checked the ignition timing which was'nt far off - was back to school day for me

 

[MOS]

If you are getting a good spark on all three cylinders and it is spitting back through the carbs, my guess is the ignition timing has moved.

Yes ... tat is wat I think as well.
Can I adjust the stator / timing without pulling off the flywheel ?

 

[iis_iis]

Yes ... tat is wat I think as well.
Can I adjust the stator / timing without pulling off the flywheel ?

Ball ended 5m/m hex wrench, through the oem magnetic flywheel holes. Though i doubt if you have a backfire, the slotted timing adjustment span is not that great to cause the amount of incorrect ignition, am suspicious there is more to the matter in hand.

Given the original fault was stop and not start again, that is not a timing issue, goes past that, BTW a DMC 1 is what? 30 plus years old? The ones i had here all died years ago, helpful or no... j

 

[Rob]

DMC1 will be using original triggers. Their wiring goes brittle and flakes off with age.
As John says, it's a very old ignition system now and worth replacing. Doubt the timing has altered given the symptoms.

 

[iis_iis]

FWIW, there are experienced eyes and hands here, very likely 1200TS Bruce has provided a possible basic issue in a different topic, with certain DMC units, high charging volts leading to shutdown or intermittent operation. Pickups, Rob has that indicated also, think he is on the money for your subsequent to shutdown, "misfire" issue, or internal DMC mis-operation. On the pickups of old, first early HKZ were Mu Metal encased, that shield was deleted (*), then the bare version, then the unfortunate plastic twin pole face removable type, mostly for BTZ the latter. Uwe Witt was a pioneer in the day, Silent Hektik ( spelling please ), Phil Todds' "Boyer Bransden" all early providers. There have been others, not mentioned, as omissive history may indicate.

Renewing the oem wiring to the potted pickups will always be rewarded with one less "unknown" when dealing with ignition issues. The DMC self test of shunting the pickup air gap to trigger a spark is basic at best. Very nice dial back timing light Like Sir Sidney has shown also a very useful tool, when trouble is multiple, confidence needs to be brought to bear on the unknowns.

(*) Basic pickup operation is magnetic field derived, the oem magnetic flywheel whirling about the pickups induces unwanted signal conditions into the timing derived dedicated pickups, had my attention also, mainly due to low cold crank start rpm, unable to provide reliable timing signal for the ignition module proper. Obvious was to redesign the pickup signal that was not cold crank rpm sensitive, opto isolated slotted triggers. Of course the oem bosch system was not compatible. Opto's served the purpose until the Hall Effect sensor trigger was perfected. Static timing was reliable with Opto and HE devices, VR ( Variable Reluctor ) need velocity to indicate timing signal validation. BTW VR have a "Natural" retard ( there is no "advance" ) slow velocity impulse cam yields near TDC signal, as velocity of the cam increase the rate of voltage rise increases and the timing "Advances" or more correctly the outward reference point ( the "max 32 degrees" ) delay is reduced...yes i know, hard to follow...a smile here.

HTH j.

 

[MOS]

Ball ended 5m/m hex wrench, through the oem magnetic flywheel holes. Though i doubt if you have a backfire, the slotted timing adjustment span is not that great to cause the amount of incorrect ignition, am suspicious there is more to the matter in hand.

Given the original fault was stop and not start again, that is not a timing issue, goes past that, BTW a DMC 1 is what? 30 plus years old? The ones i had here all died years ago, helpful or no... j

Yes I suspect the DMC 1 may be at end of its life ... but what confuses me is that it sparks the plugs when out of the engine. I tried to hook up an Innova timing light to check timing ( just using starter, plugs grounded on cam cover) ... No go ... would not fire strobe


Also I used the ball ended Hex key ... but I agree is clearly too far out .. or just not really firing when plugs are in ...

 

[iis_iis]

as it may assist, use 3 old discarded spark plugs, not for installing in the engine plug holes. Break off the frame tab arm of the plugs, to just leave the bare center electrode, fit to the HT leads, no plugs in the engine ( allows fast cold cranking ) and press the starter button. The gaped plugs will "emulate" a cold enrichment low battery volts ( starter motor drawing on the battery system volts ) minimum HV gap strike, ie if you cannot see reliable sparking on all three modified plugs, there is a problem, that will need step two, let us know the outcome of increased gap firing as demands allow. j

 

[MOS]

Hi Lav crew,

I found the culprit !
It was not the DMC1 playing up..more mechanical...
The grub screw was loose on the timing piece located on the crank below the coil and pickup plate. I should have known...strong spark ...but timing was way off or erratic. One additional learning point I figured out how to make a cheap flywheel holding tool ( $18 on fleabay...then weld on 2 nuts and round off with the bench grinder to fit the holes in the flywheel)