Jota 120 cutting out at speed

[hooksey]

Ill have a tenner on the battery. Cell collapsing and shorting out at particular revs......120's funky harmonics at play.

 

[Vince]

I went pretty nuts at Clark Rubber, fitting the best vibration-absorbing rubber I could find under my battery. I added some soft packing around it as well to remove any small movements that might stress the connections. I had a minor tipover that damaged the wiring, causing a partial break and loss of spark, which ultimately ruined a big trip. Any time shit happens, I go belts and braces to avoid it happening again.

 

[a14rgs]

Ill have a tenner on the battery. Cell collapsing and shorting out at particular revs......120's funky harmonics at play.

I'll raise you a tenner on the DMC wiring, Dave

 

[hooksey]

I'll raise you a tenner on the DMC wiring, Dave

Ooooh ahhhh, Coupla Pints it is Al.

 

[Rob]

What's involved in taking that off, do I risk screwing up its settings

Yes, you do. I'd suggest scribing a couple of lines diametrically opposite across the baseplate and primary chaincase before removing screws from the slotted holes so you can put it back in the same place.

But, especially looking at the tight cable bends straight out of the multi pin DMC connector I'd focus on that.
It is possible to remake the terminations with new pins if there's enough slack in the wiring. But a new loom is easier

 

[Otto]

When my SFC1000 with a DMC2 gave problems with running, it stops after 15 km, when cold it starts again, Gijs van Dijk tolt me:
Your photocel is broken or dirty (like Lothar said). It wasn’t dirty and other connections were right so I changed the eye. Problem solved.
It hapens more times that the eye gives problems. You can’t mesure it, just change it.

 

[iis_iis]

The DMC slotted opto device is obsolete, ( i ) did the research and matching or better specification, found an in production or vast numbers in stock replacement at low cost. Posted here some time back, no chance recalling the actual thread number... forum not fastener.

Yet to confirm what is actually on this problem engine, type of encoder trigger DMCHTH j

 

[Exmoor beast]

...more free advice paid for... you went to reserve tap and seems quickly the "fault" worsened, clue one. Some other symptoms point to electrical, many the learned friends have covered, notably DMC known issues. Then of course you may have more than one fault looking like just one fault, that is where the boys stop and the men take over... benefit of that comment has two levels, insult to boys, their gained experience is yet to come, and poor men like me dealing with faults that beguile.... easy things first, highway testing is not safe, you or others, hope that helps... or makes it worse...that is also a useful testing procedure, making it worse means your in the area of the problem.

Stationary test of the ignition system and battery volts would be a process high on list, is there a continuos spark excitation for the DMC without motor running? Not the manual tap the reluctors type? j

“Then of course you may have more than one fault…..” I would say every bike that I have had a real head scratcher has been two unrelated faults that seem to compliment each other- carbs & electrics seem to have telepathy. Another classic is ruling out a new component “ can’t be that it’s new..”

 

[Andy]

“Then of course you may have more than one fault…..” I would say every bike that I have had a real head scratcher has been two unrelated faults that seem to compliment each other- carbs & electrics seem to have telepathy. Another classic is ruling out a new component “ can’t be that it’s new..”

The “faulty new component” is definitely one of the classics

 

[charlie]

I'll raise you a tenner on the DMC wiring, Dave

Ooooh ahhhh, Coupla Pints it is Al.

I might raise you a tenner that I'm not smart enough to find the DMC wiring issue But I will give it all a go. After I try solving 18-year old car issues. There's a queue!

 

[Sir Sidney Ruffdiamond]

There is a test position on the dmc box for the box itself - by grounding the plugs and turning the ignition on the spark plugs should spark continually proving the box is ok - needs three hands - in my case it was the cable from the optical Sensor in the left hand cover up to the box - my bike would just cut out and restart occasionally and then it wouldn’t go at all - she’s running perfectly now thanks to Rob Bradbury without whom I am clueless on this stuff

 

[78jotadave]

My two bob's worth, definitely electrical, loose connection somewhere.
I had a very similar experience on my '78 1200. The bike ran fine for about 30 minutes, then after warming up or vibrating a couple of connections loose, it was like riding a bucking bronco. Would not rev over 5k rpm and had massive jumps and jerks at 30mph; it was a wild animal. Road it home took the tank off, and replaced the plug caps with Black NGK 5ohm caps, along with crimping the wires tight to the connectors on the coils.
Problem solved, bike has run perfectly since then. Start with the simple stuff first. HTH

 

[charlie]

Thanks Sir Sidney and Jotadave, simple stuff first works!

 

[oldiggy]

My 79 jota did that effect to me when I was hammering it back from thruxton on the outside lane of the motorway , frightened the poop out of me ! Turned out to be the ignition switch so the next time it happens give the ignition key a wiggle to see if it alters.

 

[1200ts]

Check the charging voltage after riding the bike for enough miles to get the regulator warm. The DMC ignition does not like voltages over 14.5 and will do many bad things. My RGS would run perfect for 20 miles and then would start to run bad and eventually die. The factory ND regulator would output over 15 volts once it was warm. Replaced with a Shindengen regulator and it fixed the problem.

But the DMC was later replaced with one of John's ignitions and it now runs even better.

Also have seen the resistor plug caps go open circuit when hot.

The DMC 120 ignition is certainly better than what Bosch made for Laverda, but not a great design. The fact they used a photo transistor instead of a hall effect is a poor design choice. On photo transitors, the LED eventually burns out. The IIS ignition is most definitely better.

 

[nick7]

Hi all...on my79 Jota running a Sachse ignition , I had a major issue like this recently. On a Rideout from Baskerville, suddenly the bike died...I started fiddling with fuel taps, then as it slowed it suddenly fired and ignited unburned fuel giving those behind a shock. Ran faultlessly for next 100 miles. But on the ride back from Wales to Kent. Failed several times including in the fast lane of a very busy M25...literally it is as if the ignition is turned off..FFS!! Each time it started up on the button. Advice was to look for a loose contact...somewhere! An e mail to Red resulted in a very kind and quick reply..it's the regulator ! The ignition box switches off in case of overcharging...replaced it, a bit of fiddle with connector block spades to ensure a match with the new part of the loom...but all seems well! No problem on initial 80 mile spin. Ideally I would have tested with a multimeter, of course. Ah. And since in my experience regulators rectifiers fail when they work too hard because of a failing battery..SP1 and VFR experiences!..the10 year old Odessy was also replaced. Having given faultless service.

 

[charlie]

Hi all, finally weather to work on it, and the result of the tenner bet is... getting close! From all the good advice (thanks), culprits in descending order may be:

1. "Check the cables on the coils"

Found RH coil cable under tank crushed, and very slightly split where crushed. More knowledgeable mate suggested it could be arcing to the tank (some evidence there). Enough to cause the bucking-bronco effect, at wilder speeds? Had enough cable length to cut and refit it to coil.

2. "Check the ignition key switch and its cabling"

There was up/down movement on the whole ignition switch within the metal key mount. Plastic threaded collar cracked. Now superglued so all tighter. Cabling is all good, so doubt movement there – if wires are intact – would have been the issue?

3. "Battery cell collapsing and shorting out at particular revs"

Battery checks out OK. Not sure how I would test it 'collapsing'. Do have a bluetooth gizmo that would show battery stats on a ride, but then I'd be in bronco mode. I could replace it, to rule it out.

4. "DMC optics"

Was going to clean the 'eye' but don't know how/what with. It's all pretty clean in there, only a small smear of oil at base. Many scribe marks! So to inspect wiring, don't know what I would mark against, or undo, if it's necessary!

5. "DMC unit"

Wiring to this has been examined, looks sound, no cracks. Cleaned and a little terminal grease applied.

6. "It's the regulator"

Not smart enough to test this. It's on the list of, if it happens again, replace it with the battery. (Easier for me to replace than test!)

7. "Fuse started dancing"

All are sound.

Plan is, ride it (gingerly) soon. And if I get symptoms, replace battery/rectifier, test again. If it still happens, presume it's the DMC eye/cable/loom which I'm not good enough to replace, so it would be a 6 hour trailer trip to Ricardo.

Cheers for any more insights,

Charlie

 

[Jo]

Hi Charlie,
working through the list sounds like a sensible approach. The multi-pin connectors on the DMCs always appreciate it if you unplug them from time to time and give the contacts a little TLC - but you’ve already taken care of that with item 5 on your list.

Now, the next test ride will reveal whether your Laverda is still a bucking bronco, or if it has turned into a little lamb.

Salut,
Jo

 

[charlie]

Danke Jo, it's been tenderized for so long it should be lamb-like by now!

 

[Andy]

That HT cable will be a problem. HT means High Tension, another term for High Voltage, and it doesn’t take much damage to the insulation before that voltage pressure leaks out, then tracks along dirt or moisture or jumps across gaps. Oily or dirty leads give problems too.