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Discussion Starter · #1 ·
Hi guys - this is a question for any of you with experience of ignition system problems on later Darmahs with stock ignition. The motor dies after 5000 rpm. It's not the fuel system and I have checked the carb needles are in their clips and the main jets are factory size. It only happens when under a lot of load, while at low revs and low throttle openings (under 5k rpm) the motor behaves as it should, nice and torquey, and it starts well.

Has anyone experienced this specific problem before?

What, in your experience, are the ignition system parts most likely to fail -– coils, pick-ups in left engine cover or ignition boxes? I haven't ruled out a poor earth (ground) connection somewhere, or a frayed wire. I also haven't ruled out getting an entire new after-market system, but I would like to test the components first. Do any of you have figures for coil resistance or methods to test the pick-ups and ignition boxes?

I welcome your thoughts.
 

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I can't help you technically, but probably the most common problem is the wiring insulation from the pick-ups inside the clutch cover rots and allows the spark to break down. What I would say is that your ignition is around 40 year's old and is definitely past it's best!
 

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Discussion Starter · #4 ·
I can't help you technically, but probably the most common problem is the wiring insulation from the pick-ups inside the clutch cover rots and allows the spark to break down. What I would say is that your ignition is around 40 year's old and is definitely past it's best!
Duccout -- thanks for that. It's good to know about the most common cause of failures and breakdown of the insulation from the pick-ups seems to be a favourite. Yes, you're right, in reality, these ignition systems are way past their design life. A new after-market system might be the best bet.

620rossco - I'm confident it's not a plug problem because I've changed the plugs and the problem persists. I do know that the fuel pumps squirting lots of juice down the inlet tracts in ordinary riding is one reason why 6 heat range plugs are often used. Blanking off the fuel pumps and using B8HS plugs is what I have done with 40mm pumpers, with no significant loss of performance and no more plug fouling. (This Darmah has 40mm carbs).
 

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dhickie: The description of your problem is interesting; does not sound like you have determined if one or both cylinders are failing under high load. When you write "dies" do you mean the thing actually shuts off or just loses power?

If all is really as well as you describe at lower power then this does not suggest an ignition issue. My first guess would be partially plugged fuel filters, most likely one or both of the petcock ones inside the tank but check the carb ones too. This would explain why all is well when fuel demand is lower. See if the problem goes away when running on reserve. This is a really common problem with older bikes.

I'd go back to basics and give the entire electrical system, including all the wiring a good once over. Get long leads for your multi-meter and perform continuity and ground checks on all connections. Don't forget the charging system and battery. It may be that under load and associated vibration, something is losing ground and compromising the ignition. Grounds must never be assumed! Always test while physically fiddling with the wires.

To answer your more specific questions; coils should be around 5ohm; my old ND coils measured 4.6ohm. The ballast resistors should be around 1.4ohm. The pickup resistances should be around 240 ohm; these can be measured at the plug connector. Look for the little "V" and "O" cable labels; the two Vs are for vertical or rear cylinder and the two Os are for orizontale or the front cylinder.

The Bosch exciters are not easy to test. If one cylinder is behaving badly then just swap the Bosch units and see what happens but I don't think this is your problem.

I went all the way with my 81 MHR and replaced the ignition system with Sachse ignition, Dynatek cables and coils, NGK caps and plugs and a Shendingen regulator to protect it all. New battery too. I also had to repair my alternator wiring at regulator end; the yellow wires are prone to overheating and getting brittle.

Good luck to you. Please report back and if you want to discuss my ignition mods further, just ask.

Rick
 

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Had a Darmah in the shop back in '87 or '88 doing close to the same thing.

It would start and idle, and accelerate O.K., but when the RPMs went over 4000 or so, it would start stuttering and not pull to the upper range.

We tracked it down to a faulty right handlebar switch. Something was loose inside and would kill the ignition under vibration. I think we unplugged the kill switch wire as a last resort to test it, and it ran fine.

Just a thought...

(Italian electrics, eh?)
 

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Discussion Starter · #7 ·
dhickie: The description of your problem is interesting; does not sound like you have determined if one or both cylinders are failing under high load. When you write "dies" do you mean the thing actually shuts off or just loses power?

If all is really as well as you describe at lower power then this does not suggest an ignition issue. My first guess would be partially plugged fuel filters, most likely one or both of the petcock ones inside the tank but check the carb ones too. This would explain why all is well when fuel demand is lower. See if the problem goes away when running on reserve. This is a really common problem with older bikes.

I'd go back to basics and give the entire electrical system, including all the wiring a good once over. Get long leads for your multi-meter and perform continuity and ground checks on all connections. Don't forget the charging system and battery. It may be that under load and associated vibration, something is losing ground and compromising the ignition. Grounds must never be assumed! Always test while physically fiddling with the wires.

To answer your more specific questions; coils should be around 5ohm; my old ND coils measured 4.6ohm. The ballast resistors should be around 1.4ohm. The pickup resistances should be around 240 ohm; these can be measured at the plug connector. Look for the little "V" and "O" cable labels; the two Vs are for vertical or rear cylinder and the two Os are for orizontale or the front cylinder.

The Bosch exciters are not easy to test. If one cylinder is behaving badly then just swap the Bosch units and see what happens but I don't think this is your problem.

I went all the way with my 81 MHR and replaced the ignition system with Sachse ignition, Dynatek cables and coils, NGK caps and plugs and a Shendingen regulator to protect it all. New battery too. I also had to repair my alternator wiring at regulator end; the yellow wires are prone to overheating and getting brittle.

Good luck to you. Please report back and if you want to discuss my ignition mods further, just ask.

Rick
Ah, some good stuff there, Rick, thank you! I'm pretty sure it isn't fuel-related because I stripped the fuel system and carbs, and all was in order. Thank you for the resistance data–very useful. In my many years with older bikes, I've usually found that it was something simple like a frayed or trapped wire, or a poor earth (ground) connection. However, I have to be prepared to check everything, including coils, ignition boxes and pick-ups.

Dukerdr – yes, indeed, the dreaded kill switch! Simple things first. Thank you. As with old British bikes, I will by-pass the kill and ignition switches, and I will check continuity of earth wires and if necesary, route them to a common earth point and thence to the battery.

When I can access the bike, I will report back. It's always good to have these experiences for the database.

Dave
 
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