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Hi folks - If someone can steer me toward clarification of my problem I'd appreciate it.
The bike - 1965 Mk3. No battery, and my charging/ignition system appear to agree with a schematic I found here in the technical info section - 40 watt AC 3-wire alternator, no regulator, no fuses of any kind, if the brake light fails the ignition quits, etc.

Also in the tech section a tech bulletin claims that after engine # 100276, all Mk3s were fitted with the Monza's 60 watt stator, flywheel and regulator. My engine number is 100458. But again, mine has no regulator.

The symptoms: After fitting a replacement carb I have been trying to sort out jetting (I live at 8000' elevation). No matter what main jet I use, as the revs climb with full or near full throttle the engine begins to miss and stutter. Rolling back on the throttle stops the miss. I kept dropping the main jet size until the plug began to look right, but the stuttering kept occurring under the same circumstances.
The bike was starting in one kick and idling fine.

Yesterday, returning from a test run, when I pulled in the clutch the engine quit. I could not restart it, and I'm not finding a spark when I lay the plug on the head and kick through. To isolate the issue, I removed the kill switch from the circuitry. No joy. Tested continuity across poles on the coil: 4.5 ohms, and something like 11K to the high tension side.
Then I checked continuity of each of the three wires from the stator (to ground). As I understand it, the wire to the lighting circuit should read 0 or near zero ohms, and both the brake and ignition wires should show open to ground but grounded when connected to each other.
I found the lighting circuit wire to read 2.4 ohms (good), the brake wire showing open, but the ignition wire read 388 ohms. I checked again: 656. Again: 10K ohms. My initial diagnosis is that there is an intermittent/partial short in that wire (or coil). Is my thinking correct? Sometimes amperes and their absence confound me.

Also, could I have the later 60 watt alternator without the regulator or should I assume I have the earlier 40 watt unit?

Sorry for the lengthy post, but with electrical diagnosis, the devil's in the details.
Thanks for any insights offered.
Bob Herman

Hi Bob,

collapse of the internal cables due to age, heat cycles, etc?

This is what i am afraid of...

I would use an analog ac voltmeter to test the voltage coming out of the stator. Those wires get brittle with age and become disconnected or grounded. Everything else you have looks OK. Its a simple system. AC to coil to spark plug. Also check the points. Make sure they are opening and closing.

The 60 watt system is a DC system and you would have a battery or capacitor, DC coil and DC points gear. You can also (for a short time only) use a battery to see if it will fire again.

Roland - that's my suspicious as well. I recall that when I first got the bike, five or six years ago, I had to patch in new wires from the stator due to crumbling insulation. I don't remember whether I replaced them entirely from the stator or not. That's where I will look now.
Mrkprsn - thanks for clarifying about the 60 watt DC system. Clearly I do not have that.
I have checked and verified condition of the points. Don't know about the condenser.
Bob

You might be chasing 2 separate problems here.

The high-speed stutter sounds similar to a 250 Rudge that I bought. I could start it and ride it around but regardless of carb adjustments and fiddling with advance/retard it would misfire when the throttle was wide open and could only manage about 40mph.

In the end I concluded it wasn't getting enough fuel and after eliminating everything else, I looked at the flexible hose from the tap to the carb.
When it had been assembled, the fitting had sliced the lining of the hose making a flap of rubber which partially blocked it.
New hose made - 60mph! I have a photo somewhere of the hose - will try and find if you're interested.

Ian P

Thanks for that, Ian. I wondered about fuel delivery too, and the hose from petcock to carb makes a pretty tight turn.
But when I opened the tab with the carb bowl removed, fuel poured freely through the float valve.

A bit of an old 'chestnut' is the petrol tank cap vent hole clear??

Romomoto wrote:Thanks for that, Ian. I wondered about fuel delivery too, and the hose from petcock to carb makes a pretty tight turn.
But when I opened the tab with the carb bowl removed, fuel poured freely through the float valve.

It's hard to explain until I find the photo, but I also had plenty of apparent fuel flow when the float bowl top was removed and the carb was filling up, but the rubber flap seemed to shut off more as the flow rate increased. When riding fast I think the demand exceeded the supply and the level in the bowl dropped.

Yeah, I'm aware of that old chestnut indeed. Worth verifying that it's not happening in this case. I've also experienced problems in the past with inline filters not flowing enough fuel under serious load conditions, but in this case I don't have any inline filters. I should chase down the spec for float height while I'm at it. This is a new, previously untested Keihin carb and if the fuel level is too low...
I've fitted a different piece of hose from the petcock, one that is stiffer and can handle that tight turn better than the flimsy hose that was on there.

Today I pulled the clutch and flywheel, and found no apparent damage to wiring or coils. And got appropriate continuity readings from the leads. I wonder if my digital multimeter was misbehaving yesterday.
As Mrkpersn suggested, I should have switched to an analog meter and retested. In any case, it now appears that the wires and connections are solid. And I have the added benefit of having had a chance to confirm that the flywheel magnets are still strong.

You know what they say: most carb problems are ignition. Most ignition problems are carburation...
Tomorrow it goes back together.

some detailed photos of stator and wiring, please?
Did you already replace some things inside?