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Engine keeps cutting out

Technical help for all early models, A, B, C, LC, D, LD, E & F models

Engine keeps cutting out

Postby AAVlover » Sat Jun 30, 2018 1:55 pm

The symptoms are similar to those of pg42, restored LD 150 AVV, runs for 3 miles, then cuts out. After around 40 minutes, it struggles to start and is then ok for another few miles. When it cuts out, the cylinder head and plug are very hot, but there is still a spark, fuel runs ok, carb has been stripped and cleaned. Coil and condenser are original, timing is 26 BTDC, plug is B6HS set correctly, points are new and set correctly. The coil tests ok for resistance when cold. Auto advance mechanism is OK, as judged by the timing strobe test and fuel filler cap hole is clear. Plug lead is new as is suppressor cap.

I have since taken the scooter out with another plug and the same thing occurred. I then changed the plug again and it didn't restart so I can discount the plug as a problem. This time, instead of cutting out entirely, as the engine fades, I could keep it spluttering for a few seconds by pumping the throttle, and then it dies completely. The Ht coil is hot, you can touch it, but it is hot. Depending on any responses, I may try substituting the condenser?
AAVlover
 
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Re: Engine keeps cutting out

Postby Steve J » Sun Jul 01, 2018 11:09 am

It's worth trying a new condenser, but just swap one item at a time. It's also possible that there's an intermittent fault within the HT coil, which only shows up when it gets hot - such as insulation breakdown within the coil, or a break in the HT winding within the coil. Check also that the brass contact breaker pivot pin is in good condition, as these can wear on one side. I once had an issue where the engine would run OK until it got hot, when the power would drop off - this was due to grease from the mag side bearing finding its way onto the contacts.

One other thing - the 12 Volt AVV with Filso ignition uses a different cam on the flywheel, which keeps the contacts open for most of the flywheel's rotation, in contrast to the cam used on the usual 6 Volt LDs, which keeps the contacts closed for most of the time. The idea is that because current from the battery only flows through the coil on the AVV when the contacts are closed, keeping them open for longer reduces the average current used by the ignition, reducing the power dissipated within the coil. If by any chance yours has the wrong cam, the ignition coil will overheat. It's also worth checking that yours has the correct 12 Volt ignition coil - those I've seen on the AVV are Bosch coils, with TJ/12 stamped underneath. (Even with the correct cam in the flywheel, a 6 Volt coil will overheat!)
Steve J
 
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Re: Engine keeps cutting out

Postby AAVlover » Mon Jul 02, 2018 11:04 am

thanks Steve - interesting. The flywheel is ok, and I've noticed the longer time the points stay open, as it is easier to check them, but did not know the reason why. I'll check the coil but as the bike is all original with only 6k miles, I'm assuming it is original too. I'll swap the condenser and coil, one at a time.
AAVlover
 
Posts: 35
Joined: Tue Aug 30, 2016 6:51 pm

Re: Engine keeps cutting out

Postby Rob Miller » Fri Jul 06, 2018 9:56 pm

Just another possibility. Try checking the fuel flow rate through the tap. They never flow fast but a partially blocked tap might cause fuel starvation.
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