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Condenser tester

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After a second ignition incident yesterday I set about trying to test if condensers are good or not.

The first time was shortly after I'd got the bike (US RD350B) on the road after restoration. During a trip the bike started misfiring and backfiring. I managed to limp home that time. As I hadn't replaced any of the ignition components I though it was time for some attention in this area. I bought a NOS set with both points and a condenser. All was well after this.

Yesterday on the way to Mum's (10 miles ) it started misfiring then backfiring on one side. Luckily I keep my classic car there so I went home to fetch timing gear, spare plugs and tools. Timing and points gap were fine. The spare plugs seemed to help so I limped home popping and farting all the way.

The only thing left to try was the condenser but how would I know if the old one I took off was any good, or if a new one would solve the problem?

I've had ignition problems with the classic car before (It now has an electronic dizzy) so I built a distributor tester. Its a bit heath robinson but I could use it to look at spark intensity using different condensers.

Initially I measured the capacitance of a few car condensers I had and the RD ones:

Car - 364, 216, 264, 202, 275, 196, 460nF

RD old - left 252, right 239nF

RD new - left 233, right 219nF

So that didn't indicate if any were bad.

Next I tried all on the distributor tester. The car condensers and the old RD one looked similar in spark intensity and waveform:

So at this point it didn't look like I was going to be able to tell a good condenser from a bad one but, finally I tested the new RD one. Right was the same as above but the left was much less intense a spark and the waveform was totally different:

After this I put the old condenser back on the bike. It now starts and runs fine. I just need to do a road test (and buy - and test - a new one to keep with the bike).

Excellent! What better way to test the capacitors.

Just testing the isolation with DMM does not put any high voltage on them.

I check the capacity with a dmm on capacity range, then the isolation with a DMM on ohms, but also warm them up with a gentle heat gun. I've seen them go leaky as they get warm.

Last leaky at hot was an AT3 one. Was on/off while riding, felt like someone kept pushing the brake pedal down, then releasing it. Started and idled fine.

I agree, heat the condensers to working temperature before measuring.

The farad value is not the only measurement with a condenser. It should not have any ohm reading as well as it needs to insulate from high voltage. You should be able to apply DC 500v without any leak current. Easiest is perhaps what is suggested in the inital post.

Are you including a 5k Ohm resistor in your HV circuit? Just wondering what that would change to the "ring" with and without.

No, no resistor  but this did get me thinking what the plug and coil actually were. The plug was a non-resistor type and the coil is a Lucas LA12 which might even be positive earth. Connections are labelled SW (for ignition switch) and CB (for contact breaker) no indication of polarity so that might be wrong.

Here is the with and without condenser video:

All this has got me thinking about going electronic again. I never understood why kits for these bikes are so expensive. The complete electronic dizzy I got for the car is still available for £69.95 and it doesn't need an external box.

They also do these:

For the price it might be worth buying one and experimenting. A bit more research first I think!


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