I had my points and condenser replaced a month ago and the bike started fine and rode ok. I had an episode with the old gas tank liner and had to remove it, sealed it with Red Kote and had the tank repainted....so after the 30 days for the paint to cure I put the tank back on, and now it has a mysterious backfire. It idles fine but anything above 3 grand or so induces a backfire that seems to come in "pulses", evenly, about every 3 seconds. Revving it is an adventure with bigger but evenly spaced backfires. I tried riding it down the street and it will rev to about 3500 but it is backfiring evenly all the time except at idle. I have cleaned the carb, tried 3 different plugs, replaced the plug cap. No change. I am not a talented mechanic. Is this the result of a bad coil or something other than points/timing? Any thoughts would be appreciated. Thanks, Tim in Florida

Not sure what bike we're talking but look at the points for gap & feel the main bearings for play [lots of up & down] but see if the points are way over the desired gap & give then a clean. Sorry i see they're relatively new. Check there connections & mount screw tightness. Look at them while running & while its playing up if it'll do it stationary & see if they spark extra bright at every misfire. I dunno i'm totally guessing & might be off the planet. Is it an electric start DT125?. If a magneto model bike does the miss change with lights on or off. Good luck.

Sorry I forgot to say it is a 73 DT3. Lights, horn all work and there is no change when turning them o/off. The strange thing is the consistency of the backfire and it isn't real loud. It has good compression when kicking it over but it doesn't kick back when I am starting it. I was just going to drop it by the mechanic that replaced the points/condenser but he is in the hospital so I thought I would try to get some observations on what to look for since you guys know just about everything. Thanks for the help!

Very strange... I've had bad condensers cause a backfire when revved... or timing way retarded will do it too sometimes. (I think what happens is the mixture is still burning when transfer ports open?) Like accidentally setting timing ATDC instead of BTDC. But it being regular is kind of strange.

If bike was fine before, I'd definitely check the stuff that was just done (timing, points and condenser)

Good audio on a video would be great as have us wondering what's going on with it. Seeing its a DT3 i don't suppose its broken the flywheel key which will put the timing out at best & not run at all at worst but could do strange things if its only moved a small amount. Remove the flywheel if you have a puller & do it up tight when you re-fit it. Crank key is a common problem although seems to be more on the 360 but maybe there's just more of them.