Maxi Has Me Stumped...

Got my maxi tuned and running great. While I was out riding, I heard a sound similar to “backfire” followed by a power loss. Now, I can’t get it to idle and it only tops out around 20mph. Before this I have a clean chocolate brown plug and a smooth idle. Here’s what I’ve done troubleshooting wise:

New plug

Cleaned carb

Adjusted needle (and then back)

Reseated the plug wire

Confirmed correct timing

Is this a bad coil or something else?

Thank you in advance!

Re: Maxi Has Me Stumped...

Backfiring is usually from an out of phase timing. So your ignition spark is happening when the exhaust port is open still. What did you do to confirm your timing? Did you remove the flywheel to inspect your woodruff key? Gap the points? Confirm that they are starting their opening at 1.8mm BTDC? That they are opening no more than 2mm?

Tuning is fun.... at least that is what I always tell myself.

Re: Maxi Has Me Stumped...

I used my Buzzetti tool to check the tiomg and, yes it’s firing at 1.8mm BTDC.


Re: Maxi Has Me Stumped...

chris eichhorn /

dont mean to make you sound stupid but did you check head is tight/gasket ok and make sure exhaust not loose.

Re: Maxi Has Me Stumped...

Probably Fred /

Possible Seized/piston/ring/cylinder damage from uncontrolled temperatures by not being monitored by a temperature gauge/advanced timing/ lean jetting/poor gas oil mix etc.

Re: Maxi Has Me Stumped...

Michael Branscombe /

were you letting off the throttle when it backfired?

Re: Maxi Has Me Stumped...

> Michael Branscombe Wrote:

> -------------------------------------------------------

> were you letting off the throttle when it backfired?

No, I was at WOT. Plug chop showed a light brown. I enriched the needle by 1 notch. The carb is a Bing 15mm clone, so I don’t know if that has anything to do with it.

Yes, exhaust is tight. Yes head gasket is tight.

It’s a stock 50cc cylinder and head. I tore down the carb and cleaned it again. It seems to run a tad better but still not the snap it had from a stand still. Guess I’ll just have to deal with it.

Re: Maxi Has Me Stumped...

sometimes the condenser acts like a grenade when it goes out. then you be wtf was that...then crappy running. new condenser time, and take a peek at your seal while u in there, oil the felt pad, and a dab of grease on the points arm, etc, etc.

Re: Maxi Has Me Stumped...

Pushrod Fifty /

2nd for condenser but did you measure the piston travel from TDC to determine the marks?

Re: Maxi Has Me Stumped...

Does it sound different?

I’ve blown a head gasket and it was exactly as you described it.

It will run. With obviously low compression, runs like crap.

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