1977 Puch Maxi Sport runs rough, second backfires

Just to clarify this is for TWO separate mopeds.

I bought both off craigslist. One needs a new read tire, no big deal, and sputters almost a backfire but not quite. Not sure where to start on diagnosing it, it also smokes a little bit more than the second one which runs smoother. Acceleration is fine, consistent power and rate of acceleration. The noise is less noticable at higher speed but I think its just masked by the louder engine noise.

The second is in excellent shape and runs great when starting cold. After it gets up to full operating temp (about a mile) it starts choking and losing power. I didn't notice the issue during the test drive because it never got up to full temp. The problem was so bad I thought I was going to be peddaling home, stalling if I had to stop and almost completely losing power. The fuel switch (for on, off, reserve) seems loose and probably needs to be replaced. It has plenty of gas, I filled it up with premium 50:1 mix on the previous owner recomendation. Not sure where to start since it could be spark plug, carb, fuel system etc. Hoping I can answer some questions to narrow it down and that its a common enough issue to not have to just start with whats cheapest.

I'm pretty good with mechanics stuff. Any help greatly appreciated.

Re: 1977 Puch Maxi Sport runs rough, second backfires

#1sputter may just be rich due to clogged airfilter or exhaust. the excess smoke supports that idea.

#2 sounds like poor fuel flow. how long until it starts working again?

what i'm picturing is a fairly common scenario where the fuel drains out of the carb during use faster than it's filling back up due to restricted fuel flow either in the lines, petcock, or the carb float or needle internally. as a result it runs after sitting because the bowl has had time to fill, but after running it dries out the carb and stops working until it can refill enough.

the other possibilities could be an airleak somewhere or if you check for spark when it dies and it looses spark when hot, that indicates the condenser is bad.

Re: 1977 Puch Maxi Sport runs rough, second backfires

Ill pull the airfilter and check. I know its more spoon feeding but is there a filter replacement thats readily available at a hardware store? I can test it by just removing the current filter and seeing if it runs smooth without it.

On the second unit Im working on replacing the condenser first. I plan on cleaning out the carbs on both and there is a kick ass vid on youtube to walk me through it. Brakes will be next...

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