maxi used to rip, now its just soso

Thompsons Mack /

I have a 1979 maxi II, and I installed a 70cc kstar kit with a hihi head on it, it has a proma gp pipe, and a mikuni vm20 carb. it was running like a beast when i first got it installed,i didnt gps it but i imagine it was topping out around 50mph. ran solid for a week, i tried to go easy on it breaking it in, and i did for the most part... ok but then i was riding and it sounded like it backfired and then it lost power. let it cool off overnight and had a hell of a time starting her the next morning, took a lot of pedaling and then it was running a lil rough and not nearly as fast - still faster than stock though. i took the topend apart and it didnt seize or anything. any ideas? i'm trying to figure this shit out on my own but its hard to troubleshoot. someone suggested the hihi head may create too much compression? is it at all possible the throttle cable has anything to do with it? THANK YOU KINDLY to any and all who give me some advice. much obliged, keep rippin.

Re: maxi used to rip, now its just soso

Austin Shores /


Re: maxi used to rip, now its just soso

your timing might be off. is your stator nice and tight?

Re: maxi used to rip, now its just soso

Probably Fred /

Backfire type sound could have a soft seize at the bottom of the piston and have low compression ( did you take off the cylinder and inspect the bottom of the piston?) with aluminum on the ring. I've seen that type of seize on those kstar kits.

Your timed correctly at 16-17 degrees, mm on the flywheel with a 16-20 thou point right?

If by chance you did hit 50mph on a za50 engine that is close to the rpm of wasting your crank bushing and clutches with stock gearing? Did you rebuild with needle crank or even check the bushing to see if in spec? You did upgear to at least 20x36 right?

You checked squish on head, prepped, chamfered ports on kit correct ?

Jetted correctly with dark plug?

Did you run a head temp gauge and keep it under 400 degrees head temp?

Did you do a break in at normal moped speeds for 300miles?

Me thinks you little or none of the above so now you have problems

Re: maxi used to rip, now its just soso

Kaj Niegmann /

Check your ring gap. Those two little overlap pieces break off sometimes and you won't notice it unless you look closely. Compression drops one of those pieces chipped off

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