1985 Marathon carb upgrade?

hillcountrymac

Hey,I have an 85 Robin 2 stroke 244cc 2pg.I've rebuilt the top end.It runs great I mix the oil 128/1,premium gas.I just can't get rid of the backfire.I have read every post I can find on the subject.Taken apart and cleaned the carb,tried different spring lengths and tensions on the throttle cable.I have tweaked the micro switch,played with every carb setting, nothing works.I live in the Tex. hill country.I drive it primarily on the street up and down hills.Everything else is great. It doesn't smoke, pulls well,overall runs fantastic other than the backfire issue. I can get it to almost disappear and then it will raise it's ugly head.I guess I will order a new carb it must be just worn out.
Couple of questions,
1.does anyone have any other suggestions?
2.if I order carb, should I upgrade to larger carb and if so which one?
3.where is best place to order?
Thanks in advance,Mac

BGW

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popatop

racin161

Hill have you checked your exh gaskets ? any leak will cause it to backfire. IE crack in pipe, loose bolts,or crack in the exh system. I really don't think the problem is the carb as you say it doeseeeeeee runs good and replacing it is not going to solve your problem. It can be trying to get everything in the exact order or the sweet spot, shout out and let us know.

hillcountrymac

Thanks,I have checked and it does close before the micro switch.I have not checked the exhaust,I will do that and hope that's it.Thank you both,I knew the forum would come through!
I'll let ya'll know.

popatop

hillcountrymac

Popatop I think you mean the screw with the spring?I have tried every setting there is.Factory(one turn past closed).Open a little, open a lot....I now have it set where it closes as much as possible(i.e. not open at all) hell it doesn't need to idle I just want it to shut.This seems to work best.I think maybe the carb is just worn out.It is 27 years old.It is frustrating because I can get it so close to right and then it will backfire. Where I really can't get rid of it is this.When going downhill and then I slow to almost a stop,get on the gas a little and boom backfire. Go to turn around in street slow get on and off the gas a little and backfire.If I floor it it will not backfire.I can drive it without the backfire by controlling how I gas it,but I don't want to do this cause I know it isn't right.If I let someone else drive, I want them to just drive, not have to instruct them how to not make it backfire.I GOT TO GET IT RIGHT>I guess new carb is next.
Thanks,Mac