Atc 70 backfires

[Dgonzo]

So I am having trouble getting my 70 to run correctly. Here is what has been done to it.
- Piston and ring replaced same as stock
- Spangler atc 70 stock replacement carb installed
- points have been adjusted to 0.35
- ignition timing is correct
- Carb has been cleaned a handful of times but nothing is ever dirty.
- At first bike would only run on choke but that has been fixed, I have a aftermarket k&n filter on it. I’m not sure what else to look at as of right now. Bike will idle fine but when you go to give it gas it boggs out. I have been told it is the jets but I’ve never went down that road. Somebody please help!!!!!!!!!

[Dave8338]

Sounds like it is running lean on the main jet, likely a result of the air filter change to a high-flow K&N. Have you oiled the filter and blotted off any excess oil? I assume it is backfiring through the exhaust? If not and it is through the intake, it is typically an ignition/timing issue. Plug gapped correctly? I'll admit to not being an ATC 70 expert however, all four-strokes run correctly with the same issues addressed.

[shortline10]

Trying to make a stock carburetor run with an open air filter will make the exact symptoms your describing .
Can try a larger main jet or buy a factory air box set up .

Their are at least 50 threads on this same atc70 topic .

[shortline10]

Try choking the air filter with a rag and see if you can get it to rev up , if yes you’ll need a larger main jet .