bike wont start

are you saying you took the heads off AFTER the problem to have the work done
or that was the work done prior to the problem of the backfire and no start.

So that pretty much rules the heads out as the cause,

You need three things to get an engine to run

1) fuel
2) spark
3) Compression

Test the compression on the cylinders, you can get an inexpensive tester at any auto parts store, you are not building NASCAR engines so you don't need one from Snap On. Be sure you are getting good compression on both cylinders.

Next, fuel, are you getting fuel, check the carb, float, take off the filter and twist the throttle, is it squirting fuel into the carb

Finally Spark, are you getting spark at the plugs,

my guess would be short of some internal problems
that you have a bad coil, or ignition module sounds like timing is off.

That sounds low. Did you have the throttle open fully when you did test?

Air filter on or off... is the carb opening 100% with the filter off make sure it's going to full open do you have a cv carb, make sure the slide is coming up all the way, so you can get all the air in when checking.

Just off the top of my head it sounds like it's got low compression figures
I would think they would be closer 150 to 160 and no more than 5 to 7 lbs off from each other. But I don't know sporty comp numbers, maybe someone will chime in that has the specs.

Low comp results from valves not closing tight or busted rings not sealing,
gasket leak etc anything that won't let it build pressure.

Does it have adjustable pushrods or did you put the stock rods back


The timing isn't going to change compression,

Have you checked fuel and spark yet.

Yes you do a leak down test, the only way to do it right is to remove the pushrods, this closes the valves otherwise you have to turn it over and be dead sure you are at TDC meaning the valves are closed completely.

Remove the plugs from the engine

With an air compressor fully charged and the trans in first gear with someone holding the rear brake and using an adapter on the air line that will allow you to put air into the cylinders, one at a time.

When you pressurize the cylinder listen for leaks out of the intake or exhaust then you have a leaking valve, if you hear it going down into the crank case you have ring issues.