Ok straight into it.

engine: max power .21 XRS on-road

Bought a rebuild kit and changed bearings couple months ago. engine ran fine no problems.

All of a sudden the engine just starts to back fire and won't clear when reving. We did some tuning and got it running yesterday for about 5mins, came in for gas and the thing just started back firing when exiting pit and eventually shuts off. Went to restart it and the whole tune changed again, just back fires and it revs like it's rich or the engine is loading up.

Tried these things already:

- cleaned carb out properly and needles etc.
- changed carb, same problem.
- pulled up engine and washed clean oiled back.
- NEW conrod ( this was installed after the problem started, problem still continued after new conrod installed )
- tried 3 new #7 plugs including one #8 ( novarossi's )
- don't think it's fuel tank, because my picco engine works fine.
- Tried 3 different pipes 2x 2015, 1x 2013 sirio. ( just to make sure the pipes were not blocked )


What could be the problem? the crankcase and crankshaft is pretty old. from 2007. But crankshaft is NOT worn, and i rebuilt it with sleeve, conrod , piston, bearings. Not even 3 gals through it yet since rebuild.

Any ideas anyone?

You might check that the sleeve is seated on the alignment pin correctly. Also make sure that you have the proper internals for your block. Some internals will interchange with some blocks, and run good. Sometimes the wrong internals will make the engine run poorly, and backfire. Also check the sleeve for cracks. I have also had glowplugs cause this when the tip of them got loose.

Right on!

After inspecting the sleeve, it does have a slight crack on one of the exhaust ports. like a slight chip off. I compared with with an old sleeve that came with the engine originally before i rebuilt it.

What causes that to happen?