My car has a low rpm stumble and backfire. I am going to reset the valves this weekend. Is it possible that the valves can cause the stumble/backfire on low rpms only? The car SEEMS fine in the upper rpms, without any stumble/backfire. The only other culprits I can imagine are the MAF, bad injector(s), or a vacum leak. The car has two day old O2's, plugs (NGK tr55), new wires (delcos), opti, and coil. The car also has hard starts, but only when it is hot, valves? Please help this car is p***ing me off. Thanks.

 

It is more likely that incorrect preload would have symptoms you would feel at any rpm. Usually more noticeable at low rpms.

 

Would the incorrect preload cause the hard starts? Thanks.