A bent valve means a bent valve stem. They usually bend near the head and this is the result of the valve hitting the piston. Only happens if valve timing is incorrect (mechanical change required for this), valve sticks in guide and piston hits it or engine was over-revved to point of valve float. When this happens the valve no longer seats and you get a compression leak. Bend an intake valve and it will backfire out the intake. Bend an exhaust valve and it will backfire out the exhaust. It will not run very well either and that cylinder will be down on power if it makes any power at all. I would have expected the shop that installed the cam and lifters to have inspected both heads to make sure all was well. It is a simple task to remove the valves and blue check the seats as well as check for straight stems.
I have no experiece with FI Urals. But I've read here that the early maps were severely lean. That would make it backfire out the carb as well as hesitate on acceleration. I imagine this is what everyone is talking about when they ask you which FI map you are running.
If you have a gasoline smell, and I mean raw gas not extremely rich mixture, then that cylinder is not firing or not firing all the time. That would make it backfire thru the exhaust because the exhaust loads up with fuel and then the hot exhaust charge from a firing stroke ignites it. Again, not familiar with the Ural FI system. But a question for those who are is, are the injectors known to leak? If so, you may be flooding that cylinder with a leaking injector. I may be wrong, but I would expect the fuel map to use the same map for both cyinders, so if one was improperly mapped, they both would be. Might be a bad assumption on my part. My point is, if that is the case and one cylinder runs fine, then the problem is downstream of the FI controller or in another system, like igntion.
I think it is time to get back to basics. You need to go thru all the systems that are required for the engine to run properly and make sure they are correct. Van's suggestion to run a compression test is part of that. The shop that did your rebuild can do one for you, or you can probably buy the gauge and do it yourself. A compression test is done by removing the spark plugs, grounding the plug wires, installing a compression test gauge in one spark plug hole and cranking the engine over with the starter while holding the throttle open. Record the highest reading. Then do the same on the other cylinder. They should both be over 150 psi and I would think 175 psi would be expected on a new engine. Others can comment on that point. Both cylinders should measure very near the same. If one is low, then you have a problem. Could be tight valve(s) clearance, bent valve, leaking head gasket, broken compression ring, seriously worn cylinder/rings (unlikely in new engine), poorly seated rings, cracked head, holed piston, and that's about it.
As you check out your systems, check everything. Make sure plug wires are well connected to the coil and to the plug cap. Check all ignition electrical connections. With everything confirmed in good condition, make sure it is adjusted properly and then functioning properly.
I've never done any FI troubleshooting, so can only guess at procedures. But if I were stuck far from a dealer and wanted to check things, I'd probably remove the injectors and see if either were leaking. I'd crank the engine and see how well they spray and if they spray equally well. I'd also look for leaky connections or restricted lines to the injectors if this is possible. Again, I'm thinking generally here since I don't know the Ural FI specifics. A question worth asking the FI experts here is, does the Ural FI have a rich, cold starting system? If so, is it possible for it to malfunction and make just one cylinder overly rich?
Proud member of the Peanut Gallery
2000 Ural Tourist
40 Pilots, 122 Mains
Before you say something stupid, always ask yourself, "What would Harpo say?".