rough running and backfire after rebuild

[MOJeff]

Hello,

I just rebuilt my CV492S (18HP, single cyl OHV, about 750 hours, installed in Cub LT1018). I had complete valve job and cylinder hone by a local auto machine shop. New battery, starter, and plug. Fuel pump and carb replaced last year or two so I'm assuming they are still good. Non-adjustable valve lash. New std size rings. New breather tube. All new fuel hose and filter. No other major parts replaced that I can think of. Used fresh 91 octane fuel with stabilizer after rebuild - same fuel source used in push mower with no issues.

The engine ran great for about 30 minutes after initial checks and carb adjustments: about 10 minutes under no/low load with varying throttle, about 20 minutes under load, then began running very rough, no power, loss of rpm, and lots of backfire (not every stroke).

No oil leaks, no oil coming out of breather.

Still cranks well with plug unplugged. I have removed the valve cover and can see valves lifting as I turn the engine by hand, but no further disassembly yet.

Any ideas of what to check? Would like to know what to look for before further disassembly.

Thanks a whole bunch for the help.

 

[Rivets]

If by chance the flywheel was not torqued properly, you could have a sheared key. That is the first thing I would check.

 

[MOJeff]

Thanks for the feedback - some delay in getting the flywheel bolt off to inspect. Will post what I find in a few days.

 

[MOJeff]

Just examined the flywheel - it is in position with key intact. Any ideas what next?

 

[ILENGINE]

Can you get us a compression reading.

 

[MOJeff]

This engine has an automatic compression release while cranking so compression test won't help. I can feel resistance while turning the engine by hand, but I don't have a leak down test tool. Will try to rent one and share results.

 

[MOJeff]

I jury-rigged a leakdown rig with a compression tester and my air compressor. Had the cylinder up to 100# (at TDC after compression stroke), throttle open, and did not hear anything leaking. I don't have the parts to do a long-term pressure leakage test to see if the pressure drops over a long period of time. Would that be worth the trouble? If not, what else could I test/replace? Could this be a lifter problem?

 

[MOJeff]

Quick update - I asked the local vocational h.s. small engine repair instructor for suggestions. He told me to check the carb. When I pulled the bowl off, the gas was a very deep orange rust color - apparently the brand new fuel filter was breaking down in the gasoline. I sprayed out the carb with carb cleaner, flushed the fuel line, and dumped a bunch of Seafoam in the gas tank. Was able to run the engine for about 20 minutes last night and it sounded good. Unfortunately I need to re-RTV the valve cover (oil leak) so can't run for an extended time until I fix that.

 

[MOJeff]

Forgot to mention - I did get a new and different brand of fuel filter.

 

[MOJeff]

Just to update everyone - engine is running fine so I conclude it was a bad fuel filter.