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I'd like to convert over my 350 small block to a propane carb setup but I am unsure if there needs to be any modification to the motor itself. When converting over to a propane solution do you need to make any changes to 350 crate motor to make sure it runs correctly? If a motor originally had an octane 87 fuel delivered through the carb is a motor rebuild in order or can you simply just replace the carb?

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    You'd probably have to mess with ignition timing but not much else. – Ben May 19 '16 at 21:51
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    Typically conversions to propane require you to reduce base ignition timing 2-3 degrees. – Moab May 20 '16 at 1:08
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I think the answer to your question depends on the current state of your engine. Are you running a really aggressive cam? Or is it a mild engine. Typically, if you're converting a fairly stock engine to propane you shouldn't have to mess with camshafts or other internals. However, you will probably wan't to adjust ignition timing. Here is a cool article about a propane conversion. They even play with ignition timing and run the vehicle on a dyno and share their findings.

http://www.fourwheeler.com/how-to/engine/131-0803-dual-fuel-propane-system-install/

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Propane burns quicker across the top of the cylinder. Therefore spark timing as well as valve timing should be addressed. Ignition timing should be close to TDC, if not TDC or better at idle . Intake valves need to be completely closed when it fires. So you'll need a different cam. Spark advance requirements will be greatly reduced.

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