Slidin Gator wrote:Flounder,
I was in the same boat as you a couple years ago, with my first AV motor. So I have been through this (and other) learning curves. I am attaching some documents that were useful to me. The timing box is useful to check the timing by yourself, otherwise you need 2 feeler gauges and a helper to move the prop.
Before doing anything on the motor, I still recommend finding a tank of 100 LL to run through the motor. If that solves the problem you will know you are on the right track here. In any case it will give you a comparison point to consider the change in performance once you get the motor tuned for pump gas.
Lycoming used 7.2 and 8.5 compression ratio pistons in the straight valve engines. Angle valve engines had 8.5 or 8.7. Higher compression ratio after market pistons are available. Lycoming rates the 7.2 for use on 80-87 octane fuel. The 8.5 is rated for 91-96 Octane and the higher compression ratio’s require 100 LL fuel. Keep in mind that Automotive fuel burns faster than Aviation fuel, backing off the timing from 25 to 20 roughly compensates for the quicker burn of auto fuel. Since Airboat engines in the South tend to run hot, I would err on the safe side of these ratings.
If the engine still has issues on 100 LL fuel, you need to check the timing and confirm that both mags are firing at the same time. If the mag timing is staggered the engine would have issues such as you describe. The mags fire when the points open. You can check this with a set of thin feeler gauges (or a business card etc.) under each point. Have someone turn the prop slowly (pull plugs to release compression first). When the points open the feeler gauges will pull free. The Timing box information attached is for a unit you can buy from aviation supply houses for about $50. With this box you can check and adjust the timing by yourself.
I suspect that the FWC boats are running low compression 540 engines to run on pump fuel. Perhaps this is why the FWC officer that gave me a safety inspection last month was stuck in his new Diamondback afterwards. He had to make the trainee get off and wiggle the grass rake to get going on dry ground.
Look at this link for details on timing, as well as Youtube for video's:
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