8hp Advancing Timing

[mitch184]

Next Issue... 8MSH used as a kicker motor on a moderately heavy boat. Currently has dual thrust 9" diam, 7" pitch prop and will only turn 4000 rpm WOT. Power is so so, but I'm going to have the prop re-pitched down to a 5" or 5.5" to get WOT up closer to 4500. At idle, in gear, the motor also bogs a bit from the prop.

However, looking to see if I could gain any power by advancing the timing slightly. Full advance spec is 35° ± 1°. I was curious if advancing it to say 37-38° might give it just a touch more power and run smoother. That's a lot of timing, but I'm imagining the combustion chambers on these engines aren't the most efficient.

I already run 89 octane in the boat because I can get ethanol free, do I'm thinking it won't ping with a few extra degrees. I also know, in general, advancing the timing slightly can bump up the off idle performance. Since the motor is a kicker and only rarely spinning up to 4500, curious if it would help a bit.

Reducing pitch only helps to a degree because then you loose available thrust. Ideally i'd like this motor to push the boat around 8 mph. Currently 6 is all it will do. The reason for the speed, has to do with *****ing while fishing. I'm going to check it again with the timing light tonight and start adjusting, but just wanted an opinion on advancing beyond spec before I did.

Thanks

[boscoe99]

Timing is usually advanced to keep up with the engine RPM. Increasing the timing while the RPM stays low (below specifications actually) won't do much I suspect. If anything, it can make performance worse. It could harm the motor. I would not do it.

That motor does not make it rated HP until it reaches 5500 RPM. You need a different propeller. Reducing pitch that allows the motor to make its rated power can result in more thrust being made by the propeller than if the propeller is over propped and does not allow the motor to run in its optimum RPM range.

Yamaha makes a standard 8.5" diameter propeller with a 6.5" pitch. The reduced diameter with the reduction in pitch vis a vis your current propeller will allow the motor to develop more HP. More HP equals more thrust. I would try that in lieu of the dual thrust propeller. The dual thrust is giving up forward thrust so that it can be a tad more effective in backward thrust. Seems to me you are interested in forward thrust.

"Reducing pitch only helps to a degree because then you loose available thrust." - Many times a lesser pitched propeller being turned faster will create more thrust than will a higher pitched propeller being turned more slowly.

[mitch184]

Turns out, the timing was down closer to 32 because the linkage wasn't adjusted right. Adjusted the linkage and the timing went up to around 34 and the motor instantly ran better. Adjusted the stops to squeek out a hair more to about 36. Had to adjust linkage again.

Overall, 36 degrees. Hard to lower the idle adjustment screw and the motor picked up 200 rpm's on the top end. No pinging and smoother in the middle. I'll probably still have the prop pitched down to 5" or 5.5", but the timing did seem to help.

Also, I didn't realize the Dual Thrust props gave up up forward thrust? They aren't advertised that way and usually the low rake is what makes them "high" or "dual" thrust. Good to know though