Mag timing for 87 octane

[lewisdj1971]

I’m having trouble getting my 0360 to turn up to 2800-3000 rpm. I’m turning a 66/36 wood stick and tested my prop on my dads 0360 and his motor turned my prop 2850 and was very smooth through rpm range. My engine seems a little rougher and tops out at 2500 rpms. The guy that had the boat before me always used 93 octane CM fuel and added about 2 gallons of AV fuel per tank (20 gals). Since I’ve had the boat I’ve always just ran 93 but I’ve never been able to get above 2500 rpms. My question is should I check timing on mags and adjust for 87-93 octane fuel or does it make that much difference?? My dads new engine runs perfect on 87 octane but I’m not sure where his timing is set.

[Slidin Gator]

Do compression and leak down tests, sounds like you might have a cylinder gone bad. Also run a mag drop test to confirm both mags are kicking properly.

22 degrees BTDC would be the max timing I would run on CM fuel, but I run AV fuel. 87 octane is just asking for trouble, run premium at least.

[lewisdj1971]

I replaced rings on all 4 cylinders about 30 hours ago on engine hours. It was burning oil bad before ring replacement. Rings improved oil burn tremendously but no change in top rpms. Compression and leak down tests are normal and mag drop is about 100 on each mag. I’ve been running 93 octane since I’ve had the boat

[lewisdj1971]

It seems like motor is just not running as smoothly as it should and seems to stumble more at top end of rpm

[Slidin Gator]

It seems like motor is just not running as smoothly as it should and seems to stumble more at top end of rpm

Stumble or pre-ignition? Stumble is no ignition on a cylinder, it's a miss. Pre-ignition will be a popping sound from the exhaust. Either will result in loss of power at top end, pre-ignition will blow the motor up in short order.

Does it run good when cold but then act up after it heats up?

[GMAC 76]

if the problem is when the engine is hot or at wot it may be a coil issue

[lewisdj1971]

It seems to run smoother when cold but still won’t make above 2500 cold or hot

[lewisdj1971]

Also seems more like stumble than pre ignition, no popping through rpm range, sometimes if I let off gas quickly I’ll get some popping out of exhaust

[GMAC 76]

what carb are you running.....10-???? on the id tag

[Gary S]

While you had cylinders off for the rings, did you happen to look at your cam shaft and check it for wear?

[lewisdj1971]

I’m gonna check out carb and may swap out with my dads to test. I didn’t check cam shaft wear during ring replacement but thinking of pulling a cylinder to see what it is. Also gonna check valve clearances

[Gary S]

Once a little rust starts and a pit forms cam wear goes pretty quick. Lycomings are more prone to do it too.

[lewisdj1971]

Ok found an issue and wanted to pass this along. I took a hard look at my #1 cylinder and noticed that the spark plugs were not discolored like the other cylinder plugs looked. Like they not ever had any heat cycles on them. I checked too and bottom plugs on #1 cylinder and they were both firing. Next I checked compression and it matched other cylinders around 90 on first hit and 130 on second hit. Then I decided to check valve clearances. That’s when I found that the intake rocker to valve clearances was way too much and the rocker and push rod tube would not build or hold any hydraulic pressure. I pulled the rocker, push rod tube and then the push rod cup and tappet and found that the spring on the hydraulic tappet for #1 intake valve was broken and the tappet was stuck in the closed position and the cup was hung up on the broken spring. I’m getting another tappet and then I’ll check valve clearances again. My question is could this have caused the cylinder to basically just be limping along and not making any power? It’s obvious by plug cooler that the spark plugs haven’t been through any heat cycles even though they were firing. Need some input please.

[lewisdj1971]

The cam was allowing the valve to open but with all the slop (or slack) with the hydraulic lifter stuck closed it seems as though the intake valve was not opening soon enough or for a long enough duration to pull in the correct amount of combustible gas to make power.

[Gary S]

Which can also lean out next cylinder, the fuel flow thru the manifold is slowed down because of the weak cylinder. Number 4 could have been running lean.

[lewisdj1971]

I replaced the broken hydraulic lifter and motor is back to normal now. Much smoother and much better power. Turning 2750 on the trailer.