Mag timing for 87 octane

Aircraft powered airboat discussion.
lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Mag timing for 87 octane

Postby lewisdj1971 » Tue Jul 02, 2019 10:05 am

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.
:proud: David

User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 636
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: Mag timing for 87 octane

Postby Slidin Gator » Tue Jul 02, 2019 10:41 am

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.
I grew up thinking I-10 was the Mason Dixon line.
1986 Airboat Engineering Inc., 14' Marsh Master. Refreshed narrow deck, SV O-540, 72” NGQ. A Bob Stossel original.

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Tue Jul 02, 2019 1:01 pm

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
:proud: David

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Tue Jul 02, 2019 1:05 pm

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

User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 636
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: Mag timing for 87 octane

Postby Slidin Gator » Tue Jul 02, 2019 8:59 pm

lewisdj1971 wrote: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?
I grew up thinking I-10 was the Mason Dixon line.
1986 Airboat Engineering Inc., 14' Marsh Master. Refreshed narrow deck, SV O-540, 72” NGQ. A Bob Stossel original.

GMAC 76
Site Supporter - V
Site Supporter - V
Posts: 1227
Joined: Sun Jan 04, 2009 11:12 pm
Location: LAKE PLACID FL.... LAKE ISTOKPOGA

Re: Mag timing for 87 octane

Postby GMAC 76 » Tue Jul 02, 2019 9:16 pm

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

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Tue Jul 02, 2019 9:38 pm

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

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Tue Jul 02, 2019 9:41 pm

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
:proud: David

GMAC 76
Site Supporter - V
Site Supporter - V
Posts: 1227
Joined: Sun Jan 04, 2009 11:12 pm
Location: LAKE PLACID FL.... LAKE ISTOKPOGA

Re: Mag timing for 87 octane

Postby GMAC 76 » Tue Jul 02, 2019 10:43 pm

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

Gary S
Site Supporter - I
Site Supporter - I
Posts: 1591
Joined: Wed Mar 03, 2010 3:30 pm
Location: Redlands

Re: Mag timing for 87 octane

Postby Gary S » Wed Jul 03, 2019 7:17 am

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

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Thu Jul 04, 2019 7:31 am

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
:proud: David

Gary S
Site Supporter - I
Site Supporter - I
Posts: 1591
Joined: Wed Mar 03, 2010 3:30 pm
Location: Redlands

Re: Mag timing for 87 octane

Postby Gary S » Fri Jul 05, 2019 6:37 am

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

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Wed Jul 10, 2019 8:15 am

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.
:proud: David

lewisdj1971
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Wed Jul 10, 2019 8:40 am

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.
:proud: David

Gary S
Site Supporter - I
Site Supporter - I
Posts: 1591
Joined: Wed Mar 03, 2010 3:30 pm
Location: Redlands

Re: Mag timing for 87 octane

Postby Gary S » Thu Jul 11, 2019 7:04 am

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
Southern Airboat Member
Posts: 24
Joined: Wed Jun 05, 2019 11:54 am
Location: French Settlement Louisiana

Re: Mag timing for 87 octane

Postby lewisdj1971 » Sun Jul 14, 2019 8:38 am

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


Return to “Aircraft Power Only”

Who is online

Users browsing this forum: Cm3gator and 22 guests