Tuning a Lean Running O-540 & 6AA / M5-4 Carbs

[Slidin Gator]

Here is your Airboat porn for tonight.

Got 2 new Narrow deck Chrome Cylinders installed with initial 25 ft-lbf torque. Will finish up torquing etc. tomorrow.







With the Narrow deck you have to deal with the hold down plates. Lycoming calls for 0.010 shims between the cylinder and hold down plate. This leaves some room for growth when it all heats up.



Manual calls for 4 feeler gauges, but I use 2.



Looks like one cylinder did not get the valve rocker pin bushings reamed, luckily I have a multi size reaming kit on hand.






Just got a new full set of gaskets for the motor in, gonna give these folks a try. They sure look like they should work better than the old 1950's paper stuff. I will report back on these later.

[John Fenner]

Chinese reamers "term for slot cut into dowel rod and Emery cloth pinched into it" do the trick.

[lilly-Pad]

[quote="Slidin Gator"]Here is your Airboat porn for tonight.

Got 2 new Narrow deck Chrome Cylinders installed with initial 25 ft-lbf torque. Will finish up torquing etc. tomorrow.



Manual calls for 4 feeler gauges, but I use 2.




Just got a new full set of gaskets for the motor in, gonna give these folks a try. They sure look like they should work better than the old 1950's paper stuff. I will report back on these later.

these should come in handy next time U are towing one of those CM Caddy boats

[Hardtail]

Slidin Gator, I always use Mystery Marvel Oil's magic in my aircraft motors, both in the fuel and in the oil, they say it helps in the seating process on breaking in new cylinders as well, 1 quart in the oil and 4 oz per 10 gallons gas. I have been using it for the past 30 years.

[Slidin Gator]

Slidin Gator, I always use Mystery Marvel Oil's magic in my aircraft motors, both in the fuel and in the oil, they say it helps in the seating process on breaking in new cylinders as well, 1 quart in the oil and 4 oz per 10 gallons gas. I have been using it for the past 30 years.

I run it in the fuel like religion to keep the exhaust valve clean. I do run it in the oil during break in but I don't like the way it thins out the oil for how I normally run the motor (hard all day). It's a trade off in my mind, the MM helps keep lifters, rings etc. clean. When the motor runs hot anyway it's not leaving a lot of carbon etc. behind to deal with. Unless the Marvel lowers cylinder temps I don't think it matters for the last failures.

At this point the crankcase is full of break in oil + MM Oil. I will report back on the old cylinders, but 2 new one's installed and I've heat cycled the motor 3 times. See you in the morning!

[John Fenner]

How about a lighter hull that will run a lot more free and not have to work so hard to do the task you do with it, after all it is 70%hull to power for optimum performance, seems it is working engine too hard to get to what you want from the package you have.

[Slidin Gator]

How about a lighter hull that will run a lot more free and not have to work so hard to do the task you do with it, after all it is 70%hull to power for optimum performance, seems it is working engine too hard to get to what you want from the package you have.

John, for some reason I missed this post, but it is very timely now, you must be reading my mind, or at least my other posts. There is no doubt that I run the boat hard and I'm not backing off the application. It is a 14x7 foot hull, but it is light as hell being an open hull and 1/8" riveted 7075. It does have 1/4" poly on it and the coonass rigging is way too heavy. Honestly I find the extra length less the deck over weight makes this thing run ground well relative to 12 foot deck over rigs with 300 Hp. Fully loaded, including full fuel and spray tanks, 2 people, a dog and a winch (that I have never used) I am sitting around 2,000 2,100 lbs based on the truck stop scales.

With this hull I find that I run the motor the hardest when I have 3 people on in deep water. In that condition I have to stay hard on the throttle, at least 2,200 to maintain plane. Skinny water is easy for sure, jump up on ground with a rider and a big dog on the bow and she runs morning dew at 2,000 with a load, the hard running comes in the afternoon, low tank, fat hog, dry green stuff and I have to get into it at times. Yes, she needs to go on a diet and that is next.

As of my last trip out (Mack's with Radtech) I noticed that she was just not sliding right, the damn saw grass seemed sticky and the boat was hopping bad. Got home and found that the poly has started to pull off the rivets, luckily the aft end of the poly is still attached, just a big bubble.

So as of now I am down and done chasing the hot motor (see the next post for diagnosis). I think I have solved the cylinder temps as best that I can for now, I might try the MA4-5 carb on the trailer before I break it all down. My final conclusion is that cylinder temps were probably in excess of 600 F before I started chasing it. I now run 450 on the hottest cylinder when running deep water with 2 folks. I did get the chance to push it hard towing Radtech's Caddy boat 7 miles to ground in area 2 last month. I towed that 16 foot alumalog on a plane at 2,500 RPM, stopping every 1-1/2 miles when #4 got to 550 degrees. I think any further improvements in temperature are a function of improving the air flow through the motor, which is part of the rigging changes coming.


I have a new hull identical to this boat sitting in the back yard and my cash reserves are back to where they need to be to move on. I am about to loose at least 200 lbs from where I sit now.



I am going to get aluminum rigging to loose about 200 lbs vs. the coonass square bar steel rigging on it now. The new hull is coated with steel flex 2000 vs. added weight of poly and I'm gonna give that a run for it's money. I plan to go with dual Lithium batteries mounted low to ditch that weight. At present I have a dual rear rider set up with the seat right in the middle of the air flow to the motor. I am switching to a single rider up higher plus an extra front jump seat if needed. I plan to paint and re-use my present motor, prop, engine stand, fuel tank, rudder and gauges on the new hull ("Ghost").

As for adding more power, that is longer term with the existing hull. From what I have seen, injected motors run cooler and that is one option being considered once the existing hull get's a refurb. Right now the plan is to have "Ghost" up and running for gator season.

[Slidin Gator]

THREAD CONCLUSION

Per my last I want to conclude this thread for now. I had hoped to implement the new boat plan while keeping the existing boat running, but the poly puts me in hurry up mode. So it is time to close this thread and start a new build thread.

Backing up a bit, 2 years ago I fitted "new" cylinders. 2 of these were weld repaired, new chrome cylinders (#1 & #2) from Jurnigan's, the other 4 were run chrome with yellow tags and certified for aviation use, bought off the internet. I also got 3 more weld repaired, new chrome cylinders from Jurnigan. Ever since putting on the new jugs it had what sounded like a small exhaust leak on either #4 or #6 (the yellow tag one's), but only at idle. That side of the exhaust always sucked in intermittently at idle too. Further, the exhaust port on #4 seemed to have been machined down previously as I had to use several gaskets to shim that cylinder to header connection. At one point it pulled the exhaust studs and I gave her a helicoil job on the boat.

It takes a lot to line up my glasses, back and a headlamp to look at the exhaust ports on the motor, so I just didn't do that when I sunk the helicoil. Here is the result on the #4 cylinder, note the break in from the tap job.





Further, we found that the ID of the exhaust port is larger than the ID of the exhaust gasket. It looks like previous machining to true up the exhaust flange opened the face ID up too much.






The motor ran fine at load, it just did not idle right. Turns out (as usual) that is for a reason. I did pull #2 (Jurnigan) cylinder also, but all appears well with that cylinder. So all I can say is she was stumbling due to failure of the exhaust gasket and pulling in cool air when the valve closed. The #2, 4, 6 bank now has a new header installed and all is good.

As for the heating issues, my conclusion is that I started with good mags and a crappy carb that was running (real) lean. I added electronic ignition with the plugs gapped to maximum and all of the sudden she was knocking bad. Like they say, lean is mean, she was just lean enough to run and the hotter ignition finished her off. Thanks to GMAC76 I have now have two good running carbs.

At the same time as the carb rebuilds, I added CHT's to monitor cylinder temp. The interesting part I learned from that is that the temp measurements I got with an IR gun turned out to be 100 deg or more less than real CHT's. After the carb replacement and with CHT's installed, running the same hard ground as prior, I had no issue with pre-ignition but still saw CHT temps as high as 600 F. But, before (with no CHT's) I was seeing oil temps stabilizing around 210 but now it was running 180.

Beyond that, I switched to cooler plugs, dropped the plug gap from 0.038 to 0.024, tested a fan and tried a shroud. I have also tested dropping timing from 25 to 22 degrees. I now have a motor running below 500 under normal operations and she takes a while to hit 550 under hard running. She will go to 600 if I let her, but we are done with that except for emergencies (1-2 per day...). Oil temp is now hard pressed to reach 180, and only after an extended hard run.

SUMMARY:
A proper carb is #1. The accelerator pump was not even operational in the original unit.

The cooler plugs and less gap are big ticket items, that made at least a 50 degree difference. It has been said on this forum before to run the coolest plugs you can without fouling, my experience backs that up.

The shroud vs. the fan is about equal, they both knocked about 25 to 50 degrees off the operating temp. I am thinking of combining the shroud and fan, or maybe a taller shroud but save that for later.

Dropping timing, with everything else resolved is still an open issue. On my last runs it did not seem to make a difference and even caused some backfires under load. So I switched it to full timing and have not done anything further there.

Time to start a new thread on the Ghost build. Hopefully the weight, friction and airflow improvements show up in even lower CHT readings next hunt season

Thanks for everyone's input and comments, it has all been worth chewing on.

[rowdy1]

You have several problems, remedies & conclusions that parallel my own experiences. For me, fuel injection was the biggest single factor in keeping my engine cool. I tried all of the spark plug and timing tricks you outlined and screwed with a perfectly good carb for several months. Yes my motor was a pile of junk parts before the rebuild, but it ran great, just hot. The rebuilt motor also ran hot. Now my new motor (as of Wednesday) runs 190 deg oil temp. The fuel injection system is fantastic, it runs and idles smoother and the throttle response gives me a "seat of the pants" feel of an extra 30 hp.

[Slidin Gator]

For me, fuel injection was the biggest single factor in keeping my engine cool.

I hear you, all the injected motors I have run with stay cool. I will be getting a new motor for the present hull at some point (along with new paint, poly, rigging etc., maybe even a high sided deck over) and an angle valve 540 with Continental injection is my baseline. But that's at least 1 hunt season away for sure.

[John Fenner]

Let me know when you're ready for the deck over can build you a hull to last a lifetime.

[Slidin Gator]

Let me know when you're ready for the deck over can build you a hull to last a lifetime.

I'm sure it would John. Maybe one day I'll be looking to build a 4 cyl. frogging boat. I got enough to keep me busy for a while for sure, but it never hurts to dream!