AeroVee EGT/CHT split Issue

[MichaelFarley56]

Greetings Everyone,

I’m hoping to run a non-turbo AeroVee situation by all of you in hopes that, just maybe, someone has seen this issue before. To fill you in, I have recently rebuilt my AeroVee to be a normally aspirated (non-turbo) engine like it was when I first began flying my Waiex. I have everything rebuilt and the engine is running, but I’ve ran into a problem with engine tuning that I can’t figure out. To make a long story short, when I try to tune the engine I’m getting a major mixture split between the front two cylinders (closest to the prop) as compared to the back two cylinders (closest to the firewall) Left side vs. right side seems okay, and at idle everything seems to be okay but when I bring the throttle up and especially when I go to wide open throttle on the ground, the front two cylinders are running very rich while the back two cylinders are running incredibly lean. I’ve tried different AeroInjector needles and mixture settings but in this case, if I lean out the engine so the front two cylinders don't stumble from excessive richness, the back two cylinders have EGTs that spike to at or over 1450 degrees. On the other hand, if I richen the mixture so the back two aren't at or over redline, the front cylinders won't hardly run. Ultimately I'm seeing an EGT split between the front and rear cylinders of 300-400 degrees (1150 degrees on the front and 1450 on the back). Of course this also leads to the rear CHT’s to spike quickly while the front CHT’s stay very cool, further supporting the readings of the EGTs.

I didn't have this issue at all when the turbo was on the engine and nothing has really changed to the core of the engine since then. I've also checked the following:

- Valve lash was adjusted and then confirmed to be set properly.
- Heads all torqued properly
- Timing is set correctly
- Engine runs give same result with either ignition turned off (both ignitions were tried separately)
- Entire intake system has been gone over multiple times for induction leaks
- Intake elbows have been removed and re-sealed twice to check for leaks.
- Intake elbow spacers have been removed and re-sealed as well.
- EGT and CHT sensors have been swapped to check for readings reliability (all is reading properly)
- AeroInjector was overhauled at the factory and is properly installed.
- Multiple AeroInjector needles have been tested (#2, #2.5)
- Same readings are found with air filter on or off.
- A compression check shows similar readings on all cylinders.
- The heads are both brand new and only have 20 minutes of ground run check time.

At idle power of roughly 1000 rpm, I may see an EGT split of maybe 100 degrees (850-950 degrees) and the engine runs smoothly. As power is increased however, this front vs back split becomes larger and larger.

So, for those of you who are flying behind non-turbo AeroVee’s, I was hoping you’d be able to chime in and let me know if anyone else has seen this issue before and/of if anyone has any ideas on other things I can check.

While you’re at it, would anyone else be willing to share what your EGT split or range is when flying at full power such as takeoff? Is anyone else having issues with front cylinders vs back cylinders?

Thanks everyone!

[gammaxy]

EGT's right after takeoff with RPMs ~2800.

1: 1150
2: 1025
3: 1225
4: 1125

After takeoff, I typically lean by feel for peak power. This increases my RPMs to ~2850. The EGTs rise to:
1: 1300
2: 1125
3: 1350
4: 1250

Obviously, take these numbers with a grain of salt. I just eyeballed them from recorded telemetry of a couple of flights.

I don't understand why cylinder 2's EGTs are always so much cooler, but I suspect it is at least in part due to it being a bit richer than the other cylinders. I think others have mentioned the same observation about cylinder 2.

[DCASonex]

I see you have checked intake system multiple times, but that is where it sounds like problem resides. Might recheck valve clearance settings if all else looks good.

David A. (not an AeroVee)

[Onex107]

I'm running the Aerovee in my Onex N2107X with about the same difference in temps. The needles don't seem to make a difference in the cyl. temps. I'm currently running #3 with excellent results, but that's another lengthy explanation of how I determined it to be the best choice. My engine has 140 hours on it but I went back to some early recordings to get this data. My #1 cyl. is the hottest and it overheats first. On climb out the EGT will turn yellow first and I richen the mixture 1/8 inch to keep it in the high 1300's. #2 cyl. EGT is always 150 degrees cooler than #1, at all speeds in the 3000 range. #3 and #4 are within 25 degrees of each other and about 50 degrees cooler than #1 and #2. I think some of that difference is due to air flow and a lot of it is due to mixture at the inlet manifold. When Bill Larson added the Turbo his temps all became nearly equal. The Turbo is a homogenizer for the AeroInjector and I believe the fuel/air mixture is more of a mist than droplets, and of course having the intake manifold pressurized causes the valves to breath more equally. The inertia/velocity of the mixture is more even at the valves. When I start the engine everything gets better when the intake manifold warms up. There may be more vaporization then. Once when we pulled the intake manifold off of a O-200 Continental that had ingested water, we found condensed water droplets all the way through. Again, my opinion, the AeroInjector does not atomize fuel, it doesn't even spray fuel, and if that happened, it would make the temps more even.

[MichaelFarley56]

So were you able to get your engine to run well and keep all the temps in check? I don't think I can do that. I did one quick lap around the pattern but my EGTs were well over 1400 and my CHTs were well over 400 as a consequence. For obvious reasons I can't fly it like that.

Thanks for the input.

[Onex107]

Getting back to you late. Been gone all day. When the engine was "new" I had over temp problems but as I remember they where mostly solved by retarding the secondary ignition about five degrees. The engine took about 20 - 30 hours before it really settled down and ran repeatedly. During that time I was fighting "the burps" as well and more flights were terminated for that. I removed the gascolator and wrapped the pipes and insulated the exposed gas line including the front end (fuel line end) of the AeroInjector.
It appears you are running lean to be in the 1400's and can't cool them with mixture. A few things I think should be mandatory before you start tuning the needle. My needle holder had .025 play in the ball/socket. That's nearly 1/2 turn of adjustment that the needle can move on it's own. Depending on where you leave it, it can go richer or leaner on it's own. That just adds to the frustration of adjusting. Start with that and tighten it up any way you can. Squeeze the block or shim the ball. I and my Onex friend in Tenn. had new holders made with a disk and slot design, instead of the ball/socket, that has "0" play. From there, any needle will work, I found the #2 required a lot of mixture leaning in order to idle correctly, The #2 1/2 required less and the #3 doesn't require any mixture adjustment from WOT to IDLE. The only time I move the mixture is to richen up a little on climb out to keep #1 EGT below 1400. The other thing I notice, when the #1 starts to get hot, if I pull the throttle back a little from WOT, it doesn't change the RPM but it slows down the EGT rise. I sense that the mixture ratio is not linear and is changing and getting leaner as you advance the throttle. You want to have a little mixture enrichment left at WOT to cool it off if it needs it.

[Onex107]

Just as a follow up, I am not knocking the AeroInjector. It is a simple, easy to operate device. Eliminating a float bowl where water can collect, carb heat, and a fuel pump is worth a lot. KISS. The needle holder needs to be tightened up and I found the needles were not all exactly the same diameter. They suggest adjustments of 1/8 and 1/4 turns resulting in linear needle movement of .007 and .015 inches. That requires a tight system. A needle that is smaller in diameter will run richer at the same setting as one that is .125. The fuel flow is also sensitive to head pressure, full tank vs 1/3 tank. Try to do all your testing with approximately the same amount of fuel. It's a dynamic fuel delivery system that works very well when it is adjusted correctly. I'm happy with mine. A new engine is tight and will run hot for a while. You should be able to keep the temps below the never exceed ones with rpm and mixture. Just sayin.

[gammaxy]

What propeller do you have? What RPM are you getting during these static WOT runups? What did you see in the pattern when your EGTs and CHTs were high?

You mention testing it on both ignitions. Have you verified that turning off both ignitions will stop the engine?

[Johns]

My Sonex that I purchased was built by a auto mechanic. He had the same problem . What he did (according to the log) was cut the middle of the intake gasket out, and that lets the air-fuel mix flow more evenly between the front and back cylinders.
It's a cheap try if that sounds like something you think will work.

Good luck, John

[MichaelFarley56]

Chris - I have a Sensenich 54x44 prop bought through Sonex and see right at 3000 on a full power static run. The one lap around the pattern I've done was quickly aborted when the front cylinders barely hit 250 degrees (1100 EGT readings) and the rear cylinders hit 415 degrees (1450 EGT). I landed so I didn't swallow a valve.

John - I actually did the exact same thing to the intake spacer. I cut out the middle section so there's a large air gap below the intake elbows. It made a slight improvement but didn't come close to fixing the problem. Thank you for the suggestion though!!