O-320 Significant EGT/CHT spread

[ultrarunner]

I have an O-320 in a Grumman Cheetah that we've been resurrecting after 10-12 years of sitting. Carb is MS-4SPA with 10-5217. Since its first flight out of the barn, it's had a large EGT and CHT split between the front two and rear two cylinders. It also seems to have high fuel consumption, but I'm still gathering data on that. ~25 hours on new cylinders & induction tubes.

Yesterday I experimented at 8500ft leaning out until EGTs fell and it ran a bit rough, then pulled throttle back until the economizer circuit leaned it out further. I then enriched about 50° ROP on cylinder 4. The result was EGTs on 1 & 2 about 1210 and 3&4 being ~1500. CHTs were 340-350 on 1&2 and just shy of 400 on 3&4.

Full throttle is better for balance, as is very low (say, < 25%) throttle settings.

Either way the carb is getting replaced (installed 1999 and sat through the 2010s), but I'd really like to know what the mechanism is here. What causes 300° (I think I saw diff 360° on the JPI) temperature spreads from partial throttle settings, with the front being so much cooler? Cracked primer nozzles? Half-clogged fuel nozzle? Weird gasket leak or butterfly bushing leak interacting with airflow somehow? It looks like the rear runners are a lot shorter than the front in the sump; streaming fuel that takes too long to vaporize?

 

[FORANE]

I've had the same problem but front two are hotter on mine. Mine have been that way since replacement of the front two. Any chance your cylinders are different front and back?

Of course induction leaks can cause this also.

 

[AV8R_87]

The result was EGTs on 1 & 2 about 1210 and 3&4 being ~1500. CHTs were 340-350 on 1&2 and just shy of 400 on 3&4.

Are the EGT probes placed the same distance from the exhaust ports?
If able, swap probes front to rear. Keep them plugged into the same channels. That will eliminate the probes as the cause.
EGT split is definitely on the large side, but CHT split isn't that bad, especially if your baffling isn't the best. Ring-type CHT probes, or bayonet?

 

[ultrarunner]

Are the EGT probes placed the same distance from the exhaust ports?
If able, swap probes front to rear. Keep them plugged into the same channels. That will eliminate the probes as the cause.
EGT split is definitely on the large side, but CHT split isn't that bad, especially if your baffling isn't the best. Ring-type CHT probes, or bayonet?

They're all pretty even. Old picture attached. CHT are all bayonet except for #1.

 

[ultrarunner]

I've had the same problem but front two are hotter on mine. Mine have been that way since replacement of the front two. Any chance your cylinders are different front and back?

Of course induction leaks can cause this also.

All new cylinders & induction tubing. I didn't want to get too excited about it until the old rusty cylinders weren't part of the equation. Cam inspected while they were off and was in good shape, plus I can vary the split with throttle so that seems to eliminate a ground down 1+2 shared intake lobe.

 

[Dan Thomas]

A look up the bottom of the typical MA-4 carb:



The accelerator pump nozzle runs up one side of the bore. Sometimes it's closer to the bore wall than in this one. It runs up the side opposite the bowl drain plug.

Now, depending on which way your carb is mounted, plug front or to to the rear, either the front cylinders or the rears can get more fuel if the primer is leaking, or isn't closed and locked. There are light check valve springs in the primer that prevent fuel dribbling out when the engine isn't running, but venturi vacuum can pull extra fuel out of that nozzle and run a couple of cylinders richer, not to mention increasing the fuel burn a bunch. You might replace the carb only to find a bad primer.

Changing the throttle setting can affect the nozzle's spray pattern. It's fuel strike the throttle plate when the plate is less than full open, and it gets dispersed better.

 

[ultrarunner]

A look up the bottom of the typical MA-4 carb:


The accelerator pump nozzle runs up one side of the bore. Sometimes it's closer to the bore wall than in this one. It runs up the side opposite the bowl drain plug.

Now, depending on which way your carb is mounted, plug front or to to the rear, either the front cylinders or the rears can get more fuel if the primer is leaking, or isn't closed and locked. There are light check valve springs in the primer that prevent fuel dribbling out when the engine isn't running, but venturi vacuum can pull extra fuel out of that nozzle and run a couple of cylinders richer, not to mention increasing the fuel burn a bunch. You might replace the carb only to find a bad primer.

Changing the throttle setting can affect the nozzle's spray pattern. It's fuel strike the throttle plate when the plate is less than full open, and it gets dispersed better.

Fascinating. It also has a bad stumble on acceleration. Just to clarify, when you say primer in this context you mean accelerator pump?

 

[Dan Thomas]

Just to clarify, when you say primer in this context you mean accelerator pump?

Yes. Exactly. Brain fart. In the case of the accelerator pump, a missing ball check spring will let the engine suck fuel past the pump and into the carb throat. I've seen it only once, and it showed up as engine roughness at full throttle, smoothing out as the throttle was close a little and the throttle plate spread it out.

The primer thing applies more to small Continentals where the primer nozzle is in one side of the intake spider, and one side of the engine will get more fuel.

I've been off coffee all week for medical reasons, and my thinking is easily muddled. Too many years on the caffeine.

 

[Gary Ward]

I Gave up caffeine Aug 30 this past summer. Gave up lots of food also. lol
Still enjoy decaf coffee.
I lost 40 lbs since Sept and 50 since the beginning of the year. Blood pressure is lower.
It's almost like gaining 20 hp in my 172.

 

[Stewartb]

What’s the temperature spread between peak and full rich? I’ve had uneven EGTs/CHTs and ultimately the solution was to increase the main jet to provide more fuel flow. It resolved the temp spread when leaned, too. If you can’t demonstrate 200° of leaning authority? That’s where I’d start.

 

[ultrarunner]

What’s the temperature spread between peak and full rich? I’ve had uneven EGTs/CHTs and ultimately the solution was to increase the main jet to provide more fuel flow. It resolved the temp spread when leaned, too. If you can’t demonstrate 200° of leaning authority? That’s where I’d start.

Well, depends on the throttle setting. Wide open, 13x0-1500° is achievable. Partial throttle, 1&2 seem to want to be mid 1200s and I'm unable to raise that much. Maybe I can get them into the 1300s; the back 2 are closing in on 1600 then. I flew around yesterday almost entirely below 5000 and CHTs on 1 & 2 hovered around 300, which starts to make me think about lead fouling & valve stem deposits. I was transitioning a bravo into a delta and didn't look, but I'm sure they got even colder on descent.

Either way, I can't find an induction leak, especially one that would apply only to the back cylinders. The primer feels like it might need an overhaul— maybe that will be next. Everything else, from the sump up, has ~25 hours on it and the only other constant is the carb. So a new one is ordered & on the way, and I'll update when it gets installed.

 

[Dan Thomas]

Check the gaskets between the intake elbows and cylinder heads. These are known to fail. They get old and hard and crack. The shrink and the elbow gets a bit loose and lets air in. They get blown out when some pilot goes to OFF on the mags during runup and then switches back to R or L or Both, getting that exhaust system explosion that can send a pressure spike and flame back through an open exhaust valve and out the intake during valve overlap, exploding the fuel in the intake too. I have found chunks of those gaskets missing, probably from that.



You will need a flashlight and small mirror to see all around the flange and edge of the gasket, and a 7/16" U-joint socket, 1/4" drive, to check the bolt torques on the flange.

 

[flyingron]

Absolute value of the EGT is meaningless. Slight variations in the placement of the probe in the stock can make substantial differences. The important thing is where the temperature in each cylinder peaks with respect to the mixture setting.