O320 Carb Bracket Bore Diameter

[krw5927]

Does anyone have a VA-149-320-PC on hand they could quickly measure to find the diameter of the big hole for the carburetor bore? This is the powder-coated steel bracket that mounts between the sump and carb and fixes the throttle and mixture cables in place.

I'm wondering what the differences are between the VA-149-320 and VA-149-360. Specifically looking for the carb diameter on the -320 version.

[Dbro172]

I have a Superior O-320 with an ma4-spa carb. The bracket is presumably for an O-320 and you'll see here that the throat opening in the bracket, which matches the carb, is smaller than the hole in the sump and the sump gasket. I believe superior uses an O-360 sump on their 320's, albeit the inside of the sump does taper up to a smaller diameter. I have pretty bad offset fuel distribution from front and rear cylinders and I think this might be the culprit. There is an old AD dating back to the 1950's that provides an insert in the bottom of the sump for it to merry up to the carb diameter... Which is my next step. I'm surprised that a brand new engine like mine would be built with such a severe difference between carb throat diameter and sump hole diameter.

[krw5927]

Thanks for posting, Derek. I have this exact same issue with my brand new O-340 stroker and MA-4SPA from Titan. The sump, like yours, has the opening for a MA4-5 carb for an O-360, and tapers to a smaller diameter inside. I'm experiencing horrible mixture distribution as well, with the front cylinders (1&2) always far cooler and richer than the rears (3&4).

I researched the issue, including Lycoming SB258, and contacted Titan. They machined and sent me a tapered insert similar to the one in the SB. They want it wedged in between the carb and sump rather than pinned in place like the SB, and I'm trying to assess if the throttle/mixture bracket opening will fix the insert in place or whether the insert could "fall" through the bracket and sit loosely against the carb without being held in place.

It seems you're saying that your throttle/mixture bracket for the O-320 has the same size opening as the MA-4SPA carb, which means if mine is the same it should adequately retain the insert.

I'll report back on results (and send you a PM). Perhaps Superior can do the same for you?

Edit: what's that spacer-gasket on top of your carb for? How thick is it? If it's 1/4" thick, you could always replace it with this, designed for the Carbon Cub which has the same problem: http://store.cubcrafters.com/Carbure...es_p_1449.html

[N941WR]

You can always have Van's punch out a custom bracket, if you need.

I had this done for my old O-290-D2 and then cut & drilled the holes I needed.

You can get an idea of the work involve by looking at Issue #3 on this page of my website. (It is about halfway down the page.)

[krw5927]

Originally Posted by N941WR

You can always have Van's punch out a custom bracket, if you need.

Thanks Bill, that's a great idea if my current bracket doesn't work out.

[Dbro172]

Wow, Sounds like we have the same issue and actually I have already purchased that insert from cubcrafters! Just haven't had time to install it yet. My rear cylinders run way leaner than the fronts too. Definitely let me know your results with the taper insert, please. The gasket/spacer came with the engine/carb combo from Aerosprort... Not sure exactly why these engine combos have the large space.

[krw5927]

To answer my original question, I found that the large hole in the VA-149-320 throttle/mixture bracket is indeed correctly sized for the MA-4SPA carburetor, which has a smaller throat bore than the MA4-5 carb and the sump opening on my engine. This allowed me to use the tapered sump insert that Titan machined for me, similar to the Lycoming Service Bulletin, and capture the insert with the bracket as Titan's engineers instructed.

I've only made one test flight since installing the insert and the new carb that Titan sent (which has a bunch of tiny holes drilled in the main jet to better atomize fuel), but results so far are extremely encouraging. The engine started much more quickly, all 4 CHTs in cruise fell in a 20 degF window, and EGTs were much closer as well. Further flights at varying altitudes and mixture settings will hopefully confirm this issue is resolved.

For anyone experiencing a massive CHT/EGT imbalance between the front 2 cylinders and the rears on their carbureted engines, I would encourage you to remove your carb and measure the diameter of the carb throat and the sump throat. If the sump throat is significantly larger, this could very well be the cause of your mixture distribution imbalance. Have a look at Lycoming SB258 - they figured it out in the 1950's. Not sure why we're still struggling with this issue almost 60 years later.

[Dbro172]

Good to know, Kurt. I'll be doing the same soon. Hopefully this also helps LOP operations too, instead of only being able to get one or two cylinders LOP

[Dbro172]

I can add to this thread that the sump riser insert offered by cubcrafters does not fit in the Superior O-320 sump. It is too wide of diameter. shame on me for not asking first.

[krw5927]

Originally Posted by Dbro172

I can add to this thread that the sump riser insert offered by cubcrafters does not fit in the Superior O-320 sump. It is too wide of diameter. shame on me for not asking first.

Before Titan machined a conical insert for me, I had bought the necessary Schedule 80 aluminum pipe and the Lycoming pins to secure it per SB258. Now that I'm not using it I'll be glad to lop off a piece and send you the pipe and a pin, if you're interested in going that route.

Unfortunately I don't have a way to get it machined to the SB dimensions, but it's an easy one to turn on a lathe if you can find someone with the machine.

Send me a PM if you're interested in the materials.