O-320 lean rear cylinders Lycoming insert OS-5372-1

[Bob_pipedream]

Hi all,

Sorry to be creating so many threads, but just found some more info online - from the Supercub guys.

It seems that there was a Lycoming SB258 (which I having trouble finding) that specified need to swap out the 10-3678-12 for a 10-3678-32 carb. But due to difference in throat diameter - a tapered insert needs to be put in the sump riser.

My sump hole diameter is about 2 1/4", but the carb throat diameter is much less at 1 15/16" creating a step. This according to one post causes the rear cylinders to run lean (which just happens to be the issue I have).

The solution is apparently this insert:

OS-5372-1 Textron Lycoming SLEEVE INSERT $56.48 EA
the dowel pin that holds it in is
STD 798 and the gasket is 66224


Which smooths the transition from carb to sump. This may also mean that I do not have to drill the jet - not sure.

Any ideas everyone - could it be that loads of people with O-320s from Superior are running around with this issue and lean rear cylinders?

[Gerald Loyd]

The rear cylinders on my Lycoming O-320 run leaner that the front also. Please post the results of you change.

[Bob_pipedream]

Hi Gerald,

Will do, have just ordered the part and I suspect that a lot of Superior O-320s are running around without this insert which was in a Lycoming service Bulletin SB258 back in 1959 stating that the A2 sump should not be used with the 10-3678-32 carb... Anyway as soon as I have the part and have tested it all, I will post back here again with the results.

[Bob_pipedream]

Hi all,

Well I thought I had better stick this on my posts so that anyone else suffering similar issues of high rear cylinders EGTs Low front cylinder EGTs and rough running on an O-320 will have the final answer.

As mentioned earlier, I found that my new Superior O-320 has been fitted (as have all Superior O-320s) with an O-360 sump. These are interchangeable however the O-360 sump has a wider diameter sump throat diameter to match the wider diameter carb. This creates a step after the carb of about 1/8 - 3/16.

Lycoming service bulletin SB258 was printed in 1959 to address rough running in Piper Aircraft. They had found that the 10-3678-32 carb (and I think all O-320 carbs of the MA-4SPA type as they will all have this step) should not be fitted to the tapered riser sump (or O-360 sump) as there would be a step after the carb that could cause, rough operation, bad mixture distribution and lean the rear cylinders. The solution is easy and is simply an insert that converts the taper into a straight riser of same diameter as the carb.

This is exactly what I had and I had tried all the other things such as timing, plugs etc.

So I had an insert made as per the SB258. Cost to have the part made is minimal ? I got some 60mm external diameter by 5 mm wall thickness aluminium tube off of Ebay and a local engineer turned it up in about 20 minutes. I got the length of the insert which is not in the SB from the engine and it is 18.9mm ? I also had an additional small 2mm x 53mm diameter step machined into the wider end so that it would locate into the Vans throttle bracket that is clamped between carb and sump (so overall length is 20.9mm).

I fitted it yesterday and took off. The difference is amazing and far beyond what I expected. It has not only cured the rough running, narrowed the EGT spread and improved performance ? it has also made her feel almost turbine smooth. At last I have an unqualified RV grin.

I did not drill the jet or do any other mods other than cleaning the flashing out of the primary venture.

So if you have a Superior O-320 (or any O-320 for that matter, it may be a good idea to verify your sump part numbers just in case someone has put the O-360 sump on there) with a MA-4SPA carb, then you WILL be requiring this insert.

Next weekend I will record more EGT data, but even at worst condition, there was only 80 degree spread which would easily lean out (it was up to 180 before). I can also now use full mixture without chugging, coughing etc.

So perhaps the ?poor mixture distribution? reputation of the O-320 is actually more to do with some engines not having the right sump/carb combo in the beginning?

[Finley Atherton]

Bob,

Thanks for unearthing this information.

I have an ECI 0-320 but it does have a Superior sump and as you correctly point out it actually has the 0-360 sump (part # SL 78915). I have got a copy of Lycoming SB 258 and can see the dimensions of the insert, however I have examined the inlet area and throat on my sump and I can't see how it would fit as my sump has a different profile to that of the insert shown on SB 258.

I have the same step at the opening so the thick end of the insert at 2.38" OD would fit correctly but after the opening, my sump tapers to a larger diameter than the 1.94" shown on the SB. I don't have an inside micrometer for a really accurate measure but instead of 1.94" the throat in my sump tapers in from the 2.38" to about 2.1"
This sort of makes some sense to me as I would have expected that the internal diameter for the 0-360 sump would be larger than the 1.94" diameter for the smaller 0-320.
So I am a bit confused how you got the insert to fit if you made it to the same dimensions shown on SB 258.

My engine runs well but the cylinder and EGT temps could be more even in the cruise so I plan to try out the insert and get one made to fit my sump but it would need to be be a slightly different shape to that shown on the SB.

Any thoughts?

Fin
9A

[Paul Y]

Bob

Your post on 11-15 said you ordered the part for this fix. But in your later post it sounds as if you had to have it machined. Was there a problem with the part you ordered? I have a Superior O-320 and have always had extra hot CHTs in the rear cylinders.

Paul
RV-9A with Superior O-320

[Bluebeard]

Hi could someone kindly tell me where on my O-320 (O-360?) sump its part number is located - I have been hunting!

I do suffer higher EGTs on the rear cylinders so this thread suggests a possible cure .....

[Paul Y]

My sump number was on the bottom toward the back under the bracket that goes between the carb and the sump. I made a sleeve, installed it and have flown about three times. Haven't seen any improvement in the low altitude flying I've been doing. My hot EGT is still about 300 degrees hotter that my coolest one when full rich. As I lean out the two cooler EGTs move up to close to the temp of the Hot two. Haven't done much testing yet however.


Paul

[Bluebeard]

So I will have to take off the carb and throttle bracket to see the part number on the sump ....

Since the engine data plate is fixed to the sump, how would the wrong sump be fitted to an engine?

[Selmax]

I have trouble finding Lycoming SB 258, it is not on Lycoming's website . Any help ?

thanks Pit