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Rough running Superior O-320 with 360 carb/sump

MikeinPhoenix

Well Known Member
All, I recently bought a RV-9 with a Superior O-320 with a 3878 carb and a 78915 sump (360). EGTs can differ between cylinders by as much as 300 degrees at times and the engine runs rough. I've looked up the 3878 carb and it shows for use on the 360 engine--not the 320. The engine was assembled by Superior so I presume they were ok with this component combination, but something sure seems wrong. At certain throttle settings, EGT temps are even across all cylinders and the engine runs smooth.

Any suggestions or thoughts would be greatly appreciated!

Mike
RV-9A
 
Engine specs?

Hi Mike,
There must be a method to there madness.Are you sure its not an 0-360 listed as an 0-320? Van's doesn't support that engine/airframe combo.An MA 4
would be the correct carb for the 0-320 and the MA 4-5 for the 0-360.The bores are the same the stroke of the crank is different.The size of the intake tubing will be larger than the 0-320 as well as the jets being too large for the 320.Have you contacted Superior or the engine builder to confirm what you have? 300 degrees is huge variance between cylinders.May I ask how many hours are on this engine?
Thanks
RHill
 
Uneven cht's

Time to hit the search button. This is a old issue and there is info available.



Don Broussard

RV 9 Rebuild in Progress
 
Carb

The current generation Superior sumps are supposed to improve mixture distribution with a carb. The current sumps should have a "bullet" at the top of the vertical passage, immediately above the carb.
One option would be to try a different carb. Different carbs of the same model and part number sometimes behave differently.
Regarding the statement on intake tube size, this is only partially true. Many 0 320's have the same diameter intake tubes as 0 360's.
You should carefully examine the intake tubes and flanges. It is common to find the flanges warped from overtorqueing. The flanges can be straightened in a vice with wooded blocks.
 
Thanks for the suggestions on what to look for. The engine has 650 hours. I did check the engine bill of materials and it's a Superior XP-O-320-B1AC2. I will also give Superior a call to see what suggestions they may offer.

Best, Mike
 
I don't know much about superior engines or 360's, but I have done quite a bit of research on carb's for my 320 and have a lot of experience with automotive carbs.

I would be uncomfortable running a carb that was set up for a 360 on a 320. The jetting on these aviation carbs seem to be highly specific to the application. I would start doing some research on the specific carb model to determine how it meters and if it runs rich or lean. Remember, you are not moving the same amount of air (directly translates to velocity) through it at full power or cruise that it was originally set up for.

All of this assumes that the carb version we are referring to is an MA-4-5 and not an MA-4.

Just some thoughts.

Larry
 
Steve's link should help solve the issue. If not, I always just call my tech for these things. Local company for me, but should be able to help you also. and they ship free online. QAA 877-833-6948

Best of luck with the diagnosis!
 
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