engine puzzle

[sksorenson]

N118SJ was restored to flight over the Covid break.  This plane first flew in 1981 and had over 1300 hours when I retired it when my Defiant was flying in 2005.  The restoration included a rebuilt O-200 to replace the C-90-8.  The new engine uses a standard MS carburetor and has duel PMags.  The engine runs very smoothly and has great power but sometimes when I'm in level cruising flight with low cruise power I get a very short hiccup every time I hit a bump in the air.  Power is normal when the usual one g comes back.  Very short period hiccups for which I have no explanation.  I have examined the fuel system and found nothing dirty and the ignitions appear to be working just fine.  I have not disassembled the carburetor to see if anything was contaminated.  I'm hoping someone out there might have some idea about that is happening.

 

Steve

    

[Kent Ashton]

No idea really but perhaps the bump is disturbing the float setting or disturbing airflow into the intake.   Maybe engine flexing in the mounts is moving the carb linkage?  Carb bolts loose or intake tubes loose and allowing engine to suck air Good luck

[bmckinney10]

Read O-200 overhaul manual, section 16-10.  Lock-O-Seal washer and "finger tight" nuts to hold intake spider.  Also, do not overtighten intake rubber coupling clamps, just snug. The whole thing is supposed to  "float".  I had all  mine too tight. At certain RPMs or bumps the engine sputtered. Too tight and the carb dumps excess fuel with vibration or jolting.  I found this on a C150 forum and loosened mine up and it's gone.

[bmckinney10]

Here's the post from the forum:

"The O-200 has an odd carburetor spider mounting arrangement. If it's not assembled correctly, rough running and big RPM loss is possible. We've dealt with this before. Here's what I have posted many times:

The Continental O-200, as you have in your 150, has a carb "spider" intake manifold. The carb bolts to it and it, in turn, is bolted to the crankcase. The O-200 is different from its older A-and C-series granddaddy engines in that the spider is mounted loosely on the case studs, with special "Lock-O-Seal" washers on the studs between the case and spider and between the spider and castellated retaining nuts. These washers are metal with a rubber insert molded into them, like an O-ring. The overhaul manual says that those castellated nuts are to be turned only finger tight and the cotter pin installed. The reason is that the MSA carbs don't like being shaken, or they'll spill fuel out of the bowl vent and into the carb throat and the engine will run rich and rough. The old Strombergs weren't nearly so prissy, and those old A- and C-series engines had much stiffer rubber engine mounts so the engine couldn't move around so much. With the carb spider a little flexible on the studs, and the rubber hose connections on the intake tubes, the engine can torque-vibrate around the crank while the carb will stay relatively still. I have smoothed out O-200s just by installing the proper washers and doing up the nuts exactly as the manual says.

So your mechanic needs to refer to the O-200 overhaul manual and look for that paragraph. This is critical for that engine. The whole carb and spider need to be able to flex sideways a bit."

https://www.pilotsofamerica.com/community/threads/0-200-rpm-loss-on-take-off.137511/

[Kent Ashton]

1 hour ago, bmckinney10 said:

Here's the post from the forum:"The O-200 has an odd carburetor spider mounting arrangement.

Wow, great advice Brian!