Re: Engine running rough at full mixture (carb problem or not?)


Are you gravity feed or do you have a fuel pump?  One explanation for what you are describing *might* be an intermittent low fuel supply flow for some unknown reason.  Partially plugged tank vent? (original plans Eze had just one)  Zero cost test:  full power ground run w/ fuel cap off.  Partial blockage in supply line(s)? Rig some sort of temporary fuel supply w/ enough gravity head directly to carb.  If your header tank output is easily accessible aft of firewall, rigging a direct line that bypasses all filters / pumps might accomplish this but check its fuel flow first.  Again, full power ground run.

Good luck,

On 8/8/2018 1:03 AM, Raymond Jimenez cyanoacry@... [canard-aviators] wrote:
Hi folks,

I've been troubleshooting the O-200 in my VariEze with no success, and I'm wondering if anybody might have some tips before I send my carb in for an overhaul or replacement.

A couple of weeks ago, after a hot day parked outside (lovely southern California heatwave), my engine ran rough at full throttle/full mixture in the runup. Leaned out and got normal performance, so I continued the flight. I didn't experience any issues at landing (kept mixture lean during approach/landing as well) and chalked it up to a hot tarmac and a high local DA. (In retrospect, this was likely dumb: the field was 300' MSL, and while it was hot, it wasn't /that/ hot.)

A week after that, I took the plane out for a late afternoon flight -- plane had been hangared, and it was cool outside (mid 80s). In the runup, again I saw rough running at full throttle & mixture, this time clearly outside of expectations. Scrubbed the flight and I've been working the problem on the ground since.

Here's what I've observed:
  • The engine doesn't always run rough at full throttle/mixture, but it can show up midway through a full-power run.
  • Leaning the engine tends to "fix" the problem. However, the max power seems to fade (can hear RPM drop slightly and continuously over a 20-30 sec period).
  • Running at "best power" mixture for 30-40 seconds doesn't seem to fix the issue, going back to full mixture can cause roughness. (Does not seem to be spark plug fouling)
  • The engine properly shuts down at mixture cutoff.
  • During start (hand prop), sometimes see roughness at start (idle throttle/full rich). Leaning for taxi fixes this.
  • External inspection of the carb and control cables doesn't show any abnormalities (no sticking, control arms move smoothly)
  • Switching ignition systems (has a Light Speed EI and a Slick mag) doesn't fix the roughness, makes it worse
  • Roughness doesn't seem to be correlated with CHT, and all cylinders are firing
  • Exhaust tubes are a little darker than usual (typically some shade of grey, but now they're pretty close to black)
I sumped the gascolator and found a bunch of crud from the fuselage tank -- may have been accumulated dirt from the last several years, given that I and the previous owner weren't routinely sumping the gascolator after fueling. No dirt in the gascolator when the fuel valve was set to wings, and no dirt from the wing sumps. (The fuel cap O-rings were pretty old, so I replaced them immediately after finding the dirt.)

I flushed out the aux tank through the gascolator until I saw no more dirt, then cleaned the gascolator bowl, yet the problem persists. The gascolator screen was intact and fine enough to capture the dirt I saw, and there didn't seem to be any buildup on the screen.

Also took the chance to inspect/replace spark plugs while the cowl was off, and they all looked fairly normal. Some sticky stuff on the outside of the plugs (caked oil perhaps, it's not the cleanest plane), but nothing that seems like an immediate cause for alarm. 

I /think/ this information narrows it all down to a carburetor issue. Does anybody know of anything else I can try to narrow down the issue, short of sending it in for inspection/overhaul?

Raymond Jimenez

VariEze N27GM

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