I've been battling a problem for 6 months and have exhausted virtually every idea/fix that has been offered, with no resolution.
I have an O-320 D2J with a Marvel Schebler MA-4SPA carb. It ran fine for 19 years, but was beginning to show play in the throttle shaft, so I traded the old carb for a factory reman (identical part number to my old carb's data plate) from Aircraft Spruce. Marvel Schebler (in North Carolina) is the company that ultimately provided the rebuilt carb.
After the carb change, I've had 2 ongoing problems:
1) After years of being able to get a 50 RPM rise as I lean the mixture at idle, I can no longer get much, if any rise with the new carb. I've run the engine with the idle adjustment everywhere in the range (from stopped out on the closed side to about to fall out because it is open so far 4.5-5 turns). The engine idles smoother with the enrichment adjustment way towards the rich side, but there is no noticeable RPM increase available at any idle enrichment setting.
2) When I pull the throttle back on descent (MP under 12-13"), the engine sputters and pops like there is an induction leak. The EGT readings (I individually monitor all 4 cylinders) don't show a smoking gun - no obvious cylinder that runs away from the other three. #'s 1 and 2 do appear to have a little EGT rise relative to #3 & 4.
The engine runs fine at >1500 RPM. It flies fine. Runup is fine. But at low MP, particularly with the engine warm, the idle is rough.
Here's what I've done:
- Sent the carb back to M/S to have it bench checked. No problems identified.
- Changed out the induction gaskets and induction hoses. No problems identified with the old ones. There are no blue stains on or around the induction system.
- Checked the sump tubes - no problems noted.
- Operated the engine with the local IA squirting solvent (brake cleaner, IIRC) at each of the joints in the induction system. No RPM increase noted.
I spoke with another IA today - a noted engine guy, who suggested I try another carb to see if the carb is the problem. Unfortunately, I don't have a MS-4SPA tree in my yard, so that's a tough get.
I'm at my (dim)wits end.
Suggestions?
I have an O-320 D2J with a Marvel Schebler MA-4SPA carb. It ran fine for 19 years, but was beginning to show play in the throttle shaft, so I traded the old carb for a factory reman (identical part number to my old carb's data plate) from Aircraft Spruce. Marvel Schebler (in North Carolina) is the company that ultimately provided the rebuilt carb.
After the carb change, I've had 2 ongoing problems:
1) After years of being able to get a 50 RPM rise as I lean the mixture at idle, I can no longer get much, if any rise with the new carb. I've run the engine with the idle adjustment everywhere in the range (from stopped out on the closed side to about to fall out because it is open so far 4.5-5 turns). The engine idles smoother with the enrichment adjustment way towards the rich side, but there is no noticeable RPM increase available at any idle enrichment setting.
2) When I pull the throttle back on descent (MP under 12-13"), the engine sputters and pops like there is an induction leak. The EGT readings (I individually monitor all 4 cylinders) don't show a smoking gun - no obvious cylinder that runs away from the other three. #'s 1 and 2 do appear to have a little EGT rise relative to #3 & 4.
The engine runs fine at >1500 RPM. It flies fine. Runup is fine. But at low MP, particularly with the engine warm, the idle is rough.
Here's what I've done:
- Sent the carb back to M/S to have it bench checked. No problems identified.
- Changed out the induction gaskets and induction hoses. No problems identified with the old ones. There are no blue stains on or around the induction system.
- Checked the sump tubes - no problems noted.
- Operated the engine with the local IA squirting solvent (brake cleaner, IIRC) at each of the joints in the induction system. No RPM increase noted.
I spoke with another IA today - a noted engine guy, who suggested I try another carb to see if the carb is the problem. Unfortunately, I don't have a MS-4SPA tree in my yard, so that's a tough get.
I'm at my (dim)wits end.
Suggestions?
