snopercod
Well Known Member
This is my first post here. I built and own a Lancair 235 and I hope you won't hold it against me. The Lancairtalk Forum is no help at all with engine problems; Most of the posters there seem to be second owners who didn't build their planes and don't know much about what's up front. So I came over here - hat in hand - where my kind of people hang out.
When I first started flying my Lancair 235 in 2014, it was suffering from high cylinder temperatures on climbout. Part of the problem turned out to be poor baffling ? which I have since fixed - but also the MA-3SPA carb wasn?t producing enough takeoff fuel flow. My engine is a Don George high compression O-290-D2 rated at 140 HP (145 during takeoff) and the carb had the stock main jet for the 135 HP engine. I had read that a rough rule-of-thumb for takeoff fuel flow is 1 GPH for each 10 HP, which would have been 14 GPH in my case. Since I was only seeing 8-9 GPH on takeoff, I drilled out the main metering jet (in stages) to #38 (.1015). I also added two #57 ?aeration holes? in the jet to better equalize the fuel distribution between the front and back cylinders. Both of those improvements worked; I now see 11-12 GPH on takeoff and, in cruise, my JPI EDM-700 shows all four EGTs at the same temperature after leaning and a little tweaking of the throttle. That?s about as good as you can get with a carbureted engine.
But now, in the heat of summer, I?m experiencing another problem. During ground operations on a hot day, if tower has me wait in place for ten or twenty minutes due to traffic, my engine overheats and wants to quit on me; I?ve had this happen several times now. The last time was 2 days ago at KAVL (field elev. 2111?, density altitude 3,500?). My EGTs were bumping up against 400 and my oil temp was bumping up against 200 F. My electric boost pump was ON the entire time, putting out 6.0 PSI. I didn?t see any fluctuations in the fuel pressure which would have indicated vapor lock (my gauge is connected to the outlet of the mechanical fuel pump, just upstream of the carburetor). I was able to restart by pulling the mixture halfway out, and kept the idle up to around 1500 RPM to keep it running. I left the mixture out on the takeoff roll, and eased it back in after takeoff. The engine ran fine when it got some cooling.
So the problem seems to occur when the temperature inside the cowling is above 170 degrees. (Yes, I have a temp sensor in there ? a digital meat thermometer LOL! ) I read where 100LL starts to boil at 140F so if the carb itself got to 170F, that would be a problem. I guess I need to instrument the carb bowl and see how hot it?s getting. All my fuel lines are insulated with firesleeve. The gascolator, mechanical fuel pump and electric boost pump have no shrouds, but I put a blast tube on the mechanical fuel pump; It hasn?t seemed to help. In fact, my problem doesn?t seem to be vapor lock in the lines, but an overly-rich mixture when hot at idle. I have a Garmin Virb video camera mounted under the tail tiedown, so I have video evidence of black smoke coming out of the stacks when the engine is quitting on me. To me that says ?too rich? rather than ?vapor lock?, but I?m just a new guy at this stuff. I?m wondering if I drilled out the main jet too much. Well, that shouldn?t be the problem because, in theory, the main jet doesn?t even come into play until around 1200 RPM.
If anybody was wondering, my idle is set to 730 RPM (warm), 650 RPM (hot), and the idle mixture is set for a 50 RPM rise on idle cutoff, which I believe is correct for the 2,000? field elevation where I am based. I?ve also experienced the problem in Denver (6,000? elevation, 8,800' density altitude.) and in Tucson (2,650? elevation) on a hot day. I?m wondering if I should lean out the idle mixture regardless of what ?the book? says.
The exhaust stack in my tightly-cowled Lancair is probably only 4? away from the carburetor. Is the fuel in the float bowl beginning to boil a little causing the float to sink and flood the engine? Does the float level control the mixture to the idle circuit as well as the main circuit? Maybe I need some insulation between the exhaust stack and carb, or wrapped around the carb bowl?
I?m leaning (no pun intended) toward boiling in the carb bowl, but am open to all suggestions. Thanks in advance.
When I first started flying my Lancair 235 in 2014, it was suffering from high cylinder temperatures on climbout. Part of the problem turned out to be poor baffling ? which I have since fixed - but also the MA-3SPA carb wasn?t producing enough takeoff fuel flow. My engine is a Don George high compression O-290-D2 rated at 140 HP (145 during takeoff) and the carb had the stock main jet for the 135 HP engine. I had read that a rough rule-of-thumb for takeoff fuel flow is 1 GPH for each 10 HP, which would have been 14 GPH in my case. Since I was only seeing 8-9 GPH on takeoff, I drilled out the main metering jet (in stages) to #38 (.1015). I also added two #57 ?aeration holes? in the jet to better equalize the fuel distribution between the front and back cylinders. Both of those improvements worked; I now see 11-12 GPH on takeoff and, in cruise, my JPI EDM-700 shows all four EGTs at the same temperature after leaning and a little tweaking of the throttle. That?s about as good as you can get with a carbureted engine.
But now, in the heat of summer, I?m experiencing another problem. During ground operations on a hot day, if tower has me wait in place for ten or twenty minutes due to traffic, my engine overheats and wants to quit on me; I?ve had this happen several times now. The last time was 2 days ago at KAVL (field elev. 2111?, density altitude 3,500?). My EGTs were bumping up against 400 and my oil temp was bumping up against 200 F. My electric boost pump was ON the entire time, putting out 6.0 PSI. I didn?t see any fluctuations in the fuel pressure which would have indicated vapor lock (my gauge is connected to the outlet of the mechanical fuel pump, just upstream of the carburetor). I was able to restart by pulling the mixture halfway out, and kept the idle up to around 1500 RPM to keep it running. I left the mixture out on the takeoff roll, and eased it back in after takeoff. The engine ran fine when it got some cooling.
So the problem seems to occur when the temperature inside the cowling is above 170 degrees. (Yes, I have a temp sensor in there ? a digital meat thermometer LOL! ) I read where 100LL starts to boil at 140F so if the carb itself got to 170F, that would be a problem. I guess I need to instrument the carb bowl and see how hot it?s getting. All my fuel lines are insulated with firesleeve. The gascolator, mechanical fuel pump and electric boost pump have no shrouds, but I put a blast tube on the mechanical fuel pump; It hasn?t seemed to help. In fact, my problem doesn?t seem to be vapor lock in the lines, but an overly-rich mixture when hot at idle. I have a Garmin Virb video camera mounted under the tail tiedown, so I have video evidence of black smoke coming out of the stacks when the engine is quitting on me. To me that says ?too rich? rather than ?vapor lock?, but I?m just a new guy at this stuff. I?m wondering if I drilled out the main jet too much. Well, that shouldn?t be the problem because, in theory, the main jet doesn?t even come into play until around 1200 RPM.
If anybody was wondering, my idle is set to 730 RPM (warm), 650 RPM (hot), and the idle mixture is set for a 50 RPM rise on idle cutoff, which I believe is correct for the 2,000? field elevation where I am based. I?ve also experienced the problem in Denver (6,000? elevation, 8,800' density altitude.) and in Tucson (2,650? elevation) on a hot day. I?m wondering if I should lean out the idle mixture regardless of what ?the book? says.
The exhaust stack in my tightly-cowled Lancair is probably only 4? away from the carburetor. Is the fuel in the float bowl beginning to boil a little causing the float to sink and flood the engine? Does the float level control the mixture to the idle circuit as well as the main circuit? Maybe I need some insulation between the exhaust stack and carb, or wrapped around the carb bowl?
I?m leaning (no pun intended) toward boiling in the carb bowl, but am open to all suggestions. Thanks in advance.