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  #1  
Old 08-28-2016, 02:04 PM
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snopercod snopercod is offline
 
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Question O-290-D2/MA-3SPA Dies when heat soaked

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.
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  #2  
Old 08-28-2016, 02:08 PM
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Mike S Mike S is offline
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This is my first post here.
John, Welcome aboard
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  #3  
Old 08-28-2016, 02:28 PM
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snopercod snopercod is offline
 
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Thanks, Mike. I never wanted to be a carburetor expert
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  #4  
Old 08-28-2016, 03:02 PM
wilddog wilddog is offline
 
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I have a 0-320 and lean the daylight out of it on the ground, just remember to push it to rich for take-off.
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  #5  
Old 08-28-2016, 03:19 PM
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snopercod snopercod is offline
 
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Thanks, wilddog. Your O-320 is pretty much like my O-290-D2 but with a MA-4SPA carb. I lean during ground operations, too, and my plugs look nice and tan. But two days ago I got stuck for 20 minutes at the hold-short line waiting for landing traffic. The engine heated up and quit on me. Have you ever had that happen?
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  #6  
Old 08-28-2016, 06:25 PM
krw5927 krw5927 is offline
 
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My bet is fuel boiling inside the carb bowl, causing the float to sink and allowing far too much fuel into the main jet.
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  #7  
Old 08-28-2016, 06:32 PM
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snopercod snopercod is offline
 
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Kurt-- That's my best guess at the moment as well. Thanks. I guess my next task is to stick a temperature probe on the outside of the bowl to confirm. What I can't figure out, though, is that the main jet isn't supposed to kick in until >1,200 RPM.
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  #8  
Old 08-28-2016, 08:29 PM
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Quote:
Originally Posted by wilddog View Post
I have a 0-320 and lean the daylight out of it on the ground, just remember to push it to rich for take-off.
Quote:
Originally Posted by snopercod View Post
Thanks, wilddog. Your O-320 is pretty much like my O-290-D2 but with a MA-4SPA carb. I lean during ground operations, too, and my plugs look nice and tan. But two days ago I got stuck for 20 minutes at the hold-short line waiting for landing traffic. The engine heated up and quit on me. Have you ever had that happen?
Quote:
Originally Posted by krw5927 View Post
My bet is fuel boiling inside the carb bowl, causing the float to sink and allowing far too much fuel into the main jet.
Quote:
Originally Posted by snopercod View Post
Kurt-- That's my best guess at the moment as well. Thanks. I guess my next task is to stick a temperature probe on the outside of the bowl to confirm. What I can't figure out, though, is that the main jet isn't supposed to kick in until >1,200 RPM.
One thing I would like to add, Lycoming recommends having the Boost Pump on for high temperature ground operation. I had HIGH temperature issues during Phase I flight testing in SoCAL back in September 1997. That is how long I have known about using the Boost Pump on the ground in high temperature conditions. After almost 19-years, I have the boost pump on for almost all ground operation even after leaving SoCAL.
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  #9  
Old 08-28-2016, 10:32 PM
SHIPCHIEF SHIPCHIEF is offline
 
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If the exhaust is close to the carb, I would put a heat shield on it to block radiant heat from the fuel system. It doesn't have to be fancy, I tested by using cut offs from steel 2x4 studs left over from constructing my hangar. Any bits of sheet metal that you can hose clamp or wire in place for testing. If you like the result, spend some quality time and build the nice one.
The mechanical fuel pump transfers heat from the engine block and the oil into the fuel.
One year I flew to the local summer fly in (in my T-18) with the oil cooler winter cover still on, and winter grade mogas in the tank. After sitting in the broiling sun all day, I started to get vapor lock on the return flight at about 3000 msl. I diverted to the nearest airport. Draining out the mogas and filling with 100 LL fixed the problem, but a few days later my brain finally put it together. the oil temp was 210F or better on the gage, so the mechanical fuel pump was hotter than usual, and the air on the backside of the engine was much hotter. Coupled with old winter grade mogas was the perfect disaster.
Maybe a blast tube to the carb? Unfortunately, blast tubes reduce the cooling air pressure drop across the engine cylinders, so too many blast tubes = poor cooling of the cylinders and oil cooler. It's a balancing act.
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  #10  
Old 08-29-2016, 06:03 AM
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First I would pull the float and make sure it is in good condition, I come from a Holly and Carter background, but I have seen brass floats get a hole and them then partially fill with gas. Shake it, weigh it. Confirm that 6 PSI is not too high and pushing the float down and flooding the bowl, confirm float height. Fuel flow should be .5 to .55 Pounds per HP per HR.
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