O-290-D2/MA-3SPA Dies when heat soaked

[Caveman]

John, I sent you a pm.

[lr172]

Originally Posted by snopercod

That explanation fits the symptoms. Which do you think I should try first: Adding another SCAT duct to the oil cooler plenum to cool down the oil, or adding a SCAT duct blowing on the carburetor? I could try either one. Either would originate from the back of the baffles behind cylinders 2 & 4.

Cooling the oil won't help the issue of boiling fuel in the bowl, if that is actually your problem (way too much heat to get rid off and too little air flow during ground ops). I would also be looking at vapor lock in the lines or pump. Fuel boiling in the bowl causes the float to drop a bit, allowing the bowl to overfill and create a rich condition. Fuel is metered by an interaction between the negative pressure in the venturi and the positive pressure imparted on the fuel due to it's weight/vertical volume (higher fuel levels in bowl create a higher pressure). However all fuel is routed past the mixture gate and can therefore be over-riden. If you can get the mixture knob adjusted for the exact fuel flow needed at your idle condition (varies based upon RPM, temp, etc.), it will be immune to an over-filled bowl and run fine (you have fixed the fuel flow manually and no longer requiring the pressure-based variable metering). Just be sure to give it a good run up before take off to stabilize the fuel in the oowl. Also, your engine won't run more than about 100 RPM greater than idle, as there won't be enough fuel flowing past the gate to support it.

Vapor lock in the lines, gascolator, pump, etc. will starve the carb and create a lean condition (carb can't replenish the bowl and level goes down, leaning your mixture). The only fix for this is to utilize a return system to keep the temps down. When I had a carb, I had a return line plumbed from a tee at the carb inlet, through and .040" orifice, back to a valve in the cabin and on to the tank. This keeps all of the fuel up to the carb cool when on the ground. I didn't think I would need this for 100LL, but knew I would need it if I ever used Mogas. The orifice keeps the flow low enough that there is still plenty of fuel flow available to give the engine 15 GPH if you forget to shut it off.

I would be doing some troubleshooting to determine if the engine stops are due to a rich or lean condition.

Larry

[Tommycat]

Originally Posted by snopercod

No, I didn't insulate the carburetor bowl, and no, I didn't insulate the thermocouple from the ambient air.

No, by all means DON'T insulate the Bowl! I was asking if you had just insulated the PROBE. To get an accurate reading of the bowl temperature. Inaccurate readings are worthless and waste time and effort. By all means get that oil temperature gauge calibrated!

Ahhh your way ahead of me. But I think we're on the same page. I'm just on the first sentence and you've read the whole thing...

And that being your picking up excessive heat from the oil sump. I'm with you as your trying to keep it at a lower temp. Had me goin there a second as you mentioned scat tube...What the... Thinking RV you know.

I suspect that you've eliminated the possibility of radiated heat from the exhaust piping being an issue. And the ambient temp in the cowling should actually be helping cool, same with the fuel. So what's left.

Is the carb heat run damper leaking hotter air into the intake?

And just a last thought.

Shouldn't that nifty evaporative cooler located in the middle of the float bowl keep temps down? Perhaps if the fuel was atomizing closer to the middle of the carb?



Regards,

[snopercod]

Originally Posted by Tommycat

Is the carb heat run damper leaking hotter air into the intake?

I'm pretty sure that's not the case. We Lancair guys had to build our own airboxes (Can you believe that???) and I modeled mine after one off a Piper Pacer. The ram air comes in the front and the heated air comes in the back with a "butterfly" damper in between to direct the chosen source up into the carburetor throat. There's really no way the heated air could "leak" into the carb unless the linkage is moved.

Here's a photo of the two SCAT tubes feeding the firewall-mounted oil cooler plenum:

[snopercod]

Originally Posted by lr172

I would also be looking at vapor lock in the lines or pump.

I've eliminated that possibility in my (alleged) mind. First, turning on the boost pump doesn't help. Secondly, I see no fluctuation in fuel pressure when the engine quits. Thirdly, I measured the temperatures of the gascolator, boost pump, mechanical fuel pump, and carburetor bowl with the engine hot. Only the carburetor bowl was over 120 degrees - it was up around 180 F. At that time, the air exiting my oil cooler was also 180F, so it makes sense that the heat source here is the hot oil.

I'm sure it's a rich condition because I have video evidence of black smoke coming out of the stacks when this problem occurs. Also, I can restart the engine only by pulling the mixture control way out.

FWIW, I did recheck the idle mixture today with the engine just warm - not hot. I got a 30 RPM rise just before it quit. I think that's in the ballpark for a 2,000' airport. I'll perform the same check again next time the engine is hot.

I appreciate your thoughts.

[lr172]

Originally Posted by snopercod

I've eliminated that possibility in my (alleged) mind. First, turning on the boost pump doesn't help. Secondly, I see no fluctuation in fuel pressure when the engine quits. Thirdly, I measured the temperatures of the gascolator, boost pump, mechanical fuel pump, and carburetor bowl with the engine hot. Only the carburetor bowl was over 120 degrees - it was up around 180 F. At that time, the air exiting my oil cooler was also 180F, so it makes sense that the heat source here is the hot oil.

I'm sure it's a rich condition because I have video evidence of black smoke coming out of the stacks when this problem occurs. Also, I can restart the engine only by pulling the mixture control way out.

FWIW, I did recheck the idle mixture today with the engine just warm - not hot. I got a 30 RPM rise just before it quit. I think that's in the ballpark for a 2,000' airport. I'll perform the same check again next time the engine is hot.

I appreciate your thoughts.

If you're sure it's a rich failure, boiling in the bowl is most likely with your symptoms. Try manually leaning at idle next time it's hot and you're waiting for a while. lean until you get some movement of the RPM. That should keep it from dying. You can also wait at 1100-1200 RPM. That will move more fuel through the bowl and keep it cooler. The engine will also be more tolerant of poor mixture at that RPM.

I like to adjiust idle mixture via the highest vacuum/lowest MAP method. It's more accurate. Given that it is taking 10 minutes to die, idle mixture is not the core problem.

Larry

[snopercod]

Originally Posted by lr172

You can also wait at 1100-1200 RPM.

Yes, if I'm in a que, I lean aggressively and keep the RPMs up around 1400. It's a tradeoff, though, because the higher RPMs cause the CHTs to climb up faster. Lately, I've been thinking, "Why not just shut off the engine?"

Could you explain the manifold pressure method of checking the idle mixture, please. I've never heard of that.

[Tommycat]

Have you done a mag check immediately after the hot engine die-out and subsequent restart?

If so...and they're both good.

How 'bout a play by play description of the engine running conditions leading up to the quitting? Fighting to keep it running? Rough running, hesitations, stumbles, rpm fluctuations, etc.

[snopercod]

Originally Posted by Tommycat

Have you done a mag check immediately after the hot engine die-out and subsequent restart?

No, I didn't think of that. I'll do one next time.

As for the sequence of events, the engine just dies. I lean aggressively to restart and it runs rough until I get the RPM up above 1,500. I have video of the latest event, but thanks to Garmin updating my editing software so it no longer works, I can't post it. I do have this old video I took in Denver two years ago. This was before I learned to lean aggressively when restarting:

https://www.dropbox.com/s/ay7prqm4i6...nnial.mp4?dl=0

[lr172]

[quote=snopercod;1182934]Yes, if I'm in a que, I lean aggressively and keep the RPMs up around 1400. It's a tradeoff, though, because the higher RPMs cause the CHTs to climb up faster. Lately, I've been thinking, "Why not just shut off the engine?"
[quote]

Not just lean agressively. Let's say your going to idle at 800 RPM. Set your throttle to 800 RPM. Once you get near the point on the mixture pull, slow down and lean until your RPM goes up by about 20-30 RPM and stop. At this point, your carb cannot deliver more than the current fuel flow, regardless of an overfilled bowl. Can't say what's going to happen when it's time to take off, but it will prevent you from going rich at 800 RPM if the bowl over-fills. You'll probably have to nurse it a bit to clear the extra fuel once your ready to advance the throttle.

Originally Posted by snopercod

Could you explain the manifold pressure method of checking the idle mixture, please. I've never heard of that.

An engine will pull the highest vacuum when it's mixture is at it's optimum level. It is a means of judging peak efficiency at idle. Highest vacuum is the same as saying lowest MAP. Warm your engine up and keep adjusting the mixture until you achieve the lowest MAP possible for a target RPM. The RPM will go up or down with each adjustment, so you'll need to adjust it back to your benchmark RPM. It is the lowest MAP for a specific RPM, not just lowest MAP overall, that you are shooting for. Remember that optimum idle mixture is temp dependent. If you plan to do this again in late fall go optimum. If you are setting a year round mixture level, set it a bit on the rich side so that you are not too lean in the winter when the air is denser and requires a higher fuel quantity to achieve the same mixture.

I use this method to set idle mixture. I do not not manually lean on the ground and have never fouled a plug in 350 hours. Manual leaning is completely unnecessary below about 1000-1100 RPM if your idle mixture is set properly. Below this level, you're running completely off the idle circuit in which you have complete control over the mixture via the idle mixture screw. Further, manual leaning via the red knob is simply not possible at anything other than at a static RPM. Once set properly, your idle mixture circuit will provide a constantly variable flow that meets your mixture setting from 500-1000 RPM with no manual intervention. Feel free to ignore this aspect, as most here consider me a heretic for saying it.

Larry