Vapor Lock maybe

[Fastwind]

Today while entering the traffic pattern at 18" MP my engine "stumbled" momentarily and then ran normal as far as I could tell.

When pulling the throttle fully closed as I crossed the numbers I heard popping sounds from the exhaust, something I've heard many other planes do while flaring but mine has never done so. As I rolled out and the engine/prop slowed, I felt the engine shudder and could tell the prop was going to stop if I did not add some throttle. I had to taxi in with the throttle open further than usual to keep the engine from getting really rough and shuddering, having to bump the throttle to bring the engine back from trying to die.

At my hangar I found that the engine ran rough on both mags at low power (but rougher on the slick mag than on the Light Speed Ignition - as it always does). I leaned the mixture and tried to smooth things out but no matter what I did if I did not keep the throttle open a "bit" the engine would stumble and I'd have to bump the throttle to keep it running. I did not note the rpm it would idle semi smooth at but of course each time the engine would stumble the MP would give an associated rise. Finally I opened the throttle to 1200 RPM and pulled the mixture and it shut off more or less normally.

When I shut down and close the fuel valve I always hold the "running fuel pressure" on the gauge and it bleeds off over the next few hours. While running the fuel pressure was normal around 4-5 PSI but a few minutes after shutdown the fuel pressure gauge had raised distinctly to about 9 PSI. It has never done this before. Vapors expanding in the fuel lines? Over time it bled down to zero as usual.

It was fairly hot today at about 90 degrees. I was hot in the cockpit once I descended so I know things were hot. But it was not the hottest day I've flown on and I've never had anything like this happen before.

The increase in fuel PSI after shutdown is quite curious to me. Is that indicative of "boiling vapors"?

Several hours later, after the system had cooled considerably I did a runup again and everything was normal, idling first at 600 RPM and 13" MP until it warmed up and then at 700 RPM and 12" all with the throttle fully closed.

FYI:
My gascolator and fuel pump are both shrouded with "blast tubes" to them.
Carburetion: Ellision TBI

Note: Throughout the anomoally I pulled carb heat several times and it only did the usual slight RPM drop.

Any ideas what is going on to cause these symptoms?

Thanks

 

[bjdecker]

Any temperature data for EGT and CHT available from any of the phases of the problem flight?

 

[Fastwind]

I noted the hottest CHT (#3) and EGT when I encountered the first "stumble" when entering the pattern. It was not that hot because I was throttled back and descending. 285
The EGT was also low because I had gone full rich for landing: 1082

Normally the CHT runs about 315 and EGT depending on how I lean but normally around 1150.

 

[oaklandaviator]

Ellison

That’s normal for the Ellison on a hot day. Turn on the boost pump and it’ll help.

The Ellison uses low pressure like a carb but doesn’t have a float bowl. On a carb the bowl is vented and allows Vapor to escape while liquid fuel is drawn up through the jet to the Venturi. The Ellison has no way of getting rid of this vapor so the only thing you can do after landing with high under cowl temperatures is turn on the boost pump to slightly increase the pressure and run up the rpm to keep the flow up. Leaning in this particular instance doesn’t help because it reduces the fuel flow. Just part of the Ellison experience!

 

[Fastwind]

Thank you for that reply. I've had the Ellison EFS-4 on the plane for about 12 years now, and have learned quite a few of its "behaviors'" along the way, but this is a new one. Describing it as "part of the Ellison experience" is a pretty fair statement. It has some pros and cons and certainly has simplicity, but sometimes the complex things, like an accelerator pump for example, have a useful function.

I had intended a pretty deep dive into the ignition and induction system prior to the next flight, but it sounds like there may be nothing to discover, except an Ellison TBI under the cowl....

Thanks again for your input!

 

[rmarshall234]

Ellison TBI

The fuel pump and gascolator have blast cooling, that’s good. Are the fuel lines firesleeved and directed away from the exhaust and do you have any 90 degree fittings in the fuel line run?

 

[Fastwind]

The fuel lines are fire sleeved and separated from exhaust as much as possible. I have some exhaust heat shields where there is any proximity.

The fitting on the EFS-4 with attached finger screen IS a 90 degree fitting though, but it is in one of the cooler portions of the cowling it would seem.

BTW Robert, you packed my rig several times - thanks!

 

[rmarshall234]

Ha! Small world, Steve. You’re welcome.

If you have available length on that fuel line I would change the 90 degree fitting to a straight fitting or at least a 45. Simple and cheap and it may make the difference. The Ellison manual says not to use 90s and I’ve found that their recommendations are pretty spot on. Here’s a pic of my installation. The bottom of that cowl gets pretty toasty with downwash from the cylinder cooling.

 

[Fastwind]

Unlike the installation pictured, mine is mounted on the Ellison "plenum box" and is then rotated with the air intake opening forward and the fuel inlet fitting facing down. The Ellison fitting with finger screen attached is actually a 90 degree fitting.

I'm not sure there is room for anything else as the bottom of the cowling is pretty close, but there may be, I'll have to look. And I'd have to get the intake fitting from Ellison that is straight, like the one in your picture.

Do you know if anyone is still in support of Ellison TBI's? And how to contact them?

 

[rmarshall234]

There is no support for the Ellison anymore, we are on our own. There shouldn’t be anything special about that fitting, if you post a pic of your installation I’ll try to help out.

 

[Fastwind]

The fitting that threads directly into the TBI (the fitting that the fuel line connects to) has a finger screen attached to it that projects into the TBI housing when the fitting is installed. Ben described this as a "last chance" filter and suggested it be inspected periodically and cleaned if necessary. The debris would be "inside" the finger screen so cleaning it can be tricky and the screen is pretty fine and fragile. Mine has always been pretty clean and easy to flush out any debris that did collect in it.

Apparently Ben used to provide this fitting as either straight or 90 degrees as mine is a 90 degree fitting and the one in your pic is straight. The distance between the fitting and the bottom of the cowl is pretty close so if the Ellison fitting were straight the AN fitting connecting to it might work it it were a 45 degree fitting, and may need to be "followed" by another 45 degree fitting in close proximity. I'll have to look at the clearance next time I'm at the hangar.