Thorp Air Command - T18.net

Steve - no CHT/EGT?? - you're a transcendentalist!!!.....

I

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http://www.dixiefriedfabrication.com/

Fraser,
I wouldn't lose too much sleep over it. I spent most of my flying youth (before my Uncle grabbed me!) towing banners down the beach in FL with an Scout 8GCBC/0-360 FP. As a young A&P/Commercial Pilot I worked on my own work airplane that I flew. The Scout was flown 6 hours a day, 7 days a week from Memorial day to Labor day. Tow speed was 60 MPH @ 2100-2300 RPM Oil Temp-215-240 CHT's 385-425 and we didn't even monitor EGT. We changed oil every 25 hours. All three Scouts went to TBO without an issue. What's my point? EGT/CHT relationship is good to be conscious of but as you well know, temperature, density altitude and mixture all have an effect. I cooled all my temps after installing an Electronic ignition on one side of my airplane.

1. Have you tried a hotter spark plug? REM37BY Aviation or Autolite 386 (Electronic Ignition)?
2. Compare all the cylinders and take an average.
3. Check your mag timing next C.I.

I personally don't think you have a problem...

Here's a good article: http://www.avweb.com/news/maint/egt_and ... 791-1.html

V/R
Smokey

Smokey - thanks for the reassurance - am running 37BYs - have been for a year - am sold on them - mag timing should be pretty close, although I know one has trouble staying within a degree - hopefully I will put on 2 new slicks soon - I only have single steam probes for CHT and EGT. Uninformed and inconsiderate children keep robbing my panel/EFIS/Engine monitor fund for stoopid stuff like, food, clothes, vehicles, horses and college funds......blasted offpring units!!!!.......If you're young, pray for short tailed swimmers!!......JK

I flew yesterday - 20 deg and 29.98 Alt.....and got those same high EGTs - ignored them - I do think I need to drill out the jet - on the 45 coming into the pattern at 190 mph and while slowly dialing in the prop, the engine started to stumble with the mixture out only about 1 inch at 5400 msl. I landed at full rich - Meethinks it's time to pull Marv, check the settings and drill out the jet.

I know this is new tech stuff, but you might consider welding a bung in the exhaust pipe and mounting an O2 sensor.
Those things are available in the racing supplier websites for cheap. Then you could tell if the engine is running lean or not.
I have one on my RV-8, works great. I use it to (help) set mixture.
The other thing, you might not need to drill out your jet, you might find a restriction, like a bit of lint, a bit of rubber from the inside of the fuel hose or some rust from the fuel level sender in a carb passage. Or an air leak in the intake system down stream of the carb.
I'm just sayin'.

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Scott Emery
EAA Chapter 326
T-18 N18TE

Good points about the possible obstructions - had not thought of that.....

Are you sure it's not a "bad" EGT probe ? They do burn out much like the thermocouple on your H20 heater . CHT good ? Oil temp good ? Before you go tearing into the carb it might be easier to try a new probe . Worst case you have a new probe . This is why I like to have "Trend" instruments to cross ref. a possible bad unit . BOOM !

RB

Now that you mention the "stumble" does your carb have the new "Blue Epoxy" float as required by the AD ? The brass and white float were replaced with the blue . The data plate will tell you . It is a number stamped on the plate , but I can't remember what number it is . Could cause flooding...ask me how I know ! While you are fishin around in there for minnows , check the inlet finger screen for goobers . I found what appeared to be small pieces of gasket material ( I think it looked like dried Permatex ). There was even a small piece swimmin around in the needle seat. Nothing from my install .

RB

Probe 1 year old - carb is AD current/compliant

I would look at the needle seat for the float . You can get a quick build kit for the Marv from Spruce that has the seat and the valve along with all the other gaskets, etc. Buying the seat and valve separate is close to $90.00 .

RB

K - took a nice de-stress flight after werk taday....plane ran great - no EGT, stumbling or running in the mixture, even at higher speeds.....time to de-cowl and take off the carb......then I got to thinking - a rare treat.....a friend stopped by and we were talking through the high speed mixture issue, and as we were chatting about the fuel system in general, he asked me what kind of pressure I thought I had, because as I was obviously running lean, perhaps rather than a carb issue, or an induction leak, I might be experiencing fuel "hunger" (it sounds better than starvation). That led to thinking about any ram air pressure on the fuel vent......

And Presto.....the epiphany hit us both at the same time.....that might be it.....so I looked at the fuel vent just aft of the firewall on the bottom of the plane and voila.

The picture is not too great, but you can see there are two (what look like) #40 holes in a tight screen soldered to the vent pipe/fitting. Run the pi and the radius numbers and those two holes are about 1/2 the area of what a vent line should be - Now, given the location in the center of the fuse just aft of the firewall, between the two exhaust exits and aft of the lower cowl carb "bump", there would be IMHO some disturbed air around that vent. My theory is the at higher speeds, the air is turbulent around that vent, that ram air is almost non-existent, and there may even be intermittent siphon. I have only a gravity feed, and that vent is obviously critical. Regardless, I think that is what is causing the "lean" issue at speed. That will get changed here very shortly and I'll let you know the results.

Pi , radius ? Sounds like complacated math even for Jethroe Bodine and he gradauted the 6th grade...TWICE ! Spellin took him 3 years ! 2 holes ? In case one gets plugged you have a spare ? Drill it out and make one big pie hole !

RB

I don't think so Rich - without a fuel pump, methinketh we gravity subjects need some ram air into that vent, especially at low fuel levels. The fix will either be a tube bent into the airflow, or I'm considering a reversed fairing to create a little pressure just in front of that vent. Film at eleven. Gotta put a screen around it as well to keep the mormon crickets from ruining my day.

Fraser...I got my ready made fuel vent from Aircraft Extras.com . It is their Aerodynamic fuel vent . Plug and play install . Had to buy 2 and you can have my extra one . Tax and tip not included ! BOOM ! You can put a skeeter screen on the front if need be ?

RB

I'm sure glad you discovered that, Frase. Remember our little "incident" during take-off while leaving Arkansas & Gary Green's fly-in a few years ago?

Back at the beginning of this thread, I think I mentioned something about the basics before doing anything else -

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE