Thorp Air Command - T18.net

1albee, thanks for the heads up. I'll use some thick fishing test to keep from scratching the insides of the bores. I also though about a bristle from a plastic scrubbing brush.

Jeff, I agree that it's most likely the carb. Now hearing what Bernie said about the nozzle I'm almost certain its nozzle size. I'm running the carb with the smaller nozzle and when I looked last night it does not have the new side holes for atomization drilled so it's an old nozzle that wasn't upgraded when the 1 piece Venturi was upgraded. I hear countless tales online of issues with the 1 piece Venturi until the new nozzle is out in and marvel has a sb on their site about it as well.

Bernie, thanks so much for that info! I have been searching for those nozzle sizes with no luck. That makes my job so much easier. Since I need to get the new nozzle type anyways I'll risk damaging this small one and see how much it improves drilling it out.

Since I have the 719 nozzle should I go to the #42 first and test it or go straight to the larger size that others have used with the 290?

I can see the light at the end of the tunnel!

I would suggest increasing jet size in small steps, simply because if you go too big, you can not drill it to a smaller size
I just accept the notion that anything done on an airplane will likely have to be done more than once to get it right.
I'm wondering if polishing the jet after drilling can be done with a pipe cleaner and some fine valve grinding paste?
https://en.wikipedia.org/wiki/Pipe_cleaner
The fuzz of the pipe cleaner would hold the grinding paste, and keep the steel wire core from touching the brass of the jet.
Should fit thru a 1/8" hole.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

So I checked everything on the carb. All settings good. Once the nozzle was pulled the problem was evident. Around the nozzle was a calcium or like looking build up that had almaot completely frozen the nozzle in the carb. I cleaned it all out and cleared all the passages. Nice and clean.

The nozzle is a 719 but it was already drilled. I slowly tested each bit inside the bore and it fits a #37. So it should be plenty rich now that the gunk was cleared out. I'll have the carb back in the week to test fly hopefully during the week but most likely the weekend.

Update and test flight:

Last night I took the plane out and test flew it. On take off I can go full throttle with no bog and my power is awesome. On climb in an upper 70* day with 1600' DA I climbed out at over 1000 fpm with engine temps only up to the low 400's. I'm getting 2750 flat out and with full throttle and full rich at 2750 the temps were low 400's. I gained over 20mph flat out in cruise. My oil temps at WOT 2500ft were going up to about 220 although my CHT's were still low 400's. Pulling back to 2500 rpm my cht's on the rears dropped down to the 360-370 range.

I'm pretty low to sea level but I still set it at 2500ft and did some leaning to see. Leaning rich of peak my full rich was still about 130* rich of peak. So my #37 drilled nozzle still is slightly lean in the lower altitudes but now that my economizer is working I get more fuel at full throttle which is helping quite a bit on temps and engine performance. My egt and cht were pretty consistent between front 2 and rear 2 cylinders with my rears still being 25-50* hotter (depending which aspect of flight) than the fronts. The egt's were consistent with those cht as well.

I don't think drilling the nozzle any further is a wise idea because it might make the engine too rich on mid range transition. The only other thing I might try if the carb comes off again is to drill the atomizing holes on the side of the nozzle to see if that helps mix the fuel better and get better distribution from front to rear cylinders. #3 is always my leanest cylinder on the JPI when I do lean find.

All in all this is the best performance I have had on this since owning it. I think the 9 hours on the new top end is the reason my oil temps are higher than they ever have been before. That is most likely the friction and because I ran camguard in my engine before which could have reduced some friction. They are still in normal range but just higher than prior to the overhaul.

On another note the new desser elite 2 retreads made a huge difference in landing performance. Plane isn't so jittery when landing. I think the wear on only 1 tire in an odd location was causing it to pull the plane back and forth.

It is good to hear things are improving.

When you mentioned the white deposits my first thought was that it was likely corrosion since aluminum “rusts” white. I have only had 5 aircraft carbs apart and the only one to show me any corrosion was one that had been running auto fuel and his source likely had alcohol in it. I bought some “100% gas” (according to the sign anyway) from a local station that tested positive for alcohol even though I put 5 gallons in the pickup before putting any in the can.

Tires can make a huge difference in handling. If the old ones had a strange wear pattern, have you had a chance to look at why? The only two options that come immediately to mind is improperly aligned and improper pressure. If you need some alignment work do not be tempted to use the wheelpant mounts as shims. The most likely result is a misaligned wheelpant which can create a lot of drag.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I have been told that you will find small amount of alcohol in all gas because they use the same tanker to deliver all gasolines.

Even though they aren’t supposed to. Same trucks move our avgas. I believe my experience was more than just a trace residue. I used the water bottle method and the fuel line moved a considerable amount. 5 gallons may not have been enough to purge the pump though. That would also explain why my Harley would never run quite right. It needed premium but if all it ever got was alcohol laden regular it would explain a lot.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I think ethanol fuel was probably the issue. Since it sat for long periods of time I could see that being left over from fuel sitting in it.

As for no ethanol fuel we have a local source and I test it frequently. Haven't found any amount measurable by regular testing methods. I'm sure on a microscopic level there is some but I don't have access to an nmr to test it.

It wouldn't take much to contaminate a non ethanol tank at a fuel station. They could have filled it wrong and residual continue to remain.


As for the tire. I though it was an alignment issue but based on the wear it looks more like a braking or landing issue. They were old and dry rot as well with no way of knowing if the prior owners flipped the tires due to wear. The plane handled quite well with landing. From what others have mentioned alignment causes poor handling with only a tiny discrepancy so I don't think there was much change. The only way to find out is to monitor the wear on new tires to see if it wears like the previous tires. Then I'll actually know the culprit. Hard to tell what madness previous owners did with this thing. I've found all kinds of maintenance neglect on this thing from the last owner. Based on the condition of his Honda Accord he used to pick me up from the airport I should have saw this coming! Lol

Glad to see you found something that worked to help what was going on. I dug through my heap-o-junque this morning and emerged with a MA3SPA 10-3346-1 from a O-290D2. I have it available if you still want to try a loaner but me thinks you have the offender on the chase. Be mindful the carb I found is in poor repair and would need at least gaskets.

Dana LaBounty

Thanks for the offer!

I had a little bog on a go around today and it's hesitating on full throttle now. It is quite a bit colder now so maybe it leaned out further. It's not near as bad as it was before and seemed to be less as the day warmed.

No more bog on take off WOT in cooler temps and if over 75*f just a slight lean out clears it up. The cooler temps and -900ft DA lately has my rear egts running lean although cht is mid 300's.

Please check my new thread and offer advice if you can:

viewtopic.php?f=19&t=8166&p=18048#p18048