Thorp Air Command - T18.net

It looks like I am barking up the wrong tree. I was flying approaches today and kept getting the stumble. On the last pass I hit mist so I called it quits before I was ready but on the last approach I pulled the mixture out a bit and it didn't stumble. I didn't think about it being rich because my tailpipes aren't sooted up and having the vent lines routed the way they were seemed a like a good possibility.

For now I will reconnect the scupper drain and add removing the tank to add a separate tank vent to the list of "do at annual".

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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

So are you thinking it's loading up on fuel and flooding with low rpm?

The next series of tests will be based on that assumption. I do get a bigger surge than I should at shutdown which tells me it is rich at idle but I carry power on approach. Maybe I am not all the way out of the idle circuit. I hate to mess with the idle mixture because the engine starts so easy hot or cold. It just seems odd I get the stumble when the tank is less than half full. I guess it could be attitude related. With less fuel the tail would be lower on approach. Float adjustment? Coincidence? As far as that goes, it could be weight related. More weight in the tank would also mean more power on approach possibly getting me out of the idle circuit (if I am not already). I don't normally look at the gauges except to check airspeed before throwing out the flaps. I will have to make note of the rpm or see if the gopro can read the tach (MGL E-1). Rain and wind keeping me on the ground today.

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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

You should only get approx. a 50 rpm rise on shut down with going to idle cutoff .

RB

To narrow down rich or lean condition, go up a couple thousand feet and setup low power simulated approach with low fuel in tank and see if you can get the problem to occur. If it does, pull carb heat and see if it gets better or worse. If gets worse, you are too rich. If better, too lean. Repeat, only lean with mixture instead. If too lean, leaning will make it worse. If too rich, leaning will make it better. It sounds like you may have discovered that it is too rich at low power. I would just lean the screw on the carb. It is not very powerful in my experience, so pretty forgiving to get it where you want it.

From reading many pilots experiences, it seems the one piece venturis tend to run a little rough at low power settings vs the two piece venturi it replaced. It seems to vary by engine and carb though.

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Bernie Fried
N18XS

Bernie, I have read a lot of the same complaints. A local A&P about 30 years my senior has told me the one peice venturi caused more problems than it solved. I know they had to issue additional SB's to change fuel nozzles to fix fuel flow on some but I don't recall any of those being for a Lycoming. Thanks for the troubleshooting tips.

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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

Years ago when I first finished my Skybolt with an IO360 fuel injected engine it would always pop and fart when taxing. I spoke with a Lycoming rep at Sun n Fun and he asked if I lean it. I said yes when I reach altitude and he said the only time you run full rich is on takeoff, lean on the ground and lean in flight. I have followed his advice for 40 years and its always worked.
A couple of years ago Bob and I built a RV7A with a Lycoming clone Mattituck 0-360 with carburetor. On takeoff it would stumble for the first couple hundred feet then smooth out and run pretty good, but you never had to lean it. We at first it was the fuel pump and after talking to Vans they said we had the wrong carburetor on it and we needed an enrichment kit. Mattituck build us a carburetor with the kit and the stumble went away and not at altitude your had to lean. Seems that just the makeup of the engine can change from one carb to another So it can work both ways.

Here where I live, home field elevation 5700' MSL, we lean pretty much whenever the engine is running, including takeoff.

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

The problem is in the transition range between the idle circiut and the main jet (1400-1500 rpm) with carb heat on. If leaned aggressively it does smooth out. Seems to run fine outside of that range with or without carb heat. I was able to duplcate on the ground. Idle mixture adjustment screw is all the way in and still giving me a 200+ rpm surge at shutdown. That is where I am at now. Field elevation is 840 feet.

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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

Jeff, That idle circuit to mid range is apparently a common problem with these carbs. I know in all my efforts to look up my problems I saw a lot of stuff in the news letters and on the forums about the 1500 rpm stumble. I think people mentioned it quite a bit as using throttle while in the traffic pattern.

I would use google to search for those. I think most of those threads popped up from the old t-18 forum. Some ppl had carb overhauls and it helped and some others said carb overhaul didn't improve much. I can't remember what they found was the issue but I want to think it was an airflow disturbance. I think some it started when they changed to the 1 piece venturi.

You are an o290 with MA3 carb, correct? Look up the guide for MA4 carb by marvel. Specifically it says idle to main nozzle transition (1100-1400) stumble causing rich condition is idle bleed clogged or accelerator pump too rich.

If you have your idle needle adjusted all the way in it might be effecting more than just that. It does sound like since it's too rich at idle and that transition is bogging the engine that you might have an idle air bleed blockage. I cut a couple bristles from a broom and used that to snake through all my passages to make sure they were clear last week when I pulled my carb. You might also want to pull the main nozzle and check all the air bleeds around it. Mine were packed full of what looks like calcium. Since doing that not only did my idle improve but my problem with the top end went away.

Once getting that all cleared up if you are getting good fuel flow you might have to move your accelerator pump to position 2 instead of position 3 in order to reduce how much fuel it sprays at that transition. I wouldn't change that until making sure the air bleed passages are open. If it was just the stumble I would say accel pump but since you are still rich and have the idle screw all the way in it seems like some balance of air bleed or fuel circuit is messed up.

I downloaded that manual and used it to trouble shoot. Here is something you also might want to read. I think this might pertain to you since you have drilled the nozzle out: http://www.kellyaerospace.com/articles/ ... ry_AMT.pdf

It sounds like when the nozzle is drilled it causes overly rich in the mid range. I guess the way to improve that is reduce the spray from the accel pump. When reading that article it looks like a clogged idle circuit would make it lean and not rich. It's still baffling that your idle is too rich. I think your change over to mid range is too much spray from the accel pump based on everything in the manuals and that article.

Engine is O-320 with MA4SPA carb but I think the information you supplied is still valid. I have not noticed any hesitation or stumbling transitioning through the range, even on a mag check. It just does not want to run smooth in that range. I may just need to alter my approach so I stay out the transition rpm range. I just hate to do it because the is where the airplane “feels right”. It may correct itself if I get the idle circuit figured out. I don’t know if the carb is new or overhauled but it has acted this way since I bought the airplane when it only had 3 hours showing on the hobbs. Of course that doesn’t mean the airbleed isn’t plugged. It just means I finally got around to looking into the problem.

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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 thought the same thing about my carb and the prior owner said it bogged the entire 4 years he owned it. On first tear down I didn't remove the main nozzle because it has a lock washer on it. This time I pulled it all down and ran that plastic bristle through all the passages. There was quite a bit clogged up. I would pull it apart and pull the main nozzle out and clear out all the passages and see how it does. If you are running rich it could have an air bleed clogged. If not then just adjust the linkage on the accel pump to the #2 and test it. I think a few others had their problem clear with the adjustment of the acell pump. Being a larger carb than what I'm using and still at a low power setting I could see it spraying too much fuel. That doesn't really answer the question of the idle richness though. I think the pump check valve could also leak but Im' not sure it would bother just that range.

Hello learned Thorpers, fuel tank vent / ram air issue here. My airplane is N118BC, Bill Cordoza's build and Lee Walton's rebuild, so you know she's a good ship. See photos attached.

Vent ram air is 1/4 inch aluminum tube facing forward. I believe it runs to the top of the fuel tank. Fuel cap is ACS brand as shown in attached photo. Here's what happens -- during takeoff and initial climb, fuel is pushed by the ram air pressure up and out of the cap. This happens even when the fuel is half tank, probably because of sloshing around during takeoff maneuvering. Enough fuel is pushed up and out the cap that it actually drips onto the floor where my passenger's knees would be. 180 hp, CS prop. My thinking is too much ram air even at standstill once cranked up for takeoff.

Add to that the additional ram pressure from forward airspeed, and here comes the fuel from the fuel cap (screwed on tight) dribbling down into the cabin. Once in cruise, the issue seems to go away unless I started with full fuel ("full" means 1-2 inches down when the tail is down). Then I smell it for a while even in smooth air although the visible dripping will have stopped. In other words, too much ram pressure into the vent -- is that right? Any suggestions to fix? Thanks in advance!

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Philip Mandel
N118BC
S/N 957
Beaverton OR

Your cap is not sealing properly. Check the rubber for deterioration. The vent is not the problem.
Cubes

Thanks, Cubes. The rubber is excellent. Fuel comes up through the center of the fuel cap, not past the O-ring. I should have mentioned it is a vented fuel cap. Where air can go in if, for example if the ram tube becomes blocked, fuel can come out.

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Philip Mandel
N118BC
S/N 957
Beaverton OR