Thorp Air Command - T18.net

This may be my pure ignorance talking.

But if it is the last 1/4 inch of throttle movement that is causing the over rich condition. Why not simply change the throttle stop limit to not go that last 1/4 inch? Or alternatively, change the mixture stops so that it is slightly leaner than in the current full rich position?

Also I have heard of problems with low speed stumbling when that idle circuit passage is plugged. This while not common issue, but frequent enough issue in Aeronca's that even repeated "overhauls" of the carb by otherwise reputable firms did not find it. Once sent to a person who knew specifically to look there and clean it out. The problems went away. It is worth a try since you said there was some varnish to try to send something through the idle circuit passage.

Also there was some problems with the float needles and whether they were metal, plastic (neoprene) or derlin. It is important to note that each type of needle takes a DIFFERENT seat. So if you have a mis-match, it can cause problems. Especially if auto gas is or has been used in the past. I cannot recall the specifics. But some of the tips will expand with the autogas additives and mess up how it seats. You have to make absolute sure of the needle and seat and that they are compatible. They look extremely identical but the part numbers have to match. Get a mis-match and problems that seem "invisible" ensue.

A final thing would be to check the float level with a proper "head" pressure of fuel. Some floats were plastic or two piece and were changed over time. However the buoyancy of a metal versus a plastic float are different resulting in the fuel level in the carb bowl to change. I think some floats had to solder some weight onto them in order to get the proper fuel level in the carb. Also a small amount of fuel if the float leaks could also affect this. So again give that a real check.

I think these things are at least worth having checked out before you spend 800 plus dollars on either a complete overhaul or a new Rotec unit.

Again I'm a bit ignorant when compared to the wise folks here and experienced A&P's etc. But it seems like these aviation carbs have been around for many, many, MANY decades and a slight over rich condition doesn't seem like it would be impossible to find, nor have to spend nearly $1,000 to get solved. These things are almost from the stone age. They are still in use because they have proven to be reliable and work. Anything that doesn't work, evolution/experience would cause them to disappear from use. So I just find it hard that your situation would be so overwhelmingly hard and costly to solve. I could definitely be wrong though. It wouldn't be the first, nor the last time I was wrong.

The only adjustment to max power would be to limit the control by making it hit the panel or quadrant before the arm on the carb can hit it's stop. My personal preference would be to actually fix the problem as opposed to hiding it.

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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

With full throttle your are at maximum fuel flow . I pull the throttle back to prevent the enrichment jet from pumping fuel, RPM and HP are not effected. Fuel burn is less. You guys are over analyzing a problem that does not exist

Although I understand the thought process this seems just like a band aid and possibly something that could become dangerous. I'd rather get to the bottom of the problem (which is most likely jetting)


No slow speed stumbling or other carb issues. Just a rough condition when the last 1/4" of throttle is advanced. Worse when the OAT is high. Fits right in with the rich condition I've diagnosed by leaning the mixture at full throttle run up.



I examined them and although I can't see part numbers they are both brass and the needle has no damage or wear on it's tip coating.



I checked the brass floats with a magnifying glass for any dents or leaks. Submersed in fuel there were no leaks and nothing inside the float after taking it out. I didn't check float height when installed. Once I had it back on I wanted to test it. I can easily add an adapter and clear tubing to check although I don't think that is the solution. From my testing it appears 100% that it's too large of a main jet (whatever they call that on these carbs) since that is the only thing operating during the last 1/4" of throttle. Since I have no numbers on those I don't think it is wise for me to start pulling jets and testing. It could also be an issue with airbox. Since it's a car airbox there might be more restriction. If I re-design and put a better flowing airbox that could easily give more air and balance out the fuel issue. If not then I'm only out my time of building a new airbox.

I'm curious why aviation hasn't adopted the use of wideband o2 sensors in the exhaust for mixture control. Seems there are tons of sensors and ways to get the "proper" lean although the most simple thing is just monitor the mixture. I'm thinking of putting my dual handheld wideband system in the exhaust and monitor it to confirm all my other testing.

I think Bill has it right on the money. The enrichment valve. So far I've just been keeping it pulled back slightly and it works fine. I would rather not restrict the travel of the throttle though.

Several years ago Bob and I built an RV7A. We installed a brand new Lycoming clone from Mattituck. On take off you would give full throttle and the engine would pop and fart then smooth out on climb out , we also found you did not have to lean the engine at any altitude. Called Matituck, there response was that's the correct carb, "you have fuel pump or fuel flow problems". Talked with the folks at Precision and there comment was you need to drill out the enrichment jet. Allen said that you could put that carb on 5 engines and it would run fine, but would not on the 6th and it was just an inherent problem of that engine/ We put an enrichment kit in and problem was solved. Now you had to lean or you would burn a lot of fuel.

I find this interesting because the entire reason for having the enrichment circuit at WOT is to provide extra cooling to the cylinder by providing extra fuel. Which gauge is reading 20° cooler? As flyingfool mentioned, there are a lot of things in a carb that could cause problems, IF there is a problem. It is too hard to troubleshoot from here. Is there documentation on the carb? If the carb was overhauled at a repair station then all ADs at that time would have been accomplished. Someone may have done an overhaul or partial overhaul using a mix of different parts. There could be some AD related items as well. For instance, the conversion from the 2 piece venturi to the single piece is known to cause a rich mixture but that would also be considered "normal". Then there is the possibility it wasn't done correctly at the shop too. We had a 0 time Continental remanufactured engine come to us with the wrong cylinders installed a few years ago.

Someone may have installed the wrong carb. There are 14 different part numbers for the MA-3SPA. Sometimes the only difference is linkage but they change jet sizes too. The proper carbs for the O-290 series are part numbers 10-3565-1 and 10-3346-1. Neither are listed as discontinued on the Marvel-Schebler website. I should have come up with this sooner because it should have been the first thing I checked on.

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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

Bill, That is what I heard from a few guys with issues like that. Had carbs overhauled a few times with no luck and then finally get it to someone that improves the issue.

Jeff, that is exactly the issue. The extra fuel for cooling is just too much. Could be from someone changing jets out or messing with things. As stated before the only way to tell is to get all the original jets in the carb then start trouble shooting from there. Even if the numbers match on jets it's hard to tell if someone has modified them. I have went through some of the logs but not start to finish to see what else could have been changed. Mine is still a 2 piece venturi. That was something I checked as well.

I have the 10-3346-1. Aircraft spruce even lists the overhaul and complete rebuilds in their catalog although they said it was discontinued when I called about it. I called the Marvel shop and they told me they didn't have any kits on that carb. If they were able to get parts the overhaul was going to cost just under 900$ plus tax. It still is listed on their website which is what I brought up when I called. I could have also been told wrong by whoever it was that answered the phone. I doubt it was a technician.

All in all paying more for an overhaul on something that they aren't even sure they have parts for seems like a poor choice considering a new throttle body system is 800$ and new parts that are still in stock with a company.

I think that the re-working of the air box and getting in sufficient clean air is worth the effort and it MAY solve the problem.

If you still have the issue and you still believe it is over jetted. It seems like an economic solution to put in a known new main jet.

If those two options don't work, it sounds like you have made up your mind about the Rotec throttle body.

I have some experience with a ROTEC TB "injector" carb.
My son put one on his RV-4 so he could fly inverted aerobatics.
I suggest you go to the vansairforce.com site and search the forum database for Rotec, Ellison and TBI.
There is a large body of knowledge there and a few of my comments as well.
You may end up over carburated with the TBI as well, the result is a loss of power and lean operation. This is controlled by limiting full throttle travel, so a TBI might not eliminate your dilemma, just change it.
However, you seem intent on changing, and it's your plane.
Another point; Carbs wear out. Usually the throttle shaft -to- bushings wear and the shaft gets loose. Air can leak in around the shaft. TBI wear is at the throttle slide and the bushing seal around the fuel distribution tube. I have no experience with this one, it was mentioned in a TBI comment. If it wears faster than a Marvel Shebler some reliability may be lost?
I bought a ROTEC with the intention of installing it on a rear induction sump in my T-18. the idea was to eliminate the chin scoop on the cowl, remove the the fuel pumps and mount the TBI as low and as far aft as possible, to take maximum advantage of the gravity feed system. It turns out the engine mount is somewhat in the way, I would have to make a custom induction tube and place the carb farther away than I preferred. I still have the sump, & TBI so it's a project 'for later' while I work on other T-18 upgrades.

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Scott Emery
EAA Chapter 326
T-18 N18TE

While I was writing my last post, Flyingfool mentioned re-working your air box.
I fully agree. My T-18 has that ugly hooked chin air intake. It has no room for an air filter.
It uses a piece of filter foam across the inlet opening. The piece is maybe 3"x 8" probably less.
I have tried different foam types, all offer too much restriction, but some really cut the airflow and reduce power. Probably effect mixture as well.
I intend to stick with my Marvel, but cut off the ugly chin scoop,and adapt an RV-6 filtered air box (FAB) which uses a K&N filter element.

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Scott Emery
EAA Chapter 326
T-18 N18TE

Could be because oxygen sensors don't play well with leaded gas. I built a mixture monitoring system for my T-18 utilizing a single oxygen sensor. It's basically a copy of a unit that was available commercially at one time. It's a simple voltage comparator circuit that triggers an LED on the panel when you reach the correct stoichiometric mixture. The advantage over conventional EGT measurement & display is its almost instantaneous reaction time, & the little light that gets your attention. I tend to adjust & fine tune the mixture much more often for changing conditions with it. The big disadvantage is the approximately 200 hour life expectancy of the sensor before it fouls with lead. The coming lead-free fuels (or if you burn Mogas now) might help to make such technology become more prevalent on aircraft.

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

It looks like MS updated their website this morning. They quit listing the kits on the page with the carb but I found it listed under "repair kits" (MINOR REPAIR KIT - 778-902--F for $147.99 and DELUXE MAJOR REPAIR KIT - 286-1628--F for $495). Not that it matters since they told you they don't have any. It is almost as if they don't want anything to do with the smaller engines anymore even though there are a lot of them out there. I think they need a rash of inquiries about availability of repair kits.

I have not looked into TBI but I think if they were all they are cracked up to be, there would be a lot more of them in use. It's not like it is new technology (1940s?). I have seen it recommended to skip TBI and go for injected in another forum. Has anyone tried an automotive EFI system? My limited understanding of them is they can be programmed and tuned to the application which, in theory, would be leaps and bounds better and cheaper than anything available to the average aviator. O-290 cylinders may not support it though.

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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 think increasing the air flow with an improved airbox will help.

I'm not dead set on the rotec throttle body injection but David Read who is somewhat local to me has it on his plane so it would be nice running equipment that someone else has. He can help me set it up and has experience and hours with it.

Going to EFI fuel injection is a big jump in price and complexity. If someone wants to upgrade then I would say going to direct injection would be the best route but I would only switch to keep from having expensive outdated carb parts on my plane that will be nothing but future hassle. It seems a waste to dump money into something that is more difficult to mess with than the throttle body.

Jeff, I'll look at those kits and see if the cheaper one has jets or if I can find someone with jets to play around with. I've just found in the past the cheap fix of just tinkering actually ends up being more expensive based on time. Since I have little time outside of my office changing to something else with less hassle is well worth it to me. I'll tinker around a little bit more with it before jumping ship though.

Ah, true, the lead would foul them faster. I used to run 100LL in my drag car. Used it about 2 years without any issues with the o2 sensors. I think for my system they are around 40$ a sensor. 100 hours a year of flying puts that at a new sensor every 2 years. Not too bad for the price of accurate fuel mixture in all honesty.

That voltage light system sounds pretty neat. If you use a generic O2 sensor you could get them much cheaper than a nice wideband sensor.

The proper carb for a 125hp 290 is a 10-3346-1. For a 135hp 290d2 it is model 10-3565-1. The only difference between the two carbs is the nozzle. The carbs metering tube is called a nozzle instead of a jet like is normal terminology on autos and motorcycles. There is only one nozzle in the carb, not multiple ones. The nozzle number is 47-719 for the 3346 and 47-728 for the 3565.

If the engine is running rich, I would first check the float level with the clear tube method. If that is correct, you probably have too large of nozzle. Unlike an auto or motorcycle, your carb is equipped with an infinitely adjustable nozzle. It is adjustable with your right hand on the mixture control. I would fly full rich, wide open at low altitude and note egt (don't worry about it being too high as the actual number is irrelevant). Then lean to peak egt and compute the difference. This will tell you how rich of peak you are with the enrichening circuit active. You should see 100 to 150 degree difference. If more than that, you have too much fuel flow at full rich mixture. You could just live with it and pull the mixture to get the proper egt for whatever flight regime you are in, or spend some time to figure out why too much egt spread.


If you think the airbox is causing problems, remove it and do a runup without it to see if problem goes away.


Then check that the right carb is installed. If the model number matches the engine, that is still no guarantee that it has the proper nozzle installed or that it has not been drilled to a larger size. If the correct carb model, I would check that the correct nozzle is installed. If the correct nozzle number, I would check the nozzle id with a numbered drill bit. The correct id for the 47-728 nozzle is #42 drill bit (.093). I don't have the size for the 47-719 nozzle (would obviously be something smaller than .093).

I wouldn't chuck the carb if it is not worn out (excessive throttle shaft clearance, leaking accelerator pump, etc). This should be a fixable issue.

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Bernie Fried
N18XS