Rotec TBI Carburetor

I am running a Rotec TBI on my FlyCorvair engine in my Cruzer. When I initially picked it up, I couldn't find any photos or help with linkages, etc so this is what I came up with. 

The throttle bracket on the carb itself is cut from a piece of 2.5" square tubing. It mounts via the two rear main mounting bolts and 3 of the diaphragm bolts on the carb. I am running a single pilot-side throttle only. The bellcrank came from summit racing. I had to machine the push/pull link to connect it to the throttle slide. There is a small safety spring attached at the front connecting to the air box. If the bell crank or anything fails, it will pull the throttle full open. 

The airbox is my own design, roughly based on the popular van's airbox utilizing a round K&N air filter. The carb heat is a combo carb heat/cabin heat design like WW recommends. Carb heat works fine, we'll see if it is enough to heat the cabin. 

The mixture cable attaches to the block using two of the rear oil pan bolts. I'll have to get some photos of that.

Anyway, I hope this is helpful to some folks looking to use the Rotec. My plane isn't flying yet, but I have broken the engine in and done some taxi testing with this setup, it works flawlessly thus far. 

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Permalink Reply by Stephen M. Eastham on April 3, 2023 at 2:15pm

Hi Peter, Does the air enter your air box from the ambient cooling air and then accelerated because of your venturi design of low pressure to high pressure in the box with filter or is there another SCAT that attaches to the front round intake? I assume it is just air entering the box from the atmosphere inside the cowling?

How did you fabricate the carb heat exchanger around the exhaust pipe? I'm just beginning to get into fab work and do not have welding skills yet. Is that TiG welded?

Permalink Reply by Peter Eiberger on April 3, 2023 at 8:46pm

The front opening connects to a NACA duct via some SCAT tubing. The NACA duct is riveted to the nose bowl below the spinner bringing fresh cool air into the carb

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Permalink Reply by Stephen M. Eastham on April 4, 2023 at 9:27am

Thanks for that. Did you have to connect a return fuel line and the shutdown drain that Rotec mentions in the manual?

Permalink Reply by Peter Eiberger on April 5, 2023 at 9:19pm

Update 2: 33hrs into Phase 1 testing.

I have transitioned to mogas (Auto Fuel) over the past 8 hours. I have been filling the left tank with 100LL+Decalin and filling the right tank with 91 ethanol free + VP Octanium Unleaded (to bump it to 93 octane). This lets me test the auto fuel but still be able to switch back to 100LL.

Things I have noticed running mogas (most likely still winter blend) on GRAVITY FEED ONLY:

  • Idling at low speed with carb heat over 50*F will induce vapor lock and cause stumbling. This is easily cleared by either purging with the purge button or simply "blipping" the throttle and removing the carb heat.
  • Engine will not idle AT ALL over 80*F ambient without constant use of the purge button if the airplane is below flying speed. This was after a flight, so the engine was at full operational temp (205* oil, 350* CHT). After I opened the cowl, the carb body was hot to the touch, but no so hot that I couldn't hold my hand on it.
  • At higher engine speeds(1500 rpm+) the engine runs just fine at 80*+ ambient, but that's too fast for taxi and the plane wants to float in ground effect forever.

I could not replicate these issues with 100LL+decalin, which should not come as a surprise given the large difference in vapor pressure between 100LL and winter blend mogas.

For me personally, running on mogas is a requirement, so I will be reinstalling the fuel pump and be adding a return line (with a check valve) back to the right side header tank. The constant flow of fuel through the carb body will accomplish two things: purging any vapor back to a header tank (where it can then escape through the vent back to the wing tank) and cool the body of the carburetor. This should fully eliminate vapor lock based on my conversations with Rotec.

My plan will be to use the fuel pump for taxi, takeoff, and landing only... like I had originally planned.

Permalink Reply by John Austin on April 6, 2023 at 7:22am

Peter, I don't know if it is relevant to your carb/fuel line configuration, but I also have a TBI carb (Sonex "AeroInjector" installed on a Jab 3300) and there are many instances of those carbs having the "burps":  Momentary hesitation of the engine due to vapor bubbles passing through the carb.  However, contrary to your experience, most of the AeroInjector Jab 3300 equipped engines will idle OK.

What was emphasized by AeroInjector support was to ensure that from firewall-forward, keep the fuel line run as short as possible and that the fuel line always must travel upwards with no downward turns that could act as a trap for vapor bubbles to collect.  It also helps to insulate the fuel line to keep down heat soaking.  (I used fire sleeve and a second layer of heat reflecting sleeve.)

My engine would occasionally "burp"/hesitate prior to this, but once I routed the fuel line properly and insulated the  fuel line, I have had ZERO "burps" running 93 no-ethanol mogas for hundreds of hours.

John

N750A

Permalink Reply by Peter Eiberger on April 6, 2023 at 8:30am

The Rotec is a bit different since it has the diaphragm to regulate pressure internally, allowing it to run on anything from gravity feed to 15 psi, but that's likely where the vapor is building up since using the purge button clears it (purge button just manually opens the diaphragm). 

My fuel line is firesleeved and I have a blast cooling duct for the gascolator.

Permalink Reply by Stephen M. Eastham on April 6, 2023 at 8:09am

I'll be running a Corvair on my 601 HDS with wing tanks only. Took out my header tank to make way for an avionics bay. Thanks for the information of different types of fuel and temps. It get's pretty hot here in Phoenix, AZ.

Permalink Reply by Peter Eiberger on April 6, 2023 at 8:47am

I seriously considered changing out the fuel selector valve for a duplex feed/return unit. A newton unit is nowhere to be found currently and I didn't want to redo the entire console panel for a different fuel valve.

Cheaper options:

Anderson

Grocco

Permalink Reply by Stephen M. Eastham on April 7, 2023 at 9:26am

I need to weld my own intake and exhaust pipes for my Corvair engine and Rotec carb. Do you know what individual piece parts and tubes I need to buy from Burns Stainless or other vendors in 304 1.5" and the piece parts, flanges, etc? I don't have the jig and fixture but I do have a set of exhaust L/R from Azalea that were from his scrap bin I can use as a port to port reference for a jig/fixture. I guess I can just eyeball the intake pipe measurements and add a 13 deg intake flange to clear my rear starter and flywheel.

I've asked welders locally but they don't want the "liability" so I am going to take a welding class and practice first. I'm not welcome at WW's anymore as a Spyder owner (spent $3K there) and Azalea screwed me after $5K spent there and owes me parts. Can't get local welders to help me in PHX AZ due to "liability." This should be interesting. 

Permalink Reply by Peter Eiberger on April 10, 2023 at 4:39pm

If you want to learn to weld for your own education and ability, great. Learning to weld is an invaluable skill, I won't discourage anyone from learning. That said, if your goal is a flying plane, you are adding a fair amount of delay and additional risk to your build. 

As I stated above, I made my flanges (on my lathe) and clamps, I don't know if you will be able to purchase those easily. 

If you want to read some 20-year old info on exhaust: http://www.n56ml.com/corvair/exhaust/

Permalink Reply by Stephen M. Eastham on April 10, 2023 at 6:59pm

Thanks. I've been through that. I'll just have someone copy the scrap flanges Azalea sent me or just re-use them since they are not proprietary. Just need the exhausts and intake for my Rotec TBI and I'll be closing her up and searching for an A&P to inspect her.

Azalea and WW will no longer sell parts to me though. Azalea stole my paid oil cooler and sent defective exhausts and WW doesn't want a Spyder owner using his site (spent $3K on common parts there with him including a new prop, Rotec and oil cooler). Very territorial this Corvair stuff.

As a retired military pilot, I don't have time for games with these Corvair guys and will solve the problem eventually.

Thanks for the help.

Permalink Reply by Peter Eiberger on June 1, 2023 at 6:40pm

Well, hot weather brings hot weather problems.

PLEASE NOTE: I do not have the Weseman/WW lower cowl, I made a custom one for myself.

Anyway, at 85*F+ ambient temps, I could easily get the right (#1) CHT over 380* measured under the head in a climb (60-65kts @3150 RPM). The highest I saw was 392*F before I chickened out and lowered the nose. The way it was climbing, I am confident it would have easily gone over 400*F. 

So the only logical course of action was to increase the exit area. I cut the lower cowl flap and opening forward of the nose gear and made a new air deflector. 

The results were worth it! Peak CHT in climb will not exceed 360, even at continuous full power climb (3150 RPM). Cruise temps also dropped 30*F, and oil temps dropped over 10*F. I haven't noticed any loss in cruise speed due to the additional cooling drag either. 

In hindsight, the elevated under cowl temps could have been a contributing factor to my vapor lock problem. I have spoken with a couple others who use the Rotec with the Weseman cowling and had no such issues. 

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