TBI and Rotax installations?

Single TBI Approach

Rotec Aerosport recommend Rotex owners simplify their engine by replacing the 2x standard Bing carburetors with a single TBI-40-3. Instead of having 2x fuel lines, 2x throttle controls, 2x chokes, 2x air filters, and all the cables and hoses to accommodate dual units. All that is required is a bridge manifold to converge both intakes. Please see the thumbnails of the Ion Aircraft.

The following is an unmodified email questionnaire to Steve Schultz at Ion Aviation.

From: Steve Schultz [mailto:Steve@ionaircraft.com]
Sent: Tuesday, 21 February 2012 6:37 AM
To: rotecengineering@bigpond.com
Subject: Re: Rotec TBI some info about your installation

Howdy Paul;

The TBI was one of the best decisions we’ve made. From the first day we had the Rotax we were frustrated with the dual carbs. Two fuel lines, two throttle controls, two chokes, two air filters, cables and hoses everywhere, etc. And although we got proficient at the Rotax carb-sync procedure, we never liked it. It was just another hoop to jump through.

1. What was involved when installing the TBI to your Rotax, was the installation overly difficult?

It was not overly difficult at all. I will send you a drawing shortly, but the short version is we fabricated a single plenum with two runners feeding the existing one-into-two induction castings. We spent more time figuring out the cable routing than we did making the actual induction.

2. What have been the main advantages when using the TBI compared to the Bing set up?

The engine runs much more smoothly across the RPM band, the ability to lean the engine, slight reduction in weight, VASTLY reduced parts count (especially moving parts) much simplified fuel and control cable routing.

3. Was any gain in power found?

No, but for the same power we are burning a little bit less fuel.

4. With the leaning feature have you established an optimum fuel burn for the Rotax?

Well, we have established that we can run LOP without trouble. We have not tried to find a particular optimum as we tend to run the throttle and mixture as the need dictates. We have found that the the engine burns much cleaner and we have seen fuel consumption reduced by approx 3L/hour for equivalent RPMs.

5. Why did you opt for one single TBI-40 when two TBI-34’s could have be used?

Single point of fuel/air induction. Two -34′s in many regards would have been similar to the Bings, i.e. two of everything, etc. We were very intent on having one body of air/fuel feeding both sides of the engine, one fuel line, one throttle cable . . . etc.

6. In regards to mixture control how have you found this in operation?

Very smooth, very simple, pain-free. We have EGT’s on all 4 cylinders, which I would recommend for best results and peace of mind.

7. Once the TBI unit was physically installed, how much did it take to dial it in?

About one half-day of farting around with the throttle stops and the idle mixture screw. No drama.

We did find that we needed to move the pressure regulator. The original location allowed the reg to pick up too much heat and was boiling the fuel in the reg (auto gas, not avgas). In a traditional airplane–not a pusher like ours–this probably would not have been a problem because we could have routed cooling air over the reg. As it was we moved it and it has been fine since. Since then, no problems.

8. Is the engine easy to start when using the TBI, what is your start procedure?

With our set up it is slightly harder to start than with the Bings, but we strongly suspect that this is specific to our installation and not indicative of a problem. Our system has a relatively large internal volume for the fuel/air mix. We think that’s why the engine rarely catches on the first try by always starts on the second try–it’s just taking a moment longer to fill the volume. Once the engine has been run, even for a moment, subsequent starts are immediate.

• Throttle to idle, mixture full rich
• Electric boost pump on, wait for fuel pressure (about 3 seconds)
• Actuate primer plunger on the pressure regulator (3 seconds again)
• Immediately engage starter
• “Blip” the primer 1-2 times while cranking

9. Have there been any down sides to using the TBI over the Bing?

None. We are very glad we did it and we would do it again. Some folks may fuss that they have to manage the mixture instead of leaving it to the Bings, but that is a short-sighted view.

http://www.ionaircraft.com/