Rotec MKII TBI

[142YX]

Finally got the new Rotec TBI MKII installed into 142YX and attempted to fly with no luck. In fact.. the engine quit on me on the takeoff roll approximately 5 MPH before liftoff (thank goodness). I had done a series of engine runs with the following results leading up to the flight attempt:

• Idles strong with no indication of quitting at ~850 RPM
• Able to achieve 2850-2900 RPM static on the ground (prince prop)
• Engine quits at full power when the mixture is pulled out, continues to run with mixture full out when at idle (expected response)
• No hesitation from idle to WOT with moderately fast throttle advance
• Some hesitation when slammed from idle to WOT (however this is documented and expected as there is no accelerator in the TBI)
• Ridiculously rich. Stupidly rich. Insanely rich. At WOT, I was pulling ~18 gph (with trends that look like it was still increasing at the point where i had to pull back for CHT's), with equally absurd values at mid throttle settings. Idiotically rich fuel burn rates validated through back smokey exhaust, and an independent fuel flow calibration test where i validated the fuel flow transmitter to be within at least 10% of reality.
• ~2-3 GPH burn rate at idle
• Never once quit on me or made me think it was going to quit (unless i was purposely trying to make it quit)

With hindsight being 20/20 - I should have never attempted the flight with fuel flow being that high - that does not make sense. I feel dumb, but hey - I am sharing this so that maybe someone else can learn from it and not repeat the same mistake. The attempted flight went like this: 3-5 second advance to full power, expected response, I noted seeing ~2900 RPM or so with all engine parameters in the green, and then it spontaneously quit somewhere north of 40 MPH IAS.

When I pulled the mixture knob out at WOT, burn rate would diminish slightly.. but it was still much to high (13 GPH) or so before it would become very rough and quit. My thinking in attempting the flight, which was tempered by the frustration of not being able to do anything more than a 10-15 second WOT ground run before reaching CHT limits, was that "so what" if it was running way to rich.. it was running and seemed smooth. Getting to altitude would both keep the engine cool and be more representative of the actual operational condition - and i could feel out the mixture setting there. Didn't work out that way.

After the attempted flight, my initial thought was that I was over-carburated for my engine. There are notes on Rotec's website about setting the throttle stop something short of WOT being a normal thing some people have to do. In another engine run done after the fact, I was able to achieve 2900 RPM at 3/4 throttle, opening the throttle even more REDUCED the RPM by ~50-100 or so. So then i re-adjusted my hard stop to that 3/4 position.. but the fuel flow was still 17~18 GPH! Subsequently, I have asked some other people and they have confirmed that they do not reduce the throttle stop at all for their installations on a 3300. I should note that I am running a TBI-40-S (40 mm throat, the recommended model per Rotec for the 3300). Is anyone running the smaller (34" mm) TBI on a 3300?

I would be very curious in hearing from anyone with a TBI on a 3300 the following data points which might help me diagnose if there is something wrong with my TBI or not:

• What size throat are you running?
• What static RPM do you get / with what prop?
• What is your fuel burn rate at WOT and at cruise?
• Do you restrict the throttle to something less than full capability?
• What range of the mixture adjustment do you use?

Any other advice or insights would be welcome!

Right now i have an email out to Rotec.. no response yet but it has only been 2 days. Right now my thought is that there is something wrong with my unit. In the troubleshooting section of the website it states that regulator spring length / size and leaky teflon seals are worth checking for overly rich conditions.. i have yet to do those steps but they are probably next on my list.

Below is a bunch of photos from my setup, as well as an excerpt of the data from the ground run series that i did. The chart here is the response when i was limiting myself to 3/4 throttle.

Thanks!

[fastj22]

I've got about 10 hours on my Rotec TBI now.

I think your idle mix is way too rich. Its unrelated to the mixture adjustment of the spray bar. Its probably dumping extra fuel in.
Mine came a bit too lean and I couldn't keep it idling. ½ turn rich on the idle adjustment fixed that. Perhaps a bit too rich as I have to lean aggressively on takeoff.

Prince P-tip.
I set mixture about ½ way for takeoff. Anything richer and it runs like crap.
I see 2800 static at runup.
10 GPH on climb out at WOT with EGTs in the 1200s (much more than I saw with the Aerocarb)
I see 5 and 6 going a bit lean at WOT and pulling back throttle a bit brings them even with the others. (over carbing?)
In cruise, I'm seeing around 6GPH with EGTs in the 1300s and RPM at 2800. (about the same as the Aerocarb)
WOT at cruise I will see 3100 RPM (never saw that with the Aerocarb).
I had intended to install a boost fuel pump, but decided to try with just the gravity feed. Its working fine with gravity only.

I did use an air straightner that Tony used. It won't effect your high fuel use, but will make your EGTs much more even.
https://picasaweb.google.com/tonyboytoo ... 7823613170

Overall, I'm pleased with it. EGTs are very even. Throttle and mixture are silky smooth. And I've seemed to have gained 200 RPM on climb.

[SonexN76ET]

The Rotec TBI documentation needs to be more thorough in my opinion.

Here are three things that are not clearly spelled out by Rotec that could be affecting your installation:

1) your engine quitting may be due to you advancing the throttle rapidly on takeoff. It takes a second or two for the fuel to run past the fuel pressure regulator to the spray bar after a quick application of power leaving you without fuel in the spray bar for an instant or two, but sometimes long enough for the engine to quit. This can be prevented by moving the location of where your throttle arm connects with the throttle cable causing you to have to make a longer throttle throw, thus slowing down the speed the throttle opens on the Rotec. Think in terms of gears. I am using a Vans Aircraft RV8 throttle and had to move where my throttle cable connects to a point closer than stock to the pivot point on the throttle arm. Another option is to tell yourself to throttle slowly but what if you forget or you loan your plane to another pilot?

2) you have to put hard stops on your mixture lever that limit the normal range of operation. These are NOT the same as the stops on the body of the Rotec at the mixture arm. My engine will hardly run at full throttle if I go to full rich according to the body stops on the Rotec. My hard stop for full rich is about 1/2 to 2/3rds of full rich. All the Rotec doucumentation says is "your full rich will certainly be less than the full rich position". This took a lot of trial and error to find the correct range.

3) like John suggested, you need to check your idle mixture separately. It seems as if you are too rich there. Also, look into an air straightener.

Two other things, if your solenoid deploys by accident depressing the primer when you are at full throttle your engine will quit then too. Second, I am concerned about your throttle cable being at an angle to the throttle slide arm on the TBI. Please consider it being in perfect line with the movement of the throttle slide. Sometimes the Rotec throttle gets very stiff and I distorted my cable and it became worthless and had to get a new cable. Also, make sure the cable can not come out of its keeper on the mount you fabricated on the TBI. On the previous page you can see an example of my mount.

On a closing note, it could also be that you just have a defective unit and need to get it replaced.

Please be careful with the Rotec until you get it all figured out.

Jake

[WaiexN143NM]

Hi Nick,
Sorry to hear you are having some issues with your Rotec TBI . I know u are anxious to get flying again. I was one that highly recommended the Rotec TBI. We love ours. Thanks to John ( I highly encouraged john to get one) and to Jake for input and spot on suggestions. Good eye Jake, I went and looked back at Nick's pics. First nick u have the right Tbi. The 40-S (spigot mount) . We have a jab 3300 w prince prop. We get 3000 static WOT on the ground . The flows are just like john's. A couple of things about the pics. First , as jake said , straighten the throttle cable with a longer bracket or rubber adell clamp. Also we drilled 2 holes and safetied wired so the throttle arm cannot come out of the slot. Also the nylon nut holding the clamp u used a longer bolt when u mounted on a/c right? . We have the mark one version , but on the regulator primer we installed a mechanical arm with a cable to the cockpit. Any chance the solenoid u installed is activating and causing extra rich mixture? I suggested to john to try gravity feed only as we have ours. We have never run the tank below approx 5 gal. Below that is there enough head pressure? Don't know. I posted once to Jake , he was worried about head pressure, also vapor lock. We fly ours in Tucson heat , never had a problem. I believe he also ran a return line to the tank with a electric pump(facet?) hang in there don't get discouraged. You'll get it solved. Also not nitpicking anything. I know u work at Scaled and your plane is impeccable. You could teach us all a thing or two. Looking forward to seeing u on the flight line and flying your 'green' machine.
WaiexN143NM
Michael Radtke

[WaiexN143NM]

Also , one more thing Nick, I ordered from aircraft spruce a stainless flow grid, about $100, is for the Ellison TBI , but is sized the same for the Rotec. The carb heat box they also sell for the Ellison was way to big, but used it as a guide to build a smaller one, and sent it back. Carb heat box on a tbi is a whole nother thread. The flow grid keeps the cht/ egt's even.

WaiexN143NM
Michael Radtke

[142YX]

Hey John, Jake & Michael, Thanks for the replies - all good pointers and much appreciated feedback.

• I will double check the idle mixture setting. Right now i believe it is about 7/8 ths of a turn from fully shut (most lean). Initially i thought i had it dialed in to give a really consistent smooth setting at ~850RPM or so, but worth revisiting. Maybe opening up the low end throttle stop, then leaning it out a bit?
• I hear ya on the rapid throttle advance.. and i did see hesitation in my static runs when i SLAMMED the throttle forward - but any reasonable throttle advance was met with a very smooth throttle response. I am certain that the throttle advance I gave on the takeoff attempt was at a lower rate than i gave it during the static runs. Below is the data from the takeoff run - it shows that the engine got to at least 2800 RPM over a ~8 second slow advance. I remember looking down and noting that everything was in the green, then looking back up to see the prop abruptly stop. Shows the fuel burn north of 12 GPH but with an upward trend that looks like it wasn't about to stop.
• On the takeoff attempt as well as the static runs (shown in the chart above) my mixture was somewhere between 1/2 and 2/3 rds or so.. but still pulling all that fuel. When i tried to lean it out i could get it down to about 12-13 GPH before it started running really rough. Hard to tell though.. as stated previously i can only get about 10-15 seconds of full power static testing in every 20-30 minutes or so before CHTs become a problem.
• No chance the solenoid was pushing the regulator override button unintentionally.. it is sized so that there is a 1/16" or so gap whenever the solenoid is not energized
• Agree that the angle of the throttle cable looks a little silly, however i got cornered into this setup when by committing to mounting the TBI vertically. It must be that close to the side of the throttle body in order to sneak by the right hand distributer cap on the back side of the Jabiru. If the cable were parallel to the side of the TBI, it would be colliding with that distributer cap. The push-pul cable claimed to be rated to 15 degrees of angular offset at the point of that swivel however, and i am using less angle than that. The slide on my TBI is smooth and there is no issue that i am aware of with the slide operation. The push pull cable is locked from falling out of its groove by the adel clamp. Michael - good eye on that bolt length, but it is actually an optical illusion because the nut is not tight in that photo. There are two or three threads showing here in the final install. Not trying to be defensive here.. just trying to explain the process i went through to end up with that. But as far as that slide operation, i trust it, even with the silly looking angle in the cable.
• I originally flew # 142 on gravity feed with the aero carb, and was having vapor issues (burps) in the Mojave heat - which was really the deciding factor for me to go to a pressurized fuel system, and the TBI. With that setup, a fuel flow test yielded only about 11 or 12 GPH flow rate for the last 5 gallons, which was not enough to convince me as it did not meet he 1.5x burn rate thumb rule. I did have the fuel flow transmitter in the line which added some restriction however, and never tested how much better it could have been if i was willing to give up that instrumentation.
• Good call on the flow grid/straightener - i just put one on order from spruce.

[Bryan Cotton]

I am curious about the flow straightener. Does it only bring goodness to a Rotec on a Jabiru? Or only on a Rotec?

[lpaaruule]

Wow, scary. I don't have experience with the MKII, but on the MKI that I helped my friend tune, we were never able to get that high of fuel flow unless the primer was pressed. We had the opposite issue, it was too lean at WOT; even with the idle mixture set way rich. Even after enlarging the holes on the spray bar, the fuel max'd out at approx 10gph.

It seems like a regulator issue. What doesn't make sense though is you said you leaned to around 13gph and the engine ran very rough and quit.

I'm also suspicious of the primer solenoid. If it's somehow activating intermittently, that could explain the cutout on the takeoff roll.

According to your RPM data, it looks like you advanced the throttle fairly slowly, so that's probably not the issue.

Sorry all I have is speculation, but it sure seems like a defective unit. I'm very interested in hearing what Rotec says about this.

Good Luck

[fastj22]

I am curious about the flow straightener. Does it only bring goodness to a Rotec on a Jabiru? Or only on a Rotec?

I think it would benefit all aero carb motors too.
Just read that VW is going to install them on their cheater diesels to get them to run more efficiently.

[fastj22]

I'm suspecting a faulty pressure regulator now.