Rotec MKII TBI

[142YX]

I'll double check that my primer solenoid hasn't somehow become mechanically jammed.. but there is no way that the solenoid was getting power during those runs. Secondary indications like current draw reported from the VPX confirm it. GoPro in the cockpit confirms that at least for the takeoff attempt which was recorded, the primer switch is in the down position. Also, every time the engine shut down, fuel pressure held (and even increased because of heat soak back) which tells me the primer wasn't depressed and the regulator seal o-ring is still intact (and also the check valves in the engine driven pump are holding well).

But next on my list is to do a teardown and check that the regulator spring is the correct one / right length. It is listed on Rotec's website as a troubleshooting step for being too rich. Agree that a somehow faulty regulator is one of my leading theories to all of this.

[fastj22]

It does sound like the regulator is not regulating properly. Not sure how you test it though.

[142YX]

I came back to check on the airplane mid this week, and found a 1-drip-per-second fuel leak coming out of the bottom of the TBI. I had left the fuel shutoff valve open, but i have done that in the past with the TBI and it had never leaked on me before. Another data point.

The more I thought about it (whilst still waiting for a response from Rotec) the more I convinced myself that one of two things could have been the problem:

1. A problem with the regulator, over regulating, but still being intact as pressure would hold with the engine shutdown, etc and it didn't leak for at least the several hours i was observing the airplane after last use last weekend. Probably on the hairy edge.. because at LOW pressure after some time, the leak developed. My theory was a bad spring, or the wrong spring. But the seals in the regulator must have been good or it wouldn't have held pressure! -OR-
2. Bad seals downstream of the regulator.. the teflon seals in the TBI housing or slide body around the metering tube.

I pulled the TBI apart today looking for smoking guns.. here is what I found.

No visible issues with the regulator assembly



Diaphragm was intact and looked to be in good shape. I noted that it looked stretched out a little bit.. but not so excessively that i suspected any problem there. All of the seals / o-rings were in good shape and held pressure.



The spring measured 14.5 mm, uncompressed



Wire diameter on the spring.. 0.33 mm



All of the metering tube seals within the slide assembly were in great shape. I tested them with ~10psi air pressure and they were all impressively tight.
This was the first time i pulled open the slide assembly on my TBI. I found that all of the throttle slide stiction (at least from mine, which was not that bad) came from excess loctite 515 that hand run down into the sides / on the slide itself. I really wish an assembly like this was not put together with glue.



So the only thing that was of any note was the spring. Rotec's website (which is down today, 12/6/2015, for some reason - but links provided below for if it ever comes back up, i hope) state that an under-lengthend spring can be the cause of both an overly rich running engine, and a source of leaks when shutdown. I think this makes sense. The second link goes on to describe that the spring should be 15mm - 17mm in length, and can be stretched out or cut to get it to the proper length.

http://www.rotecaerosport.com/faqs/tbi-rich/
http://www.rotecaerosport.com/faqs/tbi-regulator-flow-valve-spring/


So there is one documented discrepancy, my spring is 0.5mm too short, per Rotec's specification. I suppose i should be happy that i found something - but this wasn't a home run for me. That spring is so tiny and dainty - i can't see how .5mm (or even 2.5mm) of additional length are going to make the problem of 2x expected fuel flow to go away. Also.. the spring is a pain in the neck to install into its little retention boss within the regulator body.. these things make me question if it is the right spring at all. My gut tells me it is too weak of a spring overall.

Can anyone confirm that? Does the spring in your TBI look that tiny and fragile?

I have a second email out to Rotec now with that question. Waiting patiently for their response.

[SonexN76ET]

Nick,

The only people who seem to return emails at Rotec are Andrea and Paul Chernikov, it seems to be hit or miss with the others. On some subjects they did not choose to return my emails. For instance I sent three emails to ask them if they knew of an electric carb heat system that was compatible with their TBI. This was at the time when they had stated on their website to use electric carb heat or heat muff carb heat. I don't know why they chose not to respond. I also received conflicting information on the subject of a very stiff slide. I had mentioned in my earlier post that I had destroyed a throttle cable due to the excessively stiff slide. It turned out I had some metal to metal contact on the slide arm and TBI body due to the milling being slightly off. A little work with some scotchbrite took care of the problem. Not meaning to ramble, but you will need to be assertive to get some answers from Rotec.

I have not removed the spring you have in question.

There are two things I would like to share that I found out recently that may help you in your trouble shooting:

1) if you have your idle mixture too rich, your left side cylinders will run much richer than your right side. This is due to the idle jet being over on the left side of the TBI if aligned with the slide running fore and aft to the direction of flight. On my Aerovee it is a difference of over 120 degrees in EGT.

2) on my last two flights (including one this evening) when attempting to do touch and goes when I advanced the throttle to full power for takeoff I lost all power and had to pull the throttle to idle and abort the takeoff. Not fun!!! The engine did not stall and did not surge or hesitate. I advanced the throttle very slowly. It seemed to happen after I was airborne and as I was passing 3/4 throttle. I am trying to figure out how to correct this. I am considering restricting the throttle full open position to see if that fixes it. Today's flight I tried a richer idle mixture setting and that did not solve the problem so I will be leaning my idle mixture a bit to get more balanced EGTs once again. I have cold air induction and will also try touch and goes with alternate air selected to see if that helps perhaps by eliminating rapidly changing air pressure at the same time of increasing throttle.

Please keep us posted on your findings. Most of all be safe!

Jake

[lpaaruule]

Hi Jake,

Do yo know what your fuel flow is at full throttle?

My friend's rich idle mixture setting couldn't supply enough fuel at full throttle. As I've mentioned before, we ended up enlarging some of the holes on the spray bar. It fixed the problem, and he no longer had to restrict his throttle, or advance as slowly. For the Jabiru 3300 he went from 6gph to 10gph after the modification at WOT.

Your issue sounds similar.

[WaiexN143NM]

Hi Nick,
Interesting reading , please keep updating on your trouble shooting. John gillis and myself running gravity feed only on our jab 3300 w rotec carb waiex's. No trouble with ours. Nick and jake, are u running a pressure (facet pump) inline? We set our slide to be fully open at a WOT. Ground static is 3000RPM, a little more in the air. I believe the VW carb install is horizontal, the jab is installed vertical. Not sure if that makes any difference on anything. Paul told us at Oshkosh that you don't have to shut off the fuel valve like the aerocarb after shutdown , but that is not true, it will drip ! , not as much but still drips. The spring is small in the regulator. Stretch it 2.5 mm to the max of 17 . Also how are u venting your tank? We originally had a small length tube coming up vertically and bent into slip stream. Changed before first flight to a brake line picked up at Kragen/O'Rielly Auto to go down to bottom of cowl , and bent into slip stream. Small relief hole on back side in case bug etc were to hit and plug up opening. We didn't want any fuel misting out of vent to get on the lexan windscreen. Nick have you checked your vent tube ? Your plane sat for awhile awaiting your rotec, did u have your vent capped? Make sure no bugs tried to build a home. Anybody else have some ideas??
WaiexN143NM

[142YX]

Hi Nick,
Interesting reading , please keep updating on your trouble shooting.

You bet. Andrea from Rotec got back to me last night and told me they had a busy week, and that Paul is getting to the technical emails (which have backed up a bit) very soon. I very much appreciated this email letting me know what was going on. Ill post here with Rotec's answer as soon as I receive it.

I noticed that Rotec's website just changed to a new format.. and those links that go to the lean/rich, and regulator spring troubleshooting sections no longer work. Lucky me that i just came across them the day before they went down.

Hi Nick,
John gillis and myself running gravity feed only on our jab 3300 w rotec carb waiex's. No trouble with ours. Nick and jake, are u running a pressure (facet pump) inline?

I have the Jabiru stock mechanical fuel pump back on the engine, as well as a Facet pump (upstream of the mechanical pump) on there as a backup. I wanted to use gravity feed a not to violate the K.I.S.S. rule.. but as stated previously I could only get 11~12 GPH (out of the last 5 gal of usable fuel in the tank) fuel flow rate out of my old gravity feed system and that didn't give me the 1.5x margin on the engine's requirements that I wanted to sleep at night. I could have improved that a little bit by chucking the fuel flow transducer (never quantified how much that would have improved things), but.. well.. I like toys and wasn't willing to do that. I like the situational awareness of having fuel flow indication gives you. I did a fuel flow test of the current system the way I have it now (with fuel even flowing through the gallery in the TBI to be as realistic as possible), and the results were: ZERO FLOW with the pump off (head pressure from the Sonex tank was not enough to get past the check valves in the engine driven pump), and ~26 GPH with the facet pump on. No way to test just the engine driven pump, however. Both pumps put out about 4.5 - 5 psi.

Hi Nick,
We set our slide to be fully open at a WOT. Ground static is 3000RPM, a little more in the air. I believe the VW carb install is horizontal, the jab is installed vertical.
WaiexN143NM

Good data point. That's what I was hoping for in my setup as well. Other people that i have talked to have confirmed that they don't restrict their throttle opening on a 3300 as well. My TBI is mounted vertically (throttle slides downward to open).

Hi Nick,
The spring is small in the regulator. Stretch it 2.5 mm to the max of 17.
WaiexN143NM

Crap. I was hoping you didn't say that! I was hoping for a real smoking gun being that I had the wrong spring or something, aside from just being a tiny amount out of spec by length. I want to wait for Rotec's official response before I stretch out the spring.. but I will likely try this soon. If this is really the case.. it bothers me that that spring is so sensitive and critical to the unit's functionality. IMHO, If it really needs to be that sensitive, the design should have an adjustment (setscrew) that can adjust the spring's pre-load, like any other sophisticated metering device with a regulator. The setscrew would function like a high-end adjustment. Come to think of it, this could be a design improvement. Maybe I'll build my own carb! :lol:

Hi Nick,
Also how are u venting your tank? We originally had a small length tube coming up vertically and bent into slip stream. Changed before first flight to a brake line picked up at Kragen/O'Rielly Auto to go down to bottom of cowl , and bent into slip stream. Small relief hole on back side in case bug etc were to hit and plug up opening. We didn't want any fuel misting out of vent to get on the lexan windscreen. Nick have you checked your vent tube ? Your plane sat for awhile awaiting your rotec, did u have your vent capped? Make sure no bugs tried to build a home.
WaiexN143NM

I have the same vent tube setup as you describe on yours. I'll check for a clog.. but my thinking is that this would only contribute to a lean running engine, not rich?

[142YX]

1) if you have your idle mixture too rich, your left side cylinders will run much richer than your right side. This is due to the idle jet being over on the left side of the TBI if aligned with the slide running fore and aft to the direction of flight. On my Aerovee it is a difference of over 120 degrees in EGT.

I'll check that.. currently my idle adjustment was set at the high end of Rotec's recommendation. They say "somewhere between .5 and 1 turn from full shut.. no more than 1.5 turns open". Mine was set at 1.5 TFFS during all of this. I think I'll play with that again as i would rather be in the middle of the range somewhere.. but i haven't gotten a chance to note EGT's that are real yet as i haven't gotten in the air.

2) on my last two flights (including one this evening) when attempting to do touch and goes when I advanced the throttle to full power for takeoff I lost all power and had to pull the throttle to idle and abort the takeoff. Not fun!!! The engine did not stall and did not surge or hesitate. I advanced the throttle very slowly. It seemed to happen after I was airborne and as I was passing 3/4 throttle. I am trying to figure out how to correct this. I am considering restricting the throttle full open position to see if that fixes it. Today's flight I tried a richer idle mixture setting and that did not solve the problem so I will be leaning my idle mixture a bit to get more balanced EGTs once again. I have cold air induction and will also try touch and goes with alternate air selected to see if that helps perhaps by eliminating rapidly changing air pressure at the same time of increasing throttle.

Interesting. That sounds a bit like what happened to me on the ground a few times, but it would only happen when i absolutely slammed the throttle wide open from idle. Anything at a realistic rate of what i think i would do in flight produced no hesitation.

[fastj22]

Just put another 2 hours on mine. I recently recalibrated my fuel system and found I was getting 25 GPH through the fuel feed disconnected from the carb.
Reconnected everything.
Took it out and it ran very well. Was seeing around 6GPH in cruise at 2700 RPM. Bumped to 8GPH at 3000. I have never seen over 3000 with my Aerocarb, but I'm seeing 3100 with the Rotec if I really push it.
EGTs were very even, something I never saw with the Aerocarb (but I do have an air straightener on this).

If you are seeing a more than 10 GPH at WOT, something is wrong and I'd bet its that voodoo regulator.

[142YX]

I set mixture about ½ way for takeoff. Anything richer and it runs like crap.

John, Can you confirm your full rich position is at the full rich of the TBI's capability? (so when you say 1/2 mixture.. you mean 1/2 on the TBI itself?) If not.. where (approximately) do you set your full rich cockpit stop relative to the TBI's stop?

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?)

I had intended to install a boost fuel pump, but decided to try with just the gravity feed. Its working fine with gravity only.

Just put another 2 hours on mine. I recently recalibrated my fuel system and found I was getting 25 GPH through the fuel feed disconnected from the carb.

You get 25 GPH fuel flow with your gravity feed system and no pumps (the engine driven pump is not installed in your setup also.. correct?)??! Very impressive. You quoted some in-flight fuel flow numbers.. do you have a fuel flow transducer? what kind? Was it in the flow path for your hanger calibration? If i could have gotten those kinds of flows, i never would have installed the pumps. Although, per Rotec, it shouldn't make a difference...


I must say i am jealous.. the performance you are getting is absolutely what i was hoping for!