RevMaster: rough on take off only
Posted: Mon Jul 18, 2022 10:14 am
by BRS
This is a carburation issue (I believe).
I’m just getting this sonex-a flying (after lengthy rebuild and mgl avionics issues). Now that I have a few flights (3) under my belt I’m seeing a pattern.
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If I taxi out and give full throttle, the engine will stumble, once. If the throttle is then brought back to idle and again advanced, this second time, it does not happen. It’s less of an issue if I advance the throttle (rather) slowly.
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During takeoff roll, the engine will sometimes stumble but recover, but once solidly in the air this does not happen. Very disconcerting on the most critical phase of flight.
I suppose the Remaster and Aerovee engines to be similar enough for experiences to cross. Remaster’s version of the slide carb is being used. Induction is direct ram air into a cone filter (pic). I’m wondering if air might be getting disturbed at the cowl intake or some other odd phenomena.
Re: RevMaster: rough on take off only
Posted: Mon Jul 18, 2022 2:23 pm
by gammaxy
This sounds a lot like the “burps” that you’ve probably read about on this forum.
I had the exact same symptoms and resolved them by adding a fuel line directed upwards between the Aeroinjector input and the fuel tank vent. Be careful that your tee is as high as possible or you could end up like me with an unintentional syphon.
I think we have the problem because there’s nothing like a float chamber to separate bubbles from the fuel.
Re: RevMaster: rough on take off only
Posted: Mon Jul 18, 2022 6:55 pm
by BRS
Thanx Chris,
I hadn’t considered that this could possibly be the burps since it happens when things are still cool under the cowl. But considering how we shut off the fuel at the tank, perhaps that’s wen some air gets into the lines (sitting over night). Just thinking out loud, and doubting the burps idea but not discounting it either.
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 12:16 am
by gammaxy
Yeah, unfortunately I don’t think anyone has installed transparent hoses and fittings to figure out exactly what is happening, but I do think you’re probably right thinking that air gets into the lines between flights and some of it gets forced out when you go full throttle.
I had the exact symptoms you describe even when I’d expect the under-cowl temperatures weren’t very high. My fuel lines are insulated and about as short as possible. I kind-of got used to it and trained myself to be gentle with the throttle. I certainly don’t believe I’ve had it as bad as what others have described. I flew it like that for several years. Like you, it would primarily stumble when applying throttle during takeoff or runup. It did seem that I could clear the stumble during runup and it would be fine for takeoff.
Once I finally decided to try the burp tube, all symptoms went away and the throttle response was incredible. I left it like that for over a year. Unfortunately, I installed my burp tube tee low enough that one time I completely filled the tank and when I raised the tail on takeoff, fuel was able to syphon through the burp tube and out the vent starving the engine of fuel. This was confusing until I thought about it and reproduced the exact scenario on the ground by lifting the tail with the cowl off. Because of this, I removed the burp tube until I think of a better way to install it on my airplane. The tee should probably be at least as high as the vent connection to the tank, mine was about an inch lower. Now that I’ve removed the burp tube my stumbles are back. It’s possible I’m the only moron who would install it wrong, but be careful 
I have a suspicion that in addition to giving somewhere for bubbles to go, the “burp tube” also provides a reservoir of fuel that can respond to transients faster than the rest of the fuel system.
In my mind, tuning the carburetor richer could also help and might be a good first step if there’s any chance you can comfortably go richer.
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 1:40 am
by pilotyoung
Chris,
I understand you are connecting one end of the burp tube to the fuel vent as high up as possible. Where does the other end connect? To the AeroVee Injector?
Thanks.
John
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 9:55 am
by Onex107
Onex 107 is a tri gear, Aerovee with Aeroinjector, engine. I had the stumble also when starting a takeoff. I discovered that to shut the fuel off I had to use the main shutoff because the mixture shutoff did not completely do the job. It still dripped. So I shut off both when shutting down the engine. I think the slow drip from the Aeroinjector mixture shutoff allowed an air bubble to get in the fuel line. The burbs are caused by heating the fuel lines until vapor bubbles are formed and they will continue until the engine cools. Very frightening. Your engine stumbles/stops every 15 seconds. Even at cruise speeds. Insulation will stop it.
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 10:17 am
by flyingbear
My ONEX with AErovee and Aerocarb does exactly what Brock says…stumbles ONCE on takeoff only if the throttle is pushed in fairly rapidly. Does not do it with slow throttle advance and the stumble is very brief. So I just push the throttle in slowly. End of problem.
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 11:04 am
by gammaxy
pilotyoung wrote:Chris,
I understand you are connecting one end of the burp tube to the fuel vent as high up as possible. Where does the other end connect? To the AeroVee Injector?
Yes, I added a tee fitting to the elbow I already had coming out of the Aeroinjector. I used -4AN stainless steel braided teflon hose (smaller than the rest of the fuel system). Keep in mind that shutting down the engine will take a little longer because the burp tube will keep supplying fuel after the fuel valve is shut off.
Also, I’m not necessarily evangelizing for adding a burp tube. I like the simplicity of not having one, but in my experience, I can’t say I resolved all the little hiccups and stumbles until I added one, then it was extremely satisfying. After I removed it I had someone comment about a stumble they heard during a touch-and-go. It’s a little embarrassing, but something I can mostly manage by being slow on the throttle applications.
Re: RevMaster: rough on take off only
Posted: Tue Jul 19, 2022 11:14 pm
by mike.smith
flyingbear wrote:My ONEX with AErovee and Aerocarb does exactly what Brock says…stumbles ONCE on takeoff only if the throttle is pushed in fairly rapidly. Does not do it with slow throttle advance and the stumble is very brief. So I just push the throttle in slowly. End of problem.
Only happens to me on very, very hot days, and pulling the mixture out to lean it solves it instantly. If you saw how much insulation I have on my fuel lines you’d be amazed! I never, ever have burps on startup, even right after refueling on a 100+ deg day, so it’s definitely the mixture (for me).
I’ve personally never put much credence in the bubble theory. 540 hours on the AeroVee and AeroInjector with never a hiccup, except on the rare occasion noted above. But note that I don’t have a gas collator, either. By super insulating the fuel lines, and using the mixture knob appropriately, I have never reset the mixture in 7 years (I fly in 20F deg in the winter, and 100F deg in the summer). On take-off, mixture knob “in” for the winter, and partly “out” for the summer (if it burps on throttle up, pull it out a little more).
Your mileage may vary!
Re: RevMaster: rough on take off only
Posted: Wed Jul 20, 2022 9:29 am
by Scott Todd
The three Aerovee’s I have owned and the two others I have flown with Aerocarbs all do/did this. They are simply a bit rich at idle. I lean for taxi and go full rich just before mag check. I richen again if I’m not immediately ready to go. No more burps, coughs, or stumbles. If I sit for a minute without leaning, it stumbles on throttle advance.
Re: RevMaster: rough on take off only
Posted: Wed Jul 20, 2022 12:49 pm
by flyingbear
My Aerovee does EXACTLY what Mikes does- exactly. And the solution is the same- lean a bit (or advance throttle slowly). I lean my mixture at all times,even at idle after startup.
Re: RevMaster: rough on take off only
Posted: Wed Jul 20, 2022 2:07 pm
by pilotyoung
My AeroVee does the same thing. As soon as I start it, I lean it aggressively and shut off the secondary ignition. That way at low engine power the secondary ignition is not drawing power. Then I put it full rich before I do the run up. Leaning aggressively is not my idea, I got it from another member on here. By leaning aggressively, if you forget to push the mixture to full rich, when you give it power for takeoff, the engine will miss.
Re: RevMaster: rough on take off only
Posted: Wed Jul 20, 2022 3:19 pm
by BRS
pilotyoung wrote:My AeroVee does the same thing. As soon as I start it, I lean it aggressively
Thanks, ground leaning is already something I do, which I learned on certified aircraft. Seems these engines with their slide carb needs it more than the Lyc/Cont engines do.
The next thing I need to learn is how to best lean during low/idle power descents. I want to get to where I can fly base and final on idle but I’m afraid of the engine stopping. Had that happen while taxiing in once.
Does anyone with a Prince P-tip know what airspeed is required to keep the prop turning?
Re: RevMaster: rough on take off only
Posted: Thu Jul 21, 2022 12:24 pm
by Onex107
I can’t count the number of times I have heard the “mixture problems” at idle comments. I know you won’t believe me but the #2 and #21/2 needles in the Aeroinjector both had that problem. After extensive testing and needle making, I found the #3 needle moves the idle sweet spot to where it is in the correct position with the slide valve closed, at idle. I never have had to adjust the mixture during or after landing and only adjust at cruise to raise the EGT temps to a leaner mix. With the two lower needles I had to lean during taxi or the engine would stop.
Re: RevMaster: rough on take off only
Posted: Thu Jul 21, 2022 9:03 pm
by BRS
Onex107 wrote:…I know you won’t believe me but …
I believe you. Hmmm wonder if Revmaster has a different profile metering-needle.
Thanks.
RevMaster: SOLVED! (Bell Crank)
Posted: Fri Sep 09, 2022 5:53 pm
by BRS
After months of wrestling with this Revmaster “RevFlow” carb I finally purchased an AeroInjector. After a few hours of adjusting it was ready to test fly. Bottom line - it works! Not balking, no hesitation, full 3200- static rpms. EGT’s cool in flight.
Following are some pictures for the setup.
First off I needed to fabricate a cable reversal bell crank. I built it such that 3" pull on the control is 1.5" push on the aeroInjector. Sorry the pictures are so large, you’ll have to click on them to see the full image.
BellCran_b4_install.jpg (196.24 KiB) Viewed 5813 times
BellCrank_Underside.jpg (117.77 KiB) Viewed 5813 times
The bellcrank base is made from 2" angle. The radius didn’t quite fit the 1/2" Diameter engine mount tube but with some sanding it was not far off. Then the radius was liberally filled with RTV and loosely set in place to dry before tightening the hose clamps. The hose clamps are protected with Adel clamp rubber strips so as to not damage the mount.
With the engine off the control works smooth as butter but there is a bit of resistance felt when accelerating. Thinking this is normal as a sucking engine will pull that slide tight. It’s not too much, just different.
Now onto the carb…
RevMaster: SOLVED! (Carb Swap)
Posted: Fri Sep 09, 2022 6:11 pm
by BRS
The RevFlow carb is 35+mm which was replaced with the 32mm AeroInjector. 32mm works well. I noticed right away that the S flange almost fit into the Revmaster flared intake tube and that sizing was such that a rubber coupler would be problamatic. So I drilled and slit the end of the revmaster flared intake flange so that a hose clamp could be used on it. Next I turned down the S-flange by .020’ which fit perfectly…
Flange_turned.jpg (215.33 KiB) Viewed 5807 times
From this angle these carbs look about the same in size. But the AeroInjector feels about half the weight of the RevFlow carb.
Both_Carbs_size.jpg (247.61 KiB) Viewed 5807 times
Here you can see where all that extra weight comes from.
Both_Carbs.jpg (249.27 KiB) Viewed 5807 times
The drilled hole and slit (under the first hose clamp) in the intake tube is covered completely by the S-flange. It was also sealed with a bit of RTV to boot. The second hose clamp is only for the safety wire attachment.
No doubt there will be more tuning to be done but so far the Revmaster seems to be running very well - finally.