Help: Slide Carb (revmaster) settings

[BRS]

For you guys flying the slide carbs here is a question for you.

I find that I get best power with the slide partly closed (quite a bit) as in the picture below. If I open more then I get less rpm. Is this a symptom of being too lean and probably going LOP with a more open throttle?

Today I as after lining up for only the second flight the engine stumbled once opening the throttle. I aborted and taxied back to the hangar. This has happened before but it only stumbles once, subsequent full throttle attempts go fine with out the stumble. I can't have confidence in the engine when it's acting like this. Any ideas?

RevMaster Carb

ThrottleSlide Sm.jpeg (116.04 KiB) Viewed 7011 times

[sonex1566]

Hi Brock, when I was using the 2180 Aerovee with the Aerocarb I reckon that I had the slide closed almost 1/4". Anymore open than that and it would run pretty rough. I suppose the important question is what is your WOT rpm? If you're getting 3000 rpm or more then don't worry. I did have one fright which I believe was caused by a hot day, extended idling and I think fuel vaporization which caused it to stumble on take off. I've noticed that an awful lot of test pilots have grey hair........

[GraemeSmith]

I did have one fright which I believe was caused by a hot day, extended idling and I think fuel vaporization which caused it to stumble on take off.

When I have that symptom it's a signal to lean for best power prior to take off. The high density altitude means the engine is rich and it stumbles as you slide the throttle forward and it riches even more before picking up.

[tx_swordguy]

I think Graeme is right. try leaning a little on a tied down ground run and see if that helps. On an aero injector I would consider turning the needle in a little if the leaning helps. I have heard of some having to put a limit stop due to the last bit of throw not really helping and possibly allowing either more air or more fuel. I do not remember which it was they were having issues with. I do not know the exact way the revmaster works but I think it is similar to the aero injector.

I just re-read. Is this a new build or a new to you engine? If it is new to you perhaps a call to the previous owner might help

[pappas]

The Aeroinjector installation and tuning manual references a symptom where the engine runs fine upon advance of throttle until just prior to WOT and then stumbles. If that happens, the manual tells us to limit the throttle's ability to advance just a bit prior to the symptom occurring. There is a provision to do just that built into the throttle mechanism and no fabrication is necessary.

I had that same symptom on my last Waiex with a 3300 Jab. I simply limited the last little bit of throttle and never had the symptom again. There was no reduction in the capability of the engine to turn the necessary revs.

If that happens every time you go to WOT on take-off, make the adjustment. However, if the symptom only occurs on hot days with high-density altitude or you are flying from a high altitude airport, you always need to lean prior to take-off. Remember if you are sitting on the ground at an airport that has a field elevation of 3000 feet or higher you should already be leaned to the amount you would if you were flying the aircraft at 3000 ft+. Know the density altitude prior to departure. All normally aspirated engines will need to be leaned on the ground for an effective high-density altitude departure.

[BRS]

Thanks All for the replies.
Here is a bit more info.

-It's a new engine and I've no VW or slide-carb previous experience, thus I'm proceeding with great caution.

-I do have the slide adjusted so that it is at full-throttle in the pictured location. Gives me 3000-3100 rpm.

-The condition was previously much worse so I turned the needle out (rich) 1/2 turn. Helped a lot. I'm at 300' so it's not high density. Actually I believe I was going lean at full throttle. The problem had seemed to be solved but then it stumbled yesterday though I could not replicate it afterward.

-For some reason the EGT's on the MGL (MX1 & RDAC) are very slow to respond so it's hard to use them for tuning.

-I'll go read the aeroInjector manual. It should still be helpful. Joe at RevMaster was not much help. trouble shooting is hard to do at a distance.

[Murray Parr]

Your carb could be too large for the horsepower required, google Rotec TBI manual and it explains how to confirm it and to just limit the WOT position if that is the case.

[BRS]

Your carb could be too large for the horsepower required, google Rotec TBI manual and it explains how to confirm it and to just limit the WOT position if that is the case.

I'll do that.
After reading the AeroInjector manual I wondered about that, since it has a description of the symptoms as well.
Thanks.

[gammaxy]

I'm doubtful that it is necessary to significantly restrict the throttle travel of the 32mm Aeroinjector on the Aerovee engine. Mine runs fine at WOT and I suspect that's the case for the majority of us. I think the manual's mention of it is intended for people using the Aeroinjector on smaller engines, but it's not intended to be the typical solution. I could see how too course of a propeller pitch, bad ignition timing, mixture tuning, etc., could be masked by limiting the throttle, though.

From my brief research, it seems that the Rev-Flo carburetor is 36mm. This is even bigger than the 35mm Aeroinjector used on the Jabiru 3300. Not really sure what this means in your case. Maybe limiting the throttle makes sense, but I'm doubtful Revmaster would design it that way.

I assume you're getting a pretty significant stumble. I've noticed that when I do my first runup, the engine is more likely to stumble, but then it clears and repeated runups are smooth. I temporarily installed a "burp tube" that seemed to resolve this. I suspect that air gets in the fuel lines between flights (due to shutting fuel off with the fuel valve) and doesn't get completely cleared at low RPMs, so the first runup clears the fuel line. Wish there was some safe transparent fuel lines so we could see what's really happening :-) It's not a scary stumble, so I suspect what you're experiencing is worse.

[Scott Todd]

The Aeroinjectors can be too rich at the low end when they are tuned properly for full throttle. I've been flying one on a little biplane for 15 years. It has to be leaned for taxi or it stumbles when applying full throttle. It just loads up with excess fuel and it takes a few seconds to burn it off. I fly off a 4000ft runway and only need about 500 feet and its very repeatable. If you don't lean it for taxi, it will clear up in a few hundred feet once you apply full power.

My new Onex does the same thing. It needs a little more runway so I'm diligent to lean it about an inch (my set up) as soon as it starts. I go to full rich for the run-up and then take off. I'm not leaning it during flight as its a new engine but I have tested it and it seems about right. I can get about 100F increase before it starts to run rough. When I land and clear the runway, I go back to about one inch of lean on the knob. It smooths out and then idles nice around 900 rpm. I only have about 15 hours and 40 landings on it so far but it seems to be operating similar to my other one.

I don't know much about the Revmaster carb but that fuel line looks pretty small. Is it gravity feed or is there a pump?