Merco 35 with 10 x6 prop goes really rich after takeoff , any thoughts, plug has been replaced. Over propped / under propped?

  On a normal engine and tank installation, pressure or vented to atmosphere, the engine should richen up a bit when in flight due to the extra head pressure from centrifugal force, as long as there are no leaks anywhere. Like Dave mentions, how the tank is installed in relation to the engine matters Too high and you still get some gravity feed. Flying a few laps inverted usually will go leaner if that is the case.
  So, does it go so rich that you can't go inverted? If so, what does it do then? If you get it inverted, and nothing changes, you might try 5 inch pitch and up the RPM to get your lap time close. Then do some tricks and se how it responds.
  Type at you later,
  Dan McEntee

I have 3 Merco 35's, 2 of them are the original 60's version with the cast in venturi. One was sold as an RC so bored out to accept a carb and I never used that one. The last one is a Merco Black Streak made by D. J. Allen which is rubbish because the conrod was machined incorrectly so it's unrun still in its original box. Neither of the early versions were new but the first one was in a full fuse model that a guy was selling cheap somewhere back in the late 60's. The first time I ever flew in a competition was a Queensland championships which I won and the guy I bought it from came 2nd, he wasn't happy . To sum this all up, yes I've used a Merco 35.

Memory is getting a bit hazy now but I'm almost certain the prop I used was either a 10x5 or 10x6 wood but I know fuel was 75/25 all castor. Plug would have been one of those brass tipped KLG Champion? but that's beside the point because any decent hot plug will do. Your problem may of course be tank related but it sounds similar to the tune needed by my Enya 61 RE where I tune rich on the ground to give a slight 2 stroke burp but on release goes instantly into a full throated 4 stroke.
This test shows the exact same as my Merco. https://sceptreflight.com/Model%20Engine%20Tests/Merco%2035%20%281%29.html

Thanks for the reply's, its not my model just trying to help out a fellow modeller, full fuselage model inverted engine, built in tank ! We tried a restrictor on the uniflow pipe and it ran good for 3/4 of the flight then went rich. Not sure of the tank plumbing, will try blocking uniflow off, and leave overflow open and see what happens.

I had an LA46 that would do that ocationally.  Capping off uniflow and using muffler pressure on the overflow tamed it.

Ken

While I don't claim to be a great engine or fuel tank guy, I've only run into problems with tanks with leaks or some other flaw, and fixed or replaced them. I am a fan of muffler pressure, and often use it connected to the uniflow inlet, and can't imagine any reason to hook it to the overflow, or why that would help stabilize the engine run.

It seems to me that Ken's example just HAD to have been a bad tank, probably with mislocated tubes or a split in the uniflow tube. If anybody can provide any other logical reason why that would work, I'd be interested. I'm thinking it was a split or otherwise perforated uniflow tube.  Steve 

I was a plastic clunk tank.  What I did in effect was make it like things were back in the day.  Pressure outside of the fuel.  For some reason that motor never performed well on uniflow.  One of life's mysteries.

Ken

Not knowing what your tank looks like,   this COULD be a sign the tank is a uniflow that is working perfectly.   If any one of my airplanes DOESN'T respond this way I'm looking to see what's wrong.   As the engine unloads taking off then the forward air flow increases going into the tank and a pressure head forms atop the fuel which richens the engine some.   It is that pressure that helps maintain an even run through the flight.   You might indeed have the engine running pretty lean on the ground knowing the tank will respond and bring the run back to whatever you have decided you want from it.  It's very rare but if the pressure is just too much for the engine you can restrict the uniflow line some or as Ken mentioned use muffler pressure-which can be LESS pressure by virtue of the tiny inlet hole in the muffler pressure fitting and isn't determined by forward motion of the airplane.

Dave

   If not using muffler pressure, and have the uniflow vent facing forward, you can restrict the pressure there with a proper reduced sized tube and coupled to the normal vent tube with a piece of fuel line. I have mentioned many times in these discussions that if everything is correct in your fuel delivery system, you should see a richening up in flight once you come to get the first lap in and get some effect from centrifugal force on the fuel load and pressure increase from forward flight. It varies from each airplane/engine combination, and prop and engine RPM are a factor.  And again, you need to compare it to what it runs like inverted to determine if the tank height is correct early on in the flight testing. Even if it's running bad, it should run the same upright or inverted. If it speeds up inverted and or in outsides loops, you need to lower your tank as it looks when sitting on the ground. This will also exaggerate a rich condition when upright.
  Type at you later,
   Dan McEntee

It would have to be a very tiny hole in the ram air vent to reduce the total flow. Essentially you are looking at the difference in viscosity of the air going in the vent versus the viscosity of the fuel leaving the tank. As well as the line losses for each portion, which of course is dominated by the fuel feed portion due to the fuel viscosity--especially if you have a fuel filter installed. The ones I tested caused more losses than anything else in the system with the exception of the spraybar orifice. (And you do not want the filter pressure drop to come close to the orifice pressure drop!) The other thing to keep in mind is that when there is not really much air flow relative to the size of the 1/8" tubing, the ram air pressure approaches the static pressure. And static pressure would be the same regardless of the vent hole size.

An interesting exercise is to calculate the dynamic pressure of a plane flying at sea level, at 50 mph, and at 70 degrees F. I get .0434 psi. If you convert this to "inches of fuel" and assume that the fuel used is 10%N, 22% castor, 68% methanol you end up with about 1.41 inches of fuel pressure due to ram air. So you can compare that to how wide your tank is in the lateral direction. A really wide tank on a profile and an engine with poor fuel draw doesn't behave nice. And while the ram air pressure stays the same (assuming your plane doesn't speed up) as the fuel burns off, the fuel height relative to the spraybar hole keeps changing.

The entire scenario gets worse when the fuel viscosity is high, because it becomes critical that your setup must not require too much pressure differential to draw the fuel to the orifice.

I like to get the ram air vent out into clean air first and then leave it alone. Adjust the other fuel system features after that. But if it is not in clean air, you can screw around with all kinds of other factors trying to clean up the problem and maybe you can but probably you won't.

Personally, I don't like trying to tune a fuel system by orificing the ram air inlet. I'm sure it can be done, but you are going to be messing with a range of tiny holes and it is going to be very fussy. And, I don't want to have more knobs to turn in the setup sequence--I want the dominant one to be the one I'm adjusting after all the other ones are locked down. Otherwise, you risk chasing your tail trying to turn the knobs in the wrong order and find the local minima, and not the real solution. Kind of like operating the old Fox .35 "flat needle" when 20 degrees of adjustment gets you almost there, but not quite, and you have to go around a whole turn (or two) and come back to a better sweet spot in that same 20-degree zone. That's about the worst non-linear adjustment scheme going. The only reason we put up with it is that once you found the spot, you might not need to move it more than once or twice a year....

Dave, let me qualify my muffler pressure outside of the fuel head method.  If you use it on the vent tube instead of uniflow you have the same pressure on the ground that you have in the air so all you need to do is adjust for centrifugal force.  The end result is the same as an open vent facing the airstream except there is less difference in takeoff vs flight rpm and you don't have the wind issues.  I think a lot of the problems we encounter with pressure come from the huge differences in how much pressure different mufflers produce vs how much we really need.  I flew thousands of patterns "back in the day" with a stock VECO 4 oz tank with two vents sticking out the side bent into airflow.  Please tell me how having the engines lean out some in the overheads and clover was a bad thig.

Electric has removed all of these tank issues and replaced them with a new set to drive us crazy. 

Ken

Reg. recons those things shake .

Can sometimes affect air to ground , in respect of micro bubble stream .

Id try the heavy blade with the piston , at t d c .

Ifits harmonic , try a few props , might gettit out of a ' period ' .

Wonder how tough the nose is constructed . He used to use on in the Mk II T - Bird . So theyve no lack of grunt .

Back in the day when I was breaking in a new engine I would start with a perfectly balanced prop.  If I had any above normal vibration I would switch to an unbalanced prop and move it 1/8 a revolution at a time until it stopped vibrating.
The uncool part was sometimes not having a blade at 1:00-2:00 at first compression.  Sometimes drilling a hole in the spinner backplate and using it to balance can work too.

Ken