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  #11  
Old 08-29-2016, 06:03 AM
krw5927 krw5927 is offline
 
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There's no mechanism to positively "shut off" fuel from the main jet during ground ops that I'm aware of. Even with a (mostly) closed throttle plate, if the float is sinking and fuel can be pushed by the fuel pump's pressure right out the main jet during idle, the engine will suck it in.

Leaning aggressively when this first starts to happen and during all ground ops should help.

Just thinking out loud... Has anyone ever mounted an electrically-operated blower/fan to force air blowing on the carb bowl during ground ops? Would it even help, since it would be using air from inside the lower cowl, whose elevated temperature caused the problem in the first place?
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Last edited by krw5927 : 08-29-2016 at 10:31 AM.
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  #12  
Old 08-29-2016, 01:59 PM
lr172 lr172 is offline
 
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When the fuel boils, it can cause the float to sink a bit which allows the fuel level to rise. All of the mixture in a carb is based upon the fuel volume/height in the bowl. This can cause you to run rich, even with proper idle mixture settings.

The fix is a fuel return circuit. You can do some research here. When I had the carb setup on my 320, I ran a fuel return. I had a tee at the carb and added a -4 fitting with a restrictor (#40 drill size). This was fed to a valve in the cabin and then back to the tank. I never had issues with heat on the ground when I used this. It ensures that the fuel going into your carb is cool and helps to keep the boiling down or two a minimum. The boost pump is run to keep the flow up.

The restrictor insures that even if you leave it open on take-off it will not reduce your flow to the carb below the required flow.

Larry
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  #13  
Old 08-29-2016, 02:17 PM
Christopher Murphy Christopher Murphy is offline
 
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Quote:
Originally Posted by lr172 View Post
When the fuel boils, it can cause the float to sink a bit which allows the fuel level to rise. All of the mixture in a carb is based upon the fuel volume/height in the bowl. This can cause you to run rich, even with proper idle mixture settings.

The fix is a fuel return circuit. You can do some research here. When I had the carb setup on my 320, I ran a fuel return. I had a tee at the carb and added a -4 fitting with a restrictor (#40 drill size). This was fed to a valve in the cabin and then back to the tank. I never had issues with heat on the ground when I used this. It ensures that the fuel going into your carb is cool and helps to keep the boiling down or two a minimum. The boost pump is run to keep the flow up.

The restrictor insures that even if you leave it open on take-off it will not reduce your flow to the carb below the required flow.

Larry
Im pretty sure until the rpms get above 1200 or so, the carb is feeding from the idle circuit. The mixture control has no effect when you are taxiiing around below 1200 rpm or so other than at cut off. That mixture is controlled by the idle mixture screw.

Cm
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  #14  
Old 08-31-2016, 04:55 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by RV6_flyer View Post
Lycoming recommends having the Boost Pump on for high temperature ground operation.
I do that, but it hasn't solved the problem. That leads me to believe that the problem is in the carb itself, rather than in the mechanical fuel pump or gascolator. My setup is (all inside the cowling):

Gascolator==>Facet Boost Pump==>FF sensor==>Mech Fuel Pump==> Carb.

My fuel pressure line taps off at the outlet of the Mech Fuel Pump so if...say...the Mech Fuel Pump was causing the problem, I would expect to see fuel pressure fluctuations on my gauge, which I don't.
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  #15  
Old 08-31-2016, 05:16 AM
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Quote:
Originally Posted by Christopher Murphy View Post
Im pretty sure until the rpms get above 1200 or so, the carb is feeding from the idle circuit. The mixture control has no effect when you are taxiing around below 1200 rpm or so other than at cut off. That mixture is controlled by the idle mixture screw.
That's exactly what I thought as well, but couldn't figure out why, if the mixture control has no effect below 1,000 RPM, we're supposed to pull the mixture control out during ground operations to keep from fouling the plugs. Then I found this: Precision Airmotive's MSA MSA Float Carburetor Handbook. If I read this correctly, the mixture control DOES effect the idle mixture (as well as the idle adjust screw). Words just confuse me, but this link has lots of diagrams I've highlighted the important stuff:
Quote:
a. IDLE SYSTEM, (Ref. appropriate figure for a particular model). With the throttle fly slightly
open to permit idling, the suction or vacuum above the throttle on the manifold side is very high.
Very little air passes through the venturi at this time, and hence, with very low suction on the main
nozzle, it does not discharge fuel. This high suction beyond the throttle, however, causes the idle
system to function as the primary idle delivers into the high suction zone above the throttle. Fuel
from the fuel bowl passes through the mixture metering sleeve, fuel channel, power jet, and into
the main nozzle bore, where it passes through the idle supply opening in main nozzle, through the
idle fuel orifice in idle tube, where it is mixed with air which is allowed to enter idle tube through
the primary idle air vent and secondary idle air vent. The resultant rich emulsion of fuel and air
passes upward through the emulsion channel, where it is finally drawn into the throttle body
through the primary idle delivery opening, subject to regulation of the idle adjusting needle, where
a small amount of air passing the throttle fly mixes with it, forming a combustible mixture for
idling the engine.
The idle adjustment needle controls the quantity of rich emulsion supplied to the
throttle barrel, and therefore controls the quality of the idle mixture. Turning the needle counter-
clockwise away from its seat richens the idle mixture to the engine, and turning the needle
clockwise towards its seat leans the idle mixture. On idle, some air is drawn from the throttle barrel
below the throttle fly through the secondary idle delivery opening and blends with the idling
mixture to the engine as the throttle is opened, coming into play progressively and blending with
the primary idle delivery to prevent the mixture from beginning too lean as the throttle is opened
and before the main nozzle starts to feed. These Carburetors are provided with a third and, possibly
a fourth idle delivery in addition to the secondary idle delivery, depending on the application to
cover the broader idle range in these Carburetors.

b. METERING, (Ref. appropriate figure for a particular model). All fuel delivery on idle, and also
as steady propeller speeds up to approx. 1,000 rpm, is from the idle system.
At approx. 1,000 rpm
the suction from the increasing amount of air now passing through primary and secondary venturi
causes the main nozzle to start delivering, and the idle system delivery diminishes due to lowered
suction on the idle delivery openings as the throttle fly is opened for increasing propeller speeds,
until at approx. 1,400 rpm the idle delivery is practically nil, and most of the fuel delivery from
that point onto the highest speed is from the main nozzle.
However, the fuel feed of any full
throttle operation is entirely from the main nozzle. The idle system and main nozzle are connected
with each other by the idle supply opening. The amount of fuel delivered from either the idle
system or main nozzle is dependent on the whether the suction is greater on the idle system or
main nozzle, the suction being governed by throttle valve position and engine load. The main
nozzle feeds at any speed if the throttle is open sufficiently to place the engine under load, which
drops the manifold suction. Under such conditions of low manifold suction at the throttle fly, the
main nozzle feeds in preference to the idle system because the suction is multiplied on the main
nozzle by the restriction of the venturi.
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  #16  
Old 08-31-2016, 05:29 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by SHIPCHIEF View Post
If the exhaust is close to the carb, I would put a heat shield on it to block radiant heat from the fuel system.
Here's what it looks like. The carb heat muff seems like it would block most of the radiated heat, but maybe not. BTW, that redneck heat muff works GREAT! I get a 200 RPM drop during runup.

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  #17  
Old 08-31-2016, 05:33 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by krw5927 View Post
Has anyone ever mounted an electrically-operated blower/fan to force air blowing on the carb bowl during ground ops?
If it turns out that the carb is the problem, it seems like that would be an excellent idea. I'd use outside air, though. Maybe a cowl flap?
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  #18  
Old 08-31-2016, 05:51 AM
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snopercod snopercod is offline
 
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Quote:
Originally Posted by bret View Post
Fuel flow should be .5 to .55 Pounds per HP per HR.
That's about what I'm seeing now (12 GPH) and it seems about right. My CHTs now stay below 400 on climbout even at 90 KIAS on a hot day. Before, I was having to keep my airspeed up above 120 KIAS to keep the cylinders cool.

It's interesting to learn that drilling out the main jet like I did can also effect the idle mixture. (See that information I posted to Christopher).

I have the blue epoxy floats which are two years old. I guess I should check them and the float height anyway, though. More quality time laying on my back on the hanger floor
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  #19  
Old 08-31-2016, 06:39 AM
Robert Anglin Robert Anglin is offline
 
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Location: houston, texas
Posts: 636
Default neadle seat and float

This is just a guess, but what you are describing here sounds like a flooding problem after setting at ideal for a time. As has been mentioned above a weak float or needle and seat will let the fuel level in the bowl rise up to a level that will start over fueling at ideal if it dose not close off the fuel flow at its set point. Even the set point adjust angle of the float if set to high will do the same thing and allow to much fuel in the bowl. It is kind of like the float level in your potty, if you set it to high it will overflow into the stand pipe or if the shout off valve "needle and seat" dose not seal off well the level in the bowl will creep up to overflow also. If you turn the bust pump on and this is the case it will flood even more and make the black smoke even more. If you are not seeing any singes of vapor lock, I would start with a bowl gasket, needle and seat and a good new meatal float. And adjust the float to the correct level just as in the potty. Just an idea of what I would start looking at first from what you are saying here. Hope this helps, Yours as always, R.E.A. III #80888
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  #20  
Old 08-31-2016, 11:00 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Christopher Murphy View Post
Im pretty sure until the rpms get above 1200 or so, the carb is feeding from the idle circuit. The mixture control has no effect when you are taxiiing around below 1200 rpm or so other than at cut off. That mixture is controlled by the idle mixture screw.

Cm
I wasn't discussing the mixture control. I was referring to the fuel level, which is set by the linkage on the float AND influenced by the density of the fuel that this float sits on. This fuel level influences the fuel delivery at any given air flow state. The mixture control can only reduce the flow beyond what the jets have set as the maximum flow through a circuit (yes, the idle circuit has a jet too). They often flow less than what the jet allows, even with mixture at full rich.

The main, transition and idle circuits are all influenced by the fuel level in the bowl eventhough they are metered and adjusted separately. Fuel is pulled from the idle or main circuit via pressure differential. Systems are engineered to deliver different fuel flows based upon the differential pressure at any moment. In order for this to work, the fuel must have a constant pressure, which is set by the fuel level in the bowl and the atmospheric pressure.

Finally, the idle mixture adjustment is not absolute. If you look at your fuel flow when idling at 500 vs 1000, the fuel flows will be different (you are still on the idle circuit at 1000 rpm). It just influences the pressure differential that the idle circuit sees which is also influenced by the fuel level.

Larry
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Last edited by lr172 : 08-31-2016 at 11:12 AM.
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