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Carburetor Problem

rv7plt

Member
i recently overhauled an O360 and have run it. I noticed that the idle is fairly rough up to about 1500 RPM and it smooths out just fine.Suspecting a problem with the idle circuit, with the engine at idle I gave it a little shot of extra fuel with the primer and idle became smooth and then rough again which I take it to mean it is lean at idle. I can find no vacuum leaks and there is no play in the mixture shaft or the throttle shaft.

I started with the idle adjustment screw open 1 1/2 turns and opened it 1/2 turn between attempts until I reached 4 turns open and there was no change in the roughness. Anybody care to speculate on what else I can check? I hate to spend big bucks on an overhauled carb cause everything works fine above 1500 RPM including the accelerator pump. I am thinking of removing the idle screw and blowing some air into the idle orfice in case there some trash in there. I forgot to mention there is some exhaust popping an the idle range.

Comments appreciated.

John in Fl
 
About seven years ago I bought a 6A from the original builder, and had a similar problem with it's carburetted O-320. Couldn't adjust the idle satisfactorily. It would always pop when you backed off on the throttle. My first suspicion was an intake vacuum leak.
I watched CHT's and EGT's, and couldn't nail it down. I checked every gasket, part, and piece of the induction system for leaks. (or so I thought) I had three or four other sets of eyes on it. No joy, and I eventually decided to blame it on the exhaust system.
Next condition inspection I took a closer look at the manifold pressure tubing. It was complete and correct from the engine to the firewall, then it dawned on me. The engine monitor did not display manifold pressure. Turns out, the builder had not completed the tubing install to the monitor, and I found my vacuum leak up under the panel.
 
As ARTROSE said, think outside the box.
Most likely is an intake leak.
You didn't say how far you have looked for the leak.
I helped a fellow flyer look for rough running at low speeds too and eventually blue stains were found at the intake tube/cylinder junction.
I have read about sticking a shop vac in reverse with a stocking filter over the end and blow it up the carb...then use soapy water to spray intake joints looking for leaks, you will have to rotate prop to close intake valves in turn while spraying.
I have also heard of loose carbs too....but clearly you are past that point by adjusting carb and parts.
Good Hunting
 
Sounds like dirt in the idle tube/ circuit to me. But DON'T remove the idle adjustment and blow air into the orifice to clear it. Doing so can pressurize the float bowl due to the small size of the vent compared to the air supplied to blow the dirt out and that will cause the float to get compressed and likely ruin it. I guess you could remove the bowl drain and add another vent to the float chamber that way but if it were me I would take it apart and clean it out.
Good luck,
Mahlon
 
Carburetor Problems

Never had it opened up so don't know about the floats. Malon, I hate to take it apart. Would you see anything wrong if I removed the drain plug and left the plug out while I blew it out. Just do not want to hurt the internals.
 
Bob Martin had a good suggestion about blowing air into the carb intake. I used an electric leaf blower and I found a leak at the end of the throttle valve shaft so guess it needs a new bushing. No leaks anywhere else. Guess I will check with some A&Ps to see if anyone can replace that bushing. Pretty sure that takes a special tool.
 
I agree with Mahlon that a plugged idle circuit is a prime suspect, especially if the carb was rebuilt as part of the overhaul. The idle circuit is providing most of the fuel flow up to about 1300-1500 RPM. However, the vacuum is very high at those RPMs and therefore an intake leak will be most noticeable there. The carb seems the more likely culprit, but I would also check carefully for leaks first if you are not comfortable tearing down the carb (a fairly easy project if you're handy).

The fact that turning out the screw 2.5 turns had no impact lead me to believe that the intake leak is less likely. You should have seen marginal improvement, unless it is a big leak on one cylinder and the roughness is limited to that cylinder.

Larry
 
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Did the engine ever run right since the overhaul? Dry tappet clearances all correct? Does sound like an intake leak though.
 
Before doing anything rash like tearing into the carburetor, wouldn't it be wise to perform the Idle Mixture Test?
Head to the runup area (or fly it first) to warm the engine. With carb heat off and a clean air filter in place, face the airplane 90 degrees to the wind and adjust the throttle to give minimum RPM. This is typically around 650 RPM. Secure the throttle friction lock to keep it from creeping. Move the mixture control toward idle cutoff and monitor the tachometer.

As you come to the last inch or two of mixture travel, you should notice a 25- to 50-RPM rise in engine speed before the engine falters from lean misfire. Keep it running and record the actual RPM rise, whatever it is. If your lean-out gave more than a 50-RPM rise, your carburetor (or fuel injection?the same test applies) is set too rich and needs to be compensated in the lean direction.

Conversely, if you saw little or no RPM rise, your idle mixture is set too lean. The idle mixture adjustment on a Marvel-Schebler carburetor is in the form of a large knurled screw (or small slotted knob) on the throttle casting, high on the carb. On MA-3 and MA-4 series carbs, for example, find the bowl drain plug, then run your finger (or eyes) straight up the side of the carb until you come to a slotted knob with arrows on it pointing to ?R? (rich) and ?L? (lean). That?s the idle mixture adjustment. Give this screw a turn in the desired direction, then repeat the lean-out procedure described above.

Of course, alterations in idle mixture have an effect on idle speed as well. If your engine was set too rich (i.e., 150-RPM rise on shutdown) and you corrected this by turning the idle mixture screw as needed to give the desired 50-RPM rise, your engine will now probably idle about 100 RPM faster than before. Accordingly, you?ll want to adjust the idle speed to put it back in the 650-750 RPM range.
 
Finding vacuum leaks is much easier if you can find a smoke injector. Most good auto repair shops have one. I own a Porsche repair shop and we use our smoke injector at least once a week. You may have to make an adapter to inject the smoke in to the intake system but the leaks are usually readily visible.


Bob Grigsby
3B. Getting close. J3 flying regularly
 
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