Q: I’m taking a chance here to see if I can get an answer to my problem with the engine for my Van’s RV-4. It goes like this: I have a Lycoming O-320-E3D (L 46580-27A), Marvel Schebler carb MA4-SPA 10-5009, and Slick 4370/4371 magnetos. I had a rough idle at 600-700 rpm and turned to the Van’s Aircraft Forum for help.
One guy said I had an induction leak. He had the same engine and at 600 rpm his MAP was 10-11, whereas mine is 14. I started chasing leaks and I have pretty much done everything:
- Change of float (SB-2)
- Installed new throttle shaft and bushing.
- Tried another green-tagged MA4-SPA carburetor
- Adjusted the carburetor fuel level
- Checked spark plug resistance
- Checked ignition cables with high-voltage test box
- Checked the magneto e-gap
- Replaced one magneto coil
- Tested magnetos at an aviation mechanic’s shop
- Compression test result is 79/79/79/79
- Checked all the usual things, like induction gaskets, hoses etc.
All of that came to nothing.
The engine doesn’t run badly at 700 rpm, but the MAP is the same and leaning out from 1,000 rpm to idle cut-off does not give the usual 50 rpm rise before cutting out. This would indicate that the engine is already running lean, but I have tried setting the idle mixture screw from 1 turn out to 5 turns and there is no change. At magneto check at 1,700 rpm I have a 60-80 rpm drop.
I have had help from two different aircraft mechanics and they shake their heads. The don’t understand it.
What is it with my engine that makes it run differently from most other engines?Have I overlooked something? Expert advice is needed.
Hans Stråhle, Stockholm, Sweden
A: Hans, I have no doubt you’ve done an excellent job of searching for your problem, but maybe I can suggest a few more things that may help solve your problem.
I have to agree with the person who said you have an induction leak, but now you have to find it. The MAP number he gave you is correct, by the way.
Have you taken some soapy water and using a paint brush painted around all of the intake pipes at both the flange and the oil sump end while the engine was running at idle RPM?
If there is a leak, you will notice bubbling where the leak is.
Of course, you need to use extreme caution when doing this, being certain to have the parking brake set and wheels chocked.
I assume you’ve previously checked all of the intake pipe gaskets for signs of fuel stains, which could indicate a leaking gasket?
I would also very closely inspect the intake pipes where they attach with the rubber hose between the intake pipe and the oil sump. Old or deteriorating hoses in this location may be the cause of the problem.
I’d also suggest that you remove all of the intake pipes, including the rubber hoses at the oil sump, and closely inspect the steel tube that is swagged into the sump for tightness. Over time, these pipes may work loose, which will also cause an induction leak.
Should you find any of these pipes to be loose, they can be re-swagged by using a special tool.
If no problem if found in this area, then I’d reinstall everything using all new gaskets, etc.
I also recommend that you remove and block off the complete primer system. There may be a cracked primer line, which could cause an induction leak. If you remove the primer nozzle(s) from the cylinders, then you must plug the holes in the cylinders using a 1/8 x 27 inch allen plug screw.
Use caution here. Use some type of thread lube and do not over torque this plug, which may cause the cylinder to crack at this port.
Honestly at this point Hans, I can’t think of anything else to check, so I’ll leave it to you now. I’m confident you will locate and correct the problem.