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.
if a carb cleaner is sprayed in the area of a vacuum leak the rpms will increase due to extra fuel being introduced.
Make friends with an automotive mechanic that does evaporative diagnostics. They will have a UV smoke machine you can use to search for a vacuum leak. If your issue is a lean condition due to a vacuum leak it will be significantly more that a throttle bushing and should be easily detected.
Use a clean shop vac as a blower thru the carburetor, seal it well but it doesn’t need to be perfect and soapy water sprayed on the intake. Also I didn’t read what kind of primer you have but you might want to run the engine with the primer disconnected and plugged
Let’s start at that picture – get it right – it’s an F1 Evo with a TCM 550 in the front. Note the flap and aileron fairings on this one.
Seems to me that carb cleaner would tell you where the leak is. Carry on!
If you choose to use any bubble process, get the good natural gas detector bubble stuff.
No soap bubble will work as good as the gas stuff. Buy the good gas detector from a propane distributor.
There is no mention on hour many hours are on the engine, and does it not fly for some extended time, during the winter.
If it’s a high time engine, I’d look at the cam and lifters for wear.
Removing the valve covers will allow checking the valve movement and valve lift, as well as wear in the intake guides.
Check the manifold psi line for a crack and the cylinders for a crack near the intake.
The squirting brake cleaner around the intake area comment is germane … if the engine hesitates when you do this you have identified the leak.
You need a smoke tester
https://www.autolinepro.com/
I have seen the intake gasket between the flange and cylinder fail and leak, and I have seen the intake tubes crack up near the flange. If you follow Paul’s advice, you’ll be removing the intake tubes to inspect the tubes at the oil pan. So inspect the condition of the tubes for cracks or an abrasion that has worn a hole into an intake runner, then replace the gaskets at the cylinder when you re-install the intake tubes. You may also want to replace the intake sleeves at the oil pan. In the course of removing and installing the tubes, you’re almost certain to run across your problem.
Carb cleaner is best way for vacuum leaks. Remember extreme danger around spinning prop 😵
The only way to get bubbles in the intake system is to remove the carb and make a plate with a 1/4 npt thread to pressurize the system with regulated pressure and block off the exhaust pipes with tennis balls or tapered wooden plugs and then find you bubbles.
Wouldn’t hurt to check the finger screen in the carburetor fuel inlet, too. It is frequently missed at annual. I’ve had restriction there produce symptoms similar to yours with the addition of higher than normal fuel pressure.
Good idea. I’d also remove the carb bowl drain plug and catch the fuel, looking for any ‘stuff’. The first time I did this I got 20+ pieces of grit and ‘stuff’ , and flushed it to ensure no more stuff. The main jet pulls fuel from near the drain plug ,in the lowest point in the bowl, so it could pick up some of this stuff and restrict the jet.
Check to fuel pressure and then remove the fuel line from the carb, and check fuel flow to insure the min. gpm..
I have an O360 that had the same issue and went thru the same troubleshooting steps with the same results. The solution was different jets in the carburetor to match the airport altitude. This also solved the sensitivity on the mixture control.
How you going to find an induction leak with soapy water and the engine running? The bubbles will be on the inside of the induction system. To use soapy water, you need to pressurize the induction system, with the engine not running.
Finally someone on this subject making sense. We remove the carb, hook up a shop vac to blow air, block off the exhaust pipe outlets, rotate the prop to get the piston at tdc for each individual cylinder tested and spray soapy water solution on all intake connections for that cylinder. Do the easy stuff first, you’ll probably save yourself alot of needless parts changing.
Shoot short squirts of starter fluid in different areas of the induction system while the engine is at idle. Be sure to stay clear of the prop. An rpm rise will occur in the area of a leak.
Instead of running the engine, I have found induction leaks by attaching a shop vac (configured so it is blowing air out) to one exhaust pipe and covering the other exhaust pipe (I use latex gloves). I then use soapy water to spray around the areas Mr. McBride mentioned in his article. It is a simple and safe way that has allowed me to easily identify induction leaks on my IO-320.
How would it bubble,intake tubes should be in a vacum at idle…
You lost me at using soapy water on the intake… unless you are on the INSIDE of the engine to see bubbles. You want to use carb cleaner, when you spray over the leak your RPMs will go up.
The carburetor to sump gasket is another place to look. I’ve had the attach studs have too long a grip, and when applying the proper torque, the nut is torquing on the grip, not pinching down on the gasket. The carburetor will appear and feel torqued, but it is able to move several thousandths allowing a leak. This leak can be intermittent or continuous.
Find a car mechanic with a smoke machine and apply smoke to the induction. You can do this without running the engine and it will find issues you can’t see with the naked eye.
Lastly, I did not see any talk about any specific cylinder running hotter or cooler than the others, which means the leak is not contained to one cylinder, but to all. If you have a sniffle valve, check to make sure it’s closing.