What’s causing Mooney’s rough-running engine?

Q: I bought an older Mooney with a Lycoming IO-360 with Bendix fuel servo. The engine starts to run rough once the last 1/2 inch of throttle is given. An induction leak cannot be located. Would a faulty sniffle valve be to blame? Any other possibilities?


A: Jamie, with regard to your older Mooney and the engine roughness when the last half inch of throttle is applied, I’d be suspicious of a partially plugged fuel nozzle.

I’m not certain if the older Mooneys had a fuel flow gauge or not, but if you were to have a partially plugged nozzle, it should show up on the gauge as an excessively high fuel flow at full throttle.

This is a result of the fuel servo unit allocating four cylinders worth of fuel — which is what’s be being called for at full throttle — being sent to the flow divider on top of the engine. The flow divider’s job is to simply distribute that fuel to the cylinders equally.

However, when one cylinder has a partially plugged or blocked nozzle, then either a reduced flow or no flow goes to the cylinder that is having a problem. This is normally noticed by a higher fuel flow on the gauge because the fuel that’s not able to go to the cylinder with the problem will now go to the three remaining cylinders, causing a rich condition in those cylinders.

What I’m trying to say is the other three cylinders are actually receiving four cylinders worth of fuel, which could cause some engine roughness and unusually high fuel flow.

There are other problems of a more serious nature that could come into play if this is what is actually occurring. The cylinder that is not getting the proper amount of fuel could possibly experience a failure. With insufficient fuel to a cylinder with a partially plugged fuel nozzle, the end result could be a burned piston. Should that occur, then the possibility of metal contamination throughout the engine is possible.

I’d suggest you do some simple troubleshooting before you decide to give up on this poor old Mooney. Let’s do a flow check on each fuel nozzle, which doesn’t take long and may give us a lot of information.

Remove the top cowling and disconnect each fuel line at each fuel nozzle. Then remove each nozzle from the cylinder. If they are the newer type two-piece nozzle, make certain you don’t lose the insert. Reconnect each nozzle to its original fuel line. By bending the fuel line and nozzle assembly slightly, place each nozzle in a clean small container like a baby food jar.

The object is to observe the fuel flow from each nozzle so the container should be positioned so it can receive the fuel without spilling any. The test can be conducted by turning on the master switch, with throttle and mixture full forward, and then turn the aircraft boost pump on. You should then observe the nozzle spray pattern on each cylinder as it flows into the container. The spray pattern should be a nice pencil-point stream and not a wiggly air bubbly type spray. If the latter is observed, that nozzle may need to be replaced.

After 30 to 45 seconds you may bring the mixture control back to idle cut-off position and close the throttle and turn the boost pump off. It’s ideal to take all of the fuel containers and place them on a level surface and hopefully you should see the same quantity of fuel in each container. If each container has the same amount of fuel, then all of the nozzles are functioning correctly. Should the quantities be different with one nozzle showing a lesser amount of fuel than the others, then this is the nozzle that has been causing the problem and will either need to be cleaned or replaced.

I’d try cleaning it first by blowing clean shop air through the nozzle in the direction the fuel flows and reflowing it with the others. Use caution here if you have the newer style two-piece nozzle and be certain to keep the two pieces together, but blow each piece with the clean shop air. You may also clean the nozzles with a good cleaning solvent. I prefer to use Hoppes No. 9 gun cleaning solvent. Again, if you have the two-piece nozzles, keep the two pieces together because they are a matched set. In other words, just don’t throw all the pieces in the Hoppes together for cleaning. DO NOT use a piece of safety wire or any other solid object to clean the nozzle.

You mentioned the sniffle valve, so let’s think about that. If the sniffle valve is allowing an induction leak, then I’d expect to see a higher manifold pressure at engine idle of 600 to 800 rpm. Typically, on a normally aspirated engine, the manifold pressure in that idle range would be about 10 inches. If you have an induction leak, then I’d expect to see a slightly higher manifold pressure of 12 inches or higher.

Just for the heck of it, with the engine at normal operating temperature and at idle, pull the mixture into the idle cut-off position and watch the rpm. If the engine has an induction leak, you probably will not see a 25 to 50 rpm rise just prior to the engine quitting, which may indicate a lean mixture as a result of an induction leak. If the fuel system and induction system are working as they should, I’d expect to see that slight rpm rise just prior to shut-down.

Jamie, I hope this gives you a couple of things to check and hopefully discover where the problem lies. I don’t think it’s anything too serious, but I’d check into it as soon as possible.

Leave a Reply

Your email address will not be published. Required fields are marked *