Since starting the engine for the first time, I’ve run into a bit of a problem.

It runs okay at higher idle speeds but it doesn’t want to run at speeds that the book says it should run at. When I first ran the engine, I couldn’t get it to run under about 1,200RPM. And I had to pull the mixture to about 1/2″ short of cutoff.

So I borrowed the fuel pressure gauges from the factory hooked them up.

The engine manual says that at 600RPM the fuel pressure should be 8-10 PSI with the mixture full rich.  At 1,200RPM, I was seeing half that… and I had to have the mixture pulled out just to get that.

I increased the fuel pressure by a half turn and leaned the idle mixture by a half turn then ran the engine and checked the pressure. The pressure went up, the mixture didn’t have to be leaned as much and the engine could run at a little bit lower RPM.  Finally after FOUR turns on the pressure and and four turns on the idle mixture, I can run the engine at about 800RPM with the mixture full rich and a fuel pressure of about 11psi. If I go any further, the engine doesn’t run as well.

Since I was close, I decided to try a full power run.  The book shows full power being 2,700RPM.  But stationary on the ground I should only hit about 2,650RPM.  But when the throttle was pushed in all the way, 2,100RPM was as good as it got.

Clearly something is amiss.

I talked with Continental when I was at Sun-n-Fun and they suggested having a mechanic that’s familiar with Continental fuel systems look at it. But there isn’t one in Sebastian. The nearest one is in Sebring and he can’t make the two-hour drive over until the end of May.

I was talking with my A&P of many years and he said “Don’t pay anyone to look at the engine until you’ve had the fuel pump sent out and checked.”  And he was pretty adamant about it. I started looking for shops that do this work and the first one I hit basically said that since the engine has been sitting for four years the seals in the pump and manifold can dry out and get cracks which can cause some very strange behavior.

I pulled the fuel pump and fuel manifold (spider valve) and shipped them to Aircraft Accessories of Oklahoma.  Ken called me the day the parts arrived and said the fuel pump was “perfect”.  The only thing wrong was that the pressure was dialed up too high. I asked him if it was about four turns too high. He paused and said “uh… yeah. How did you know?” :-)) The manifold has a rubber diaphragm and it was “a little stiff” so it was replaced.

He said that I probably will never get a smooth idle at low RPMs because of the lightweight prop.  He also said the only other thing that could be wrong is the throttle body (fuel metering unit) and that if it didn’t run properly after reinstalling the parts that I should call him.

When the part arrived, I installed them and hoping for the best, fired up the engine… with the exact same results (low unmetered pressure, inability to idle at lower speeds and running very rich). I talked to a bunch of people and the consensus was that the metering unit was allowing too much fuel into the cylinders (making them run rich) which did not allow the pressure to build up (low pressure). So contrary to the official Continental Service Information Directive, I began adjusting the idle mixture by leaning it a half turn and running the engine.  The engine ran leaner, the pressure was higher and it was able to run at a slightly lower RPM.  Seems like the right track.

EIGHT TURNS later, the engine is idling at about 800RPM (a bit rough) with the mixture full rich and about 10.5psi unmetered pressure.  Well, I’ve been here before.

So then a full power run was done. The engine hit about 2,650RPM but then dropped to around 2,575.  Because the RPM dropped after peaking, it indicated the prop governor was now limiting the RPMs.  I backed out the prop governor and did another full power run and was able to maintain 2,650RPM.

That’s great… But the fuel pressure was very low for that power setting.

Now there’s an adjustment for this (high pressure adjustment).  But that adjustment will throw off the low pressure adjustment.

At this point, I’ve been fighting this since March.  I’ve spent about 18 days working on this and pretty much nothing else. It’s time to call in experts. Another builder had similar problems and called in Leading Edge Aviation from Tampa. I put a call into them and we made plans for Jonathan to meet at the plane.

When he came in to work on the other builder’s engine, he arrived at 9am and spent the day here.  But he told me that he’d arrive around 5pm!  I asked why and he said that it wouldn’t take very long (the other builder had a twin turbo-charged version of the engine). I was still surprised… and skeptical so I made sure that I had plenty of lights available.

So the day came but he was delayed a bit and ended up landing at around 6:15. He bought his own gauges and (small) handful of tools.  So we pulled out the plane and started it up. While he did get a pressure reading at 600 RPM, it was pretty ugly. He was working the throttle to keep the engine from stalling and he RPM was bouncing around so much that he would have to yell “NOW” whenever the engine touched 600RPM so his co-worker could read the pressure. Which needed tweaking along with the idle mixture. But not much.  At least I got that right.

Once that was set, we ran the engine up to full power and then the metered pressure needed adjustment… Which required quite a bit of adjusting. But that required going bach and adjusting the unmetered pressure and idle mixture adjustment. After only about an hour, it was finished!

Bob (another builder who’s building a twin) was on hand to observe pointed out “I told you that you should have called them two months ago.” And he did… And I should have. So I bought him dinner. Next time I’ll listen to Bob.

Time to begin taxi testing.