Properly Adjusting the TSIO-360 Engine - M20K 231
ShopTalk – MAY 1999

I would like to discuss with all M20K (231) owners the need and importance of proper fuel flow and turbocharger adjustments. So many times I have seen 231s come through my shop that have the potentially catastrophic setup of too lean fuel flows and too much boost. This combination is a premature engine overhaul or a burned-up engine just waiting to happen.

As we get into the nuts and boles of how to set-up your 231's -GB or -LB engine correctly, keep two things in mind. Number one, this is a setup for your mechanic to make. Never attempt to do this procedure your­self. It takes special tools and instru­ments that you will not have (the air­craft's instrumentation is not good enough for this setup). Also keep in mind that your mechanic must follow this exact sequence to get the setup right. He can't make up his own proce­dures here. If any step is performed out of sequence, the entire procedure will be compromised. This procedure requires both the top and bottom cowl halves to be removed, so take them off and store them in an out‑of‑the‑way place while doing the ground runs.

Since this procedure involves high power runups of the engine on the ground, be sure to make special precau­tions. Keep everyone well clear of the airplane while running it up. Try not to run the engine for longer than two or three minutes between adjustments. This will be long enough to get the readings you need. Allow some time for the turbo to spool down and cool off before shutting down the engine between adjustments. Do not be aggressive with the throttle. Gradually add and reduce power.

The first step in properly setting up the TSIO‑360‑GB or ‑LB engine in your 231 is to check the propeller governor setting at full red‑line manifold pres­sure. Do not trust the tachometer in the panel. Use a calibrated prop strobe. You will probably find that your pro­peller is not developing full‑rated RPM per the strobe at max power. If not, adjust your prop governor for full RPM. One turn out (counter‑clockwise) on the adjusting screw will yield about a 25 RPM increase.

Now that we know the engine is devel­oping a full 2700 RPM, the next step is to hook up a fuel pressure gauge to the engine. Use a gauge calibrated in one psi increments from 0 to 60 psi. With this gauge, we are going to measure unmetered fuel pressures at max and idle power settings. On the TSIO‑360‑GB or ‑LB, a fitting is provided next to the mixture control on the top of your engine. Unscrew the AN‑4 cap and install a ‑4 aircraft flex hose wich fir­tings at both ends. Run enough hose so you can read the gauge while sitting in the pilot seat with the engine run­ning.

Next, pressurize the line and gauge. Bleed all the air out and verify you have no leaks. Start the engine and gradual­ly increase the power while watching the fuel pressure gauge you just installed. Write down the gauge reading at 40 inches of manifold pressure. Next, reduce the power to idle (make sure it's the correct idle RPM per TCM specs). Write down the gauge readings. These readings are your unmetered fuel pump pressures at max and idle power.

Now find your engine model number on TCM SID97‑3 and verify your unmetered fuel pressures are within limits at idle and at full manifold pres­sure. Let's talk about the term "within limits". I have found that on the TSIO‑360‑GB and ‑LB engines, it is best to set the idle pressure right in the middle of the limits, but the full power pressure should be set at the maximum pressure setting. The reason? When the engine gets hot, the metered fuel flows tend to drop causing a lean engine just when you need a richer (cooler) engine. Even if you have a new engine or a recent fuel pump overhaul, don't think you are okay. Never have I seen a TCM or any other overhauled fuel pump set up for maximum pressure at maximum mani­fold pressure.

During this check of unmetered fuel pressures, remember not to run the engine so long as to get the cylinder head or oil temperatures above one‑third on the gauges. When the engine gets too hot, the metered fuel flows drop and you will be chasing your tail and doing damage to your engine. The bottom line is to let the engine cool off between runs. I generally do ground runups first thing in the mornings when the OAT is low. Now that you have adjusted the fuel pump pressure correctly, remove the gauge and line and reinstall the B/nut cap on the engine.

The next step is to run the engine at 1000 RPM and very slowly screw the mixture control towards idle cutoff. Write down how much the RPM increased (this is your idle mixture setting we are checking). Adjust the idle mixture so at 1000 RPM the engine has a maximum of 25 RPM (sea level conditions) increase when the mixture is slowly screwed out toward the idle cut­off position. More then 25 RPM is too rich, no increase in RPM is too lean.

The next step is the fun part, let's go flying. Our flight test will be to check the engine's so-called "critical altitude". Critical altitude is defined as the standard day pressure altitude at which your engine will no longer maintain 40 inches of manifold pressure, even at full throttle. If your engine comes out below the recommended critical altitude, it is not performing up to its capabilities. If it comes out above the recommended critical altitude value, it is could be experiencing stresses above limits and could be damaged. We can adjust the critical altitude value of your engine by screwing in (clockwise) or out (counter‑clockwise) the bolt in the wastegate bypass valve.

Note: If your engine compression test (while hot) is not at least 60/80 or higher in every cylinder, then don't bother adjusting the bypass bolt. Over‑boosting your engine by screwing in the bypass bolt is certainly not the correct way to solve a low compression prob­lem. Get the compression fixed first.

Before we go flying, count the num­ber of threads exposed on that waste­gate bypass bolt. Now, go flying and get the pressure altitude (29.92 in. Hg) and OAT where your engine no longer will maintain 40 inches of manifold pressure during a climb. Use the OAT to calculate a corrected altitude for standard day conditions. The ‑GB and ‑LB engines should no longer be able to maintain 40 inches of manifold pres­sure at full throttle at about 14500 feet MSL. Engines modified with Merlyn automatic wastegates should get about 5000 feet more. If your engine is with­in 1000 feet of these specs, don't mess with it. You probably can't get it any closer than this on a repeatable basis anyway.

If you do have a need to adjust the bypass bolt to change your engine's critical altitude, this job can be a real nightmare. The threads of the bolts will be clogged with exhaust by‑products which have probably caused the bolt to seize. After taking consid­erable time to free up the bolt, remember to adjust the bolt out to lower critical altitude or in to increase the altitude. After adjusting the wastegate bolt, another test flight should be accomplished to verify the desired improve­ment.

Remember to pay attention to your climb speed during your test flight, always maintain at least 110 MPH to keep enough air flowing through the engine to keep it cool while adjusting the throttle to maintain 40 inches of manifold pressure at full‑rich mixture settings. Always keep the engine tem­peratures within the green arcs during the climb.

On most 231 airplanes I see in my shop, the normal bypass bolt position is one where about 10 to 13 threads are showing (fine‑thread bolt). If you have the newer coarse‑thread bolt, there will be fewer threads showing. Always remember to tighten the jam nut and safety wire both bypass bolt and jam nut after any adjustments are made.

Another item to check during your test flight for critical altitude is magneto operation. While at critical altitude and straight and level, fly around a little to make sure that your magneros don't cross‑fire of miss fire at altitude. Early 231s did not come with pressurized magnetos. Those early unpressurized Slick or Bendix magnetos would build up bronze material inside the distributor cap and at higher alti­tudes (thinner air) the mags would cross‑fire causing the engine to slow down when the wrong cylinder would fire. If your mags have over 1000 hours SMOH, get them replaced (Slick) or rebuilt (Bendix). Install a magneto pressure kit at this time.

One last check. If you have a fuel com­puter in your aircraft, it should read 24.5 to 25.3 GPH at 40 inches of man­ifold pressure as you are climbing out on a standard day from sea level condi­tions.

That about covers it, but remember if your compression is not 60/80 or better in every cylinder then get that fixed before you try setting up your engine.

As always, if you have a question about this or another article, contact me at our aircraft repair facility via e-mail: or the old-fashion way via phone: 307‑789-6866.

All ShopTalk articles can be read here at Until next time, enjoy flying your Mooney.