carbs, mags and certified engines

Builders, I wrote the comments below to address a guy who put a 65 continental on a new homebuilt, rebuilt the carb himself, and couldn’t make it even slightly hint of running in two hours of hand propping. The man was not a mechanic, never built a plane before, and had never soloed a plane. He went on a net discussion group to ask others how to start his plane, and got some advice on starter fluid. the comment below was to hopefully get some builders to look at the bigger picture, that reliability isn’t cast into the metal of certified engines, it is in the attitudes and decision making of the people working on and flying them.


I have been an light aircraft mechanic in Florida for a long time. One of inspection tasks that is occasionally done is looking over a single engine plane before it flies to the Bahamas. The gap from West Palm to West End is 56 miles, and smart pilots, particularly those renting, get another set of eyes on the plane before they stick their family in it. When given 30 minutes to evaluate a certified engine’s condition on the ramp, my focus is on the Mags and the Carb, as there two are the most likely sources of taking a swim. A slightly low compression cylinder is not the same trouble as a failed mag on a 95 degree day with four people in a C-172. If the Mags and the carb are working perfectly, odds of other trouble are quite low. The slightest hint of issue from either is a good reason to delay the trip.
The exact same logic applies to Experimentals, and I can make a statistical case that flying the 40 hours on a new homebuilt, even one with a certified engine, is greater risk than flying for a week in the Bahamas. If a neighbor chose an A-65 Continental for his newly built Pietenpol, I wouldn’t be concerned that the basic engine had 800 hrs. on it. If it has consistent oil pressure. it is not likely to throw a rod, but I would advise him to stack the deck in his favor and make absolutely sure that he had a perfect Carb and Mags on it, as they are the likely source of any issue.

When looking at the O-320 headed to the islands, I look at the logs to make sure that the last people who touched the mags and carb were in a repair station, or the factory. After visual inspection for leaks and security, I run the engine to full power and try to make it misbehave with the throttle and mixture. A critical test is full static power and slightly leaning must show an rpm increase. Carb heat must work, and cutting off the fuel and letting it idle must cause a 25-50 rpm rise before it quits. Engine must idle as solid as a rock. Turn the prop and feel for low compression and listen for impulses to click at the same time. The 1/2″ nuts holding the mags are checked for torque. Hands on mags to make sure they are secure. Leads traced to look for cuts, every 3/4 nut checked. Engine is started and the key is messed with to make sure a worn switch will not short. The run up is performed with the engine heat soaked, because mags have trouble when they are hot, not cold. Zero tolerance outside of limits on mag drop. The goal is to find the circumstances in which it misbehaves, not to show that it runs ok. Any discrepancy on mags or Carb, even one that is hard to quantify, is cause for the delay of the trip. If I bring any issue to the pilots attention and he responds with a variation on “It will be alright” I never fly with him nor work for him again. I am not a cat, I don’t have 9 lives.

If a newly finished home built has a used certified engine on it, and the builder is having trouble starting it, odds are the trouble is with the Mags or the carb. If it is stored in a reasonably dry place, a piston in a bore can happily wait 20 years to be re-stared, but the points in mags don’t like this and carbs don’t like fuel, especially auto fuel evaporating from them. (The sole common exception to the mags-carb rule is the camshafts on Lycomings left to sit often corrode and if the engine is run without correcting this the grind the lobes off in a few hours and pump the metal through the oil system.) A homebuilder is allowed to fix his own carb and mags if they need attention, and there are manuals and parts lists on the net, but I can make a case that this isn’t always smart.

Looking at the carb: aircraft carbs are deceptively simple, and they look far easier to rebuild than a four barrel. Here is the hidden issue: Many carbs on engines for home builts are 60 years old and have had long periods of inactivity, previous owners mix and match parts, and people who like to drill out jets. A skilled guy in a FAA fuel system repair station can spot all of these, but a homebuilder is likely blind to them. I like aircraft carbs, and I teach people to use them after sending them to a professional. Maybe 3 of 10 NAS3’s or MA3’s sold at fly marts have mix and match parts inside. Hard starting is not the worst thing about poorly tuned carbs. First, a carb that is set too lean or has a malfunctioning enrichment circuit will damage the engine in flight. Second, ones that don’t run smooth will often quit at idle. Put this on a hand prop plane and combine it with the fact that many pilots don’t fly every pattern power off, and the new homebuilt ends up 100 yards short of the runway threshold. For more info on carbs, look at this link:


I do not trust mags that have no logs, were repaired by amateurs, or have had a decade with no inspection of any kind. My neighbor owned a Mag test bench that could run all brands and evaluate them with proper loads on the leads, a tool you find in a Mag repair station. He just sold it on Ebay and got $4,000 for it. If it was actually possible to properly evaluate, repair, overhaul and test aircraft mags without this tool, then it would not be possible to sell it for $4,000. It is legal for a homebuilder to ‘repair’ his own mags, but no rational person who make the argument that a first time amateur without the test device could do as good a job as a professional with the correct equipment.
Now lets think about a new Pietenpol getting ready for it’s first flight: Plane is built by a nice guy, but planes are a hobby, not a career. A tech counselor looked at it, but that man’s experience was building one RV-6A, and all his “looks good” offered was a false sense of security. It passes the FAA exam, with a DAR that charges $400 but didn’t even ask to see it run. The plane is out of rig, but no one knows this yet. The low time pilot’s time in type is two trips around the pattern at Brodhead. He got 3 hours of tail wheel in a Cessna 170, (a plane that could land itself) but he was not allowed to solo it. The pilot has never flown anything that has the short glide ratio of a Piet. At his last Biennial the CFI allowed him to drag the 152 in with power and plop it down on the runway. He is nervous enough even without the video cameras, but there is a growing group of spectators adding pressure. Under these conditions, does it sound smart that he is also flying the first aircraft carb that he has ever ‘rebuilt’?
A small continental is an easy engine to troubleshoot if you are trained on them. This training can come in many forms, but the most effective is learning them in person, from someone who knows them. Theoretically you could learn to fly by reading a book, but everyone understands that in person flight training works. I only make the same point with maintenance, that instruction is best, person to person. On a relative scale, making one run that is reluctant to start is very easy compared to doing an airworthy job overhauling a carb or a mag without specific tools or training. Everyone is entitled to their own opinion about this, but what ever difference in opinion is, the wager riding on the opinion is the same, the whole value of the plane and the lives of the people in it. Place your bet carefully.

I like Continentals, and have a lot of time flying behind them. Their primary quality is reliability. This well earned reputation was made seven decades ago, when homebuilding was still illegal in the US. The Continental reputation was built on relatively new engines, installed at factories, and maintained by trained, licensed A&E mechanics, in a era where people had longer attention spans. Seventy years later, anyone expecting that the same reputation magically lives in the metal is deluding themselves. To get the same results, you have to get as close to the original format as possible. But the issue is that the parts can be old, the details of the installation on a homebuilt can be weak, and the guy working on the carb may have never built one before. Is the issue beginning to make some sense?

To even get close to the original reliability, One must spend some money on parts, the used parts must have a history and be within limits, and critical items like mags and the carb should be done or at least checked by a repair station. You can choose to do otherwise, but it is not possible to then argue that you can expect the full reputation for reliability. Anyone who thinks that you can have the reliability of a certified motor when you buy one that is advertised as “no logs” or “experimental only” is mistaken. You don’t get to have it both ways. Continental’s reputation was not built on engines made of junk and spray painted. If the engine was just as reliable with out of spec parts, then they wouldn’t be out of spec would they?

There are always people who argue that they have to have “a reliable certified engine” and that they will not fly auto engines. Then the first thing they do is go out and look for the cheapest collection of parts bolted together that are masquerading as a “certified” engine, made of out of spec parts. That behavior isn’t rational, but people who are compulsively cheap often are satisfied with the illusion of reliability instead of the real thing. Want to know who isn’t fooled by this? Our old friends Physics, Chemistry and Gravity. If the FAA considers the engine un-airworthy in a certified plane, it is just as un-airworthy in an experimental one. Physics, Chemistry and Gravity don’t care if the plane was built in a factory or your garage. An engine built of out of spec parts doesn’t magically become airworthy when it is bolted on an experimental.

I am an Embry-Riddle trained A&P with 24 years of continuous work on light aircraft. I am qualified to work on virtually any part in GA planes, but that doesn’t mean I am reluctant to hire other mechanics with greater experience and better tooling. When the right mag had excessive drop on the C-85 in my wife’s Taylorcraft, I could have replaced the cracked coil myself, but instead I took both mags to a repair station and waited while they were overhauled. In the last 10 years we have finished several home builts, and I could have overhauled each of the carbs myself, but I elected to send them all to a certified repair station. The difference between ‘fixed’ and ‘Yellow tagged’ is often hundreds of dollars. It sounds like a lot of money until you have lived through two plane crashes and attended a few funerals. 90% of the people reading this make more money than I do, and 95% have less experience with aircraft engines. If those people are trying to save money by fixing a mag or a carb themselves, when I would send the same part out, they should rethink that plan.


My known specialty is training amateurs to build aircraft engines for experimental aircraft. It doesn’t matter that the hardware is mostly Chevrolet and not Continental, It isn’t about metal, it is about the capacity of builders to learn, and I am not speaking of turning wrenches, I am speaking of learning to make good decisions in a very unforgiving environment. No one has to agree with my perspective, but I have been doing this for long enough, with enough homebuilders that it is worth considering carefully. Homebuilding, including building engines, can be done with reasonably low risk, but only when the builder makes good decisions. -ww



About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at and in more than 50 magazine articles.

4 Responses to carbs, mags and certified engines

  1. Tom Porter says:

    Hello William and Grace, I started my engine on my airframe for the first time yesterday. It fired up on about the second rotation and ran smooth. I finished the mods on my 601xl and added the fifth bearing from Dan and his father. I have most of your parts and followed most of your instructions and advice.I know I’m slow, but I hope to have my project at a local airport ready for inspection by Brodhead. See you soon. Thanks, Tom Porter

  2. I read your words with the same intensity that I read the Bible. I consider you the Apostle of Reliability. A sycophant I am not; but my ‘flight plan’ shall follow your words and I choose to deny myself flight rather than violate them should that choice present itself. I hereby declare my accountability. I gladly submit to your correction should that opportunity ever present itself. Long life and prosper.

    • Jackson,
      I appreciate the thought, but the concepts I write about were all taught to me by many good men who have been my mentors in aviation. If I have any talent at all, it is just making the wisdom of these man available to people who didn’t have a chance to know them in person. I accept your thoughts on their behalf. -ww

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