Keep your Engine Running Right

Keep your Engine Running Right

I’ve been selling, servicing and repairing model engines for close to 50 years. During that time, I have come across quite a few strange engine-related problems, but I recently came across one that I think I should share with you all because it was definitely a first for me. I had one of my Custom K&B 61s for servicing, and it had been out of use for a while. When the owner decided to use the engine again, it flamed out after takeoff, and his aircraft was slightly damaged. Thinking that there must be a problem with the engine, he sent it back to me. Needless to say, guys, if an engine dies unexpectedly, it is usually fuel-system related, i.e., foreign matter in the fuel clogs the carburetor; there’s a hole or split in the fuel line or pick-up line in the tank; something is blocking the muffler pressure line so that the tank can’t vent, etc. I disassembled the engine but didn’t see any particular problem other than that the engine had been run lean at some time (there was a little piston scuffing that had varnished over). I checked the carburetor for foreign matter but didn’t find any, but I did find that the high-speed needle valve only opened ? to turn. This was puzzling, as at this setting, the engine shouldn’t even have run. Perhaps the owner had turned the needle in, but if he had, why? Thinking that the root of the problem was probably in the fuel system, I cleaned and reassembled the engine and soaked the muffler in hot water and detergent to clean off the goop buildup. While drying the muffler with the compressed air gun, I discovered the cause of the problem. I blew through the manifold, and only a little air exited the tailpipe. Further examination revealed a wad of cloth jammed tightly halfway inside the tailpipe. Evidently, the owner had plugged the tailpipe to prevent oil from dripping out, but we have no way of knowing how it got jammed halfway in. As I write this, my note to the owner explaining the problem and the service charges is still unanswered. The plugged tailpipe explained the almost closed needle valve; the high backpressure it caused had increased the tank pressure, and in turn, this fed the fuel under pressure to the carburetor. This muffler restriction probably also affected the engine’s top end, but the owner didn’t mention this. So the lessons here are:

If an engine has been out of use for a while, always check the exhaust and intake for plugs before you run it.

Whenever you have to turn the high-speed needle valve more than one turn from its usual setting, there’s something wrong. Find out what’s wrong before you attempt to fly the aircraft.

Updated: July 14, 2011 — 12:50 PM


  1. Hi Clarence, It’s always great hearing from you. Dan Reiss

  2. Dear Clarence:

    Just a note to say “thank you” for the years of great information and detail regarding the two strokes. The younger guys often give me a hard time about doing things the old fashioned way. I break in all my engines before flying them as per your recommendations maby back as far as the 1950’s? Memory is not to good at 76. My general response is “just how many engines do you have that are till running well over 10 years old?

    I will drag out a O & R, Brown, K & B Torpedo, Veco and others once in a while just to prove a point. Just because they are clean and look new is just part of the fun. Thanks again for many years of enjoyment.

  3. Hi Clarence:
    Do you still modify Veco engines? I have a Veco .61 that is in excellent condition but it has a very low (I think) compression. Cosmetically, it’s appearance is almost like new. The previous owner says it’s been used very little. Never having owned a Veco .61 before I have no idea what the compression is supposed to feel like, whether it’s a little on the “soft” side or whether it’s supposed to be “hard” like a lapped piston engine of similar displacement. Any way, I’d like to know if you can bring it up to your superb Clarence Lee standards and if so, for how much coin of the realm.
    Thanks you for your time and reply,
    Steve Emery

    1. Dear Steve,
      Ringed engines will seldom have the hard compression of a lapped
      piton engine due to the ring end gap allowing some leakage when
      cold. However, they should still have a some what snappy feel,
      i.e., not feel mushy or soft.
      I still service the older Veco engines providing they are new
      or near new. I don’t accept old ~orn out junkers that a lot
      of guys are purchasing on e-bay now days. In 1972 I updated
      the engine for K&B and the Veco name was carried through 1974.
      These engines had a larger bypass that allowed for the PDP
      modification. The charge for Customizing a 1972/74 Veco is $28.50
      plus $750 shipping. If the engine is prior to 1972 I can not
      incorporate the PDP mod but, I can give it the Standard
      Customizing procedure for which the charge is $24.5~ plus
      shipping. The 1972 and later engines had a single expansion
      ring. 1969/71 had a Dykes ring, i.e., the ring at the top of
      the piston. Prior to this the engine used two expansion rings
      and I no longer service these engines. Any parts required such
      as a ring are at list price less 20% If the engine .has a Dykes
      ring it will require updating which will also require a new
      piston and rod.
      Clarence Lee

  4. Dear Clarence
    this post deals with the valve clearance post in the most recent “Engine Clinic”

    I am relatively new to glow engines, but not new to engines. This caused me to wonder about the valve adjusting advice in the most recent “Engine Clinic”.

    My experience with valves in an air cooled 4 stroke engines is that the need for adjustment comes from the valves seating more deeply into the head over time. This mode of wear would lessen the clearance between the valves and result in no detectable loss of performance until the lash gets so small that when the engine runs and heats up the lash goes away and you prevent the valve from seating properly. This would in turn prevent it from shedding heat which could result in extreme cases of the valve stem loosing temper at the head and the head popping off. in less extreme cases exhaust gas blow by could burn the valve to seat mating surface resulting in a burnt valve and poor performance and a repair or replace situation.

    I thought these small engines run much harder than most motorcycle engines, so suspected they would be in more danger of this type of failure than other 4 stroke engines. I could be wrong.


    1. Mike, there is a big difference between the valve wear in full-size 4-stroke automotive and motorcycle engines and model 4-strokes. In the course of replacing stripped glowplug and exhaust port threads, I have a dozen or more 4-strokes pass through my hands every week. All the wear has been in the valve train that increased the clearance. –Clarence Lee

  5. A friend of mine and myself have been looking at useing 2 cycle engines that are used in weed eaters/blowers. After taking a few apart and looking over the engines it seems possible to make them adaptable to a RC project. We are also venturing to desgn, draw out, and plan our own RC project instead of purchasing plans/kits. The question here is what formula does the manufacturer use in determining what size gas/electric motor to use for a plane. Do they use just weight. Size. Or both

  6. I still like going to the field with a glow engine, the sound and the length of the flights are better then electric. Yes I fly some electrics but glow is my favorite.

  7. I had the privilege of having my K&B 61 Ported by Clarence and even had him sign the box, his work and professionalism are well known in the RC engine industry might say a legend. I have used Veco, K&B and many Cox engines for many years his knowledge and sharing has my thanks and appreciation.

  8. Dear Mr. Lee: I’ve read every word you’ve written in every model magazine I’ve ever been able to find your advice in. That’s why I’ve never written to you about a problem. I add your advice to what the manufacturers of my model engines say in their instruction manuals. So each of my engines is clean, firmly mounted, and connected to a fuel tank located properly, cushioned, plumbed with proper tubing, filled with good filtered fuel of the recommended oil and nitro proportions, etc. Prop size/type as recommended, balanced and kept tight; recommended glow plug installed; muffler pressure rigged; carb adjustments set for reasonable idle at low throttle, just slightly rich at full throttle. I do what you and the manufacturers tell me, and guess what? I never have any problems, and I expect I never will. Best regards, –Scott Kruize

  9. I wonder if you are able to help, I am trying to source a carburettor assembly for a Veco 61 I have tried the internet and a lot of English model shops and I can only find a complete engine on an auction site in America. I appreciate that this is not within the remit of your letter but I would appreciate any help you may be able to provide given your extensive experience. I am a member of our local flying club and the BMFA

    1. Mike, Do you still need a Veco .61 carb? I might be able to help you ( free)

  10. I had a Veco .45 RC engine. One time I think I read in your column that they were machined backwards. Tight at the bottom of piston travel and loose at the top. This engine would not accelerate, and the suddenly race ahead, as I remember. With this engine in an old free flight design, plus a controlaire pulse 6 channel radio, it crashed every time I tried to fly it. Any comments. JWM A few years ago I checked it and it was frozen solid. Tossed it out.

  11. What options are there for modifying a K&B 4011 these days? Is it worth doing compared to the power you get from the ABC version? I have quite a few 4011’s that I would consider for modification as long as it is not cost prohibitive. I am not looking to change the RPM range of the motor but just want to modernize the scavenging to turn a larger prop at the same RPM. Any list of options would be appreciated. Thank you

  12. Dear Clarence,
    Just a quick note to say thank you for the many years of service and information you have provided to all of us over the years. I have a number of your ‘custom’ engines in my engine collection. Perhaps three of the most significant are your custom Lee 51 and 2 Custom Lee 45’s. The Lee 51 was built for Phil Kraft back in the 1960’s. You traded it for a Kraft radio. I plan to donate one of the 45’s to the Craftsmanship Museum in Carlsbad Calif this summer. I am still looking for a Lee 49 to complete the set. In closing, thank you for your wisdom and terrific insight. God Bless. John Carlson, BC Canada

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