Text Box: WARBIRD NOTES # 19  10 Jul 95  (22)      PRIMER SYSTEMSDelving into the systems on a recently acquired PBY “Catalina” caused us to uncover a few things.  These discoveries engendered some comments such as, "I sure didn't know that" and "Wow, that's really an old system"!  I'll run one of them by you below and you can see how the particular aircraft you happen to be flying might fit into this picture.      


You really need to know before you initiate the start just what kind of primer system you’re dealing with.  And it’s probably (but not necessarily) going to depend upon how old the airplane is!  The systems that I’ve encountered are of three types – (1) manual, (2) electrical (“spider”) and (3) electrical (“throat”).  I’ll describe some things about each of these types below.


One concern of mine is the silver-soldered connections, as well as the nipples and hardware on some primer lines. As far as I’m concerned the “spider” systems on larger round engines (see below) should be banished, banned, run out of town, AD’d out of existence and done away with – period!  Do you get the feeling here that I don’t like them?  You’d be absolutely right in that assumption!  I think they’re a fire hazard of the worst kind, this is from personal experience!


  While learning to fly in the Aeronca 7AC “Champ”, my first primers were the manual “pull-out and push-in” Lunkenheimers/Kohlers.  My N-2S Stearman also has one of  this type, but it’s mounted externally near the inertia starter’s handcrank.  I’ve also used it on the T-6 and the P-47 Thunderbolt.  The BT-13 had one and the pilot’s manuals for the B-17s show them also.  Seemed to me that invariably they’d leak around the packing gland and spray some small amount of av-gas into the cockpit!  Due to the limited capacity of the manual plungers, the output was delivered only to several upper cylinders on the engine via what I’ve always referred to as the “spider” system of small stainless steel tubes.  On the systems of this type but with electrical power, they still delivered their fuel only to the top cylinders.  Obviously, while this made it impossible to run the engine on that system, it represented a vast improvement over the miniature funnel and/or oil-can systems that the early aviators utilized!


I always kept one “shot” in the manual primer when starting.  By this I mean that, after I‘d finished with my priming, I pulled the plunger fully out once more and left it that way during the start in order to have a “shot” of fuel available for the almost invariable cough or hesitation as the engine is first started.  Sometimes I‘d need to (hastily) recharge the primer and repeat the priming process when the engine wanted to die after the first few seconds of running.  Obviously, the problem “lurking in the weeds” here is that you may forget to push the primer in and lock it after completely started.  If left out or unlocked the engine will burp and snarf and – in general – run rough.  And – ease it in when you push the plunger in to lock it, don’t just abruptly shove it totally in, thereby momentarily flooding the engine!


Now, regarding the later electrical priming system.  With this system, a spring-loaded (momentary) switch in the cockpit opens and closes a solenoid operated valve to control the flow of pressurized fuel through the priming system.  That fuel then is sent to the engine via either the “spider” system or the “throat“ system, whichever is installed on that particular airplane.    


The installation that I’ve always referred to as the “throat” system furnishes fuel to all the engine’s cylinders.  Actuating the primer switch enables the fuel (assuming you have the boost pump switch ON) to be delivered through two or three adapters on the carburetor throat to the impeller where it is atomized and sent to all the cylinders through the normal induction system.  I’ve always tested this system just before starting to ascertain that it working.  The method I use for this is to simply “flick” the primer switch and while noting a corresponding fuel pressure flicker (assuming again that you have the boost pump switch ON).  Of course, it should be noted here that this only guarantees you that the primer is functioning at that time and it’s possible that it might not during the start.


Our airline’s Convair 340s/440s all had the “continuous prime” selector switch installed on them. I’ve also seen this on the DC-6s that I flew.  Most likely other airplanes may also have had them – but this is outside the range of my experience as I write this.  This selector was to be utilized if it ever became necessary to operate almost totally on the fuel furnished by the primer system if the carburetor malfunctioned or became affected by “bleed icing”.  I’m aware of at least one time that this system “saved the crew’s bacon” when they encountered a severe case of bleed icing near the Duluth airport on a scheduled flight.  With this system the primer delivered a fixed quantity of fuel, the throttle needed to be adjusted so that the engine would keep running. In general, it would support approximately 2,000 or so RPM.


Even if the airplane does not have the “continuous prime” system installed, many times you can obtain a usable amount of RPM from it (in the event of carburetor failure) by asking the co-pilot to actuate and hold the primer switch continuously (if it is of the electrical type).  One particular B-25 that I’ve experimented with will support about 1400 RPM on the fuel furnished by the primer alone, a considerably better situation than feathering – plus allowing the hydraulic pump, vacuum pump and generator to continue operating.  Otherwise all of these would be lost when that engine was feathered.


Several years ago I was administering a type rating on the PBY Catalina out in California.  The applicant correctly performed all the procedures required to start the left engine, things were going well.  However, as soon as it fired, it backfired – even though I could see that he was not using an excess of throttle, thus not an excess of air from that!  He tried it again with the same result.  At this point I intervened and tried a start myself.  Same deal!  After xxxxxx

I purposely haven’t included the “how to” use of the primer for engine starting in this article.  For a detailed discussion concerning that, you’ll find the subject completely covered in Warbird Notes #18 (CONTROLLED QUANTITY ENGINE START), also some mention is made in #12 (ROUND ENGINE BACKFIRES).  If the primer system is inoperative, then looks like you’re going to be faced with doing a “mixture start” or something – and, again – that’s outside the discussion in this article.


Things yet to write: 


C-46, number of adapters and technique, results



R. L. Sohn    1995   ©



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