To prime or not to prime that is the question?
BlackRV7 said:
OK, I've been sitting on the fence with this primer install deal. Basis here, O-360 A1A configuration, carb, dual mags, standard vans electric fuel pump with mechanical. Primer or no primer installed on panel???? Care to throw out your thoughts and experiences?? I have a new primer I bought at SNF still sitting in it's box. Complexity (can't be much), photos of install would be helpful.
I have a O360A1A for my RV7 project and plan on going with out a primer system to start. I have flown O360's without the primer system and had no problem starting them. My O320 RV-4 had an electric primer and did use it on first start of a cool or cold day. Sometimes I did not use the primer.
As you probably know the carbs on O320's do not all have an accelerator pump like the larger O360 carb. The MA4-5 has an accelerator pump to feed the larger engine when going from idle to WOT, like on takeoff. This circuit squirts extra fuel in the carb throat by fwd movement of the throttle.
I assume you're talking about the electric solenoid type primer. Not many are using the manual pump much these days. I can speak for the electric method, since I used it on my RV4. Unlike the hand pump on a Cessna, you can over do it. I used my electric primer very sparingly, pushing the button for just a second before cranking. Sometimes I'd jab it during cranking. You can over do priming with any system; if you do, fuel will drain down the manifold, into the air box, just like the pump prime method. There is some small fire danger with primer lines. A damaged primer line could pump raw fuel into the cowl and start a fire. Unlikely, but its a maintenance item and not unheard of.
With a priming system you're pumping raw fuel right into the intake ports, so raw fuel can get directly into the cylinders. This can wash oil off the jugs, causing extra wear. I don't know if using the upper primer port is worse than the lower ones for getting raw fuel into cylinders. I know some guys are using the upper ports for priming because the routing of the lines is nicer.
Complication of a primer system? Well its all those little primer line runs to each cylinder. Metal lines (usually 1/8" copper), fittings at each cylinder port and some kind of distribution block or "Tees" does make it more complicated. Running little primer lines from the lower ports across or near the intake and exhaust pipes is a pain. It also looks bad and is hard to support. Go out and price all the little special primer fittings and AN fittings; they are expensive, but the copper line is cheap. The special primer fittings have a reduced orifice. If a primer line breaks in flight the engine can run rough. If you used AN fittings with out the reduced openings, the engine would run scary rough, since a vacuum leak causes a lean condition.
Maintenance? Something more to go wrong. You have to wire the solenoid, wiring, circuit protection and a button. The little primer lines look terriable and can be a maintenance issue.
Going with out a primer system is lighter, cheaper and there is no maintenance.
Down side of not putting a primer system on? Well it may be harder to start?
From my experience and what I hear, that's not the case, at least if the temps are not sub frigid. I have always lived in temperate climates, so that is my perspective. If you go on the advice of one of Van's senior guys and RV builder, he said he wished he would not have put the primer on and ended up removing it. This was documented in the RVator.
This debate goes on and on. People with out primers seem satisfied. People who opt to install the primer are also happy. The primer system is a fairly complicated system. By leaving it off I save parts, cost, weight which are fairly significant. Solenoid, wiring, CB/Fuse, switch, AN fittings, primer fittings, lines, brackets, clamps, nuts and screws, I'm sure I am missing something, all adds up. That's my story and I am sticking to it.
![Big grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
If you want pictures email me I'll send you some pics of primer line installations. It is not horrible and if you want to jump off the fence onto the primer system side, that's OK. A lot of builders do install the primer.
PS: So what is priming like with out a primer?
The technique of priming the O360 using the accelerator pump, is pretty simple, pump the throttle once or twice before starting or during cranking. Two things people do wrong is pump the throttle too many times, once or twice max, at least at first. Since raw fuel can build up in the air box, don't over do it. The other technique is waiting a short period of time after priming before cranking. I usually prime while I'm finishing pre-flight, letting it set for 10-60 seconds, say while I close the canopy or copy ATIS. This allows the fuel to vaporize, which is really key in cold weather. The colder it is the longer you should wait. The time allows the fuel sitting in the air box to vaporize. If it's NOT COLD you can skip priming before cranking and see if its starts or pump once during cranking, if it does not catch. Remember to always go back to cracked open throttle while cranking.
Don't over do the priming because you could make a fire. When you move the throttle don't be abusive and just move it steady but not super fast or abrupt. If you have never seen what or how it works, take the airbox off, look under (not directly of course) and move the throttle slowly. If the carb has fuel in the bowl, you do NOT need the electric fuel pump. You will see a stream of fuel injected into the carb throat. Its pretty impressive amount of fuel. If you get too close you will get fuel injected into your throat.
A primer system may be handy in really cold weather? I rarely fly when it's that darn cold. If I do fly on a crisp day, its from my hanger and I have heated the engine compartment. I don't plan on doing winter time X-C's to cold climates, leaving the plane tied down on a ramp in a snow storm in sub freezing temp. If I fly in the winter its South. Even than, in cold weather, I don't think the O360 is impossible to start without a primer. The trick as I mentioned is to pump, pump and wait to let the fuel vaporize a little. It takes a minute or so to let the fuel vaporize in cold weather. Really that applies to a primer system as well. You want vapor not raw liquid fuel.
One thing people worry about using the carb to prime is there's a possibility of a stack fire. It's not a bad thing to worry about, but I don't think its a big deal. When was the last back-fire you had? You do have to be very careful not to pump gobs of raw fuel into the air box, where it drains into the cowl and puddles under the plane on the ramp. Just use common sense. My engines have never been that hard to start. I guess that is a good point. The engine has to be in good repair, good battery, starter, ignition and fuel system. May be a primer system may help a weak starting engine.
I am going with out at first and plan on not looking back. If I need to add I will do it later.