Primer Modification on the O-300

[blueldr]

On the O-300 engine, the stock primer system primes the lower end of the left bank of cylinders. I used to have a sort of rough start in really cold weather, so I modified the system.
Inasmuch as each cylinder has a 1/8th in. pipe plug at each intake valve, I removed the plugs and installed primer fittings with the very small orifice. From an aircraft junk yard I got a six position fuel spider from a fuel injected engine and connected the primer discharge line to it with a line to each cylinder, thus priming all six cylinders simultaneously right at the intake valve. The original primer hole in the left intake manifold was plugged off.
This system made for much smoother and reliable cold weather starts.
I know of other cases where the primer dischatge line was split and used to prime the bottom of both intake manifolds. I was told that this also helped in cold weather though I suspect that it would require a little more cranking than my system.

[Greg Bockelman]

Just out of curiosity, what does it take to make that modification legal? Just a logbook entry? STC?

[blueldr]

If you're concerned about the liability of that well hidden primer modification on a sixty five year old airplane, and you're allergic to sleeping pills, don't even think about doing it. You're probably only going to fly on a nice Summer afternoon when a primer is superfluous anyway.

What it takes to make that modification is a few simple tools and a desire to make a diifference in an operation that needs improvement.

[cessna170bdriver]

With all due respect, I think that's a solution looking for a problem. In 33 years I've never had a problem using my single point primer and/or accelerator pump to get enough fuel to start a cold O-300.

[blueldr]

If it had not been a troublesome problem for me, in very cold weather, I would never have made the conversion effort.

[c170b53]

I've had my primer lines crack more than once. Having said that, when its cold, you need a few shots.

[GAHorn]

Houston Northwest Airport, 2002, OAT 28 degrees F.... I gave it 4 or 5 shots (I forgot how many) of primer...and then re-loaded the primer and gave it a crank.
It caught on about the 4th or 5th blade and barely running...I gave it that last shot of primer S L O W L Y... and it finally started. It took almost a full minute before the oil pressure came up ...and then it went to the TOP of the gauge...and stayed there while I idled it for about 10 or 15 minutes before it came down below 60 psi.
It only came off the bottom of the Oil Temp gauge after about 20 minutes of fast idle... when I then accomplished a run-up.

I've always pre-heated my engine since then with a hot-air heater called a "milk stool heater" with some 8" aluminum ducting. (Total cost less than $25.)

If it's really really COLD...a non-pre-heated O300 is hard to start. I believe Dick's mod is likely eligible to be called a "minor alteration" since TCM provided the primer-ports and the GO300 used the associated hardware in a similar manner.

[hilltop170]

My C180 has the exact primer system Dick described going to each cylinder. I thought it came from the factory that way since there is no logbook entry or 337 for it. It works great and I have had to start the engine unheated a couple of times in really cold weather, as in 10°F.

[GAHorn]

An issue with the standard primer set up is the fuel is introduced to the left intake manifold spyder...which can allow fuel to run back down the carb and into the carb heat box where an intake fire can occur. (An outside observer with a fire extinguisher is a good idea when priming a cold engine. Also, if an intake fire is suspected, KEEP CRANKING to suck the fire up into the engine. Also, an intake fire calls for an inspection of the carb air box, air filter, etc.)

The primer-ports in the cylinder at the intake valves is less likely to cause such a problem, although the trade-off is that the intake valve lubrication is easily washed off with excess fuel.

[blueldr]

Richard P.
The induction system on the O-470 engine in the C-180 is a completely different system using intake runners rather than an intake manifold such as is on the O-300. I believe all the O-470 carbureted engines are equipped with six cylinder priming.

[BeeMan]

Some years ago I owned a C182A converted to conventional gear and flew it on skis in the Alaska winter. As I recall, our 0-470 was initially a single port primer and we had our A&P modify it to the 6-port primer because it was so rough starting in cold weather. After modifying the primer system it started easily and ran smoothly in cold weather. Just my experience, YMMV

[4BravoWhiskey]

I can see how blueldr's setup would help. I had trouble starting mine in AK late this last summer when it was around 30F and even around 40F. It would take a lot of primer and cranking to start, and then not keep running. I got a tip from a local A&P that helped immensely. Pull carb heat on to help keep it running after startup. Two strokes of primer, and then pull the primer out. Start cranking and push in the primer. Then remove carb heat after engine is running good. This worked every time, usually just 2-3 turns of the prop at most. I have the original primer setup. Maybe this is more or less standard in colder climates (and sounds similar to what George described), but being from Texas I wasn't familiar.

I've noticed (actually others outside have told me) that with more than 2 shots of primer I'm getting fuel dripping out of the carb, and hence the fire potential.