Priming an engine
 

The Seidel 770 starts easiest if it's been liberally primed before flipping the prop. On the test stand I've been injecting the fuel directly into the open carb with plastic pipette. The question is how to accomplish this once the carb is buried in a cowl (and right up against the firewall).

I made a tight 90 degree bend in a 4mm brass tube, cut one end off short and slipped on bit of fuel tubing for protection. The end of the tube would sit in the "throat" of the carb and the other end would run up through a slit in the cowl. At least that's my thinking at the moment. But I'm wondering if the fuel line from the tank might actually be in the way. I'm not sure about that; I'll have to check.

Is there any other way to mechanically "suck" fuel into the carb? For example, if I block off the exhaust tube and rotate the prop (a few times) would that do it?

When my gasser has been sitting for a while and I know it may be difficult to start I will pressurize the tank to get gas up to the carb. I do this by pinching the vent line while fueling and rocking the prop back and forth against compression a few times. Works great.

If you have muffler pressure fitted - yes.

If not do what I did with my Gypsy Moth with an Enya 80 four stroke which was put a small plastic squeeze bottle on
the fuel tank vent line & squeeze gently until you see the fuel travel along the tube & up to the carb.

Which way does the throat of the carb face, up or down? Enyas face down, if you force too much fuel into the carb
it just runs out unless the engine is inverted in which case you run the risk of flooding the cylinder. On all my models
with inverted engines I just wind them over with the starter until they suck their own fuel then start when they are ready.

John.

The carb faces rearwards and there is probably less than 3/4" clearance from the firewall. The way the ARF is setup there is an 800ml fuel tank the end of which protrudes through a 1" diameter hole. So the plumbing tubes are going to be very close (and maybe a bit above) the carb opening.

And, yes, it's have a vent line to the exhaust. I've never had a model without a vent line and don't really understand how that would work.

The idea of an additional line on the fuel line sounds worth pursuing. My usual method is very primitive. I've always just have three lines: 1) fuel to carb 2) tube to exhaust and 3) a third "fill" tube that dangles more or less free.

Have you tried opening the throttle and covering the exhaust with your finger and flip the prop a good number of times; no glow on of course.

The only starting procedure I have tried (and that works every time) is directly priming the carb. This was the advise of Speutz and he seems to know what he's talking about. On this scale models using the 770 he also used a brass tube to inject the fuel. But with this setup on this ARF that seems hard to manage...just no space.

Had same issue as you with my Seidel 770. Made a long "J" shaped tube from K&S 1/16" brass tubing. Soldered a short piece of the next size down brass tube to the short end to form a nozzle and soldered a flat piece of brass to the bottom of the "J" to allow the piece to be bolted in position to the firewall. The long side of the "J" comes out to the front of the engine between a pair of cylinders. If you baffle the engine (which you should), the tube can pass through a grommeted hole in the baffling which provides good support for the long end of the tube. A squeeze bulb with some fuel tubing makes it easy to inject the prime. I cap the long end of the tubing with black rubber cap I found at an automotive store (brake line cap???). With the black cap, painted black brass tubing, and black baffling, the fitting really isn't visible except with close inspection. I'd post a pic but I'm in the middle of a house move so everything is in storage.

And by the way, I've found the original Seidel (not sure about the UMS version) runs better without muffler pressure.

I'm surprise that you used such thin brass tubing (1/16"). But maybe that's all that's needed. I used 4mm tube on my "first attempt." Many thanks for your response though; I'm sure I'll end up doing something similar. I wouldn't have thought of running it "forward" towards the the front. Also could you comment a bit more about the need for baffling? I don't see why this would be necessary (particularly on the sort of installation I have where the rear of the cowl isn't fitted to the shape of the fuselage).

What do you mean about the 770 running "better" without muffler pressure? Do you mean running smoother. running better at idle, runner better at high RPMs? Or do you mean running better without an exhaust ring entirely? I'm a little afraid of running any engine without muffler pressure to the tank since that's not something I've ever done.

Too bad they did not install a choke on the carb.

What does a choke do? Is it sort of the equivalent of putting a finger over the exhaust?

It is more like putting your finger over the intake. It sucks fuel into the carburetor. The smaller the venturi size of your intake, the more fuel draw the carb will have. Fully closed off (or mostly) sucks up fuel optimally.

I will go through my starting procedure for a gasser.

Full throttle
closed choke
ignition on
rock prop back and forth 7-8 times (gets pump pumping fuel)
Flip prop through compression hard and fast, usually 3 -5 times, until engine fires, then it will die. Now you have fuel all the way up.
Open choke
Lower throttle to high idle.
Flip prop hard and fast, should be running within 3 flips.

The engine will not continue to run if the choke is closed. I highly recommend looking into adding one, and hopefully there is something available for your engine. On one of my old gasser carbs that had no choke, I put a cork (from a tequilla bottle) on a music wire linkage, and choked with that. It worked great.

I am going to assume you do not have a choke. I think your best bet is thumbing the end of the exhaust. If that technique does not work, running a tube shooting straight into the intake may be the way to go. Feed it with a syringe or something.

The carb on the Seidel/UMS is just the same kind you'd find on any single cylinder 4-stroke. The tough part on this installation is that there is only about 1cm between the back of the carb and the firewall.

Don, why not drill a small hole in the firewall for a small 2 or 3mm inner tube and lead that to the side of your model so you can prime your engine with the push of a small squeez bottle.

Teus, that's what was rolling through my mind last night! It would have to snake its way around the tank through.

Many of the newer rear intake gas engines almost require a hole in the firewall, as they tend to get too close. I also like to use velocity stacks, so On many of my aiplanes the intake air is from the other side of the firewall. This gives the benefit of good clean air that is not turbulent or overheated from the rear muffler as well.

It would seem a priming tube would work well for you, and a syringe could give an exact, prescribed dose.

The problem with THAT (and there always seems to be a catch to each suggestion) is that the ARF design has the 800ml tank seating precisely behind the firewall with it's stopper sticking through a fitted hole. Of course, I could move it back but then I'd just be creating a new problem of how to route plumbing. What I'm leaning towards at this point is a combination of the ideas suggested here: A 2mm brass tube mounted to the front of the firewall that I would inject fuel into with a syringe.

While I ponder this, I've been making a new set of longer connecting wires for the glow system since current ones are too short to reach the Microsens controller. One by one the problems present themselves and one by one they get solved. Such is RC modeling.

It's been a busy week at work, but I finally got the fuel priming tube made and intalled. I rummaged around in my box of 100yen ($1) goodies and found a syringe. The needle fits perfectly into a 2mm brass tube. After putting a few bends in that I silver soldered it to a short length of steel strap (the same stuff I used for the cowl brackets).

So now I just open up the throttle, squirt in a measured amount of fuel, and voila!

Perfect!

Abu you may still benefit from a hole in the firewall just in line with the carb opening and bigger than the carb hole by about twice the diameter. With the wild air from the engine running it may have problems without a bit of relief in the area. Running it will tell. I think I recall you have run this engine on a stand so if it doesn't perform as well on the model give the hole a try. Gas engines(which this is certainly not) with Walbro carbs (and similar) have a fuel stand off situation and really wont run up to their potential if there is a restriction (wall) near the carb opening. They also benefit from the use of a velocity stack, though I'm not at all sure this engine would, four stroke and with 3.5 cylinders drawing fuel/air each revolution it would have a rather constant flow into the carby.
Doc

Doc, I'll have a look at this issue when I do the final plumbing. At the moment, the plug for the tank fits a hole in the firewall so that only the ends of carb, vent, and fueling tubes protrude. This is almost directly in back of the carb opening and I'd guess there is about an inch of space there Maybe a bit less. I could enlarge this hole but can't move the tank (without completely rethinking the setup). I think I'll just test it like this. I have been planning some extensive on-the-ground but in-the-model engine testing anyway.

Still lots of little stuff to do.

Hmm...today I was mounting the engine, hoping it would be the last time, with the throttle linkage, tank and all tubing in place, when I realized there is still one more problem to solve: Access to the needle valve. I had been thinking this would be a matter of an easy-peasy extension (and a slot in the cowl) but just now noticed that the the needle valve is directly in line with the ring exhaust. In other words, the ring blocks a direct line to the needle valve.

This is going to take some thinking. Is there such a think as a "remote needle valve?"

I wonder if I could use flexible wire cable as the extension.

Yes, there is such a thing but buy a good quality one. I tried the Fox remote needle, all three I
received from Tower were faulty. Gave up & bought the OS unit:

http://www3.towerhobbies.com/cgi-bin...?&I=LXCR66&P=0 for top quality or cheaper:

http://www3.towerhobbies.com/cgi-bin...?&I=LXMW56&P=0

Haven't used mine yet, bought it for an OS FT300

John.

So that goes on the fuel line? That's neat but there wouldn't be any place for it with the present setup.