2 stroke twin won't start

[petertha]

..you should be able to get away with a carb with a 6-7 mm bore and have it run fine. A lot of my engines have a bore of 1.00 (25.00mm) and run just fine with a venturi of .210 ( 5.33 mm) The small diameter won't prevent the engine from running it will just limit the rpm range of the engine.
gbritnell

Good writeup George.

1. did I get the carb throat size correct on your actual engine?

2. specific to your V8 & manifold/timing is it fair to say the 1 carb on common manifold is serving X cylinders 'equivalent' at any given time? If so, how would one figure this out? ie. work out valve open timing across multiple cylinders based on overlapping timings & firing orders etc.?

3. curious about your V8 rpm & compression ratio

 

[XD351]

There could be a problem using a diaphragm type carby such as a walbro as they use induction pulses to operate the diaphragm pump inside the carby and as there is a blower between the carby and engine these pulses would not exist but there may be induction pulses on the engine side of the blower that could be utilised.
That being said i have seen them used on model engines mostly V8 four strokes and would like to know if they have been modified in any way ?
The carby used on a nitro engine usually uses exhaust pressure to pressurise the fuel tank so I'm wondering if the engine is not getting fuel that is atomised but is getting fuel that is dribbling through the carby , do you have the fuel tank set up above the carby ?

 

[petertha]

..could be a problem using a diaphragm type carby such as a walbro as they use induction pulses to operate the diaphragm pump inside the carby

Very good point. I'm not clear what the 'equivalent' of that is on his particular engine induction cycle.

You are 100% right about typical RC muffler/pipe pressure tap to the fuel tank. I view it as kind of a push-pull thing. The tank pressure helps push fuel especially in installations where the tank cannot be vertically positioned on the carb inlet, or where the model is in different flight orientation which changes the effective head. Once running, venturi suction is quite efficient at drawing fuel = the pull. If the engine stops, it is possible to flood with positive head tank fuel. But its amazing how small the spray bar gap is with needle valve in mixture running position. Its more like a slow drip. The way to test is temporarily remove the silicone tubing from carb. It will flow 'high volume' through the tubing at the same tank hydrostatic head. Hook it back up the carb nipple & flow is substantially less.

 

[gbritnell]

Peter,
I'll answer your questions with a short answer then and add a little bit more information at the end.
1. Yes you got the carb size correct.

2. On all of my engines, 4 cyl., 5 cyl., 6 cyl. and V-8 I have one carb feeding all the cylinders. The V-8 was a concern with the single plane manifold but in all actuality the distance the fuel/air travels is no farther than having a carb centered on a 4 cylinder engine. For this engine there was no hard numbers to follow it was just an experimentation over the years that gave me an idea of what size to use.

3. The V-8 can be made to idle at 8-900 rpm but that involves adjusting the carb and timing. As has been discussed many times on these forums one of the biggest limitation to slow running is flywheel mass. This is lost when scaling the engine down. Compensation for the flywheel can be made by reducing the compression ratio, probably in the neighborhood of 4-5:1.
The way my V-8 is set to run has the timing set at 25-30 degrees advance with the carburetor adjusted and not needing any further adjustments while running except as the engine get hot (quite hot) the carb will sometimes need to be richened up to compensate as the mixture gets lean. Only on two occasions have I brought the revs up slowly to maximum with the fixed settings and that produced rpm readings of around 7,800. Free revving it gets some pretty high rpm's but that's not a good indicator of how the engine is set. The compression ratio on the V-8 is 8:1.

Now some information on the use of Walbro and Walbro type copy carbs. When I was experimenting with different type carbs over the years I had contact with a couple of the fellows who belong to the Bay Area Engine Modelers who use these carbs. At this point suffice to say that several modifications need to be made to use these carbs on an engine. There are different configurations of these carbs for there intended use.

1. Some have an internal pump which is nothing more than an enlarged area in the gasket which will allow the gasket membrane to pump using the pulses from the engine crankcase. Now this is not the diaphragm enclosed under the chrome cover on the other side of the carb. That gasket/diaphragm is hooked to a small needle valve which controls the flow of fuel through the carb once the engine is running. This diaphragm has a large surface area and is controlled by the vacuum produced in the venturi of the carb to open and close the needle valve.
The internal pump is generally used when the fuel supply is lower than the carb. On engines with the fuel supply above the carb sometimes these aren't used. Some of these engines have a primer bulb. This is used to prime the carb by pumping gas to and through the carb and then back to the fuel tank through a return line. Once the carb is primed the internal pump will continue to supply fuel to the carb.
2. When these carbs are used on 4 cycle model engines the builders remove all the pumper pieces (not needed nor will they operate properly) One fellow did use this setup but made a small air pump hidden inside a dummy generator to provide the pulse signals. This was Paul Knapp, the rimfire spark plug designer.
So to operate these carbs a small electric automotive fuel pump (2-3lbs) is housed in the box below the engine. It provides fuel but will not overpower the carb needle. This is the setup that the BAEM fellows use.
Ron Colonna uses one on his Challenger V-8 but has a small hand pump to prime the carb.
3. I have a couple of the tiniest Walbro type carbs ever made and even at this the fellows who use them usually make a bushing for the venturi to reduce the diameter.
4. The needle valves on these carbs have a very steep taper because on a larger engine they don't need the fine control that we do for our model engines, so the needles need to be modified.

Will these carbs work, yes but with modifications. First of all unless you're building a 1/3 scale or larger engine these carbs look out of place. A simple air bleed carb can be used and it's outside shape can be made to represent a full sized counterpart. I use them, Steve Huck uses them and many others use them on engines ranging from radials to Snow tandem compound engines.
gbritnell

 

[grizelli]

Golly chaps, I'm learning a ton of stuff here! In answer to one of the points raised, the bluish flame at the exhaust ports I don't think is necessarily an indication of poor timing (although the timing may well be off). Its more that the ports are very short, directly visible from the outside and the flame inside the cylinders shoots out the ports when uncovered by the piston. TBH I was more interestd in the colour as an indication of the level of combustion.

Someone asked about the fuel tank level. It sits a little lower than the carb to prevent fuel draining into the engine, and the transparent fuel pipe lets me see how the fuel moves. I originally had an exhaust pressure feed to the tank, but had to change the exhaust to separate the cylinder ports from each other (flame from one cylinder finding its way past the back of the other piston and into the crankcase is generally a poor idea, especially when fuel mix from the other end of the cylinders has found its way past the pistons and into the same crankcase....) and dropped the pressure feed - without it, the suction through the carb seems to draw the fuel through just fine, and I don't get any change in the engine firing if I apply pressure to the tank.

I've done some more digging on the carburettor size that might be needed, and found amongst other things that the general rule on 50cc scooter engines is to use a 12 or 14mm diameter carb. Although I realise that these are very different engines, this seems to equate fairly well to the requirements for my approx 45cc per cylinder beast, so I'm inclined to build a 12 mm carb and see what happens - I have all the materials in my junk box, and it shouldn't take more than a few days to put it together. I have looked at the availability on ebay of carbs used on scooter engines - they seem to be either Dell'Orto or chinese copies, or Mikuni types - but they are pretty big, and probably uneccessarily complex for my needs.

I'm thinking of a slide type carb, with a tapered needle sliding through a fixed jet, and with an adjustable needle to regulate the fuel feed into the bottom of the jet. Working out the taper of the needle is proving to be a little difficult - I can get the taper dimensions for needles for bigger carbs, but I'm not sure how best to scale them down to suit this one: any suggestions are, as always, very welcome!

Its a very valid point that two carbs may work better than one, if only because the two cylinders will inevitably not have the same compression and the twin carb setup would compensate for this as well as generally improving the breathing.

I'm sketching up the plans for this carb as we speak, I was brought up on pencil and paper so it may prove difficult to upload my ideas, but I'll try. Time to make some more swarf

 

[Kasey]

I'm thinking of making a 2 stroke engine with 4 stroke lubrication via splash with no use of crankcase induction, but with a 2, 3, or 4 vane type supercharger turning slower than the crank but timed to match the porting instead, to pulse charge the cylinder. Is this a possibility provided the charger is made big enough?