Carb for single cylinder engine

I've made a couple of IC open crank engines that will easily lift fuel 2" but they do need a non return valve so that the fuel does not drain back on the miss stroke, you may get away without it on a throttler but worth adding anyway. The faster running Stuart Lightweight will also lift a small amount of fuel out of the bottom of the float chamber which is at least 2" lower than the venturi and that does nor have a NRV.

Best thing to do is set the engine up with a temporary tank and flexi fuel line then you can see what effect raising and lowering the tank will give.

You might also want to turn up a quick restrictor to put into the carb to reduce the venturi dia being a slowish reving engine you won't want as big an area as a performance engine

Have you considered replacing the carburettor with a simple spraybar as used in a model aircraft engine in order to establish a calibration datum? This would help you to get the engine running nicely before spending a lot of time going down a series of trial and error blind alleys to find a solution.

Another show and tell This is my 25mm bore 50mm stroke engine running with a 4.3 mm bore model aircraft type barrel carb. I could get a higher output with a bigger carb but it was more 'touchy' The tank is around 30 mm below the carb.

**LINK**

Thanks to everyone who has replied, I think the simplest idea would be to put a non return valve in the line. Would this be an ordinary steam type clack valve with a ball? I'll also follow up on the suggestion to attach the tank with a flexible pipe and try adjusting the level. The one in your video Roger runs a lot more smoothly than mine does. Mine runs more like a 'hit and miss'.

I've done them with just a ball or flat disc sitting on a ledge so either should work

Thanks Jason, using a flexible fuel pipe and raising the tank as well as making non return valve did the trick. I had it running for about 10 minutes with a rev range of 750 to 1500 rpm. So I am thinking of either making an 1831 carb which has the jet about 2 inches below the intake or modifying this one by moving the spraybar to a similar low position. I think I will get away with the tank in the original position then. I measured the lift I was expecting and it was nearer 6 inches than 2! I raised the tank by 4 inches and it ran ok at that.

Another issue that showed up though is the head joint is leaking. What is the best way of sealing this joint?

Good to know it was a reasonably easy cure.

I tend to use a smear of liquid/instant gasket usually from Halfords but plenty of choice on the net.

What is leaking through the joint?

Gas, or coolant?

If Gas, the ideal joint might be some copper shim folded over, but difficult to make as washer, or the irregular shape around the valves.

You could try bending some solder (Will probably have to be cored, unless you can get some SMALL diameter uncored ) that can be formed to shape and use as a Wills ring. The ends will need to butt together or slightly overlap.

It might be worth carrying out a trial to see what thickness results after tightening down. A thick joint will reduce compression ratio, not that it is likely to be high, so a small loss could be a big percentage change.

A little more torque on the fixings, equally distributed, MAY cure the leak.

You may need to retorque the head after a little running, as everything settles.

Depends on how many head studs you have, but if only four, or even six, be wary of over tightening. You could worsen a blow rather than cure it, if the head flexes.

A screw thread, even a relatively small one, can apply a surprisingly large force!

(I have seen this happen with a substantial cast iron head clamped by six 9/16 UNF bolts! )

Howard

Hi Howard, thanks for the input. I think the cylinder water jacket needed skimming, I think it was an uneven surface I was bolting down to. So I've skimmed it and will try again with a little blue Hylomar jointing. I'm making an 1831 carb for it. The advantage is that the fuel inlet is much further down and it won't be trying to lift the petrol more than 2 inches which I found it would do easily.

Looks like you are getting better running now Any chance of a video?

I will be interested in how you get on with the 1831 carb. Like the road roller engine it seems that very few 1831/Wallaby engines are actually used in locomotives and vehicles

Hi Roger, I will post a video but I don't know how to do it! Do I have to put it on Youtube? I am making the 1831 carb from stock bar as I don't have a casting and I have made it slightly bigger because this is a 30cc engine single cylinder whereas the 1831 is a twin. It looks a better design than the roller carb being a little more sophisticated.

Yes Youtube or similar will be needed

I've uploaded a video on YouTube. MVI 7332 Rumely Oil Pull model engine running. - YouTube

Edited By JasonB on 17/04/2021 13:14:33

Very nicely done, I like the sound of it when ticking over slowly.

Thanks Jason, I wonder if you can suggest what the problem may be. It will run slowly like that and starts very easily but as soon as I try to open the throttle it just dies. I am wondering about that 1831 carb with the automatic air valve. Have you ever tried to make one like that? I'm sorry about the bad quality of the video. I need to get a better camera from somewhere.

Nice work John!

Sounds quite prototypical.

If it dies when you open the throttle, it probably needs some extra, temporary enrichment.

Opening the throttle admits extra air, but if the fuel isn't there, the mixture becomes too weak to run.

In full size, more recent carburettors, there would be a capacity well or accelerator pump, or in the SU and Stromberg Constant Vacuum carbs, a damper for the piston, to provide the temporary enrichment.

Any chance that you could provide a volume of fuel to surround the Main Jet to act as a capacity well?

Howard

Thanks Howard, I'll have to think how I could provide a capacity fuel well. Meanwhile I can easily try enriching the mixture.

The air bleed should allow you to set one mixture for running and then by adjusting the air bleed you don't run too rich at slow speeds. I've not built the 1831 or one like it but have done carbs with air bleed on aero engines.

Hi, I've made a float chamber for this carburettor but I've come across an unexpected problem.. I followed E T Westbury's design for the 1831 carb. He said make the float out of cork and shellac it to stop the petrol soaking into it. I did that but the petrol just soaks in anyway. Then a news item this week said they were going to increase the amount of ethanol in petrol to 10%. Apparently lead free petrol has 5% ethanol in it which is alcohol so it attacks the shellac. I tried a nail varnish, it attacks it too, humbrol enamel , same problem. Has anyone made a float recently?

If convinced it's the ethanol then consider some 'aspen fuel'. Or it might be worth trying to mix a small amount of petrol with water, shake it up and then let it settle and siphon off the top part. I don't know if it'd work but ethanol dissolves in water quite well (vodka).

Another thought would be to paint your float with thin epoxy as a fuel proof assuming you don't get into weight issues.

pgk