Carb for single cylinder engine

My engine for the Rumely oil pull tractor I am building uses a scaled up version of E T Westbury's road roller design. The carb is also a scaled up version of that one. The air is controlled by a spring loaded slide which has no damper on it and it just bangs up and down as the engine runs. I notice that a similar design carb of his for his 1831 engine has a damper incorporated in the air slide. Has anyone any experience of these carbs and can suggest a mod to make it more controllable please?

Could you add a damper like that b used on the SU carb. Just a close fitting piston inside an oil filled bore.

It should provide a more constant air:fuel ratio so that the engine runs more steadily, but may need to be set up slightly differently from the undamped version.

May even run over a wider speed range, if that is needed.

Howard

Hi Howard, yes that's what I'm thinking of trying, I'll get back to let you know if it works.

Could it be that the spring is too soft? (i.e. the air sleeve is massively over-shooting the desired setting, causing oscillation). If I read the original article correctly, it suggests a spring of 12 to 20 turns 1/2" OD 32SWG which I've calculated as 0.008-0.014 lbs/inch. If the design has been scaled up, the starting point would be these stiffness values multiplied by the square of the scaling factor. (Simply scaling the spring diameter up will result in a softer spring).

I would expect a damper would undoubtedly help though.

Edited By Andy_G on 28/02/2021 13:43:44

I would go along with howards idea of something using the SU damping arrangement. Noel.

There is (was) a tendency in the motorcycle world to prefer plain Amal-type slide carbs for singles, but (sometimes) to use an SU carb (or a CV type) for a multi. The erratic suction of the single causes flutter of the suction-operated slide of the SU, so it is never in the intended position, whereas the continuous suction of the multi tends to make the Amal-type slide stick as it is continuously pushed sideways in its bore.

If the engine has a heavy flywheel, and gentle valve timing, so it is not snappy when opened up, you might find an SU with thick oil in the damper will be OK, though. Of course, this is the last sort of characteristic needed by the average motor bicyclist.

And incidentally, the SU damper was not 'just a close fitting piston ... etc' The outer edge of the piston fitted accurately in the outer sleeve, and within the inner steel sleeve was a brass piston held loosely on its rod, with a valve in the base of the piston so that resistance in one direction was greater than the other.

Hope this helps

Tim

Quite right Tim!

The extra damping in the upward direction of the slide provided mixture enrichment for acceleration, and unlike the rubber diaphragm in the Stromberg CV, the slide did not have a spring rate to add to that of the spring above.

The other advantage of the SU was that it was impossible to fit incorrectly oriented.

An AA man messed up my father's Viva HB by fitting the slide so that the two vacuum ports were on opposite sides of the jet bridge!. The car went much better when the slide and diaphragm were fitted in the right place!.

Howard

I have been running an 5.5mm bore SU type carb on a 12cc twin. The piston doesn't have a damper but is fairly stable:

**LINK**

I made a damper to fit inside the sleeve today using a piece of 1/4 inch brass tube and it has improved matters. I think getting the spring right will take quite a bit of experimenting.

A hint: Early (preWWII) SU carbs did not rely on springs to assist in returning the needle and piston assembly. Instead, the weight of the piston was altered (in development) so different models of car got different weights. This was often a steel washer rivetted on top of the piston flange. The main effect of changing weight or spring is response to snap throttle openings - too light and you got hesitation, too heavy and you got sluggishness. But perhaps with a tractor the need to snap the throttle open is reduced?

So, rather than finding lots of spring wire and diameters etc, it might be quicker to use a selection of washers on top of your carb piston. Much quicker to change and modify. And once you find something that works, it can (if necessary) be soldered etc in position.

Cheers, Tim

I've found that the engine will only run very rich. To get it to start I have to inject a small amount of neat petrol into the intake and the exhaust is visible. If I try to lean it out it stops. The air slide is pretty much shut, if I open it the engine stops. I've tried messing about with the ignition timing and find it runs best if the spark is at about TDC or just after. It will run continually but only with the air slide shut. This is the first IC engine I've made, I've been a steam loco man up to now.

Thanks Tim, I didn't see your post. I could try that but at the moment it doesn't like the slide open at all!

Have you checked the valve timing? The exhaust should open towards the end of the firing stroke and close just after the next top dead centre. The inlet should open slighly before the exhaust closes and close as the compression stroke is starting. If you are out a tooth or so you can get pressure rather than vacuum in the inlet pipe which rather upsets things.

I don’t know the actual size of the original, nor 1.4 times it, but I suspect that the venturi effect is insufficient to draw and atomise the fuel (from the jet) at low speed

If the steady state mixture is correct, but opening the throttle causes stalling, the slide needs to be damped in some way so that the increased air flow enriches the mixture while the engine accelerates, and then lifts the slide.

As NDIY says, the air velocity over the jet bridge is insufficient to atomise the fuel.

The float level should be set so that the fuel is only JUST below the jet bridge.

As a guide, a previous post says that a 5.5 mm bore carb ran a two cylinder 12 cc engine. (Only one cylinder breathes at a time, so think 6 cc single cylinder! )l

Howard.

Hi, there is no float so the fuel level is over 2 inches below the jet bridge but that is the same on the road roller design. I don't know if the road roller runs satisfactorily as designed, there have been only a few made and I can't find any report on running it. To have a float would require a fuel pump. Could it be that the spark is weak? I have Jan Ridders ignition system which only runs on a 1.5 volt battery. I have ordered a 'proper' ignition system.

IIRC this engine is around 35cc If you are looking at slow speed running a 5 or 6mm venturi will be fine. For power you might need something like 8mm but that will depend on the valve sizes and cam timing.

Asking the carburettor to lift fuel two inches is asking a lot! No small wonder that there are problems atomising the fuel!

That explains why starting is difficult, and it stalls when the throttle is opened.

You are requiring the air flow and venturi to produce a depression of 2 inches of petrol. Full scale engines don't do that, which is why carburettors have a float chamber to keep the fuel at the right level in the jet.

No need for a fuel pump if you can arrange a gravity feed?

If you are departing from prototype by using a constant vacuum carburettor, you may as well carry on and fit a float chamber.

You need to bring the fuel level upto that of the jet bridge, by some means..

Before worrying about the ignition, since the engine runs at steady speed, with the present one, you need to put the carburation in order first.

There would need to be a weak spark, marginally so, to cause the stall when the throttle is opened, if the mixture supply was correct..

Howard

I think you are right about the fuel lifting, I think it's a bad design. The prototype doesn't allow for a gravity feed tank. I don't mind having a float chamber but from what i've read they can be tricky to get working.

Just double-checking that this is the design being discussed:

http://www.modelenginenews.org/etw/roller/page52.html

To correct my spring numbers above - It's 1/4" OD, so the relevant stiffness figures are 0.07 - 0.12 lb/in

(If the scale factor is 1.4, I think the spring wire should be about 0.015" dia ).

Have you run it long enough for it to warm up? (Not unusual to have to lean out the mixture considerably over the first minute, or so.)

What happens if you open the throttle? (I presume you mean the spring loaded sleeve when you say 'air slide ' ) - Apologies if I have got the wrong end of the stick.

Edit to remove random smiley - have just seen latest info.

What size of primary choke are you using? (The hole in the side of the carb)

If the primary choke is too big, or there isn't a good seal between the air sleeve and its cylinder, the carb won't develop enough vacuum to draw the fuel through. Might be worth trying with a bit less than 2" height difference.

Edited By Andy_G on 01/03/2021 18:17:54