4 stroke single

[Allen Smithee]

I am using standard pump petrol (95 Octane).

What about oil content, or does this engine have a separate lube system?

AS

[Roger B]

Brian, That was the conclusion I was coming to. The tank is currently much lower ~5 cm / 2". I will make a new bracket and try again.

Allen, It is straight petrol. The bottom end and camshaft are splash lubricated with a restricted reservoir on the side of the crankcase. The rockers get a few drops from an oil can.

[Roger B]

Looking at George Britnell's Holt carburettor thread the fuel tank appears to be level with the carb (may be the camera angle). It seems that I have set mine far too low  . I was assuming it would work like my battery charging set where the fuel tank is ~20 cm below the carb.

[Admiral_dk]

Nice piece of old gear.

I'll bet there's an fuel pump somewhere - it might run on the crankcase pressure ....

[Allen Smithee]

Static petrol engines like that usually have a diaphragm pump in the carb, operated from the pressure pulses in the crankcase.

AS

[Roger B]

There is no pumping mechanism at all. If I remember correctly there is a non return valve incorporated in the petrol tap. The carburettor is a simple venturi with an adjustable needle valve. The air cleaner rotates around a selection of holes giving various restrictions, Choke (fully closed), Cold start (small hole), Hot start (bigger hole) and Run. The starting pulley is quite wide allowing 8-10 turns of starting rope.

It with charge a 12V Lead Acid battery at 6-8A.

[Admiral_dk]

Well the valve is a give away - to me anyway (I might be completely wrong), but to me there's no way that the engine could lift the fuel that much by normal venture vacuum and run efficiently.

I wouldn't be surprised if the diaphragm is located between the crankcase and the tank ....

OK - I just googled carburettor venture fuel lift and discovered that Brigs and Stratton had a third system I didn't know : A system where the pump is inside the carburettor and operated by the vacuum in the venture. So there's still new (to me) ways to skin that cat.

[Roger B]

The engine sits on the fuel tank separated by 5 springs. The only connection is the fuel pipe. The crankcase breather is the sort of trumpet thing on the side of the block. It is not high powered, 80W from around 30 cc but it would run 8 hours a day 7 days a week charging the batteries that kept our camp alight at night.

As this engine would run with such a high lift I assumed that mine would. This could well have been a mistake 

[Admiral_dk]

Thanks for the extra pictures - an interesting "model" indeed.

OK - I'm guessing that the venture vacuum can lift the fuel for a short duration of time and the one-way valve ensures that the fuel does not return between vacuum pulses + that the choke really shuts of almost all air, to create the necessary vacuum to get enough fuel up to start it. Never seen anything like it, but it just proves that the simplest solutions often are the best for stuff that has to work 24-7 under primitive conditions.

[Allen Smithee]

The presence of the non-return valve makes me wonder if somewhere there is a pressure tapping in the crankcase (possibly timed, but it's not essential) which is pressurising the tank to feed the fuel. A small, high-performance 2-stroke single can develop as much as 11psi from an untimed crankcase pressure tap (and more from a timed one) - that's 22 *feet* if water pressure, so it wouldn't surprise me at all to find that a low performance one can develop enough pressure to lift petrol through less than a foot. It could also use exhaust pressure, although that's much less likely on a petrol-fuelled engine than a methanol- or paraffin-fuelled one.

But to lift petrol through a foot of height using pure venturi suction only needs a manifold depression of about 0.3psi - easily achievable but vulnerable to large mixture changes with small variations in conditions. If the problem with your engine really are related to fuel starvation it's far more likely to be due to the variation in suction that just insufficient suction.

It's also possible that something in the tank. plumbing or carb design is simply preventing it flowing *enough* fuel. A classic "bum-bite" im model aeroplane fuel systems is to fit on-board fuel filters that simply won't allow enough fuel to flow for full throttle use (ie the filter is more restrictive than the needle valve!). Have you check your fuel system for constrictions, leaks or FOD?

AS

[Roger B]

I've made a new bracket for the fuel tank to bring the fuel level closer to the jet. There are no obvious blockages/restrictions in the fuel supply. The tank has an air vent, when I remove the pipe from the carb to drain the tank there is a steady flow that seems appropriate for the size of pipe and head.

Hopefully I will get a chance to run it at the weekend.

[Admiral_dk]

Sorry Roger - but that is too high (if you fill it to the top). You want the top of the fuel to be slightly below the carb.

[Roger B]

Typically I half fill it so it should be just below the jet, but I have a reasonable range of height adjustment available.

[Brian Rupnow]

If you forget and overfill the tank, the gasoline will gravity feed thru the jet orifice, pool around the base of the engine, risking a fire, and hydrolock the engine.

[Roger B]

This tank is just a trial set up. If having the fuel level closer to the jet works I will make smaller cylindrical tank like I have seen on several other engines on the forum. As the bracket is slotted I can see the effect of varying the level while the engine is running (assuming it runs for long enough  ).