# Stihl 4 Mix Engine Question



## Vintage Engine Repairs (Jan 1, 2023)

On the compression stroke, what stops the charge going back through the bypass port and then in to the carburettor / venturi picking up a second charge.


----------



## trains (Jan 1, 2023)

Um, there will be pulsing in the motor, but overall, due to the volume of air being drawn in it dosent matter much.
the valves open, the vacum in the cyld draws in the mix, then the valve closes, and stops the mix entering the cyld, that causes a pressure pulse, but then the valve opens again and more moves thru.
Not sure what you mean by the bypass port and then it going back into the carb, its just got inlet and exhaust, and the valve train is open to the intake side of things so it gets lubricated by the oil mix.
edited to add, there would be some movement of the mix from the crank causing pulses too, im sure thats how they designed it so there was positive flow of mix for lubrication, and its one area that gets blocked up and needs cleaning.


----------



## Vintage Engine Repairs (Jan 1, 2023)

On the intake stroke yes the charge can be drawn in from the carb and also from the crank case charge (once it has gone back past the cam, push rods, in to the space within the valve cover then finally get in to the intake past the valve). However on the power stroke with the valves being closed and the piston lowering in the cylinder and the mix transferring back, there must be fuel that goes back in to the carb to an extent.

In this video they show that it just stops going in to the bypass port (left of intake valve stem) on the power stroke, but that doesn’t make sense.

Edit:


----------



## trains (Jan 1, 2023)

well yes, that happens on all engines, I called it pulse.
they have a circuit for want of a better word that enables mix to go over the valve assy, cam and crank and lube like a normal 2 stroke.
Looking at the vid, I would imagine that the crank helps move the mix thru, and it is then drawn into the combustion chamber.


----------



## Chevboy0167 (Jan 1, 2023)

I would think there would be a reed petal valve of some sort to prevent backflow??? I tried looking for a good pic or diagram to show such a thing but came up empty handed. I did find a YouTube video showing some top views and I saw nothing of the sorts over/near the port.

Full disclosure, I have never worked on a 4 mix or even ran 1!!!


----------



## Vintage Engine Repairs (Jan 1, 2023)

trains said:


> well yes, that happens on all engines, I called it pulse.
> they have a circuit for want of a better word that enables mix to go over the valve assy, cam and crank and lube like a normal 2 stroke.
> Looking at the vid, I would imagine that the crank helps move the mix thru, and it is then drawn into the combustion chamber.


Hmm of course conventionally a valve will stop back flow in a 4 stroke and the piston skirt or reed stops it in a 2 stroke, in this case the back flow isn’t stopped by either lol. Will call you later


----------



## Vintage Engine Repairs (Jan 1, 2023)

Chevboy0167 said:


> I would think there would be a reed petal valve of some sort to prevent backflow??? I tried looking for a good pic or diagram to show such a thing but came up empty handed. I did find a YouTube video showing some top views and I saw nothing of the sorts over/near the port.
> 
> Full disclosure, I have never worked on a 4 mix or even ran 1!!!



That’s what I thought too, you’d expect there would be a reed valve or other one way valve in the intake manifold.


----------



## trains (Jan 1, 2023)

guys, your getting all wound up on things that dont really matter.
there is an inlet and exhaust, its a 4 stroke, and the normal rules of 4 stroke applies.
but it has some 2 stroke design being its lubed by the fuel mix, so it needs the typical lube a 2 stroke gets from the fuel mix.
so they have designed a port that allows fuel mix to enter the valve and crank area like a 2 stroke, well sort of, you know what I mean.
as its small, and also influenced by the crank pulses from the crank counter weights, ie crank stuffers, it all works.

go out and spend time with your significant others and enjoy the day.
Ok, just watched that vid, looks like the crank direction dictates the flow from the rocker area, does that answer your questions.

never really bothered to think that hard about it before.


----------



## Vintage Engine Repairs (Jan 3, 2023)

I have done some more digging and for anyone that comes to this in the future wanting an answer, they do indeed spit back more than normal 2 or 4 strokes - there is no valve.


----------



## Vintage Engine Repairs (Jan 3, 2023)

The more I think about it the more I realise you can’t have a valve in the manifold, it has to be open firstly to improve flow and lubrication and also as the piston goes down on the compression stroke in a sealed unit, it will put pressure on the crank seals, valve gasket and even the intake and exhaust valves.


----------



## frank_ (Jan 3, 2023)

my understanding is its a very low pressure backflow because of the crankcase volume, and once running the inlet tract flow/momentum is a strong pressure against the feeble backflow, so it is minimal and helps lube the engine anyway


----------



## Vintage Engine Repairs (Jan 3, 2023)

Thanks Frank, that’s very true, it must be because you’d limit lubrication / movement of oil mix if it was valved. I’d be curious to run one without the AF and see how much spitback does occur. Edit : clearly not enough to matter, but it answers my question!


----------



## frank_ (Jan 3, 2023)

Vintage Engine Repairs said:


> Thanks Frank, that’s very true, it must be because you’d limit lubrication / movement of oil mix if it was valved. I’d be curious to run one without the AF and see how much spitback does occur. Edit : clearly not enough to matter, but it answers my question!


the kubota 130cc engines i do for the bowling green mowers have a very crude decompressor that jabs the inlet valve open a tad just before tdc, and they suffer blowback, with fuel being sucked up the mainjet twice, and if the valve clearance is too tight it dumps too much compression, and that soaks the air filter and makes the engine very laggy i have to grind some off the cam followers when they get too bad


----------

