If you’re embarking on a top end rebuild for your 2 stroke engine, I think it’s important to run through the 2 stroke cycle and then we can move on to the exact steps to follow to rebuild your top end.
I want you to be able to understand what you are doing and why you are doing it. This way you will be starting your top end rebuild with a full understanding of how the process works and hopefully this will help you to build a better engine. When I started my side hustle back in 2016, I was working with a friend who was a mechanic and I wanted to learn everything he had to teach me – believe me he knew a lot more than I did at the time and I was in awe of just how much knowledge and skill he had. When I started working on my own in 2020 I quickly realised how many of the fundamentals we’d got wrong. From that point on my learning accelerated massively. I’m sure partially because I was working on my own but also because I’d re adjusted my understanding and then everything just slotted into place for me.
I really want to share the correct knowledge so that you don’t have to make the fundamental mistakes we did and that will save you having engine problems. Hopefully you won’t have the costly engine problems we did back in the early days and that will mean that you’ll have more success more quickly.
Starting with the fuel source as this really is the beginnings of the cycle. It’s a critical part of the cycle and changes to it can make a significant difference to our engines especially 2 strokes. I’m not going into Tuning in this guide though I do have plans to bring out Tuning guides on phoenixmotos.com so I’m going to focus on standard readily available fuel from the garage forecourt.
Here in the UK in 2023 we’ve recently had E10 fuel forced upon us, E10 fuel contains 10% Ethanol. It’s in use because the government wants to reduce our use of Fossil fuels. There are a lot of horror stories with E10 and 2 strokes, a lot of which I don’t completely believe if I am perfectly honest. That said, it is more unstable when left, attracts moisture more than fuel containing no Ethanol and it is well known that Ethanol is not good for a lot of rubber products. With this knowledge I’d strongly recommend that you use an E5 fuel in the UK. If you do a little online research you will find that some oil companies are providing E5 fuel with 0% ethanol content at the moment. E5 is typically also a higher octane premium fuel, this is generally a good thing for your 2 stroke.
Fuel goes off so if it’s been sat for a while don’t expect it to be as good as it once was. If it’s been sat a couple of months it’s probably not worth using it in your 2 stroke engine and better to swap it for fresh.
1 thing to bear in mind with a 2 stroke is that a change in the fuel is likely to need a jetting change. just to clarify what a change in fuel could mean:-
- A change in the Octane of the petrol / gasoline
- Different Petrol:oil mixing ratio e.g. using 32:1 instead of 40:1
- Change in altitude
- Air temperature
Jetting can be complicated especially if you have moved away from standard specs; big bore kit, aftermarket pipe, porting etc… If you’ve had tuning work done then to get your jetting nailed the guidance to follow will be covered in the 2 stroke tuning guide on phoenixmotos.com
General guide for jetting is to start from stock and work from there. The job of the jetting is to try to keep the air to fuel ratio as close to perfect as it can be for your specific engine.
This is what the jets & slide do:-
- Pilot jet: allows fuel to pass through the carb even when at 0 to 1/8 throttle
- Air screw: allows air to pass through the carb even if the slide is at the bottom at approx 0 to 1/8 throttle
- Needle jet: controls the fuel flow via the main jet from just above 0 to fully open throttle
- Slide: controls the air flow through the carb from just above to fully open throttle
- Main jet: allows fuel to flow through the carb at above 1/8 to fully open throttle. At fully open there is no restriction to fuel flow caused by the needle jet.
I see a lot of comments on social media which show me that there is a bit of confusion about what rich & lean actually mean.
First of all the terms rich an lean refer to the ratio of fuel to air getting through to the cylinder. A rich condition is where there is too much fuel to air & a lean condition is too little fuel to air. Richer conditions run cooler but they cause the engine to run badly e.g. the power delivery will not be smooth whereas lean conditions run smooth but run hotter, too lean equals very hot which is very very bad. The aim is to run in the sweet spot.
I see & hear a lot people talk about a rich or lean fuel to oil mixture. It can be referred to as this but this is often confused with the fuel to air ratio. A couple of things to note:-
- The oil lubricates your bottom end so it’s good to have enough! Ideally go off the manufacturers recommendations. Always run high quality oil.
- Running a higher oil ratio e.g. 24:1 instead of 40:1 is actually a leaner fuel to air mixture (leaner) and therefore is likely to need different jetting (richening up).
Every time your piston travels up the cylinder, the pressure in the bottom of the crankcase drops. This ultimately results in your air to fuel mixture entering the bottom of your crankcase. The mixture could have entered directly in a piston port configuration or via a reed valve.
The Reed valve is a more advanced mechanism of metering the Air and Fuel Mix “AFM” entering the crankcase. As the pressure in the crankcase is low (piston travelling up) the reeds open as the AFM is drawn via the carb into the crankcase. As the piston passes the top and travels down the cylinder the Reeds close.
In the case of the piston port, the mix entering the crankcase is literally controlled by the piston travelling up opening the inlet port and allowing the AFM in and down closing the port. Of course this mechanism is not as effective as the Reed valve and what happens is that as the piston is moving down to close the port that a proportion of the AFM is pushed back out of the carb. Imagine the effect of a bellows, you open them and they suck air in, you close them and the air is forced back out well this is very similar.
Imagine now that the piston is at the top of the cylinder, the bottom of the crankcase is full of AFM ready to be transferred from the bottom end to the top end as the piston moves back down and uncovers the transfer ports. For this to work the bottom of the crankcase needs to be in a sealed state. Or our AFM could escape!
It is very important that the bottom end is sealed air tight. The cases are sealed either by a gasket or sealant, the cylinder is sealed onto the crankcases by a gasket and the crank is sealed with crank seals (rubber rotary shaft seals). Both ends of the crank are used, 1 end for the drive and ancillary side which basically spins the clutch, the gearbox, water pump (if water cooled) and power valve governor (if there is a power valve). The other side of the crank runs the flywheel and basically produces power and the spark. It is critical that the bottom end is sealed air tight, lots of issues can occur if it isn’t!
Assuming the bottom end is sealed as it should be then the AFM will not escape. As the piston travels down the cylinder producing high pressure in the bottom end, it eventually uncovers the transfer ports and the AFM “transfers” from the bottom end to the top end.
It’s important to understand the port layout in the cylinder at this point. The intake is simple it is responsible for the AFM entering the bottom end. When the piston is at the bottom of it’s stroke the exhaust port and transfer ports are fully open.
Just think about that, the ports from which the AFM enters the top end are fully open as is the exhaust port where the burnt exhaust gases leave the top end. As this is the case, a percentage of the fresh unburnt AFM is lost out of the exhaust port. Don’t worry though some of the unburnt AFM is drawn back into the top end as the cycle continues, a bit similar to the piston port pumping action as described earlier. In order that the unburnt AFM is not completely lost down the exhaust, the transfer ports are directed away from the exhaust port which means that the fresh AFM should force the majority of the exhaust gases out of the exhaust port and only a minimal amount is lost.
It is normal for the exhaust port to be open sooner than the transfer ports and therefore stays open longer. In engines with a power valve at a lower RPM the power valve will be closed lowering the exhaust port and reducing the time which it stays open. As the engine RPM increases the power valve opens increasing the time the exhaust port will remain open. At full RPM’s the power valve will be completely open.
Reducing the size of the exhaust port improves bottom end and mid range whilst opening the port up improves top end. The power valve has made power delivery of the 2 stroke more linear, smoothing it out and reducing the surge of the power band. Though in 2 stroke engines with power valves there is still a very noticeable power band as ultimately it has allowed manufacturers to tune the 2 stroke for low mid and high range power.
The power band characteristic of a 2 stroke is what we all love about 2 strokes!
There are a couple of other types of ports, a boost port which is effectively an additional transfer port and a Boyesen port which is a different story and is covered in the 2 stroke tuning guide on phoenixmotos.com
Now the AFM has transferred to the top end, the piston moves back up the cylinder and starts compressing the mixture. You’ll have heard of high compression pistons and the method of creating a higher compression ratio can generate greater performance to a point. People often want to increase compression ratio in their engines but this has a more significant and simple power increase in a 4 stroke engine rather than a 2 stroke. Running higher compression in a 2 stroke should also mean the use of even higher octane fuels and more tuning to make the engine perform to the desired level. There is a lot that can be done with a 2 stroke but it is a more complex topic and is covered in our 2 stroke tuning guide on phoenixmotos.com
Once the piston nears the top of it’s stroke (commonly approx 2mm Before Top Dead Centre) the spark plug fires igniting the compressed AFM which forces the piston back down for the cycle to begin again. it takes a little time for the mixture to ignite by which time the piston has continued it’s journey and should be just past top dead centre and on it’s way back down. If the spark plug fires too early (advanced) the ignited mixture trys to send the piston back down before it has reached the top – BAD and if too late (retarded) the piston is already too far back down the cylinder so there is a huge loss of power. Finding this sweet spot is really important both in terms of preventing engine damage and also production of power. Like with jetting the best place to start is at the stock position & then alter from there. This topic is covered in more detail in our 2 stroke tuning guide on phoenixmotos.com
Hopefully this is enough of a description of the 2 stroke cycle to give you enough insight into each stage of the process so that you will have a strong understanding of why we are doing each of the stages of the rebuild. Like I have already said I want you to wholly understand what we are doing and why we are doing it so that you can do a good job of your rebuilds.