Welcome back friends and Happy new year! I can’t believe it has been another year without me finishing a god damn bike! Having said that, 2018 was a year full of learning and tinkering so all is not lost.  I mentioned previously the high-level reasons that led to scrambles becoming another project bike but I spared the detail until now…So if you don’t want to continue reading poorly explained motorcycle engineering theory please go read a much better-written article about a talented builder on http://www.bikeexif.com instead. For those that stuck around, thanks…prepare to have your brain melted 😉

This engine shitting the bed taught me a lot… I learned a lot about engines, way more than I had initially intended (which is actually a great blessing in disguise), I learned a lot about myself…like how much of a GOD DAMN IDIOT I WAS BUYING A BIKE ON A WHIM & NOT DOING THE CORRECT CHECKS (possibly…).

To help you understand the issues I had, we need to go way back to basics, primarily to how an engine works…to save getting bogged down too much I’ll try to summarise the best I can through the medium of poorly drawn sketches.

An internal combustion engine burns fuel and air in a compressed environment to cause a change in state (big boom), which generates power and is subsequently fed through the drivetrain to the back wheel

All of this takes place within the engine. There are several components which make up an engine and feed into the process of generating power.

Top end

Carburetorsee carb post – this controls the level of fuel and air mixture being sent to the combustion chamber.

Cylinder head – The cylinder head is made up of the below components

Valves & springs – there are two valves: an inlet and outlet valve – one allows fuel air mixture into the cylinder (inlet) and the outlet valve allows exhaust gas to exit the combustion chamber. Some engines have more than two valves but they serve the same purpose, to provide a flow path in and out of the combustion chamber. 

Camshaft – The camshaft attaches to the top of the cylinder head and presses the valves open and closed at the correct time in the combustion process. To ensure the timing is correct and in sync with the rest of the engine components, the camshaft is driven by a chain off the crankshaft (see below)  or via a pushrod & rockers – Timing is something worthy of its own detailed guide, so I will not go into too much detail here (/i should probably learn a bit more before I try and blag how it all works…) but a key point….if this is wrong, your engine will not be happy and will probably try to eat itself from within. 

Exhaust header – Exhaust gas exits the cylinder through the outlet valve and through the exhaust ports – noise follows….BRRRRAPPP

Bottom end

Cylinder block – The cylinder block is where the magic….I mean science happens.  The below components all play a key part in the combustion process (or in the case of my bike, some of the parts kind of worked together to attempt a combustion cycle)

Cylinder – The cylinder contains the combustion and provides a pathway for the piston to travel along. The top of the pathways is known as Top Dead Centre (TDC) and the bottom is called Bottom Dead Centre (BDC). The volume within a cylinder is used to calculate the capacity of the engine. i.e. for the TT600, the volume that the piston passes across within the cylinder would be = 600cc.

Piston & conrod – the piston and conrod are connected to the crankshaft. the piston travels along the cylinder bore and provides the change in environment to allow combustion to happen…see combustion cycle below.

Crankshaft – The crankshaft attaches to the piston and transfers the linear force into a rotational force (which goes through the flywheel, clutch, gearbox chain to the rear wheel). There are bearings which connect the conrod to the crankshaft (conrod bearings) and crankshaft to its housing (Main Bearings).

The 4 stroke combustion cycle

The simplest way to understand the internal combustion process is to follow the fuel and air mixture through the system of components mentioned above. This can be broken down into 4 simple steps:

Suck > Squeeze > Bang > Blow 

Suck – The piston at the top of its cycle (Top Dead Centre or TDC) starts to descend. The intake valve on the cylinder head opens and fuel-air mixture is sucked into the combustion chamber and fills the void as the cylinder moves downwards until…

Squeeze – …the cylinder reaches the bottom of its cycle (Bottom Dead Centre or BDC)  starts to move upwards again (due to crankshaft doing a full rotation). The volume occupied by the fuel-air mixture starts to decrease and mixture is compressed, both inlet and outlet valves are closed at this point.

Bang – As the engine has reached TDC again, the spark plug ignites the fuel/air mixture and causes an explosion and rapid expansion within the chamber. This forcefully moves the piston back to BDC. This movement (the power stroke….lol) is what is used to drive the bike forward via the crankshaft. 

Blow – once BDC is reached, the piston starts to travel upward. The exhaust valve opens to allow the ignited mixture to be expelled from the chamber leaving it in a state to start the process over again…

This process happens several thousand times a minute…..maybe it will make you think twice about revving hard from cold ( I’m speaking about you Mr scumbag that blocked me in the other morning in your POS chav mobile) 

This 4 stage combustion process is what denotes the name 4 stroke engine. and yes those scribbles took fare longer than i would care to admit.

I hope this explanation helps build a picture that all the parts of the engine ARE REALLY FUCKING IMPORTANT and have to be working in perfect harmony. 

On my bike, they were not working their best:

The Symptoms – The bike always felt a bit sluggish. I mean a 600cc scrambler should be pretty terrifying in the low gears, even with my fat ass on top of it.  I always thought this to be the carbs needing cleaned/tweaked slightly so I had planned to do this. One morning, when warming the bike up before a long run, I gave it a quick rev and noticed smoke coming out of the cylinder head, rather than the exhaust (see video)

This got my alarm bells ringing as my detailed diagrams show the exhaust gas is meant to exit well… through the GOD DAMM EXHAUST. This is an issue. With an improper seal around the cylinder the engine is not running efficiently at all, so not nearly enough power being generated and can only get worse (wearing head gaskets).

I have genuinely had what can only be described as a complete and utter shit storm of issues trying to remedy this problem, and I will save that to the next post….trust me…everything that could have gone wrong did go wrong so next time we can rejoice in my misfortune as a team…

In the background, I am progressing with this build and have another few posts lined up…I am also trying to get my head around some video editing software for some youtube how to’s …. first things first though..time to freeze my nuts off and get working on this bike!

to be continued,

Struan

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