YX 125cc Engine – Kick Start (Manual) – Kit 4


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SKU: C00 00 00 YX125-4 Category:

The new generation of 125cc engine from the YX factory. With the factory fitted big valve upgrade and aluminium cylinder, the factory claim 12bhp at the flywheel from this engine. The YX 125 engine is one of the most reliable learner legal horizontal lay flat engines on the market.

Being the top spec 153fmi engine model, this engine has the extra power from the bigger valves, fitted as standard with the YX140 type alloy head, alloy cylinder barrel, wide ratio gearbox for increased top speed and long term reliability.

Why are our engines different to other sellers engines? 

Other seller engines:
Engines are sold boxed, un-opened and unchecked and with no adjustments carried out which if left unchecked could cause potential problems and furthermore serious damage.

Our engines:
We carry out a full inspection to ensure all components are present and make all adjustments "Which are always required" to timing and valve clearance settings. We drain the transport oil which is not suitable to use for running your engine.

What does this mean for me?
This means your engine can be fitted straight into your bike with the peace of mind that a professional technician has set it up to run perfectly for its breaking in period, which is the most important point of the engines life....... just add good quality 10w/40 semi synthetic motorcycle oil and away you go.

Engine can be used to upgrade the following:
Pit Bikes, Honda Dax, ST, SS, CUB, Monkey, Z50, Mini Trail, Chaly, ATC 70, ATC 90, the list goes on and on!

Comes complete as follows:
-Complete Genuine YX 125cc engine 12 HP
-4 speed manual gearbox - 4 up (N-1-2-3-4)
-Removable side cases
-Output sprocket

Complete with:
-Inlet manifold with spacers, gaskets and bolts
-Kick start
-Gear selector lever

These engines are 440mm long, 280mm wide. Top mounting is 93mm wide with an 8mm bolt hole, Rear mounting hole is 82mm wide with an 8mm hole. They also have the 4 M8 threaded holes on the underside which are 88mm apart left to right and 44mm apart front to back, These are to hole centres.

Please note: All our engines need to go through a braking in procedure.

Please follow the engine installation guide we include as this will helps you in setting up and getting the most from your new engine.

Here is a message from a happy chappy who purchased one of the Honda clone engines for a near 2000 mile European trip.

"Just thought I'd let you know, I have just completed an 8 day tour of Europe on my Honda Cub fitted with one of your YX 125 4 speed engines. It ran perfectly for the 1,939 miles of the trip. Speeds were mainly between 43-50mph. It is still as quiet as a mouse and used no oil (I did have an oil cooler fitted). Countries visited, in order, were :- France, Belgium, Luxembourg, France, Germany, Czech Republic, Germany, Austria, Germany and finally France. Was I worried it wouldn't make it? YES. Am I worried now? No. Another YX engine also made it, one of your 150 manual clutch jobbies. Thanks for a great engine, Bogger"



Before you use, please note the following:

The engine valve clearances have been set and transport oil drained by us before dispatch.
Recommended replacement engine oil is Putoline DX4 10W/40 from our store.

Engine Data:
Oil Level – on most pit bike engines they hold 700 – 750ml of oil, please ensure engine is level and flat with the bike not resting on the side stand. If using a dipstick then remove, clean and push back in without screwing in, then remove to look at the oil level reading. If using a sight glass, then the oil level must be around halfway.
Valve Clearances – all 2 valve new engines use the same valve clearance settings (engine cold) Inlet = 0.10mm & Exhaust = 0.15mm.

If you are unsure on how to time your engine correctly and how to set your valve clearances, then please follow a YouTube tutorial.

We would always recommend installation by a professional qualified person.
Please ensure the engine breather pipe is fitted to the crankcase and the rubber blanking plug removed (if fitted). Ensure there is no kinks in the breather hose and check you can feel the pressure escaping when the engine is running.

Oil Coolers:
We would always recommend using an oil cooler when possible. Using an oil cooler will keep the engine below 190 degrees, even though it is an air-cooled engine, if used under racing conditions then they can become very hot. The cooler will lower the oil temperature which will prolong component lifespan inside the engine.

Running In Period:
We firstly recommend the engine has a few heat cycles before use, this is where you would start the engine for around 5 minutes and then turn off to cool down completely. This allows the piston to bed in and for you to check for any leaks around your oil cooler lines.
We would then advise about 1 hours use (with breaks) of gentle riding which doesn’t exceed half throttle.
We would then finally advise for an oil change and to check the valve clearances and adjust if needed.

We offer our standard 30 days warranty by UK Trading Law on our products. This only is valid on manufacture faults. Issues caused by the user such as incorrectly installed, abusive use, deliberate damage and race use will not be covered.
30 days begins on the date of purchase.
Regardless of if you have used the engine for 1 hour or 0 hours, once the 30 days has expired it has expired.
Race use - We fully understand there is a market here in the UK for motorcycle racing using pit bike engines. Please note anything used under racing conditions is void in warranty as you’re a putting the components under huge stress. These engines were never designed for competition use so please bear this in mind.
Claims – If you think there is an issue with your engine then please contact us first to discuss your issues, 99% of the time these are always resolved without the need to return the engine and is usually 1 of 2 problems.
1 – Incorrect wiring loom and cdi used for that engine (if using your own loom).
2 – Incorrect carburettor used.
In the event of a claim, it is the buyer’s responsibility to return the engine in the original engine moulded box and to cover postage charges.

Upon the return, the engine will be inspected by us to see if it is covered under a manufacture error.

Items Not Covered:
Below are some examples of items not covered under warranty as it is related to rider misuse.

  • Kickstart gears (caused by excessively kicking & incorrect carb setup)
  • Clutch basket centre gear (caused by excessively kicking & incorrect carb setup)
  • Barrel & piston wear
  • Gearbox dogs (caused by not using the clutch)
  • Premature valve damaged (caused by over revving)

Please use common sense when using these engines, please bear in mind these are still Chinese engines and Chinese quality therefore expectations should be made for Chinese standards.


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