Does anyone know what has been done to the later model 4rt's to reduce engine braking ?

Is it a mechanical method or ptb programmed differently ?

There is  a one way reed valve in the crankcase.  Simple but affective. 

Edited February 16, 20179 yr by billyt

What is this magic? any pics?

Here you go right out the owners manual.  

Introduced (I think) in/around 2015, for the 2016 MY bike - along with a re-map and revised fuel pump.

 

I'm reasonably technical but have wondered how does this work.

i always believed the 4t engine braking was due to the extra stroke taking momentum out of the engine- i.e. Power required to push out exhaust gasses and compress the fresh charge.

without lifting a valve or reducing compression how else is engine braking reduced unless my thinking is completely wrong.

3 hours ago, billyt said:

Here you go right out the owners manual.  

"One way reed valve" - as opposed to ?

I don't understand this either. Clearly there is a lot of pumping going on in the crankcase of a single cyclinder 4 stroke. A volume equivalent to the swept volume needs to be pumped in and out of the breather every stroke. If the breather isn't big enough there will be a fluctuating pressure in the crankcase. Adding a one-way valve simply means that the breather flow in one direction is enhanced. I can see this being a benefit for stopping the engine pumping oily fumes out through the breather, but I don't see what it does for engine braking.

That reed used as a "breather" must be mounted to the crank case and any build up pressure will be released by the reeds out.

Very funny Billy.

While the sump reed-valve may reduce engine braking, it isn't the change that may have additionally reduced engine braking in later models.

I say that because the same reed valve has been in place since the very first 4RTs. I believe its primary purpose is for moving oil from the sump to the screen-filter / oil drain cavity that's exposed when the left side cover is removed.

There are other changes in the 2014 and later bikes. One is an orifice in the left crankcase half that vents between the crankcase and left side cover (stator & timing chain). A similar orifice has been used for several years in the CRF250/450R, and went through some sizing changes over the years on those engines. It could be that this is helping reduce engine braking a little because it provides some venting on the upstroke to reduce vacuum -- vacuum created because positive pressure on the down-stroke opened the reed-valve.

Another change is the cam, which has longer open-times for both the inlet and exhaust valves. This could reduce engine braking. See http://www.montesa4rt.nl/  (Tuning | HRC | Camshaft) for the difference between the new cam (HRC cam) and original cam profiles.

I believe that some of the systems to reduce engine braking vent some of the crankcase pressure back into the intake side of the motor.There is info on the internet as there are aftermarket products available for high performance street motorcycles.

2 hours ago, oni nou said:

Air flow restriction [closing the throttle] is a major factor in the engine braking effect

Yep - Not much engine braking when the throttle is open!!!

According to the Honda blurb it is the crankcase decompressor that has brought about an additional reduction in engine braking on the 2017 model.

I have an 09 and wouldn't want any less engine braking than that offers,  but then I ride in an old fashioned style.

 

I know what the blurb says but is thete an actual crankcase valve that lifts the valves. The old tlr had a cable to aid starting. I see nothing external on the later bikes and nobody has shown anything like a vaccum operated valve lifter etc on parts list.

is it in the cam and the rest is smoke and mirrors as the later bikes definitely run on more.

dont think it is in the mapping as have run both new and old throttle bodies. 

I'm not questioning the effectiveness just trying to understand the mechanics if it.