RM250B deleted reed valve

[noisy toy]

Hi all. I still havent figured out why my 250b engine has been previously assembled without the reed valve fitted.  It has had some extensive port work done, but i didnt think they would even run without one. I didnt hear it run but was told it did. This one has done enough hours to wear the piston a bit, any old school guys out there who can help?
I want to put it back together but i dont know wether to re-fit the reed valve or not? I suppose it is only 6 nuts so i should try it with and without. Any suggestions?
Cheers ,Shane

[DR]

never heard of this Shane. Whilst it may go without my gut instinct says it would be far more tractable and ride-able with it fitted. I can't imagine it would have much bottom end power without the valve but up top it may not be an issue and may actually produce extra neddies. If it were me I'd 'have to try' with and without for curiosity. Starting it could be a problem though, I know if the reeds are stuck open or broken the bike normally becomes a pig of a thing

[JC]

Shane,

Many years ago I came across an RM250 barrel in a wrecker's that had the bottom inlet tract (to the reed) completely welded up so it was piston port only - I have no idea why!

But that'd be the only way it'd start & run properly w/out the reed it seems to me.

[noisy toy]

Thanks for the replies, I picked up a reed valve assembly today and have put it back together. Just swap with the PE engine and hopefully good to go! I will let you know how it goes. Spoke to Wayne Leonard today and he hadnt heard of this before either. They have a dyno so maybe worth a play one day. I have an RM 400 pipe but the port angle seems to be different to the PE. Thought i might try a swap there too but dont think it will fit. I might give the bike a bit of a scrub while the engine is out, it looks pretty crappy.
JC where are you in NQ? I am at edmonton.
Cheers 

[DR]

A good mate of mine spent a few years working at Wayne Leonard's some time back. Spoke highly of him too I'm surprised the Institute hasn't given us their thoughts on this one..Loz??

[crs-and-rms]

with out the reed you will lose primary compresion which means when the piston comes down it compresses the air fuel mixture in the crank  case ,this pressure is what pushes the fuel mixture up the transfer ports when the piston opens them . with out the reed it will also push this mixture back throught the carbi

[Lozza]

OK first one is a decenting piston cannot "push" anything out the transfers("i'll explain that one day), the deleted case reed would just be a an exercise in detuning and kick starting practice. Without the reed there would be nothing to create the low pressure in the cases as the piston ascends. I would like to see the port work though..................... As half pregnant the idea is would still be much better than the floor port welded up and no reed at all.

[crs-and-rms]

well it dose it compresses the fuel air mix in the crank case its called primary compression as the piston moves down more it uncovers the transfer ports the positive pressure in crankcase  [7psi ]then pushes the fuel mixture throught the transfer ports this pressure also helps expell some of the exhaust gases  as these gases exit the decrease in pressure causes a portion of the energy to changes sine from a positive to negative wave energy when it hits the exhaust port creating 7 psi of negative energy so now we have a total of 14 psi sucking and pushing the charge into the cylinder , then the remander of the sound waves continues till it hits the converging cone the energy waves are reflected back as a positive energy wave as it reaches the exhaust port it pushes back the fresh charge lost into the cylinder

[Lozza]

All that was true in the 1960's when high case pressures were invogue ,since pressure transducers were used all that was killed all that stone dead. If I have 7psi in the cases and I attach a compression tester to the plug thread and get 150psi without the mixture being ignited you quickly see that even after the blow down period is finished the is much much more pressure above the ring than below it. That is why case pressure does not push anything, anywhere at anytime.
The rapidly outward bound exhaust gasses(fast high pressure) have a large depression(-ve pressure) trailing it when the diffuser angle an diameter is correct this is timed with the case pressure equalising.Drawing everything toward the depression because that has the lowest pressure in the engine.
All very easy to see on a simulator.

[crs-and-rms]

would love to see this or read more about it as this is what they still  teach at tafe  and this ist still what they teach at  yamaha,as for 150 psi when doing a compresion test that presure is made after the exhaust port has been closed ,what is your source of informaion  cause id like to read it ,i will quote the yamaha2 stroke traning once the piston has reached tdc and moves down wards the mixture in the crankcase is compressed  by the piston the mixture cant escape back through the intake port  because its blocked from doing so by the piston or the read valve  as the top of the piston continues its downward movement the cylinders transfer ports are uncovered the compressed fuel mixture in the crankcase is forced throught these ports into the combustion chamber

[noisy toy]

Hey thanks to all who have offered comments, this is very interesting to me. I may have access to a small dyno i can strap the engine to on a bench, so i can fart around with reeds or without, change pipes, and fitting pressure sensors and an air/fuel meter would be interesting.
Lozza, what is this simulator? I would love to check it out.
Bring on the discussion!
Cheers, Shane

[Lozza]

Nothing in the crankcase is forced  anywhere by a decending piston.END OF. There are literaly 100's of SAE papers on the subject.If there is 150psi in the combustion chamber before ignition it is probably 1500psi after ignition. Yamaha ditched that dumb corporate ethos and then started to win championships about the turn of the century.
Reeds close about 80deg after TDC right about the time the exhaust port is ready to open, then again a reed petal is opening and closing 100 times a second at 6000 rpm. So when does all the pressure build up? It doesn't, what stops reverse flow is the intake inertia and the equalising of the pressure in the cases and inlet tract.

Shane AFR meters are useless. Simulators give a snapshot of the pressures inside a 2T engine. Just about everything is measurable and quantifiable.

[crs-and-rms]

well i will stick with what ive been taught at tafe and at yamaha schools they are still teaching it right now put up some links or post some info before i will change my mind

[LWC82PE]

Yeah i dont think it would work with no reed valve. I had one come loose on the mounting holes on my TS185 once which i didnt know at the time, but it just started running terrible, so terrible that in the end i had to push it home!. When i took the top end off it was plain to see that the 2 mount screws had come a little loose and the valve wasnt tight on the gasket.

So i re-loctited the mounting screws back in and it was good as gold.

[micks]

noisy. i too find it interesting. sorry for the hi jack, but can we really quote 150psi etc as my understanding is as a 2st rev`s higher it will loose compression ??