OzVMX Forum
Clubroom => Tech Talk => Topic started by: noisy toy on November 18, 2010, 10:38:03 pm
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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
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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 :-\
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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.
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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 :)
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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?? :)
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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
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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..................... ;D As half pregnant the idea is would still be much better than the floor port welded up and no reed at all.
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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
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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.
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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
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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
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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.
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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
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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.
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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 ??
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So i have put the RM engine in the PE frame, found a couple of things. The 250 engine came out of a very bodgy 125c conversion . An RM 1250? ;D The carby that was on the engine was 32mm, maybe from the 125 engine? I have put the 36mm from the PE engine on it. OK now to start. Nothing. Check spark. Nothing! Bugger, maybe thats why they stopped riding the dangerous POS 1250 before i got it. I think i will change to the PE CDI unit, or i may have to put the PE flywheel and coils on it, as the RM harness plug is different to the PE plug, so i may as well change the whole lot. Is there much difference in the curves ets between the PE and RM? I have an RM 370 CDI unit too , worth a try? Lots of questions, sorry....
Cheers, Shane
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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
I'm not out to change your mind you can believe whatever you want to believe. If you believe a basic Tafe course will teach the detail that A Graham Bell does( who inspired and contines to inspire a generation of two stroke tuners around the world) then keep beieving that.His book Two Stroke Performance Tuning continues to be my first point of refernce for any two stroke related matter. Plenty of free downloads on the net or Graham still sells them via his Maitland based website.
Pg 49
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0002.jpg)
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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
I'm not out to change your mind you can believe whatever you want to believe. If you believe a basic Tafe course will teach the detail that A Graham Bell does( who inspired and contines to inspire a generation of two stroke tuners around the world) then keep beieving that.His book Two Stroke Performance Tuning continues to be my first point of refernce for any two stroke related matter. Plenty of free downloads on the net or Graham still sells them via his Maitland based website.
Pg 49
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0002.jpg)
You are taking that out of context. All he is saying is that they have gone away from high crankcase pressures in building engines. It still needs crankcase compression to move the air/fuel mixture through the transfers. No longer are they seeking a high pressure though. The modern two stroke still has the free spaces in crankcases and crank removed or stuffed. Stuffing the crankcases and crank can still work on early two strokes and does give a good power increase.
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so the decending piston still compresses the air fuel mixture in the crankcase and moves the air fuel mixture throught the transfer ports which is all i said in my first post and id rather have a tafe certificate than just a graham bell book
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The key word there for me is 'HIGH' crankcase compression.
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Must be the month for old wives tales to do the rounds again ::)
Sure your not taking the text 'out of context' ?It states they have 'got away from the IDEA' nor does it say case compression is required for transfer flow to occur. If high case compression doesn't push fuel through the transfer how will low case compression ever do that? The decending piston increases case compression by a factor of 0.1ie you have 1.3 case comp the decending piston moves it to 1.4, however above the ring we have far greater residual combustion pressure long after the transfer ports have opened. How is it possible for air/fuel that is at best under 5psi pressure overcome pressure that is still well over 100psi? It cannot without the aid of the pipe,as it's not untill BDC does mass bulk flow from the transfers occur( which again proves the decending piston pushing anything at anytime as crock of shit).Bulk flow in the transfers occurs at BDC because the lowest pressure in the engine is in the header pipe, the highest in the inlet tract.
Stuffing cases really only brings the power lower down the rev range it cannot increase output of an engine, the effect is similar to high secondary compression(that increases the output).Works lower down the rpm range as rpm rises the engine struggles to overcome the pumping losses.
Here's a snapshot from a basic widely used and trusted simulator. Shows the transfer duct 1 with the port 1/4 of the way open and air/fuel velocity in that duct to be -100 ft/s
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG-4.jpg)
then we have velocity in the same duct at BDC
(http://i155.photobucket.com/albums/s297/Lozza85_2007/IMG_0001-1.jpg)
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no need for any disputes guys ::)
Whilst I do sometimes refer to reference books and take others opinions aboard there is only 1 'real and for sure' way to know the answer.... TRY IT!
You can speculate all you want with facts and figures but a few minutes on the bike will answer every question. Easiest way is to simply take it out and ride it ;)
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Hey Lozza, that simulator looks like an interesting tool, where can i get hold of one? I am still having spark issues with the old girl, but i do intend to ride it with and without the reed, just for the hell of it. I am not sure if the results will be the same as before, as it had a 32 mm carb on before and i have fitted the 36. I may as well try both there too. A dyno would be good so i could post up some curves, i see what i can do.
I am surprised no one else has responded that has actually tried this?
I will try to take some decent pics of the porting when i have the barrel off next.
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I got the old PE running properly , finally. I swapped coils and rotors and still no success with the no-spark issue. So i thought bugger it, i will remove the RM engine, swap the barrel and piston to the PE bottom end and chuck it back in. Bingo! A gentle test ride, but still enough to fill my jocks. It goes quite well! It even idles and everything.I gave it a bit in the drive way, it took me by surprise and stepped out sideways a bit then took off toward the shed! It was about now i remembered the front brake was not connected yet,(it is now)and the rear,well,... I may as well have held my hat out for a similar effect.But i missed everything and made it back in to the shed to fix the last few things.Will report after the first proper ride.
Cheers. ;D
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It was about now i remembered the front brake was not connected yet,(it is now)and the rear,well,...
We have had some ugly moments on the PE list where people have taken a PE around for a quick blast sans brakes. Good to hear you have now fitted the front brake!
Luke
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While skimming through this post I saw the comment re stuff learned at T.a.f.e etc,
While not wishing to denigrate tech trade courses I came to realise that a lot of what I was taught at tech in the auto mechanic course was 'simplified'.
Natural trade curiosity on my behalf drew me to the conclusion that the primary aim was to educate students to a point where they had enough info to provide confidence in making accurate diagnoses of faults.
While saying this I am also convinced that some teachers didn't understand what was going on.
A classic example was the bulls*** explanation of the Kettering ignition system [common on everything prior to true electronics]. e.g. "the condensor is nothing but an electrical shock absorber to stop arcing at the points", and also some of the truly weired explanations of cam design.
My conclusion is not to take for granted that which you learned yesterday is gospel, research more if you wish to learn. cheers pancho.
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I should have added that my knowledge of two stroke engines is very limited having being taught at tech in the days when 2/'s were strictly for small low performance industrial engines and considered by lots to be useless for anything else.
Thats why I find these discussions so informative cheers pancho.
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Hey Lozza, that simulator looks like an interesting tool, where can i get hold of one?
HERE (http://www.iwt.com.au/MOTA.HTM)