Wow what a lot to reply to. I would have replied erlier but i was at the rc swap meet. I ended coming home broke but i got rid of a bunch of stuff I was never gona use and gained a zenoha g63 for 50 bucks. he says the crank is bent and needa a new one. this crank is a three piece crank all pressed together just like a motorcycle crank. Its not to hard to straighen it back out as it just slipped on the rod journal pin. I then needed a plnae to put it on so i scored a new in the box sig sun dancer arf for 225. man is my wife gona be peved lol. needless to say im broke again and this sure wasent the intent for the day but I just could not resist.
Dieco thanks so much this is the kind of comparision I would very much like to see.
I do not have a cylinder with the side decompression slots so i cannot do any comparison on this set up. I just dont see how this could be good for performance. If a larger gap on the rings of .030 makes a difference than i cannot see that the slot would not make a difference. they are in affect leaking compression back into the crankcase just like a larger gap in the ring would do and we already know this is not good.
I have difficulty getting a stable reading of RPM but I think it gained about 50 RPM.
this is interesting as I also have difficulty getting a stable reading and I am wondering if the ignition is affecting the tach. I am thinking a thrust weight testing might be more accurate for this but i have no way to do this and really dont want to take the time to fab something up. I would like to see how you do your test this way. I ussually have to work at it for a while till i can get a stable reading and go with that number. it takes a little fooling with but i feel i have been pretty consistent in my method.
I next put on the old style cylinder with the single runner transfer ports and the decompression slots cut above the transfer ports. The piston was a two ring and the crankcase the later configuration designed for the two runner cylinder. By your observation the cylinder head volume may be smaller too ie greater compression. The performance was abysmal!! It lost 630 rpm and required a lot richer mixture but boy did it idle nice! It got down to 1300 RPM before it started missing, but it still didn't quit! Needless to say idle is not the operation point we are most interested in is it! The obvious conclusion is that a cylinder with decompression slots makes a good paper weight!!! I'm not confident enough of a positive outcome to try filling the slots with anything.
now this is very interesting as I had a pretty large gain in power. now this is some good comparison thanks so much. The only thing is I donot have the decompression slits in mine. so i cant really tell what the difference is. Now im wanting one of these cylinders with the slots just for comparison sake.
The results of your testing point to a bigger carb as the very first thing I need to get improved the power. BTW when you all speak of carb size, eg. 11 mm, are you talking about venturi size? Also I noticed that even on the stock carb that came with the WW that the intake manifold dia is greater than the carb bore. You'd think that it would be of great benefit if the two were the same although with the aft facing step there is almost no loss in flow coefficient although the flow will decelerate.
Yes i do mean the venturi size. If you go back through this thread you will find where you gan get the size from the carb number. I think I folow you in the loss of flow coefiecent. I agree it would be of benifit if they were the same size as this would give a more consistent velocity. Just as long as the venturi is not larger than the intake runner. I just dont see where this would work. if you get the 11 mm carb you will notice the outlet end of the carb is the same size of the intake runner port.
Also, what do you suppose the benefit of cutting a waist in the reed is? More flow area when the reed is lifted? Also I notice that, at least for the backplate that's apart on my bench, the reed seat is not flat. The reed is suspended over a concave surface so that when closed there is space between the reed and the seat in the middle of the flat reed. In this configuration there would have to be some crankcase pressure to close the port to the carb. Is this the way they're supposed to be? I would think that if the reed were completely closed at static condition it would work better, but then again this is all a guess. I see in your most recent post that on your second reed test there was no difference in the modified reed so perhaps the question is academic.
As far as cutting the waist are you reffering to the modified reed valve i tried in the second part of the reed valve test? if this is the case the idea was to reduce the pressure it took to unseat the valve and causing a decreased restriction to flow. i dont know if it would create more flow area or maybee it would if the valve were to lift higher. It needs to be understood that to move the air gas mixture there has to be a difference in pressure. Sorry but there is no such thing as vacumme. except outer space and in your wifes closet. so it is not vaccume that draws the air in but it is atmospheric pressure that pushes it in as it is at a higher pressure than the pressure in the crankcase caused by the upward stroke of the piston. the upward stroke also causes a pressure differential. things in nature flow from high to low in the path of least resistence. So the idea was that with the modified valve it will take less pressure differential to open the valve and would cause more air to flow as the valve opens sooner and the air will flow with less resistance. remeber the path of least resistance. so in affect the pressure of the crankcase will be higher when the piston starts to compress on the downward stroke than if the valve causes a greater path of resistance to flow like the un modified valve. That was also the idea of the rubber valve as it is supper easy to open and has a very small path of resistance to the airflow. Does this make sense or did I not explain it good. Im sorry if thats fuzzy i know my comunication skill at times need work. If you have two reed valves modify one and make a rubber one also. if you put your mouth on the intake runner where your lip covers the very small pulse port for the fuel pump and try to blow through each one you will see what I mean. This is how is what i did and the difference is quite visisble although it dosent taste to well lol. I dont think the no power output results to be academic as i take it to indicate that there is a resrtriction to flow elsewhere where this mod makes no difference. it may make a difference if this flow problem is found and found to be a problem so it is good info to put in the cranial file cabnet and may yet be usefull latter.
I also had a reed tah was not flat on the backplate and i adressed this in the beginning of the post.
The second discovery is that when I bent the reed valve stop it bent close to the screws which relieved the stop out towards the intake hole. If you look at it from the side it does not allow the reed valve to completely sit flat with the back plate and you can see a small gap between the valve and back plate. The back plate has kind of a swoop up cast into it where the reed valves sits. It is impossible to get a picture of this as I have tried lol. I don’t know how I didn’t catch these problems to begin with. Just not paying attention I guess.
Ok so where do I go from here. I think my first step is to modify the back plate that goes with this engine without any performance mods. This will give a stock and correct crankcase volume. I then need to bend and modify my reed valve stop to where it holds the valve flush with the back plate and has the 1/8" gap. I think if you’re going to bend this reed valve stop the best thing to do is to put it in a vice as close to the bend as possible and bend it there not up close to the screws. Check your reed valve when you do this to make sure it still sets flush. I will then test run this engine again in this configuration and get a good starting point. From here I can begin to modify.
form flathead
If you haven't guessed by now, my emphasis has been on "Stuffing" the crankcase. That is why I have mentioned the different back plates and reed valve arraingments. I cannot determine to date if smooth back plates, or partially inclosed reed valves are best. I do know that the early crankcase gives me best performance to date. I am adding additional material to the backplates from the "STIHL" labeled units because are smooth and easy to add material. I am fighting a severe infection at present, so my testing is on hold, but it don't stop my thinking process. I am at 3650 ft elevation, which makes a difference also. Keep up the good work, and keep us informed. FLATHEAD 4
Those kind of numbers at 3650 ft is pretty damn impressive. Yes i can see you are wanting to stuff the crankcase lol. Im right there with ya on that one. When you say erlier crankcase do you mean the one that has the smaller dia and the shrouded reed valve backplate. If so i have said in my opinion this may be the hot set up. the smaller dia crankcase = less crankvolume. the shrouded reed vanlve means there is more backplate stuffed into the crankcase making less crankcase volume. also look at the counterweight on the crank. its the one on the left to me this screams less crankcase volume.
I think this picture speaks volumes. the one on the left is quite visibly smaller. if the end of the reed stop is the same depth or didstance to the rod then the one on the left definatly has more material in the crankcase ==ing less crankcase volume. I know it looks like it wont flow as well but from what your numbers indicate the not as good flow is offset by a large gain in less crankcase volume. also notice the relief cut in the backplate where it clearances the cylinder. the one on the right is cut at an angle and would seem to me to have lees material to take up crankcase volume. this is just my opinion and i would really like one of these motors for testing.
In your post about the reed valve mod, I think that the transfer port size is holding u back. On the newer jug, the ports are very narrow. On my jug, i removed the metal between them completely. I have not been able to test the mod yet (its just too dam cold here in canada) I have been told that the mod might cause problems with the rings not being properly supported. As soon as i can get this engine running, i will post a video. The only other mods to my engine are bigger carb, modified muffler (just drilled two 1/2" holes) exhaust changed to approx 150 degrees, and the transfer port mod.
To stuff the case, if you were to machine a new backplate, you could have a recessed area that would clear the conrod and crank pin. Just be careful when stuffing. I blew up a homlite 25 doing that. the plate that i used was too close to the crank pin, and when the metal expanded, the pin gouged it and the metal pieces jammed and broke the ring. It was spinning at about 9000 rpm when it did this, so it made one hell of a bang.
You very well may be right on the transfer ports holding me back. I have looked very closelly at the cylinder with the two runners per side and have concluded that even with the runners being smaller colectively they are very close in volume to the single runner ports. I really dont understand why they did this and would really like to hear it from the engineers mouth on this one. I am very interested to see how you rengine responds to the removal of the metal between the runner ports. I hope you dont take this wrong But i think even if it does improve its not gona be as good as the single runner cylinder as it has a higher compression ratio. in my opinion widening the runners of the higher compresion cylinder may be the way to go. Point well taken and appreicated on the bang warning lol.
Before worrying about other methods for stuffing the crankcase, I would make the bearing/seal change and see if the engine responds well first. You can't move the backplate inward on the engine much because the reed valve stop will hit the crank pin.
Once again, with the ring gaps you are running you would see a nice power gain with a Frank Bowman ring. 12 bucks isn't that much in the grand scheme of things... (Just my two cents here.)
Im right there with ya on this. it sounds like a good idea to try the seal first and see where that goes. the more i think about it i am convinced the next step is to do some crank stuffing. this stock set up has such a large crank volume it just cant create a very large pressure differential. it very well may be that there is not enough of a pressure differential created for atmospheric pressure to push the air into the induction sytem at a velocity to where a change in the reed valve restriction to flow would make a difference. Anyhow as far as moving the backplate inward on a 2 inch dia a small move should make a large reduction in volume so i dont think it will take much.
I so agree with the ring but that may have to wait a while as i am now broke lol. darn swap meet. oh poor me i know lol. anyhow the rings are 12 apiece so he told me it was 24 for the 2 ring set up delivered.
I am also wondering if i could set up two ports on the crankcase and use a one way valves one in and one out. I could use this set up along with a u tube manometer to measure the pressure differential created with a given port, valve and crankcase size configuration. this could be very interesting.
Wow I hope i didnt miss anything lol.