Brian, why such a heavy spring? I'm also curious about the gasket that I think I see between the head and the muffler. What do you typically use for gasket material?
Throttle governed engine
- Thread starter Brian Rupnow
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Combustion chamber hasn't changed at all, other than the location of the valves and sparkplug. Biggest change of all was the shape of the rocker arms. They went from simple "straight' pieces of steel with rollers on the center to the new ones with the cantilevered rollers at the end. In test mode, with the adapter screwed into the sparkplug hole, the cylinders sealed up tight up to 50 psi. of air pressure No leakage of air out the exhaust pipe nor out of the carburetor throat, nor past the rings. The valves sealed great under "test" conditions, but engine had no (or very little) compression when cranked over. Nothing gets past the Viton o-rings, no leaky head gasket. Conclusion is that the valves aren't seating properly. They are lapped, and do seal under "test" conditions but not in normal "start the engine" mode. Normally, if I can get an engine to fire at all, the force from igniting fuel will force the steel valves against the brass seats hard enough to make a perfect seal. Tried to start it already, and there simply wasn't enough compression for the fuel to ignite. It's rather coincidental that Boomer has been talking about valve springs.--That is where I ended up going anyways as that's about the only thing left to try. Ignition and valve timing hasn't changed since the last time I had the engine running. I can push the valve off the seat, against the new spring pressure when gripping the head between my thumb and finger. It is noticeably harder to do that now, but my cams, lifters, and rollers on the rocker arms are all heat treated 01 steel. I did have to make a spring compressor, because it is very finicky work trying to get the spring compressed, the keeper in the right spot, and the c-clip on without something to compress the spring and let me work with both hands. The push-rods are 1/8" diameter about halfway up their length, then are reduced to 1/16". I will probably make new pushrods tomorrow that are 1/8" diameter over the full length. I use .031" thick waterpump gasket for my head gaskets. I have never had a head gasket burn or "blow out".
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SO - what is your valve timing? Especially at BDC where intake closes and Compression starts?
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I found exactly what I had set it for. Both exhaust and intake lead by about 20 degrees.
I haven't installed the head with new springs yet. I'm ready to go, but I want to mount a 0.060" spacer under the rocker arm bracket but wife isn't up yet. This has become my morning routine. Wake up, get dressed, make a coffee, then head down to my computer and look at the forums to see what everybody else has done.---Quietly. My bandsaw is out in my main garage right under her bedroom. I am going to saw the spacer out of 1/16" plate, and the bandsaw makes a Hell of a racket. I'm putting the spacer under the bracket to raise it up 1/16" because as you can see in the picture, clearance between the adjusting bolt in the end of the rocker which adjusts the valve lash is uncomfortably close to the head of the bolts which hold the intake and exhaust flanges in place.
Thanks for the explanation. So, if I understand correctly, you are using the stronger springs to help force the valves closed in order to get compression. Question: once you have run-in the engine, fully seating the valves, would there be any advantage to going back to softer springs? I don't know enough to know what the impact is, but wondered if the heavy springs would leach some power from the engine - ??
Awake--I have completely reassembled the one cylinder with the heavy duty springs on the valves. If I see an improvement, I will do the other side to match. Will it hurt the performance of the engine? Probably, as it takes more power to lift the valve off it's seat. I don't really know. I've never used springs this heavy before. I have about fifteen i.c. engines that I have built, and none of them have springs heavier than 0.020" wire diameter. I am in "Try it and see what happens" mode.
I await the results with interest! I just made the springs for the modified Webster that I'm working on; the plans call for the intake spring to be .013" wire and the exhaust spring to be .022" wire. The former is really light - I've wondered if it will be too light, but we shall see. I'm almost to the point of assembling the valve cage, attaching to the head, and seeing what sort of compression I have ... or not!
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I was thinking more in terms of when the intake closes. That would be part of the cam lobe duration.
If the intake is late closing it will be robbing some compression. Also late close will result in some charge being pushed back out the intake.
Large overlaps (and duration) are really racing engine type configurations where the rpm and mass of the moving air figure into things. You're better off with next to no overlap (maybe 5 deg at most) at TDC and get the intake closed at or very near BDC.
More like the "gas saver" cams commonly available for vehicles.
The 0.013" spring is just about perfect for an atmospheric valve which doesn't use a cam. The 0.020" or 0.022" is absolutely right for the exhaust valve which is operated by a cam. I have about 16 i.c. engines I have built with those size springs and they worked fine.
So, you're saying it is going to run on the first try, right? 
Awake--The most critical thing with these small i.c. engines is that the valves seal completely. If you get the valves and seats right, then your engine is almost guaranteed to run. Might not run good---might not run continuously, but it will run. If the valves leak, and you don't get good compression, then you will just make yourself crazy trying to figure out what is wrong. These small valve springs work great, but they depend on a very smooth and frictionless interface between the valve guide and valve stem, and a perfect lapped seal between the valve face and the valve seat. Using a steel valve and a brass seat, even if its not a perfect seal, if you can get the engine to fire at all while you are trying to start it, the pressure from the exploding fuel will drive the valve face against the brass seat hard enough to make the brass seat conform to the shape of the valve. That is why many of these small engines run better and better for the first half hour of running. If you absolutely can't get the valves to seal, you can resort to other things like much heavier stiffer valve springs (such as I just did on the twin cylinder I'm working on), but I don't recommend it.
Thanks, Brian. Soon, soon I will be at the point of testing compression ... meanwhile, let me not further derail your thread. Did the stronger springs produce the desired result?
There is no joy in Mudville---I have no good news to report. The engine is reassembled with my new cylinder heads (the uber strong valve springs are only on one of the cylinders). With my adapter screwed into the sparkplug holes, the cylinders both hold 60 psi air pressure, with no air escaping past the valves. I have tried to start the engine and it isn't happening. I even went so far as to set it up on my bench and drive it for half an hour to see if that would seat the valves a bit better but it didn't seem to do anything. I am going to go out today and buy a socket and pigtail for a 12 volt bulb and wire it up through the points so I can get a really good concept of the ignition timing. I will also do one more check of the valve timing. This time I will check valve timing on both cylinders. The cams are made as four individual components and Loctited to the camshaft. I doubt very much that they have spun on the camshaft, but I will check it. I know a thousand things to check for on engines that won't run, and I've used up 998 of them. If this engine doesn't run after a final ignition and valve timing session, it's going up on the shelf. I have an idea for a new and different engine in my head that I would like to spend some cad time on, and I'm ready to move on.---Brian
If you put it aside, don't stop thinking about it. Taking a break to think about something else is often a good way to get the creative thoughts flowing again. At least, it always has been for me.
George Punter
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Good Day Brian,
I have been following your engine construction and it brought back many memories of the first engine that I designed and made ( also a flat twin of 20cc). Like you I encountered problems but gradually eliminated them one by one. It’s all part of that learning curve. On some of my engines I use bronze for the inlet seat/ guide and cast iron for the exhaust and leave the all the seats (valves included) as machined and do not use ant lapping paste. As your engine has good compression there are a couple of points that you could check - is the spark on the compression stroke and you could also try more advance. I am sure your engine will burst into life soon. One time I was working on a Velocette LE 200cc that would not go only to find out that the chap had the plug leads on the wrong cylinders! Keep up the good work.
George.
I have been following your engine construction and it brought back many memories of the first engine that I designed and made ( also a flat twin of 20cc). Like you I encountered problems but gradually eliminated them one by one. It’s all part of that learning curve. On some of my engines I use bronze for the inlet seat/ guide and cast iron for the exhaust and leave the all the seats (valves included) as machined and do not use ant lapping paste. As your engine has good compression there are a couple of points that you could check - is the spark on the compression stroke and you could also try more advance. I am sure your engine will burst into life soon. One time I was working on a Velocette LE 200cc that would not go only to find out that the chap had the plug leads on the wrong cylinders! Keep up the good work.
George.
Okay--An update. this afternoon I checked and reset the ignition timing. I even wired in a 12 volt light thru the ignition points so I could see exactly when the points opened the circuit. Stuck a hex wrench down the sparkplug hole and turned the engine over by hand, one hand hanging onto the hex wrench which was in contact with the top of the piston and the other hand slowly turning the vaned flywheel. I didn't really like the ignition timing it was set at, so I changed the timing a bit until I was happy with it. Then I slowly turned the engine over by hand , still hanging onto the hex wrench, and got a good reading of when the intake and exhaust valves began to open. That was right, so I then moved over to the other cylinder and repeated the act. Valve timing was good on that cylinder too. So---the cams haven't slipped on the camshaft. Tried to start the engine, and it didn't start, however the right hand cylinder was puffing smoke and getting warm. The left hand cylinder was too (that's the side with the heavy valve springs) but not as much as the right cylinder. I put new fuel in the tank, although I don't really think that Naptha goes bad with age. Tomorrow I'm going to set the engine up on the bench and drive it with the electric motor, until it flies or dies. I'm not going to work on this anymore. Now, an observation::--This is fairly important. The crankshaft has bronze bushings on it, not ball bearings. There is some runout on the crankshaft, but not much. The thing is, and you'll have to believe me on this---If there is any runout AT ALL on a crankshaft, it's going to squirm as it rotates in the bushings which support it. Consequently, the bushings will be worn away on a taper on both sides of the bushing centerline. Then it is only supporting the revolving crankshaft on a thin "line of contact" in the center of the bushing, which quickly wears away. The result of this is sloppy fit between the crankshaft and bushings. This won't prevent an engine from starting, but it will clunk and clank while running, and if your ignition cam is supported on the crankshaft, the points gap will not be stable. HOWEVER---And I wouldn't have believed it if I hadn't seen it---A single race ball bearing will tolerate a small amount of squirm without damage to the bearing or the shaft. I have found this out by my own personal experience, and will never again build an engine with bushings supporting the crankshaft. Again, on a 4 cycle engine this will not keep the engine from running. It will screw up a 2 cycle engine because a squirming crankshaft will quickly wear out the crankshaft seals and the crankcase will no longer be "air tight". That's it folks. I'm done with this. If the engine does decide to run tomorrow, I will shout "hurray" and post a video of it, and tell you all how clever I am. If it doesn't, it will go up on the shelf and stay there.---Brian
