Brian builds Atkinson Engine
- Thread starter Brian Rupnow
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Good show so far Brian. 
At least it's running for a period of time. That means your real close to getting it. I can't offer any suggestions as I don't have any experience with these sorts of engines. But, I know that you'll figure it out.
Todd
At least it's running for a period of time. That means your real close to getting it. I can't offer any suggestions as I don't have any experience with these sorts of engines. But, I know that you'll figure it out.Todd
The compression is improved a lot, now that the engine is back together. Re-lapping the valves with my valve lapping machine did help a great deal. While I had it apart, I used some gasket sealer on the cylinder head and intake gasket. I have had the engine running again on its own, and the runs are getting much longer. Not quite video worthy yet, but getting very close.
I am rather at an impasse right now. The engine has vastly improved compression after re-lapping the valves. I removed the Traxxas carburetor, because although the engine would fire and run for a short time, it wouldn't stay running, and I had no good way to tell if the carb was set too rich and was flooding the engine out. I went back to my vapour carb set-up, and now the engine fires consistently and happily when turned with the electric drill, but it still won't stay running when the drill is removed. This is the maddening time of engine building, when everything indicates that the engine should run, but it won't. I have advanced and retarded the ignition timing, but really, it works best at the 185 degrees from TDC just as Jan calls for in his plans. I know that the exhaust and intake cams are correctly positioned and they are built to Jans specifications. I have a few tricks up my sleeve yet. The gasoline I have been using is mixed at a 40:1 ratio with 2 stroke oil for my chainsaw, and is about 6 months old, so the volatiles in the fuel may not be what they should be. First thing today I will try it with new, unmixed fuel. The sparkplug does not protrude into the combustion chamber, but sets in a slight recess in the sparkplug hole. that didn't seem to make any difference with my other I.C. engines, but I may try and find a sparkplug with a longer nose to get the spark closer to the combustion chamber. I want to take a set of feeler gauges and check the clearance between the push-rods and the end of the valve stems. Those push-rods were a tad too long and were holding the valves open by a few thou, and when I shortened them to get some clearance to ensure that the valves closed all the way, I may have shortened them a bit too much. If all of that doesn't result in a running engine, I have one other trick left, but I don't want to resort to it. The engine was originally designed to have a 25mm bore. That is 0.984". I had a .875" reamer, so I built the engine with a 0.875" bore. I don't see how that could make any possible difference, but I am going to have a running engine here, come Hell or high water!!!
Okay--It wasn't the gasoline. Put in some brand new gas this morning. I rechecked the ignition timing by holding the coil wire in one hand, setting the timing template I made up, and rotating the flywhell by hand. When my hair all stood up on end and I lost control of my bladder, I knew the coil had fired. I checked to see where my timing mark on the flywheel was in relationship to the hole drilled thru the timing template at the 185 degree mark, and to my surprise the spark was occuring about 15 degrees in advance of when it should have been. I reset the timeing to a point where it was "right on" the 185 degree line. The engine liked that even more, and began firing even more consistently as it was rotated with the drill.--Still not running by itself though.
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I feel your pain, Brian, I know how frustrating this part of the journey can be.
I doubt that your problem is caused by anything in the ignition. It sounds like it simply doesn't have enough power to keep running. That either means too much friction, not enough fuel/air, or not enough compression or a combination.
I calculate that the displacement of your engine, with the smaller bore, is about 79% of Jan's original design. The might be exacerbating your problem but is probably not the single cause.
For all the beauty and simplicity of the vapor carburetor, I could never get my plumbing parts engine to really run well with it. I think the right carburetor would get your past your problems, but I'm guessing the Traxxas is too big. I finally built a carburetor that really works well with my engine. I plan to post a drawing of it at some point and will do so sooner rather than later if you're interested.
Chuck
I doubt that your problem is caused by anything in the ignition. It sounds like it simply doesn't have enough power to keep running. That either means too much friction, not enough fuel/air, or not enough compression or a combination.
I calculate that the displacement of your engine, with the smaller bore, is about 79% of Jan's original design. The might be exacerbating your problem but is probably not the single cause.
For all the beauty and simplicity of the vapor carburetor, I could never get my plumbing parts engine to really run well with it. I think the right carburetor would get your past your problems, but I'm guessing the Traxxas is too big. I finally built a carburetor that really works well with my engine. I plan to post a drawing of it at some point and will do so sooner rather than later if you're interested.
Chuck
Thanks Chuck---does anybody out there have access to a sparkplug thread chart? I am currently using an NGK plug #CM-6 with an M10 x 1 thread. It is about 1/8 to 3/16" shorter than I like down at the business end. I need to source a sparkplug with a longer thread, but which is still M10 x 1
ninefinger
Well-Known Member
Brian, one quick thought. Are you still using the check valve in the fuel tank? Try hooking up a different fuel tank to the Traxxas carb or remove the check valve for the vapor carb and replace with a screen. I suggest this cause it sounds like the engine is running on the "prime" and when it is consumed it stops..
Good luck. Looking forward to the run video.
Mike
Good luck. Looking forward to the run video.
Mike
When I was trying it with theTraxxas carb I was using a seperate fuel tank with a line directly from the carb to the tank. Incidently, regarding size, I have the same Traxxas carb on my Webster which has the same 7/8" bore as this Atkinson, and it works fine on the Webster.
AussieJimG
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Jan seems to have a special talent for minimising the friction in his engines which then run on very low compression (at least I think that is what happens). I could never get my flame licker to run freely as his did and my version of his Debbie two stroke would not run on its own with the compression ratio from his drawings.
After much frustrating experimentation like you have been doing, I increased the compression ratio to increase the power and suddenly it burst into life. I think I might have overdone it but, as they say, "If a job is worth doing, it's worth overdoing" so I am not going to change it now.
Since it fires consistently when turned with the drill, and it takes (at least some) of the load off the drill, the problem seems to be lack of power to overcome the friction. You have optimised the spark timing, varied the fuel arrangements, checked the valve timing, minimised the friction and ensured that there are no leaks from the cylinder.
Is it worth building a new piston to increase the compression ratio?
Jim
After much frustrating experimentation like you have been doing, I increased the compression ratio to increase the power and suddenly it burst into life. I think I might have overdone it but, as they say, "If a job is worth doing, it's worth overdoing" so I am not going to change it now.
Since it fires consistently when turned with the drill, and it takes (at least some) of the load off the drill, the problem seems to be lack of power to overcome the friction. You have optimised the spark timing, varied the fuel arrangements, checked the valve timing, minimised the friction and ensured that there are no leaks from the cylinder.
Is it worth building a new piston to increase the compression ratio?
Jim
I've been running around all day with good wife picking out new yard lights, haven't had much time to play engine. I had to buy a new set of feeler gauges as I couldn't find the ones I bought 40 years ago. In checking the gap between the push rod and the end of the valve, I have about .015 clearance. This is too much, and in addition to not opening the valves far enough, it has some bearing on the valve timing as well. I doubt that in terms of actual distance, the valve lapping changed things by more than .001 or .002". The head design does "follow the plan" but this engine is not a high compression engine anyways, so I am not sure of the net effect of reducing the bore by 1/8" from the original specification. I will keep fixing perceived shortcomings untill everything I can think of has been done. Tomorrow I build new pushrods to cut that clearance down to .002-.005" and see what that does.
Well, for the nonce, I've ran out of ideas. This morning I made new valve pushrods and was able to hold a clearance of .002" between the end of the valve and the end of the pushrod when the pushrod was not up on the cam. All my pivot points are on sealed ball bearings, so I have very little friction anywhere in the system. The valves are sealing well, and I have good compression. I rechecked the valve and ignition timing this morning. The engine fires when turned over with an electric drill, but it just doesn't want to "take off" and run on its own. I have advanced and retarded the ignition, with no real difference in how the engine performs (or fails to perform!!!) I have tried both the Traxxas carburetor (which performs just fine on my Webster engine with the same bore) and the vapour carburetor, and the results are about the same. The engine fires more consistently when using the vapour carburetor, but still won't take off and fly on its own. I am going to call NGK in the morning and see if they have a different sparkplug with an M10 x 1 thread which has a longer nose to get the spark out farther into my combustion chamber. I did a quick calculation, and my reduction of bore size from 25mm to 7/8" (22.22mm) makes for an overall reduction in cubic inches of about 26%. a 25mm bore with a 1.66 stroke has a volume of 1.26 cubic inches. A 7/8" bore x 1.66" stroke has .998 cubic inches. If nothing else transpires, I may bore the cylinder out to 1" (25.4mm) and make a new larger piston. I don't know if reducing the diameter of the bore has anything to do with my current plight or not.
110samec
Active Member
I'd say you should bore it out. It seems quite a power reduction. Been following your progress with interest and must say its looking fantastic
Sam
Sam
Jerry--As far as I know, there are no Pep Boys in Canada.--At least not in my part.-- I will call NGK tomorrow.-Brian
A little insight into what went on today---The Atkinson is a strange beast, one of the strangest things being that it has two different stroke lengths due to the mechanism which replaces a crankshaft. On one stroke the piston travels right up to the end of the cylinder closest to the cyl. head. On the next stroke it only approaches to within about 3/8" of the cylinder head. Now on every i.c. engine I ever worked on before, you always set the piston at top dead center, and went on from there for all the engine timing. Jans instructions start off by saying to "Start off at 0 degrees upper position of the piston" and goes on from there. and I blindly followed right along. It wasn't until sometime this afternoon when I was trying to figure out why things wouldn't run that I had a sudden epiphany---if I start out from the top dead center closest to the cylinder head, and the spark doesn't occur until 185 degrees into the revolution of the flywheel, then Damn---I was getting the spark at the top dead center position farthest away from the cylinder head, which would mean it was firing on the stroke that gave the lowest compression!!!---So---I started over, beginning with the top dead center position farthest from the cylinder head, so that 185 degrees into the flywheel revolution, i would be firing at the top dead center with the highest compression.--Of course this involved resetting the intake and exhaust valve cams as well as the ignition cam. This excited me a great deal, as I expected a runner as soon as I made the changes.--And it DID run much better. Just not enough better to keep running. This being the case, the only possible answer I can think of at the moment is that with even higher compression (which would be achieved by opening the cylinder bore out to 1" diameter from the current .875") it would develop enough "Oomph" to keep the flywheel turning and keep the engine running.---By the way---Tried the propane bottle trick. Same result as on gasoline---It fired but wouldn't keep going. Of all the things I've tried, my longest runs have been with the Traxxas carb and my new timing.
techonehundred
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Here is a link to a NGK plug chart
http://www.ngksparkplugs.com/docs/racingspecs2010.pdf
http://www.ngksparkplugs.com/docs/racingspecs2010.pdf
AussieJimG
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You will get greater "Oomph" with a larger bore because the volume of fuel ingested will increase. But unless I am missing something, the CR will not change. I think the calculation is:
(d+s)/d where d is the distance from the piston to the end of the cylinder when the piston is at TDC and s is the stroke.
both d and s vary as the square of the bore.
Jim
(d+s)/d where d is the distance from the piston to the end of the cylinder when the piston is at TDC and s is the stroke.
both d and s vary as the square of the bore.
Jim
