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Member postings for Jim Greethead

Here is a list of all the postings Jim Greethead has made in our forums. Click on a thread name to jump to the thread.

Thread: Jan Ridders Pressure-controlled Two-stroke engine
19/02/2011 21:10:19
Confusion all round, and all my fault. Sorry about that. I had forgotten about the earlier 2008 article but I remembered reading John Woods Forum thread.
When I had problems, I simply tacked onto the thread without checking. For some reason, I just assumed that "Pressure-controlled", "Simple Two Stroke" and "Debbie" were alternative titles for the same thing.
In regard to the name, I didn't know that the Simple Two Stroke Engine was called Debbie until John Somers mentioned it. And mine is not called Debra to make it sound superior despite John's mischievous suggestion. After I made some small changes to Jan's design of Debbie, I felt that I had also to make a small change to the name.
Anyone seeing Jan's original and John's superb and beautiful copy would agree that if they were all real people, they would not have anything to do with my scruffy and very lower class Debra as shown in the Colortune videos.
If Debra runs, maybe I will tart it up. If it runs.
18/02/2011 21:00:48
Terry - Thanks. One problem down, one to go. I will use a different browser next time.
Jim - A good thought. I don't have an adapter but the heads are so simple that I just made another one with 14mm spark plug hole. But the Colortune does introduce additional volume so I will change the heads and try it.
Jens - I will test for the "poof" when I have the head off. I get compression at the entrance to the expansion chamber but it will be interesting to see what happens at the cylinder.
Richard - You are a legend. Who would have thought of keeping the steaks cool until lunchtime with evaporating petrol and then just throwing a match in?
Overall, I am not sure how critical it is to have a perfect seal at the ball. Four stroke valves have to seal to stop them from being burnt by the hot gases but we don't have this problem here. And we have all had four strokes that continued running while waiting for us to do a valve job haven't we? A static test (by blowing into it) shows that the balls are seating very well. We just don't know if they are doing it at the right time.
Not beaten yet.
18/02/2011 16:48:06
I tried creating a link with Google Chrome. I arrived here with the target URL on the clipboard.
Then I highlighted a word like (here) and clicked on the unbroken link above.
Everything went into a light blue mist and stopped. I had to kill out using right click/back and then come into the Forum topic again.
That is what was happening before as well and I thought I was doing something wrong. Maybe I was. Maybe I still am. Maybe it doesn't work with Google Chrome.
First the engine, now this. I am not having a good time am I?
18/02/2011 06:28:12
Well, I made a new ball valve exactly to the dimensions in the drawing except that I included the extra air tube in the valve body (using the correct dimensions) and added a screw to adjust the lift of the ball.
I have uploaded photos of the new valve including the inside and some video of the result and you can find it at Despite Terry's advice, I could not figure how to give you a simple link - sorry about that.
The tank and ball valve combination works alright on my 30cc four stroke engine Bonza both with, and without the ball. But the engine faded away after about 30 seconds. The tank had become quite cold from the evaporation and found that Bonza would start again and keep running if I held the tank in my hand to keep it warm. So maybe Richard was right about the cold. But that is clearly not the only problem.
I made an adapter to fit the valve assembly to the much bigger Bonza tank and connected it to Debra. The tank did not get cold but the engine did not run any differently.
The Bonza test shows that the ball is lifting ok but, because Bonza is a 4 stroke with its own inlet valve, it does not show that the ball is seating reliably or in a timely manner.
I have now designed a poppet valve assembly but I will not have time to construct it until Monday. It will be interesting to see what happens.
I am convinced that friction is not the problem at this stage. The Colortune plug clearly shows that the engine is not firing correctly. When this problem is overcome, I will worry about friction if necessary. Incidentally, unless my maths is wrong, an engine with 24 mm bore and 24 mm stroke has a capacity of 10.0cc (or am I calculating it wrongly).
Please keep the comments and suggestions flowing, I need all the help I can get.
16/02/2011 20:33:28
We all seem to be agreed that the reason that the engine will not run unaided is that it is not firing consistently. Once this is sorted, we can worry about reducing the friction if this still causes problems.
And the most likely culprit is the ball valve. As you suggested John, I started off with a square edge and tapping the ball. This is the normal approach. But a couple of times when I sucked/blew into the tube to check the seal, I found the ball had stuck. This could be because I used a 4mm diameter hole and the ball sat too far down and wedged.
But relieving the edge with a small chamfer did not make any difference.
I did try a light spring but this did not help. Perhaps I did not pay enough attention and this might be worth another try.
I also tried an acetal ball with is lighter in case there was not enough "suck" to lift the ball high enough. This was when I was still wondering if the engine was getting enough mixture to make a big enough bang. No difference.
You and John Woods both used Jan's design without success. So how come this design works for Jan and not for other people?
Today, I will build one to Jan's design but with a screw adjustment through the top to vary the maximum ball height. I don't think my machining skills will permit me to get it right otherwise.
My other thought is that since this ball valve is doing the same job as the automatic inlet valve on my hit & miss engine (Bonza), I could use a spring loaded poppet valve. I am still playing with arrangements for this to try to get one that looks a bit elegant. But I might have to forget elegant and just make one. We shall see.
El Illegitima, nil grindem carborundum!!
16/02/2011 04:46:45
Ah! So that is how it works. Thanks Terry.
16/02/2011 04:02:41
I have now posted a photo on this forum site of the Colortune test setup if anyone is interested.
16/02/2011 02:25:48
Thank you Jens, I am glad you found the video. I took the following measurements just before I did the test:
Exhaust port opening 162 degrees around BDC
Transfer port opening 112 degrees around BDC
Compression 25 psi
Ignition timing 2 degrees BTDC
The port openings were measured on the rim of the flywheel with steady pressure on the piston crown. They are probably withing 5 degrees either way. I can't believe how many different values I measured, those are the best.
Compression is very low but the piston fits closely in the bore. One reason for the low compression is that the piston has a 6 mm radius fillet all around the crown which limits the minimum volume at TDC.
The ignition angle was measured with a timing light at cranking speed. Ignition is by RCEXL CDI which has electronic advance.
15/02/2011 21:44:47
Sorry about that, it looks like the link does not work. But it is there if you go to AussieJimG and look for Debra Colortune.
15/02/2011 21:42:41
I have now posted video of the Colortune plug test on YouTube at
I just hope this link works. I am not sure how else to do it. Comments and advice will be most welcome.
15/02/2011 09:20:37
Thank you Jan, I will certainly send an email and look forward to seeing the changes.
I spent a lot of time today trying to reduce the friction in the engine and I am convinced that it now as good as I can make it. It is still not as free-running as yours.
I also took some more Colortune video which shows that even with the correct mixture, the engine does not fire consistently. So I am wondering if the problem is the ball floating as you suggest.
I also have not fitted the exhaust to the engine and maybe that is part of the problem. I know that the inertia of the gases is important in two strokes and that the exhaust can have a big effect. So I will do that next.
14/02/2011 23:02:50
John Somer's engine is much prettier than mine. After hearing of his problems, I took a few short cuts (sorry Jan) in regard to the purely decorative parts of Jan's beautiful design. If I get it running, it might get a coat of paint.
Thank you Jan, for your continuing interest in our attempts to replicate your beautiful work. I would be really interested to know how you made the teflon bearing so smooth and yet still sealing. I took a lot of care with this but it is still not as free-running as your engine.
And I am very interested to hear about the ball valve. I assumed that it was just another version of the reed valve on two stroke engines but it seems that yours is different. Can you explain the difference please?
And it can't be the cold. Yesterday was about 23 degrees and today is forecast to be the same.
Richard: I have not previously thought of using a birthday candle like that. But I have been using a Colortune transparent spark plug that lets me see the spark and the flame in the cylinder. The flame colour varies with the mixture from yellow (rich) through bunsen blue (about right) to almost white (lean). I will try to get some video of it to put up so all can see,
I did take some video showing how free-running the engine is (or isn't ) but I hesitate to put it up because Jan will see the changes I made to accomodate my limited skills.
Thanks everyone, I will keep you posted
14/02/2011 00:45:00
Can't be the cold, it is still summer out here on the fringes of civilisation. Temperatures in the mid to high 20s. Winter, of course, is a different matter.
I keep telling myself that it can't be all that hard. After all, it is just suck, squeeze, bang and blow. And two strokes are so simple. Aren't they?
13/02/2011 10:34:29
So, John, did you get the engine to run? I followed this thread last year with a view to building one of these as soon as I finished the current project.
I have now completed the pressure controlled engine and, like you, can not get it to run.
From your experience, I built in an RCEXL CDI from the start so I know the ignition is ok. I have checked the ignition timing and it fires at 5 degrees BTDC which seems about right.
I have checked the mixture using a Colortune plug and I can adust it from rich to lean through the correct "bunsen blue" and the engine fires but will not run unaided.
It is not a free running as Jan's, but it is certainly not excessively tight - about normal for an engine in fact.
Compression is 34 psi which is lower than I would hope, but not too bad.
Can you offer any advice?
Thread: Rina and T&K drawings
09/11/2010 10:52:07
Hello Graham, I have just checked the dimensions and I agree with Stick that the CM-6 will have the required reach. It should just drop nicely into the hole when the washer is compressed to half size.
A couple of things to note about the CM-6 are:
1. The thread is M10 x 1.0 which is neither metric coarse nor metric fine so you have to ask specifically for the M10 x 1.0 tap,
2. The cap is not the usual screw top so you need a special connection. I have seen a spade terminal used (not very pretty). The RCEXL ignition (as in Part 8) has a CM-6 version with a connection that fits right over the plug if you push hard enough. 
02/11/2010 12:14:54
I understand why Jim would make the conversion shown in Fig 8 if the machine was already constructed but I can't help thinking that there might be a better way to incorporate the CDI system than by modifying the ignition lever.
Since we no longer need the lever to adjust the timing, a more unobtrusive arrangement should be possible. I will give it some thought; anyone else have ideas? 
31/10/2010 21:45:31
No John, I don' t think you sound too niggly. I agree that we can probably sort out the missing bits, given time, but some advice would be helpful.
As a newcomer to IC engines, I am very grateful to Jim Service for the effort he has put in to this project; without people like him, I would be unable to get started. And without people like you, I would probably be unable to finish.
 Your drawing of the carburettor is superb! I must confess that I have not had a close look at it but there are features in your drawing that I had not noticed.
You mentioned that details of the inlet valve are missing. Other people to whom I have spoken have mentioned difficulty (with other engines) in getting the spring tension just right. This has led me to speculate about some spring tension adjustment: maybe a threaded collar or similar. What to you think? 
29/10/2010 06:20:15
Your solution is much better than mine John, simpler and more elegant. Thanks.
I note that you have omitted the fuel tank arrangements and the muffler which were also omitted from the original drawings. I wonder if Jim Service could be persuaded to offer some suggestions for these items.
Thread: Hints and tips
29/10/2010 05:59:04
Sorry about that, I still have most of my marbles but sometimes it takes a while for them to roll into the correct holes.
Conversion to and from metric is most easily done with a conversion chart. I have one such chart laminated and conveniently displayed in my workshop. Apart from other information, the relevant column headings are: millimetres, fractional inches and decimal inches.
It is low tech but faster and much more convenient than calculation.
29/10/2010 01:23:57
To go the other way, divide the figure in mm by 25.4 and multiply by 64 to get the imperial dimension in 64ths which is probably about as small a fraction as you would need.
Then just simplify: 32/64 = 1/2 etc. You could go to 128ths the same way.
Might need to round off a bit when you get 32.2/64 it is probably 1/2 or close enough.
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