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Nemet - Lynx

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Doubletop17/11/2013 09:52:56
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I'm in the process of building the Nemet Lynx from the recent "Best of Vol3". I guess that with the original articles being published in 2006 any errors didn't make kit to the online forum, or the forum didn't exist back then..

So apart from

  • the "Best of Vol 3" not having the complete set of cam tables
  • the drawings for the cam box in the 2006 version being completely wrong (not an issue in "Best of'" )
  • the stated offset of 1mm for the bearing holes in the cam box being wrong in both "Best of" and 2006 (it meant to say find the centre and offset by 0.5mm)
  • The spacing between the outer faces of the cam lobes being 19mm not 13mm as stated in both versions

Is there anything else I should be aware of?

Regards

Pete

 

 

Edited By Doubletop on 17/11/2013 09:54:27

Edited By Doubletop on 17/11/2013 09:55:24

John Stevenson17/11/2013 10:22:55
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Pete,

Can't answer these questions never having built it or studied the drawings with a view to doing so .

 

However the build up of the cam always seemed a bit of a kludge to me where everything had to be assembled inside a manhole and then the bearings fitted.

It would have made more sense to have the drive side bearing large enough so an assembled / one piece camshaft could have been threaded in and then the bearing fitted.

 

Food for though on a new build ?

 

John S.

Edited By John Stevenson on 17/11/2013 10:53:44

JasonB17/11/2013 10:46:39
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Pete, regarding the offset on the cam box which is 15mm deep, if you place the hole 8mm from one side as shown on the drawing it will offset it 0.5mm from centre line. So only the text is wrong.

Where are you getting your 19mm between cam outer faces from or the 13 for that matter. I make it 21.5 by the time the spacer has been added between the two cam lobes, though 19 would suit the valve spacing better.

J

 

Edited By JasonB on 17/11/2013 10:52:38

Traction man17/11/2013 15:40:25
37 forum posts

There is a correct table for the cam cutting, on a previous thread on this web site.

As to having to assemble tha cams inside the housing. As far as I know that is not correct, that is why the bearing outer diameter is as large as it is, so that the cams will pass through.

Traction man17/11/2013 16:44:33
37 forum posts

Correct Cam cutting table is in the thread "Best of Model Engineer 3" on page 7 posting by Katy Purvis pdf's for down load by Jason Hope this helps.

David

Doubletop17/11/2013 19:29:27
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Thanks guys I think I have most of this covered off. I'll address each on in turn so it all here for others.

John

It is possible to assemble the camshaft 'out of the (cam) box'. The cam blank is 12mm and this is machined down by 2mm to the base circle. So it ends up as 10mm to the lobe peak so will fit through the 10.5mm hole. BTW I did my hole 11mm which makes the bearing housing 11mm and a bit more robust when pressing in the bearings.

Jason

Yes its the text I was commenting on. If the builder follows the text then the hole is 0.5mm too low and the cam will probably not be able to rotate.

Spacer? There is no drawing for a spacer although the photo shows one. As you say the valve spacing is 13mm and with the 6mm cams the outer edge of the cams is 19mm. The width between the bearings is 29mm minus 19mm (not 13mm as the text) that makes the outer spigots 5mm (measure from job). With 19mm to the outer edges of the cams the inner edges are 7mm apart and therefore the inner spigots are 3.5mm as per the text. (One of our club members had started this build from the 2006 articles when he showed it to me the cam box looked all wrong. When we checked the 2006 version in the online archive the dimensions were all to cock http://www.model-engineer.co.uk/members/digitaleditions/popup.asp?t=8&i=444 Page 137 Not the case in the "Best off" )

David

Got that thanks, and copied here to consolidate the data.

On the missing cam data the direct link to the online copy of the original issue 4281 is on page 262 here.

**LINK**

 

There was one other thing that isn't an error but a change that I've made to the design. I wasn't happy with the valve seats and tappets running directly in the aluminium head. Malcolm must have come to the same conclusion with the Bobcat/Jaguar as he has designed a bronze seat /guide. There is plenty of meat on the head of the Lynx to do the same so I've copied the combined seat/valve guide from the Bobcat (to Lynx dimensions). I've also made the counter bores for the tappets 11mm vs the 10mm in the drawing and pressed in bronze sleeves. I'll do some photos later as I'm working on the valves at the moment.

Is there anything else not covered?

Pete

 

 

Edited By Doubletop on 17/11/2013 19:30:36

Edited By Doubletop on 17/11/2013 19:36:05

Doubletop13/12/2013 05:52:54
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Here are the promised pics of the valve seat inserts and tappet sleeves


They're made so the internal dimensions remain as the original design, there's plenty of room in the head to accommodate any suitable external dimensions you may choose. They are just pressed in. Take a look at Malcolm's Bobcat drawings and write up to get the idea.

Hope that helps somebody

Pete

Doubletop23/12/2013 10:29:23
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409 forum posts
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Its a runner!!

All the parts. (I've cheated a bit on the carb. It's a Chinese knock-off clone of a Walbro)

And the ignition is another knock off of a RCxel

>

A bit more work but its close

Pete

Edited By Doubletop on 23/12/2013 10:32:36

Edited By Doubletop on 23/12/2013 10:33:45

Steve Withnell23/12/2013 12:10:10
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Very nice - but you have forgotten the blue smoke emission system -

**LINK** laugh

Steve

Merry Christmas!

Doubletop23/12/2013 22:06:32
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Steve

I've got one of those installed, I think it’s an inherent part of the design. In my case the feed tank may have run dry.

I’ve now done this and the Bobcat, in both oil seems to come out of places you’d least expect.

Today is strip down and see how much oil is actually left in the sump and how much there is in the silencer .

Pete

Edited By Doubletop on 23/12/2013 22:08:00

Doubletop20/01/2014 06:43:04
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An additional note while its still front of mind. For the Lynx, and the Bobcat I built previuosly I used an RCexel ignition system They are self contained and cheap

The ignition seemed to work fine but I had starting problems on the Bobcat which I put down to being a first timer. Once I got it started it wouldn't rev as high as expected and the engine cut out. I put that down to the home made carb. On the Lynx I used a Walbro carb so didn't expect any running problems. I was wrong the performance was much the same.

In order to understand the problem I hooked up with a group on the RCgroups.com forum who are in the process of producing a similar system to the RCexel but more flexible and with more features. To cut a long story short I've got involved in helping write the code for the PIC micro controller used in the system.

These ignitions use a micro controller to measure the time for each engine revolution and then apply the necessary advance to the spark dependent on the current RPM. I have realised that all the systems rely on the the hall switch being on the crankshaft, Both the Bobcat and Lynx have the sensor on the camshaft so the microcontoller always thinks the engine is going half the actual speed and applies the wrong advance for the revs the engine is doing.. Although the hall switch is set at the right firing point before TDC, it calculates the wrong delay and fires the engine late. So although its possible to get the engine to fire (with difficulty as I found) the engine is never running at its optimum and at some point in the rev range it just stops.

If it works as planned the system we are doing on the RCgroups forum will be aware of the location of the hall switch (crank or cam) as well as enabling the user to configure their own advance curve for their engine.

So if anybody is struggling using one of these ignitions, like i did, that's the reason

Pete

Edited By Doubletop on 20/01/2014 09:34:48

Stub Mandrel20/01/2014 20:42:57
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> all the systems rely on the the hall switch being on the crankshaft,

Designed for 2-strokes?

Neil

Doubletop20/01/2014 20:47:21
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409 forum posts
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Yes but will work with 4 strokes albeit with wasted spark

Pete

alan-lloyd24/01/2014 16:15:14
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Hi bit late now but there are no errors in the cam box drawings from 2006, and there are no spacers on the cam shaft, the cams lobes are 6mm wide with bosses each side, one side 3.5mm wide the other 4.7 wide, the 3.5 bosses go together in the middle, this gives you your 13mm centres same as the valve centres, with the cams on there shaft the overall dimension is 28.4. the cambox is 45mm long with a 30mm long slot, 7.5mm each end, the bearing housings are 8mm wide therefore protrude .5mm either end into the 30mm slot, which will leave you with .6mm end float if your measurements are that accurate. The cambox is 15mm deep and the cam bearing holes offset towards the head by 1mm. Regards Alan.

Doubletop24/01/2014 20:27:31
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409 forum posts
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Alan

I agree the full set of drawings will be fine, I doubt if they would have been changed since 2006. The problem is a transcription error from the drawings to the magazine article in 2006. The cam box dimensions are all wrong. I only noticed it when a club member showed me the engine he had made from the 2006 articles. Something didn't seem right so I checked. There are also errors in the text that have been carried over to the 'Best of ME' edition.

Just a case of builder be aware, and check when working on this part depending on your source of information.

Pete

Traction man25/01/2014 08:20:18
37 forum posts

Hi Pete

At the risk of let every one know I've lost the plot. Looking at the cam box drawing in the Best of Vol 3 page 38

the cam box depth is 15mm and the offset is given as 8mm giving 0.5mm. But the text says 1mm as does Alan above.

Can you clarify which is correct. If it is 1mm then the bearing housing is only 1.5mm from the head face.

Thanks

David

JasonB25/01/2014 08:34:53
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My drawings are rev 3 dated 26/7/2007 and show the offset as 8mm giving 0.5mm from ctr line.

Traction man25/01/2014 10:30:52
37 forum posts

Thanks Pete

I must say your job is very impressive, I liked the video. Did you find that it used much oil and was that collected in the silencer. I have noted that very few small engines have oil control rings I am not sure why this is, perhaps it is to keep the piston short.

David

alan-lloyd25/01/2014 12:08:07
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157 forum posts

Hi sorry I have lost the plot, Jason is correct, the offset is .5mm offset towards the head.

Traction man25/01/2014 16:19:22
37 forum posts

Hi Jason and Alan

Thanks for the conformation and appologies for the posting mixup.

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