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I have just built a 240 lump, it’s using a wiseco h2 piston, mugello/Monza central squish head, squish is about 1.1mm

It starts well but takes a real swing on the kickstart to get it over the compression stroke, I can literally stand on the kickstart and it won’t go past tdc..

I’m thinking compression is too high and as well as a pulled hamstring it’s going to end up with a hole in the piston.. any words of wisdom anyone can offer to support or debunk my theory or suggestions welcome..

Cheap option is put a thicker head gasket on it and raise the squish or dearer option is take it too a tuner who can lower the comp on the head

I assume it kicks over no problem with the spark plug removed?

Thanks
Yep I’m thinking replace or alter head.. or perhaps slightly large squish, don’t want to go over 1.2-1.3 really though..
And yes turns over ok with no plug in..

Squish is a very small factor in compression, rather, you need to consider the volume of the combustion chamber first. Squish volume, plug void and other elements will influence compression ratio but combustion chamber is the biggest factor.

Once you know the volume of the head you can start to calculate geometric compression ratio, and then bring the exhaust duration into account and calculate the corrected ratio. Many people will only calculate the geometric, as it is easy, but corrected is something that really needs considering when dealing with a tuned motor. Thankfully, there are a number of websites that you can use to help (e.g. Eden's Lambretta Images website) and this will make your job a lot easier.

Good luck getting this sorted.

Adam

thanks, better get myself a burette..

Just trying to sort the very same problem myself on the J Range 230. My squish is 1.1 also. Going to try a thicker head gasket first.

Rich'

Going on from Adam's reply, I think you will need to know what the port timings are at as the height of the exhaust port plays a major factor in the corrected compression ration...think this is also referred to a 'blow down'?

I’d have thought that without getting to carried away with combustion chamber volume that if you have a squish of 1.1 I’d add a 0.3mm head gasket either instead of not having one at all, or as an addition to the one you already have then test the compression pressure which should be somewhere between 110 and 125 psi.
If it’s still no good then look at combustion chamber volume.

I have a Casa 210 on a 200cc engine (big brother to the 185 kit)I had the same problem,it wouldn't kick up without me worrying that I was going to either break the crankcase side or the teeth on the kickstart shaft.In the end I put a larger cylinder base gasket on (it doesn't use a cylinder head gasket )and I have been running it for over a year now without a problem.

if its a 240cc, how did you get it too that?
presumably 71mm dia x 60 stroke?
1.1mm squish is minimum for a 58 stroke crank when warmed up, please read taylors tuning on cranks / squish featured in scootering a couple of months ago, you might find that its closer to 1.2mm min for a 60 stroke crank
being in tooooo many laybys when the pistons tw*tted the head

2 strokes, are by design, a variable compression engine. It's largely the action of the exhaust /expansion chamber that affects this.
Static compression is calculated as SV+CV/CV. Where SV = Cylinder Swept Volume and CV = Head Compression Volume.
For arguments sake:- Assume SV = 240cc and CV = 25cc.
240+25/25 =10.6:1
10.6 x 14.5038PSI (1 atmosphere of pressure) = 153.74 PSI.
153 PSI is a figure you would commonly measure in a 4 stroke engine. NOT a 2 stroke. Why?

The 2 stroke only starts to compress its contents once the exhaust port is closed when carrying out pressure tests.
So, again for arguments sake:- Assume the exhaust port duration = 180 degrees, therefore there is now only the other 180 degrees left available of compressible volume.
So, this effectively means the engines usable compressible volume is only half the Swept Volume = 120cc.
120+25/25 = 5.8:1
5.8 x 14.5038 = 84.22 PSI.

Using most 'off the shelf' compression testers will introduce an error here because you have to consider the fact that the plug hole adaptor, the connecting hose and the Bourdon tube within the gauge itself has volume. This volume becomes part of the Compression volume of the head. So, your 25cc head is now maybe 26cc? The reading you get will be lower than it really is..........
For this reason I use an electronic 20 bar Peizo sensor in the plug hole and measure the voltage output with a bit of software on a laptop (Anal.... but damm accurate)

Squish isn't really associated with stroke. It is however associated with conrod length. Assuming the use of a good quality conrod, then 1% of the conrod length is a good guide to minimum squish measurement which would be 1.07mm for a standard length rod.

thanks to all for responding, that's what I like about this forum (and is difficult to replicate on a facebook or WhatsApp etc group)
you ask a question, you get a number of replies, some practical, some theoretical and some food for thought and you can look it up late and other people can search on it..

I now have a few theories of my own as a result of the above I shall be pursuing when its warm enough to go back in the garage..