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A partial rebuild over the Winter (new Wossner piston, replaced worn out Li150 gearbox with a better one, final drive altered to 4.7 17/47) has resulted in the scooter being almost impossible to start. It feels like, suddenly, the compression ratio has increased. If I take the plug out, it kicks over easily so I am fairly confident the problem is with the top end.

But, other than a new piston, which is in the correct way round, I have fitted the same 3mm packer (58/110 crank)and .75mm head gasket as was on before. So, I don’t know why it is so hard to kick over and almost impossible to start as a result. The squish is 1.4mm.

That’s why I have, for the first time ever, done a calculation as to the swept compression ratio. By my calculation, it is 10.15 which, as it is an AF Mammoth head, is in keeping with what it is advertised as.

Is a swept compression ratio of 10.15 likely to be my issue?

Just to add, previously it had an 18 front sprocket.

Downsizing the front sprocket will make it more difficult to kick over especially if you have a light flywheel.
Change it back to an 18 and see what difference it makes and go from there.

That is a good shout. Thanks. The flywheel is a mid weight Indian. Thinking about it now, I also changed the flywheel and stator to ones I had on the shelf and not off the previously running engine. I suppose, potentially, the flywheel could be a bit heavier but both flywheels were mid weight ones.

My problem with that is that the gearbox and exhaust I have (Li150 and Ron Moss Avanti) is better suited to a final drive ratio of around 4.7 and not the 4.4 I had previously. That 4.4 was a good match with the CST3 I had fitted a year or so back.

I do not especially want to go back to the CST3 but I wouldn’t discount this entirely.

Since my post, I have measured the corrected compression ratio as best I can with the engine still in the frame and using an online calculator. I don’t entirely trust my measurements as being flawless but it is indicating that my compression may well not be too high.

As the crank is a 58/110, I am going to have a mess around with cylinder base and head gaskets and squish in the next week, or so. I could make the compression worse by going down that route but I’ll have a better understanding of where the issue lies, hopefully.

Just out of interest..... What is the measured volume of your combustion bowl?
I assume this is a 70mm piston?

Yes, 70mm piston. 31cc but, as I say, there were differences depending on whether I used two stroke oil or diesel or tried to use eyesight to gauge the surface tension of the fluid rather than “seal” the chamber. 31cc was the sort of figure I got most of the time.

31cc would be low compression!

Quick geometric check
..
(31 +225) ÷ 31 = 8.26:1 geo, which will only become a smaller number when calculating the CCR (corrected).

If you're measuring just the combustion chamber on the bench then allowing for squish volume would also reduce this geo figure.

How are you trying to measure the volume? Not the whole area incl squish band I hope as that's virtually impossible to do by eye on an upturned head.

IMO better to measure combustion volume only, then calculate the squish that you're looking for when assembled... adding this as volume of a cylinder to the calcs.

Measuring assembled is, without doubt, the best way to do it but the repeated assembly, measure, clean, rework head, repeat, repeat is tiresome. I'd rather work and measure the combustion chamber until I have the required volume to make the calcs give me the GCR and CCR that I'm looking for... seems to have worked ok for me over the years.

Seeing your 31 figure, I'd want to revisit your measurements or calculations.

BTW - If you're measuring combustion chamber but not taking squish volume into account then this could be where you're going wrong... e.g.


(223 + 31) ÷ 31 = 8.2:1

... but add in 1.5km squish 5.77cc at 70mm

(223 + 31 + 5.77) ÷ (31 + 5.77) = 7.06:1

... both geo figures are relatively low (first ok for tourer) and calculating corrected should give a smaller number still.

I used Eden’s calculator in this last calculation of corrected compression but added the squish to the length from the top of the exhaust port to the top of the barrel. That got me this:





https://lambretta-images.com/tuningh/po ... alculator/

I would agree with Adam's calc's resulting in a 7:1 ratio, which is very low. The corrected ratio (which is what you'll feel when trying to start the engine) is likley going to be around 4:1, which should take no effort at all to kick over.
31cc for the combustion bowl (excluding squish) is a huge figure. What method did you use to arrive at this number Paul?
I can't see how you arrived at a figure of 10.15:1 for the CR.

To be right Paul.....you need to add the squish volume to the combustion bowl volume. Don't add it to the swept volume.
So.. in your case, with your squish height of 1.4mm, this will add an extra 5.4cc of volume to the combustion zone, making the total 36.4cc

Eden's port calculator works out the same 8.2:1 as my geo calc figure then, with CCR lower too as I mentioned it would be... neither close to the 10+ that you mentioned.


Using any of these new figures suggests that compression is far from 'high' so I would look for other issues if it is hard to kick and/or feels high.

I used a 24cc medical syringe that I found at home. I filled it with diesel. I made an adjustment of 1cc for the fluid in the plug hole. Given that it is a medical syringe, I’d have thought it was fairly accurate itself but, as I have said, I did have difficulties.

My first swept compression ratio was calculated using the head of the piston sealed against the head with plasticine, and making an allowance for squish, as per the way steelweasel did it on his YouTube video “TS1 Part 12 Head Volume.” The head volume doing it that way was 22cc.

What was the combustion chamber volume that you measured, without an allowance for squish?

Clamp the head in a vice so its plumb and level.
Fit the spark plug.
Fill the bowl with some liquid (It can be virtually anything that isnt too viscous)
Carefully place the piston plumb onto the cylinder head squish band and allow the excess liquid displaced to overflow.
Clean up your spills and then carefully remove the piston allowing any drips to drop back into the combustion bowl.
Withdraw all the liquid using a medical syringe (I use a 5cc one that used to refill my printer cartridges) and measure as you go.
Add 5.4cc to your final tally and this is your total combustion volume.

Adam_Winstone wrote:What was the combustion chamber volume that you measured, without an allowance for squish?

31cc which does appear at variance to the 22cc when the piston crown and squish was brought into the equation.

It does go to show how important it is to have a method of measuring and calculating that you're happy and familiar with.

In which case the figures are still not suggesting that high compression is likely to be your issue.

Fast n Furious wrote:Clamp the head in a vice so its plumb and level.
Fit the spark plug.
Fill the bowl with some liquid (It can be virtually anything that isnt too viscous)
Carefully place the piston plumb onto the cylinder head squish band and allow the excess liquid displaced to overflow.
Clean up your spills and then carefully remove the piston allowing any drips to drop back into the combustion bowl.
Withdraw all the liquid using a medical syringe (I use a 5cc one that used to refill my printer cartridges) and measure as you go.
Add 5.4cc to your final tally and this is your total combustion volume.

Thanks for that. I’ll see what I can rig up

Adam_Winstone wrote:It does go to show how important it is to have a method of measuring and calculating that you're happy and familiar with.

In which case the figures are still not suggesting that high compression is likely to be your issue.

After I have measured the head volume as FnF suggests, I may well just put it back together virtually as it was (the only material difference will be the insertion of a 3mm packer on the standard TS1 Reed block).

If it still is ridiculously hard to kick over, I’ll put the 18 front sprocket back on with the CST3.

Failing that………

You could also just put on some extra weight and wear heavier boots Paul.

Fast n Furious wrote:You could also just put on some extra weight and wear heavier boots Paul.