Some fairly dumb timing and engine setup questions.

[tippo88]

Unless you dowel the head so that it aligns correctly every time, the squish will vary as the head is free to float a little, only being aligned by the cylinder studs. As such the difference of 1.5 to 1.3mm is perfectly in the typical 1.0 to 1.5mm range (I tend to aim for 1.5mm) and that's fine.

Whilst we're talking about the head, don't assume that the volume is correct for the compression ratio to suit your motor and/or run reliably on today's poor quality fuel. Many of the smallblock kits come with a head that started as a 150 head and have simply been opened up to suit the larger piston, whilst the combustion chamber is still the 150cc size (!). You should never assume that the head that comes with a kit is actually suitable, which is a shame, and you should measure the volume of the combustion chamber PLUS the volume of your 1.4mm (average) squish band, which combined will allow you to calculate the geometric compression ratio. You should be looking for a figure of about 8:1 geometric, which would then give you a reasonable correct compression ratio with your intended port timings.

Have a look on Lambretta Images website and you will see that Eden has kindly designed and shared a Lambretta Port Timing Calculator, which will allow you to calculate compression ratios (geometric and corrected) as well as check port timings.

Good luck,

Adam

Thanks Adam, I got a 4 mm base spacer and had the cylinder top skimmed 1 mm and transfer ports lowered 1 mm.
At bdc piston sits level with bittom of transfers and top ring is beliw the bottom of exhaust.
This gives me port timing of
exhaust 171
transfer 135
blowdown 18.24
inlet 140

With no head gasket i get
squish .75 one side and 1.33 the other
compression ratio 8.75 uncorrected 5.74 corrected
With head gasket
squish 1.45 one side 1.95 the other
Compression ratio 8.05 uncorrected 5.32 corrected

Am thinking of going with the head gasket as best setup ir woukd it be ok and more powerfull without?

Cheers

Tippi



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[Adam_Winstone]

With the head gasket gives you the squish that I would look to use. The only drawback about using a head gasket is that it introduces one more thing to fail. Whenever possible, I would always look to run without a head gasket (as per Vespa P-range) as I've suffered plenty of head gasket failures on various motors over the years. Taffspeed used to actively promote the use of 'no head gasket' on their website as TS1 owners were regularly suffering head gasket failures.

NB: Building it with a head gasket should be fine for a reasonable length of time so this may be the way to run, test and evaluate this engine. I certainly would not build it without = 0.75mm!! Nor would I look to add to the base to change adjust the squish as 135 transfers is already very high, especially for the other port durations.

Adam

[tippo88]

Thanks again Adam. Will report back once its running.

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[tippo88]

The last question, I hope, is what exhaust to use with this setup. I'm using a pwk 28/30 carb. Ideally want to keep looks standard but would sacrifice looks for significantly better performance /price balance.
I don't want to spend big bucks on the exhaust and have been looking at these options :
Available off the shelf in Malaysia
KBA/AF Rayspeed clubman
Scootrs stainless expansion
Taffy copy by Saigon Scooter Centre
Have also been considering :
Scootopia Ancellotti clubman
Original tino Ancellotti clubman
Or something else?
The easiest thing would be to chuck on the KBA clubman but would there be a significant difference in performance compared with on of the Ancellotti's ?

Thanks
Tippo


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