Port Timing Numbers

[Dimitrios_231]

Can you comment on these numbers?

Exhaust 184
Inlet 165
Transfer 140
Blowdown 22

What type of use can this engine have?
Best exhaust/carb that will match this porting?

Stroke 62mm
Bore 72mm

Cheers.

[Dimitrios_231]

Forgot to add

110mm conrod
2.5mm base packing plate, plus 2 gaskets, plus 0.5mm alloy gasket
1.5mm head gasket

squish minimum 1.1mm

[Dimitrios_231]

I have closed the engine now,
strange thing is that I measured the ports 3 times.

On the first take,the transfer was 130 and with the addition of the 0.5mm alloy gasket went to 132.
I wanted to get the squish right,
didn't have a thicker head gasket so I added one at the base.

I'll have to remeasure it,
but I'm too lazy now .

[Dimitrios_231]

Measured again today the trasfer duration,
it is 140 degrees.

How much should I raise the exhaust port?
How many degrees should I aim for?

One last question,
when in BDC,
does it matter if the piston crown is further down the bottom of exhaust port?
Should it just line up with the bottom of exhaust port?

Cheers.

[Darrell Taylor]

WHAT CYLINDER DO YOU HAVE,is it std or tuned?

WHAT WOULD YOU LIKE TO USE THE ENGINE FOR?

The cylinder can be set up with a combination of settings and exhaust systems to achieve your end goal although the 4 speed gearbox in the lambretta pretty much dictates to some extent how far you can go

achieving a good squish is not the main concern as its best used as a baseline for whats more important which is the compression ratio that you end up with but ideally getting the correct compression and squish clearance will prove most effective

using base and head gasket combinations u can move the cylinder around to achieve a better set of durations for exhaust and transfers

without knowing the cylinder in use a set of durations are difficult to give as it could be a cast cylinder with 2 transfers(little transfer area) or say an rb with 6 or 7 transfers (large transfer area) so very different durations would be used

[Dimitrios_231]

It is a Rapido 250,I would say mildly tuned,
everything matched and flowed at the entries.

This is the second year that I use it,
I run it with a TMX 35,Avanti 48mm and SX200 gearbox.

[Darrell Taylor]

id go with a 3mm head ally gasket leaving 1mm squish providing the heads compression is not too high
which should work out that just the 2.5 base plate should be about right
giving 178.5 133.5 bd 22.5
then raise the ex port back to 184(approx 1.5mm) where it was before giving you approx 25 b/d if your after maximum drivability from it
ideally it needs more ex duration but thats the least amount to raise it that still retains good drivability which im presuming your wanting from your choice of a rapido 250 and avanti 48
or was you after more of a road ripper?

[Dimitrios_231]

Thanks for the input Darrell,
to be honest,
I like it as it is,
I just find it hard to jet it perfectly at low revs.
I have also used a PHBH 30.

It pulls nice from mid revs and upwards,
the Avanti might not be as powerful as other expansion chambers,
but it revs quite high.
It is also on a 60s themed lammy so an expansion is not an option.

I have used it in the past with 4.5 gearing,
but acceleration is better with 4.8.

I mainly ride in fourth as it stays easily there, and pulls strong from 3500 revs upwards.

I always need/want something extra,
but I know that I can't have everything.