Squish and port timing..

[Thompo]

Hi folks

Putting a whole new top end on my mates 03 300. New pistion replated barrel of unknown year, exchange service.

Last top end on a 2t i done was a beta trials bike when i was 14 around 20 years ago lol. I was happy it worked and knew nothing of squish and port timing etc.

Roll on today and i want to do this right...
Ive fitted a single 0.5mm base gasket.
Pistion is around 1.0mm above the exhaust port at bdc.

Squish measures 1.8mm at cylinder wall increasing to 2.7mm, 12.5mm from cylinder wall.

Squish seems a bit large from my reading.

Reducing the base gasket to 0.3mm should help but will lower the exhaust port further.

Whats best compromise?

Hes new to offroad riding (in at the deep end on a 300 2t lol) so peak power isnt going to be an issue. Cutting the head isnt a option for a few months.

Cheers

 

[Jakobi]

I'd be whacking 2 x 0.5mm base gaskets in there to get the piston only 0.5mm above the base of the exhaust port. Then you'll have a nice big squish clearance; send the head to milled down to 1.3mm and have the compression ratio adjusted through chamber volume.

 

[Zman]

You can run the 0.3mm base gasket now if you do not have a ridge on the cylinder and get the head cut later. The piston spends so little time at the bottom of the stroke that the piston overhang does not matter too much for flow but the port timing change will move the power band to the lower rpms. Some people prefer that while others like the stronger top end.

 

[Jakobi]

True that Zman!

Personally I find a 300 with the power curve moved down can become a bit snatchy and responsive to small throttle inputs.. Then again, everyone knows I prefer the delivery of a 250 as well.

Was the pan to cut the head, or just find your best compromise working with what you have?

 

[Thompo]

Possibly will cut the head in the future but for now i just want to get him riding before winter hits.

I think id want to aim for as mellow a power delivery as possible especially off the bottom end to keep him out of trouble, he is unlikely to use the power band off road for a good while yet.

With that in mind i should raise the port at expense of squish to keep the hit up the revs a bit?

Thanks

 

[Jakobi]

Given what you have said,

I'd run the single 0.5mm gasket, 1.8mm squish clearance, jet it the best you can and then let em at it.

Most stock bikes have squish clearances in this range. It's not ideal but also not necessarily excessive.

It won't be too wild and if it is you can always gear some snap out of it or retard the timing a touch.