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 Post subject: gt380
PostPosted: Tue Jun 12, 2018 5:23 pm 
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Still in the Driveway

Joined: Sat Jun 02, 2018 5:20 am
Posts: 1
Country: United States
Bikes owned: gt380 gs550
Hi everyone, this will be my first 2 stroke build, I currently build performance 4 stroke engines, mostly doing stupid head tricks... anyway, I'm looking for more power from the engine without going too overboard. It's a piston port with a pretty mild temper, I have no problem altering the port timing and sizes and building expansion chambers won't be hard for me, I am however, trying to find a reed valve I can adapt to it, the cylinder studs are pretty close together and I'd like to eliminate as much dead air between the piston port and reed valve as possible, anyone have suggestions? I heard yz85 valves might be a good fit, but I don't want to keep ordering different valves to finally find one that works.


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 Post subject: Re: gt380
PostPosted: Thu Jun 14, 2018 1:16 am 
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Expert racer

Joined: Mon Dec 01, 2008 2:52 am
Posts: 1109
Location: Manchester, UK
Hi and welcome.

As you say, the 380 is a very mildly-tuned engine, relying on the heavy crank and modest porting for torque. From my own experience, just shaving the cylinder heads by 0.5mm / 20 thousands and fitting chambers makes a BIG difference to top-end power without affecting the sweet mid-range.

The stock Suzuki pipes are as heavy as lead and only have flat baffles inside - no cones for resonance - so chambers make a BIG difference. I had 70s J&R chambers which are similar to Bassani pipes - long and relatively slim in the belly. I also cut the one-piece head into three individual heads, this totally cured annoying, persistent weeps from the head gaskets.

One thing that is worth looking at is the exhaust port shape that Suzuki added to the GT250A and B models. They added supplemental exhaust ports either side of the main port, to speed up blow-down without risking the rings getting snagged. I've got some porting tips on my work PC which I'll post later.

It may be worth trying all that stuff first, and adding a set of 28mm carbs from a GT550, and see how it goes before getting serious with adding reed valves :up:

_________________
1976 GT380 - wounded by me, and sold on
2006 SV650S. It's got cams, and valves, and stuff


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