Moderate performance modification to 72 GT380?

[JFISHSOLEVIBE]

Howdy guys!

Thought I'd post a pretty general questionnaire here regarding building out my 380. Suppose I should say: general questions seeking specific answers.

My bike as it stands is stock... Was wondering what sort of things I could do to get some more power out of her.

I've read about guys taking 1mm or so off of the heads to Increase compression, and adding VM28's to introduce more fuel and air.

Has anyone got any specific numbers on the amount of material to take off of the heads for a slight bump in the compression? Has anyone achieved this result by running thinner copper gaskets?

Is there an overbore recommendation for such a "stage one" compression boost?

Ideally I don't want to get TOO crazy and would prefer to stay away from porting if possible.

Any help or guidance is greatly appreciated!!

John

[Craig380]

I ran a lightly skimmed head on my 380 for a while, I had 0.5mm removed. Sharpened up the low end and mid-range a little, so it was good for street use where you tend to spend most of the time in the 3,000 - 6,000rpm region.

To be honest, the best performance mod is a set of chambers. If you retain the stock airbox and filters, you shouldn't need to rejet much (if at all). You get a healthy boost in power and save a good few kilos in weight, not to mention the sound. The SOUND

[JFISHSOLEVIBE]

Craig you read my mind! That was my next question... Any known suppliers besides Higgspeed? Theirs certainly look nice but a little too rich for my blood at $900 American!

[tz375]

The 380 is essentially 1 1/2 GT250 twins in terms of ports and head shapes and pipes that may give you some ideas.

You could contact Jon Easton at JEMCo in TX and see if he has dimensions to make you a set. Do not take his T250/T20/X6 race chambers though. They are full on race kit pipes and not suitable for a street bike. The biggest performance increase is from the weight you lose when you go from stock pipes to chambers.

Raising compression helps the bottom end and midrange and doesn't hurt the top end at that level of tune.

Back in the day the hot ticket upgrade was a set of carbs off a GT550. You could probably use a set of 28mm RD350 carbs but you will need 1 1/2 sets of course. Bigger ports would help too, but that requires a fair amount of work or investment, but you could easily raise the exhaust port and widen it slightly.

Electronic ignition helps to keep the sparks happening at the right time and is more or a long term reliability issue than a performance boost.

All of the above assumes that the motor is in sound condition of course.

[JFISHSOLEVIBE]

Thanks man!

Yeah the motors in great shape. Rebuilt the top to stock last year. Standar rings and pistons, new pins and bearings. I have the newtronic ignition on there just looking for some more juice from my little triple. As far as I know, the 500 carbs were an iteration of the VM28's so maybe ill just pick up 3 of them and skim the heads a bit for now while I hunt for some chambers.

[Craig380]

The others making street chambers in the UK / Europe are Gibson / Allspeed (gibsonexhausts.co.uk) and Jim Lomas (http://www.jl-mxshop.com/" onclick="window.open(this.href);return false;).

Worth asking those guys, as you're in the US you should not need to pay UK purchase tax (we call it value-added tax or VAT, it runs at 20%) which would go a long way to covering the shipping costs ... however you're still looking at nearly $1000.

70s-manufactured chambers do come up on eBay from time to time ... took me a year of patient bidding until I landed a set, but they are out there.

[JFISHSOLEVIBE]

Thanks guys! Yall are all really helpful.

Craig, Ill keep an eye out to see if anything pops up. I sent an email to Jon and Jemco to see if he has a jig for the 380, as that would be ideal - his price is reasonable and hes in Texas.

TZ: Do you have any data/info on raising and widening the exhaust port as you mention? I havent done port tuning on a 2stroke before and dont wanna go at it blind. I thnk the carb upgrade may be a good place for me to start, since its essentially a bolt-on. Curious to see what level of increase id get going to the larger carb.

EDIT: Curious as to if I were to use some 550 carb as opposed to the stockers, I believe stock at the intake manifold is 28mm and the 550's are 35mm... how do I get around the sizing variation in the intake boot? Seems like that could be the only hangup.

[tz375]

Raise the roof by 2mm keeping the same shape as stock and widen the port by 2mm on each side. That port can be even taller but we need to keep it smaller for street use.

You could probably increase teh compression by machining .040" 0f the head and having new squish bands cut into the heads