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 Post subject: GT750 tuning info
PostPosted: Mon Jan 09, 2017 2:32 pm 
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Around the block

Joined: Thu Dec 05, 2013 12:32 pm
Posts: 82
Country: Austria
Bikes owned: GT750
Hi, in my GT750 BIMOTA replica (see separate thread) I am planning to use the following engine setup:

- Close to TR-spec engine (exhaust 34mm, transfers 49mm, 3.3mm spacer+2nd base gasket
- piston skirts shortened 3mm at intake side
- head milled 1mm, 1mm copper head gasket
- close ratio gearbox
- VM34 carbs with individual pods
- JEMCO pipes

I wanted to go for a cruiser type of bike, not a racer, but I had that spare TR spec engine laying around. I am concerned I might get a pretty peaky engine now, especially with the pipes only being 91mm in the mid section which is really small for 250cc cylinder volume.

I hope some of you have tried a similar setup and can share their experience with me. In worst case I might still try to find a stock cylinder and run close to stock porting. I definitely want to keep the JEMCO pipes since the look exactly right for that bike.

Hoping for your comments

Wolfgang


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 Post subject: Re: GT750 tuning info
PostPosted: Mon Jan 09, 2017 5:06 pm 
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To the on ramp

Joined: Mon Mar 05, 2012 1:22 pm
Posts: 234
Location: Wales UK
Country: Wales
Bikes owned: RG250 GT750L GT750 DRAG seelyt500 tr500MK3
It may well be a bit peaky,My concern would be that the first gear would be to tall for street use.


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 Post subject: Re: GT750 tuning info
PostPosted: Mon Jan 09, 2017 11:36 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5982
Location: Illinois
I like that plan, because it will be very race replica. Funnily enough my thoughts on the transmission are the other way around.

All GT750 gear clusters have a huge jump from first to second which really hurts acceleration unless you over rev it in first or let it drop in the hole. With a stock motor that isn't much of a problem but with a tuned motor the gap becomes more obvious. The cure for that is a higher first gear and for the street the next issue is first too high and it is until the overall gearing is dropped with a larger rear sprocket.

My bike is mildly tuned and nowhere near Wolfgang's proposal and with a 15-50 gearing it hits over 111 at the end of a quarter mile. With a slightly taller gearing, it would be snappy up to 120 and easy around towns and country roads but not well suited to a drive up the autobahn or motorway for hours on end.

One complication with the proposed porting package is that JEMCo pipes are really not designed to rev to where that exhaust duration suggests it should go (8000 or more). They peak closer to 6,500 and the ports. It will work OK but the small diameter of those pipes will restrict top end. So starting with the end in mind and using JEMCo pipes, how about an exhaust open timing of say 90-92 degrees ATDC and with 30-32 degrees of blowdown as a place to start?

Lane (rngdng) has a similar set up to what you are planning. He has 34mm carbs JEMCo pipes and a 3mm (I think) spacer plate and his bike runs just fine. There are many paths to the same end point. :wink:


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 Post subject: Re: GT750 tuning info
PostPosted: Tue Jan 10, 2017 4:51 pm 
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To the on ramp

Joined: Mon Mar 05, 2012 1:22 pm
Posts: 234
Location: Wales UK
Country: Wales
Bikes owned: RG250 GT750L GT750 DRAG seelyt500 tr500MK3
Maybe my use of the word to tall has been misunderstood,
My meaning is exactly as you describe,The gap between 1st and 2nd is to big.


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 Post subject: Re: GT750 tuning info
PostPosted: Fri Jan 13, 2017 5:30 am 
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AMA Superbike
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Joined: Tue Oct 14, 2008 6:47 pm
Posts: 1722
Location: Blythewood, SC, USA
Yes, that set-up is pretty close to mine. I believe my spacer is 2.75mm, I took 3.5mm off the top of the cylinders, and 3mm off the rear skirts. I also cleaned up all of the ports. Running 34s with Jemco pipes, it revs to 8K, which is fine because GT750 pistons don't like to rev much higher. I run 14/45 sprockets as I am geared for The Dragon.


Lane

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If you stroke it more than twice; you're playing with it.

Too many bikes, too much time, ENOUGH SPACE, FINALLY! Never enough money.........


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 Post subject: Re: GT750 tuning info
PostPosted: Fri Jan 13, 2017 6:24 am 
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Around the block

Joined: Thu Dec 05, 2013 12:32 pm
Posts: 82
Country: Austria
Bikes owned: GT750
Hello Lane,
sounds interesting. Do you happen to know your exhaust timing or distance from exhaust port upper edge to TDC or cylinder deck? The spacer alone does not give the "radical" TR timing that my cylinder has. How is your low end grunt compared to stock engine - is it OK for relaxed street surfing?
I still have a bone stock barrel which I could rebore and prepare instead if my engine turns out to be too agressive.

Wolfgang


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