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Discussion Starter #1
I picked up a 76 GT750. The engine was froze so it is now towrn down for the overhaul.

2 Questions at this point.

1. Should I go with stock oversized pistons or aftermarket? I am going for speed and durability.
2. Looking for a good machine shop near Dallas Texas that can do cylinder boring and port mods?

Thanks,

2Stroke
 

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You should wait until you get inside the motor and measure it before you can really make a decision on oversize pistons, unless you are doing that to increase displacement and that really isn't an ideal way to make more power in most cases. Best practice has always been to oversize things as little as possible so you can blow it up again and still go to the next size up before you start replacing barrels.

as a side note: there are cylinder coatings that can be applied to old barrels now using technology that did not previously exist.

+ I bet you could polish up the ports yourself, but it's a piston port 2-stroke, so putting reeds on it would be a far superior performance modification imho.
 

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Discussion Starter #3
Thanks TrialsRider. I have the engine apart now and the pistons are 70.5. My next step is to find out what pistons others have had good results with. That is a good though on the reed valves. Any information along those lines would be good too.

2-Stroke
 

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Thanks TrialsRider. I have the engine apart now and the pistons are 70.5. My next step is to find out what pistons others have had good results with. That is a good though on the reed valves. Any information along those lines would be good too.

2-Stroke
If there is no measurable wear on your cylinders the best pistons are the ones that came with the bike, if the cylinders are worn and scored you are going to need to over-size the barrel. If you are still thinking about boring the cylinders larger to gain horsepower :/ you are setting yourself up for a disappointment, one or two oversizes on a 2-stroke piston isn't going to buy you any appreciable performance increase.

I know of people that have fitted a reed cage and larger Mikuni carb onto Bultaco motorcycles with some success, still using the stock barrel and round intake hole. The modification won't buy you much top end horsepower but reeds can be made to improve low and midrange power considerably. Because the motor does not operate as a piston port 2-stroke after the modification, builders are free to modify the piston skirt with additional holes (windows) to promote better scavenging.


.... you can't have those extra holes in the piston if you need the piston skirt to close off the intake port, that requires a port valve (reed or rotary disc valve)

Expansion chamber exhaust will always be the best way to yield crazy 2-stroke horsepower gains, but the bike becomes race only real quick because of the noise. Cops would have no problem following your sound and smoke trail.
 

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Discussion Starter #5 (Edited)
Hey TrialsRider Thanks again.

I still torn between making it a street bike or a track bike (Would be my first). The expansion chambers I have are the TR style that goes through the frame. The engine was frozen when I got it and has some pitting on one cylinder. It has to be bored. So I have been looking at Stock, Wisco and Woosner's, At almost $400 I don't want to make a mistake in getting something I am not going to be happy with. Here is my plan so far.

1. Get the crank rebuild and balanced.
2. Get the Cylinders bored and whatever else I decide to do to them.
3. Replace the points with a SS ignition.
4. Replace coils with Dyna's and wires.
5. Jet the carbs with Six Sigma kit.
6. Fit GS1100 forks and swingarm to frame.
7. Clip on bars.
7. Pods.
8. Make a seat to replace the UGLY LARGE King and Queen seat that came on it.

103607

103608
 

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Discussion Starter #7 (Edited)
Very nice. Some day in about 20 years I may get there. You just don't find TRs anywhere. So maybe it will be my target the hard way (GT to TR).

What type of frame is that?

I am sure it would cost more than I am willing to spend. I know it is great advice though.
 

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I wouldn't put reeds on track race bike either, you're going to have it wide open throttle most of the time, low and mid-range is going to do something for street riding and only if you can make if work. OP has to decide if he's building a street bike or a track bike, something tells me he's not a racer.
... and ya the stock GT frame and suspension was trash for racing.
 

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Discussion Starter #11
I wouldn't put reeds on track race bike either, you're going to have it wide open throttle most of the time, low and mid-range is going to do something for street riding and only if you can make if work. OP has to decide if he's building a street bike or a track bike, something tells me he's not a racer.
I am not a racer but love fast bikes. Here is my GS I built. But it has changed since this picture. Top speed 135.
103611
 

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Hey TrialsRider Thanks again.

I still torn between making it a street bike or a track bike (Would be my first). The expansion chambers I have are the TR style that goes through the frame. The engine was frozen when I got it and has some pitting on one cylinder. It has to be bored. So I have been looking at Stock, Wisco and Woosner's, At almost $400 I don't want to make a mistake in getting something I am not going to be happy with. Here is my plan so far.

1. Get the crank rebuild and balanced.
2. Get the Cylinders bored and whatever else I decide to do to them.
3. Replace the points with a SS ignition.
4. Replace coils with Dyna's and wires.
5. Jet the carbs with Six Sigma kit.
6. Fit GS1100 forks and swingarm to frame.
7. Clip on bars.
7. Pods.
8. Make a seat to replace the UGLY LARGE King and Queen seat that came on it.

View attachment 103607
View attachment 103608
Holy man I seen a lot better project bikes at Woody's today, that is one rough looking water buffalo. I would work on getting the motor running on a bench before you put money into the rest of it.
Those bikes Dirk posted, nice, but your bike didn't stand a chance of being that bike even when it was new. If you had a brand new engine in a crate looking for a frame, ya then I might buy a racing motorcycle rolling chassis to wrap around it.
 

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prices for gt750's are high. you could probably find a runner for about 4-5K and put the chambers on it. how much do you think its going to take to get those pile of parts into a running bike? and time?
 

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Discussion Starter #17
Holy man I seen a lot better project bikes at Woody's today, that is one rough looking water buffalo. I would work on getting the motor running on a bench before you put money into the rest of it.
Those bikes Dirk posted, nice, but your bike didn't stand a chance of being that bike even when it was new. If you had a brand new engine in a crate looking for a frame, ya then I might buy a racing motorcycle rolling chassis to wrap around it.
Yeah, it is but I got it for $300. Believe it or not besides the cylinders needing to be bored and the water pump replaced most everything looks like it had low milage but has been sitting for a couple hundred years. Clutch plates are barley used and bolts look like they have never been touched. However I know it has been bored up .5. I am saving that picture for a before and after photo.
 

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Was just looking at photos and spotted this one, Sean Bird riding a vintage OSSA with reeds added.
Guaranteed if it didn't work, he wouldn't be riding them, you should see this guy ride.



BTW: Not suggesting you should run out and buy reeds, mission one should be to just get that old bone running. It has to function before you can make it perform any better.
 
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