Project GT750 finally off the ground

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Mgmark
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Suzuki 2-Strokes: ‘75 GT750, '19 Triumph T100
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Project GT750 finally off the ground

Post by Mgmark »

After returning home from the Deals Gap meet this year, and spending time on my newly purchased red GT750 from Roy Siggins (thanks Roy, love it!), I decided to move ahead with my custom GT750 project bike I've been wanting to build for a long time. I'm thinking along the lines of a 1970s AMA Superbike.

I am quite happy with the way Roy's red bike handles with his mods, so I thought with more weight savings a GT would be even more fun to ride. So far I've been building up the bike with the titled '75 frame and bits I already have, adding to it Lane's donated monster Pile O Parts and '75 motor, and adding an alloy GS1100 swingarm, GS1000 spoke wheels with wider 18" alloy rims; 2.5" front and 3.5" rear. The plan is coming together. I shortened the alloy 1100 swingarm to near stock GT length so the steering won't suffer and mounted some adjustable dampening Hagon shocks. I also copied Roy's rear disc brake conversion he did on the red bike since that works perfectly. I will eventually run VM34mm Mikuni carbs but am starting out with stock '72 32mm slide carbs (thanks Tim Steele) since they bolt right up and I can change out the carbs once the bike is running. I try to limit things to one big change at a time.

The plan is to have the bike ready for next year's Deals Gap meet. I want to have the bike running soon, then tear down for paint and engine work. The frame has quite a few unused or needed brackets that can come off to save weight, as well as deleting other things like the electric start bits like Lane did on Blue Boy. It's a start.

Mark
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tz375
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Re: Project GT750 finally off the ground

Post by tz375 »

Nice start. If I may ask, where did you find those fat rims?

How do you like that rear wheel? I picked one up last week in WI and nearly caused a hernia. I could not believe how heavy it is, so I stripped it completely and weighed it all and of course every individual part is heavy. I thought I'd fit a small thinner disk but even a small diameter drag race type rear caliper hits the spokes, so that needs some thought to drop a kilo or two (or more). Alloy sprocket and bearing spacer and thinner disk and a small caliper would be a good place to start. Titanium axle would be nice but hardly a cost effective solution. :(

Any ideas on what you will do to the motor to perk it up some?
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Mgmark
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Suzuki 2-Strokes: ‘75 GT750, '19 Triumph T100
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Re: Project GT750 finally off the ground

Post by Mgmark »

I weighed that rear wheel assy and it came in at 42lbs with a 140/70 tire, axle, spacers and caliper hanger, everything but the caliper. There is plenty of room in that 1100 swingarm for the 140, with another 1/2" to spare on each side. I got really lucky and found the GS1000 front and back wheels on fleabay. Maybe old drag race stuff, the rims are DID dated 1982, and have Buchanan's stainless spokes. I got each wheel with old tires mounted for less than what a new rim alone would cost. Just lucky on the timing.

As far as the motor I will probably go the same route Lane did with his Blue Boy. Nothing extreme but following generally accepted modifications in the "route to 100 hp" thread, but I won't go that far. The Jemco pipes are good enough for my needs and sound great. The set weighs 15lbs vs the complete stock exhaust weighing 42lbs.

The next goal is to have all of the brakes rebuilt and custom braided lines installed this week, then try to start the bike up and see how the motor sounds. If it sounds like it won't grenade immediately I'll do some laps around the hood and see how things fit and feel before it comes apart again, although if all is well I may swap to the 34mm carbs first and dial them in.
If I had an unlimited budget I'd love to go the TR750 route with a Curtis Racing TR replica frame and serious engine work with the dry clutch, close ratio trans, ignition and porting. I discovered my hands can't do clipons anymore so I'll stay with the superbike version. I kinda like the stock bodywork look and a TR750 isn't too street friendly anyway.

Mark
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tz375
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Re: Project GT750 finally off the ground

Post by tz375 »

I played with a similar idea until I woke up to that fact that having built it, I would basically have nowhere to ride it. My bike in many ways is heavily influenced by lane's Blue Boy. It just makes sense.

I checked and my wheel without tire or tube but with the caliper and hanger and a stock 2.15 DID rim weighs in at a hefty 36 pounds which I think is about 2 times what a GSXR wheel and brake weighs

You did well to find those wheels. Nice catch.
rngdng
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Re: Project GT750 finally off the ground

Post by rngdng »

It's funny that you got carbs from Tim Steele. The engine came from Tim Steele via Mark Haase!!! :lol: :lol: :lol:



Lane
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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Mgmark
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Re: Project GT750 finally off the ground

Post by Mgmark »

rngdng wrote:It's funny that you got carbs from Tim Steele. The engine came from Tim Steele via Mark Haase!!! :lol: :lol: :lol:

Lane
I'll know who to donate the left over pile O parts to when I'm done!

Hey Lane, I sent you a pm about jetting.

Mark
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Mgmark
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Re: Project GT750 finally off the ground

Post by Mgmark »

I'm making some progress. I have all of the front brake bits rebuilt, custom lines installed and the system bled. The gas tank has been sealed with Caswell, and the carbs are holding gas. It takes some fiddling with details to turn a pile of parts back into a whole unit again.
The bike started up fairly quickly without too awful much kicking. The tacho drive is non-functional, and the bike won't start with the button, just spins over. The bike does kick start fairly quickly now that it has been run a bit. I haven't had a chance to check the charging system yet. The clutch of course is stuck. Maybe tomorrow I'll have a chance to do the old "ride it around in first gear with the clutch pulled in, while stomping the rear brake" to get it freed up. I don't want to get into engine work until the bike is apart again.
I'm calling it a bike.

Mark
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Suzukidave
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Re: Project GT750 finally off the ground

Post by Suzukidave »

I hope the tach problem is in the gauge or cable as the water pump also runs on the same shaft as the tach and that runs off a nylon gear under the left hand side of the motor .
the older i get the faster i was
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Mgmark
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Re: Project GT750 finally off the ground

Post by Mgmark »

Suzukidave wrote:I hope the tach problem is in the gauge or cable as the water pump also runs on the same shaft as the tach and that runs off a nylon gear under the left hand side of the motor .
I saw early on that the cable wasn't spinning while the bike was running. The end of a cable was rusted into the drive when I got the motor but I was able to work it loose and clean out the drive end.
So after today's short ride to release the clutch plates I decided to go ahead and remove the electric starter bits and see what was going on with the tach drive since as you say it also shares the water pump drive, and I wanted to be sure that was working.
Here is what I found:
The tach drive shaft is well and truly seized in the housing and is sheared off, and the water pump drive gear is rather badly split. The pump cross shaft down in the cases seems to spin easily so hopefully all is well inside the bottom end itself. I didn't want to split the motor until I had the rest of the bike better sorted and it comes apart anyway for cleanup/painting, but it seems that is the only way to access the lower part of the tacho drive so it may wait.
Are there any better options for the plastic gear or just go with a new Suzuki part?
I'm going to post this over in the repair section also.


Mark
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rngdng
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Re: Project GT750 finally off the ground

Post by rngdng »

Mark, I will email my water pump gear repair option.


Lane
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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Suzukidave
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Re: Project GT750 finally off the ground

Post by Suzukidave »

I am amazed the gear didnt shatter with the strain of that seized tach drive :shock: , Lane's gear repair will get ya going but truely the age of these engines has that gear pretty breakable . I would suggest if you can afford it to throw down for a new gear . If your lucky a strong magnet may pull the remaining piece of the tach drive out of the case .
the older i get the faster i was
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tz375
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Re: Project GT750 finally off the ground

Post by tz375 »

They all do that sir. Lane's repair is a good one. Not as light as an OEM gear but cheaper.

With that tacho drive, if you need spares for the sleeve and drive gear let me know. I have a few here somewhere from someone else's pile O bits.

BTW those are 74 L model forks. Is the frame and airbox also L model by any chance? Only reason I ask is that some parts are one year only. For example: Complete airbox can be replaced with MAB but end caps cannot be changed without also changing the box.
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Mgmark
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Re: Project GT750 finally off the ground

Post by Mgmark »

Hey Dave, good idea on the magnet on a stick. I was lucky and able to fish out the broken gear end and you can see where the teeth are ground off from it being stuck.
Also I drove out the broken piece of shaft from the housing with a long punch. The top 1" of the shaft was badly rusted and siezed into the housing pretty hard. If my luck holds the tach gear is the only thing to suffer from this whole thing. I'm really hoping the drive gear on the cross shaft down in the cases hasn't lost teeth also.

My frame is a '75, as is the engine. The instruments are early without the gear indicator. The rest of the cycle parts could be anything. Some came from Lane, some from the Barber swap meet, some left over from my previous GT750 ownership. I do have a set of later model forks also if they are a better version.

Mark
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rngdng
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Re: Project GT750 finally off the ground

Post by rngdng »

tz375 wrote:They all do that sir. Lane's repair is a good one. Not as light as an OEM gear but cheaper.

With that tacho drive, if you need spares for the sleeve and drive gear let me know. I have a few here somewhere from someone else's pile O bits.

BTW those are 74 L model forks. Is the frame and airbox also L model by any chance? Only reason I ask is that some parts are one year only. For example: Complete airbox can be replaced with MAB but end caps cannot be changed without also changing the box.

I think he got the 74 fork legs from me as well. I don't remember what all I gave him, but I knew I wouldn't use them....
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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Mgmark
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Suzuki 2-Strokes: ‘75 GT750, '19 Triumph T100
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Re: Project GT750 finally off the ground

Post by Mgmark »

I've made some progress on small jobs. I replaced the broken tach drive gear and was lucky in that it was the only piece that was damaged in the gear train when the shaft seized in its housing. The tach works properly now. While I had the cover off I did an elecric starter system delete on the bike, removing the starter motor and related gears, and I copied Garry55 modification of removing the starter clutch off the crank. That mod alone saved almost three pounds off the crankshaft, and about 10 pounds total for the entire starting system. I sealed the starter hole with a 30mm freeze plug.

For the cooling system I removed the remote radiator filler, overflow bottle, hoses etc and moved the filler neck to the top tank on the radiator. I also removed and plugged the thermostat bypass hose and fittings, adding simplicity and saving weight. I will control water flow through the thermostat housing to maintain temperature.

I pulled the alloy swingarm out for finish welding and to finalize the lower shock mounts. I stuck a stock GT swingarm in place with a stock GS1000 wheel to keep the bike a roller. Just for my own information I also tried the wide 3.5" wheel and 140 tire and it fit right in the GT swingarm with quite a lot of room to spare. So I added the GS style brake caliper anchor mount to the GT swingarm, and won't worry about changing it until the alloy arm is ready. The alloy swingarm saves 4 pounds over the steel one.
I've also been doing some general cleanup and polishing on engine covers while they were off. That's about it for now.
Mark
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