Suzuki GT380 Top End Rebuild Project

[alexwlwsn]

Hey y'all! I picked up another project bike this previous weekend.

It's a 1972 Suzuki GT380J (first year) which is a 371cc 2-stroke 3 cylinder motorcycle. Apparently, these are super rare and pretty coveted in the old-school motorcycle community so I'm real excited to get it on the road.

It's not factory stock, in the sense that it's been painted, has a different seat, handlebars, etc. It's somewhat dirt tracker style if you had to give it a name.

The PO bought it with a blown top end and also had purchased a spare with a seized bottom end for parts. With the bike, I received a nice bin with new (used) clean heads, spare transmission gearset, and more. He started work on it but just ran out of time and decided to off-load some projects.

On to photos...
Here she is in my little shed where I also rebuilt my CBR 600F4i:


Shot of the gauges:


Clean head(s):


Badass exhaust the PO apparently imported from Japan:

The exhaust has a silencer in it so don't worry, the bike shouldn't be deafening to ride.

and the typical trailer shot:


All in all, I'm hoping for a simple rebuild. I've found 3 heads that have clean cylinder walls that likely just need a hone to get to brand new quality. I still will need to measure them though to make sure they're within factory spec before I get a set of cylinders as I may need to bump up to an oversize if they need a real bore/hone from a machine shop.

Top-end rebuild guide

Planned costs so far:
$60 for a set of 3 cylinders/rings/etc. LINK
$22 give or take for a full engine gasket kit LINK
$cheap for a cylinder honing tool

All in all, she should be up and running soonish depending on how much time I give it. Once it's all assembled I'll get to learn all about tuning 3 carbs at a time and making sure it runs right to avoid another top-end oopsies.

Stay tuned!!

If you're good at tuning 2-stroke engines, feel free to hop in whenever to give advice when you see fit.

 

[JohnnyTightlips]

Exhaust does look pretty bad ass. Looking forward to seeing this turn out.

 

[alexwlwsn]

Exhaust does look pretty bad ass. Looking forward to seeing this turn out.

I'd LOVE to get an actual expansion chamber 2-stroke pipe made for it but I feel like that is way way outside of my budget.

 

[Db09ms3]

Is that your Focus ST? Sweet vintage bike, carbs are fun to tinker with...

 

[alexwlwsn]

Is that your Focus ST? Sweet vintage bike, carbs are fun to tinker with...

That belongs to a local buddy of mine. It had a real droney exhaust which was just wonderful to drive 4hrs towing a trailer with. I ordered a hitch for my fiance's Ford Ecosport but UPS was silly and postponed the delivery.

Also, looking like for my bore/hone I might just want to go to a machine shop. It seems like the hone ball tools can potentially mess up the 2-stroke windows in the cylinder which I obviously don't want. Since it's only 3cyl it might just be cheaper to have someone else do it. I'll have to call some folks today and see what the effort on that would be. Might be able to get away with going to a lawnmower shop.

 

[mangosmoothie]

port it out lol

 

[alexwlwsn]

port it out lol

I have 2 sets of barrels and heads so that's a possibility for the second set. That's also a pretty intense rabbit hole to venture down into based on my minimal knowledge.

I have found a calculator that'll help me make expansion chambers for it though so that could be a fun future project. Port the spare barrels & custom fab a nasty 2-stroke pipe and turn her into a real ripper.

 

[mangosmoothie]

That's awesome you have 2 sets. Can just get it running at first, then play with everything later.

I'm not sure what bike it is, but the guy that tuned my triumph plays with a suzuki 3 cyl 2 stroke. He always brings it out when he does dyno days. I think it makes 60 something at the wheel.

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[alexwlwsn]

That's awesome you have 2 sets. Can just get it running at first, then play with everything later.

I'm not sure what bike it is, but the guy that tuned my triumph plays with a suzuki 3 cyl 2 stroke. He always brings it out when he does dyno days. I think it makes 60 something at the wheel.

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That thing sounds like a ripper for sure. There's a lot of mods for these bikes that help get them to around that 60whp figure. People toss in larger dirtbike pistons, double up gaskets, larger carbs, etc. to fatten up the power.

This will be a simple build though. I basically just want it to run as-is. I'll be able to learn a lot about carb tuning and all so this will be a good project.

It's neat digging into all the 1970s documentation for these bikes though. I accidentally cracked a hose last night (it's hella old and was a hard plastic) and after research, learned about the Suzuki SRIS system that takes the pool of oil under one cylinder and allows the next cylinder's vacuum pull it out and into that cylinder - LINK - All the documentation on it is for different bikes but I guess this one has it too. Luckily it's just lengths of 1/16" ID tubing so it'll be easy to replace I assume.

Still trying to find someone to bore out my cylinders but my local motorcycle shop was closed yesterday so I couldn't make much headway on that. I'll have more of an update later today. Odds are I'll just go with the +0.5mm piston and get a bore/hone and it'll be perfect.

 

[alexwlwsn]

Double post but I have an update:

I called a recommended machine shop and they quoted me on $40 per hole for a total of $120 for a bore & hone to spec with 0.5mm oversized pistons. It would be fun to measure out the cylinders, hone them, etc.. on my own but for that price, I think I'll just have someone else take care of it.

Edit -
Ordered +0.5mm pistons from a company in Japan so they'll be here mid-December, along with a gasket kit from Amazon.

Price total so far -
$60 + $12 shipping for pistons
$22 for gasket set

Planned -
$120 for bore/hone
$~20 for misc items?

Estimated costs -
$230-250 on top of the bike.

 

[Easter Bunny]

Should have overnighted the parts from japan

 

[alexwlwsn]

Should have overnighted the parts from japan

I bet they would have came in a Fast and Furious manner if I had.

Budget is looking pretty wild though, not sure if I can sustain this build:

 

[SharksInSpace]

Have you considered not being A Poor?

 

[alexwlwsn]

Okay friendos, advice hour - regarding the SRIS system (uses one cylinder's vacuum to pull excess oil from under the previous cylinder)

The oil lines are about 1/16" ID hard(ish) plastic and unfortunately, I cracked the end of one test fitting it to a cylinder. They can be replaced although the bottom end seems to be stuck in the crankcase. See below image for where it's mounted to the block. The base is held in using a 7mm nut and it's a little tighter than I felt comfortable turning.



It then routes through a hole between the crankcase and the trans to the top of the engine where it mates with its respective cylinder. Below is the barb it attaches to (one hose for each cylinder).


Here's the cracked hose:


SO, seeing the small hose, the barb, the difficulty of the bottom nut removal, and knowing hot oil passes through the line... what do we think it the best route? I could trim the hose 1" shorter and attempt to extend it, maybe find an oil-resistant heat shrink tubing that
s slightly larger and use it to seal to the cylinder and hose?

 

[Easter Bunny]

Throw some heat on that nut and crank on it

 

[alexwlwsn]

Throw some heat on that nut and crank on it

It's a brass 7mm lil guy. I could try for heat but it's so tiny and already seemed to not like torque.

 

[Easter Bunny]

more heat then

 

[alexwlwsn]

more heat then

Always more heat!

Actually, researching on the interwebs, it seems the hoses are just on barbs and held on by a small metal clamp. I knew the bottom was a one way valve so I wasn't sure if I needed a special hose in case it was some push to fit fitting. I ordered some 2mm ID high temp chemical resistant tubing from McMaster Carr and I'm going to give that a go. Once I have that I'll experiment with cutting the existing line off and we can go from there.

 

[alexwlwsn]

Small update. Spent an hour or so working today.

First thing was replacing the SRIS hoses with the new hoses from McMaster Carr. It went well, just went one by one routing the tubing the same way as the original cracking hoses.

Started at the bottom...


and routed through the block to the top...


Pretty simple design. Right cyl goes to the middle, middle to the left, etc.

Then I swapped out the red handlebars with a black set he gave me, put on the grips and the nice bar end mirror.


All in all I'm super happy with how it's looking. The black bars match the bike really well.

Cylinders are also at the machine shop and getting bored/honed this week. (the cylinders you see in the photos are a spare set I'm using for mocking up the SRIS lines. Fingers crossed this week they'll be done and we can try to get the ole girl started next weekend. I'll keep y'all posted.


Snuggly puppy tax:

 

[alexwlwsn]

Well, somewhat unfortunate news just arrived.

The machine shop called and apparently the center cylinder I had given them was already +1mm over bored. I had 2 sets of cylinders to begin with and delivered the 3 cleanest I could out of the sets to the shop. I guess the previous owner (or even before that) had already bored one set out. Looks like the bike won't be running until 2020 but I can live with that. The machinist is also on vacation until the 2nd after this week so I must now be patient.

Oh well. It is supposed to be 50 degrees at the beginning of next week too so I guess I'll just do everything I can up to the point of reassembly. This will give me the chance to clean the carbs better or something.