Welcome to my man cave

Man cave
A proud owner of a new Man Cave

Welcome to my blog where I describe my journey in Suzuki GT restoration.

Hopefully at help for myself an others with the same interest. Scroll down for for new posts.

The main idea with this blog is to use it as a reference guide for Suzuki GT 750A  and GT 380J model.

Don’t hesitate to give me feedback and guidance if you see me doing it wrong.


Before and after refurbishing

The coil wiring was in a bad shape like the rest of the electronics. Wrong colors on the wires and missing connector.

If you don’t know how the coils and ignition is working, please have a look at my post i did 8 years ago when working on my GT750:

How to start: All parts were dismounted and cleaned. I was not able to clean the coils properly so I decided to paint them white.

The metal brackets holding the cables were sandblasted and nickel plated.

I had to use impact driver to loosen the screws. Don’t forget to use JIS tools ( Japanese Industrial Standards)

The bracket was sandblasted and polished.

All of the wires had to be extended or replaced. Each individual strand in the wire must be sanded before soldering. After soldering the joint was covered in epoxy and painted.

The other end of the wire got new terminal and was crimped.

This is the color coding and placement of the wires into the connector.

The orange wires are all connected to a common +12 supply and the placement on the upper row does not matter. On the lower row, it’s very important to do it right, if not the bike will misfire on wrong cylinders.

Mounted with new wires and connector.

Before mounting I also did a spark plug test to verify all of the coils. Connect 12V to the orange wire and short the other wire to GND( 0v). The body of the spark plug must also be grounded. When you release the wire from ground the park plug will fire. ( Just tap the wire on/off to GND and you will see the spark) Do the same on all coils to verify the function.

The resistance in the coil winding should be around 4,5 ohm. The current in each coil when grounded will therefore be 12V/ 4,5 ohm= 2,7 Amp. That’s the reason why you drain the battery very fast when leaving the ignion on and the bike is not running.

Wiring harness GT380

Hmm, a lot of questions but few answers.

According the the parts manual it looks like the wiring harness should be installed on the right side of the bike, or… not, right side must be wrong. I’m told it right is right, but based on feedback with images I have only got photos from GT380 owners with the wiring harness on the left side of the frame, and it’s the same on my GT750 too. The drawing above must a guidance of how the wiring are connected, not which side it’s fitted.

Based on photos I have got I have some idea about how it should be done. Some advice can also be found on YouTube, but you can’t trust them. Some might be right and some are wrong, and others are horrible wrong.

So, to summarize: I’m not sure, and in some case I don’t have a clue, but I have to start and will use common sense when I have to make decisions. I think a look at mye GT570 will be at a good help. As long as I make it all in compliance with the wiring diagram it should be fine.

Connectors and tools:

I bought a kit from China with a lot of different type of connectors + the crimping tool needed.

In addition I got hold of the upper and lower wiring harness and only need to make some few extra cables to fit into the harness.

The cable from the alternator is already done, please see my previous post. The same for the cable from the ignition electronics (the points )

Next up was the battery cable. That became a bit tricky. Was not able to find the old cable anywhere and how to make a new one. The corresponding connector is made of rubber and how can I make that ? Same procedure as before. I drew the part in Fusion360 and fired up my 3D printer with rubber resin. And here is the result:

I found a picture of the cable on internet and did the measurements on the mating connector from the wiring harness. After printing the part I glued the terminals and added heat shrink to the wires.

The STL file for printing can be downloaded for free:

Next step:

I inserted the main cable for the upper harness through the lower hole in the headlamp and the upper hole will go to the clocks and and switches on the handlebar. I will use adjustable cable ties for clamping the cable onto the frame so I can easy move and do changes as I continues with the wiring.

That’s enough for today. Will continue another day 🙂

Mounting the oil pump

For about two months ago I tested several oil pumps in the test jig I made. I decided to go for the early version of oil pumps with suction. That was the one giving best result when idling. I got scared about the others giving little or noting at low throttle.

All o-rings were replaced and the same with the nylon spacers.

The 50 years old plastic and the oil-lines are very fragile, and I didn’t do much to clean or make it look nice. I knew it all worked since I had it in the test jig with good results. To drive the plastic base all the way down I prefitted the screws for holding the oil pump.

Torque settings for the banjo bolts is difficult to find. I did some investigation back in 2016 when rebuilding my GT750A and found a number of 2,5Nm. That’s lower than I today have on my smallest wrench. At that time I stole a key from my work, but this time I used common sense and was gentle to the bolts.

By using a syringe I was able to inject the oil all the way to the end of the pipes.

I have the original screws for mounting the oil pump, but I went for 5mm bolts with hex head instead. Much more easy to mount and the screw are not visible when the covers are on.


Clutch Release Assembly

The parts were cleaned, greased and refitted.

Mounted in the cover and fitted onto the bike:

See the service manual for the procedure how to hook up the clutch cable and do the adjustments. I will also update this post later on when I do my own clutch adjustments.

Side cover:

The bike so far:

Neutral Switch GT380J-K

The picture above shows the function of the neutral switch. The shifting switch (Part 44) is attached to the gear shifting cam. In neutral position the switch will reach the contact point inside the housing (part 39), this will lead to shortening the blue wire down to ground. Since the lamp in connected from the blue wire to +12V, the bulb will light up in neutral position.

Make sure the contact points are cleaned and polished at the inside and outside of the housing. I added a new washer and screw too.

Before mounting, put the engine into neutral position. Make sure the switch is mounted with the lip into the groove at the end of the shifting cam. If so, the contact point in the switch will meet the contact point in the housing.

To verify the function, measure the conductivity from the witch down to GND. Should be a short (close to 0 ohm) when the engine is in neutral position.

When you add the wiring harness, the blue wire from the alternator cable should be fastened to the switch.

Indicator Relay

The original indicator relay was missing on my GT380J model. A ugly replacement relay had been fitted, but was not properly fastened in the rubberband due to its square shape. I had to go on Ebay to search and found a round type of relay, a bit smaller compare to the real thing but looks nice.

Because it’s smaller I can’t use the old bracket with the rubber mount. I made a resin 3D printed bracket so I could use the rubber mount that came with the new relay. Like this:

Resin printed and cured with UV light.

The STL file of the bracket can be downloaded from the link below.

New terminals to the wire so it will fit into the original wiring loom from Suzuki. I also did a test of the relay to verify the function.

Important to use the correct color coding. Orange to the + wire (orange wire is the + battery voltage after the ignition switch) and blue to the indicator lamp. ( the casing of the indicator must be grounded. Note! the blue wire to the indicators will not be blue all the way to the lamps, please see the wiring diagram for details. The 2-wire relay is connected in series from the battery (after the ignition switch) to the indicator lamps. Controlled by the switches, left and right lamps will then get the voltage applied from the blue wire on the relay.

Mounted together with the rectifier and the regulator relay.

Note! The rectifire in the picture above is mounted different compare to how it came with the bike. I turned the recifire upside down and made a proper ground to the negative part of the diode casing to achive a better ground down to the frame. The + part with the insulator is therfore at left side of the picture. Please see my previous post about the rectifier:


Speedometer file for 3D printing

This is how I made the inner housing for the speedometer. I did the print in white PETG, able to withstand higher temperature compared to PLA filament. The STL file can be downloaded for free from the link below. Can be printed in with support to the build plate.

The original housing had a cutout for the reset-knob for the trip counter. Don’t know why, but it is probably done because difficulties when mounting the clock with the knob attached?

For the rubberpart sealing the reset-knob shaft, I will make my own fix. Can’t be bought so I will try to make something from a rubber tube and see how it works. If I face any difficulties when assembling and need to modify the 3D model, I will upload a new STL files on this site.


The orange wire is the +battery voltage, but after the ignition switch. The green wire is the +voltage to the rotor and the white and black wire is the GND wire.

The regulator on GT380 and GT750 works in the same way. The purpose is to control the rotor current and thereby adjust the magnetic field so the generator (alternator) and rectifier gives about the same outputvoltage regardless of the rpm.

The case got sandblasted and repainted. The contact breakers in the relay were sanded a bit to achieve a good connection and the current flow was thereafter tested with a power supply.

The pictures below shows the different stages from 1-3 depending of the voltage to the relay coil.

The diagrams are a bit difficult to read, they are only meant to be a reference for the different stages in the pictures above. Details about the diagram and how it all works are explained in my previous post, the Alternator.



GT380 schematics

The figure above is a copy from the wiring diagram. It shows the 3-phase rectifier with three yellow wires as AC input and the plus (red wire) and GND(ground 0V black and white wire) as DC output.
The wiring diagram also shows the housing (one of the heatsink) is grounded.

Note! I had a look on my GT750A model. An other type of rectifier (the smaller one) and the black and white GND wire is missing. All the grounding is done direct through the mounting bolt. Only four wires, three yellow and one red to the connector. A bit confusing, because the detailed wiring diagram for GT750 shows the GND wire, but not in the owners manual for GT750A. I assume it’s done different from 72-77 models.

GT750A version, no black & white GND wire.

If you are uncertain how the rectifier works with all 6 diodes, please review my previous post about the alternator for GT750 and GT380. There you will also find link to YouTube explaning how it all works.

The rectifier in the image above is from the J model. You can might find a different package on later models with smaller heatsink, but they all works in the same way.

You have six diodes, three with the anode connected to the case and three diodes with the cathode connected to the case.

Please note, the rectifier has the heatsink splitted in two parts. One is the pluss output ond the other one the minus output (GND) . If you scratch off the pain from the positive heatsink and by accident short the heatsink to GND you short all of the voltage to ground on your bike. Note! the heatsink is connected to plus, not the bolt. The image shows how the bolt going thrugh is insulated. I did a small mod using making the insulator in peek material to improve the design a little bit.
The other part of the heatsink shall be grounded to GND according to the schematics. Mine was not and I’m not sure if it was wrong mounted or it is common not to do do. Anyhow, both options will work, but if not grounded all of the current to GND has to go through the black and wire to GND. I did it in my way and made better connection to GND from the heatsink.

Remember to remove paint to achieve a proper connection to GND.

All of the connectors where polished and the red wire who was broken got replaced with a new wire and connector.

To remove the terminal from the connector, use a flat screw driver to bend the lip down and pull the terminal out from the rear side. Remember to bend the lip back to normal position when inserted.

Diode testing:

When using a multimeter for testing, switch to diode testing and do the measurements. Positive terminal(red test lead) on the anode and negtive terminal (black test lead) on the cathode. Measure all six diodes, but keep track of anode and cathode. Since the three of the diodes have the anode to GND you can also measure to the heatsink insted of the black and white GND wire/treminal

The instruments shall give you a value for about 0,45V, and no voltage if you swap the terminals ( positive on cathode and negative on anode).

Measure all six diodes, but keep track of anode and cathode. Since the three of the diodes have the anode to GND you can also measure to the heatsink insted of the black and white GND wire/treminal.

If you don’t have any multimeter you can use a small 9V battery as input and and a test lamp at the output. When you swap the polarity on the 9V battery applying power to the yellow 3-phase input the lamp shuld still light, and do the same betwen all phases.

Done !

Oil pump

This post will be about the oil pump, for both the GT750 and GT380, old and new models.

The information below is my understanding based on testing and iformation given to me. If something is wrong, please let me know and I will updatate my blog.

Some basic informations:

As the picture above shows we have an early model of the pump using one extra piston for suction. On later models the gravity from the oil tank above the thank do the job, and Suzuki decided to make it more simple with only two pistons.

Note! Never,ever go away from using the oil pump and only mix oil into the fuel, if so the crankshaft will not be lubericated.

Are all pumps interchangeable ? Yes, all of the GT380 to GT750, old and later models, but there are some differences. Later models for GT750 using the vacuum carbs have a different arm connecting the throttle cable, see the picture below:

The punch mark and alignment marks are at different location (order) on J-K (GT750 models and all GT380 models) compare to the latest GT750 models. The alignment procedures are also differnt, see the picture above and the service manual.

Test setup:

The speed of the pump is controlled by a DC motor and most of the parts are 3D-printed.

The shaft running the pump is made of brass and locked in position by two ball bearings.

Lesson learned after the testing:

At normal operation the cylinders get more oil compare to the crankshaft, but one important observation:

At the first punch mark there are almost no oil coming to the cylinders, only to the crank. On my test with two later models with only two pistons and no suctions, this was the case. A bit scary to watch. On the early model with three pistons and suctions I got a bit of oil to the pistons at the first punch mark as well, but no much. As soon as I increased the throttle it all worked as normal. Most of the oil to the cylinders and less to the crankshaft.

In the video below the pump is running between the first and second punch mark.

On my GT750 the oil consumption is close to 100ml per. 100 km, as it should be. In test above the rpm of the pump is 1hz ( 60 rpm) and refer to the engine that will be about 4000 rpm. The consumption from the test looks very much the same as I see on my GT750, but the 3-piston pump with suction gives a bit more.


I ordered new gasket from E-bay, but to start the testing I made my own by scanning the old one and cut new with my laser cutter:

Seems to work fine. Since I have paid the money for new ones, I might swap the gaskets when they arrives.

Copy from the manual: