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.
Gasket:
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.
Time to build a rear wheel for my GT750. Was able to get a new original Suzuki rim, spokes and a used hub, and now it’s time to get the job done. The wheel mounted on the bike has some broken spokes and a dent in the rim I can’t repair. Better to rebuild a new one and keep the old as spare.
Removing the bearings from the hub.
A bit sanding and polishing of the hub.
Mounting new bearings
Next step, make sure you have all the parts organized: 18 inner side spokes and 18 outer side spokes
Brand new Suzuki rim 🙂
And what to do now ? Not sure, but I have done this before. Therefore I’m jumping to some old post on my blog:
I will follow my own procedure shown in the links above and will post an image of the rebuild rear GT750 wheel when I’m done later today.
Step 1 (after studying my old GT380 post)
Please notice. I have some old scratches in the hub from the old outer side spokes. If you want them to be hidden with the new ones, remember to mount the inner side spokes at the opposite directions as I did in the photos above. Inner and outer side spokes have always the opposite directon of mounting, and you have to do the inner sides first. And once again, don’t do this for the first time if you have not study my old GT380 post about this topic.
Done:
Adjustments: Once again, I checked out one of my old post about the topic:
The explanation below will be valid for both GT380 and GT750. The alternator wiring for GT750 can found on page 94 in the service manual and on page 44 for the GT380. It’s very well written and explained in the manual, but I will do it in my way and perhaps it can give you a better undrestanding of how it all works. Both manuals can be downloaded from the documents category on my blog.
Wiring diagram from the service manual:
I will use images and measurements from DENSO parts, not KOKUSAN. The differences are explained in the service manuals. The figure above from the service manual is a simplified schematics. The alternator has 36 coils (12×3) in the stator and 12 coils in the rotor (6 north poles and 6 south poles). Please see the figure below:
Abbreviation and definitions:
DC: Direct Current
AC: Alternating Current
DC Voltage : A voltage with fixed plus and minus polarity and the current will flow from plus to minus if you have a closed circurit. ( the flow of electrons are the oposite way, from minus to plus ) Ex: The 12 Volt battery on your bike is the DC voltage source and the minus pole is grounded to the frame.
AC Voltage: An alternating voltage where the polarity changes with a given frequency. On your bike the rpm + the numbers of coils in the rotor and stator will define the frequency.
Stator: The stator in the alternator above is a 3-phase generator giving 3-phase sine wave voltages phase shifted at 120 degrees.
Rotor: Ther rotor is a rotating DC controlled magnet giving magnetic flux varation to make induction of voltage at the stator windings (coils).
Regulator: The voltage regulator in the diagram is a ordinary relay with 3 positions. In postion 1 and 2 DC current will flow from the + pole on the battery through the rotor windings and down to ground.
Position 1: No restriction of the current into the rotor windings. If the voltage is too low to activate the relay, it stays in position 1, shorting the resistor. The rotor will get the maximum current and gives the maximum flux from the coils.
Position 2: The voltage has rise and ther relay activate and and the resistor be in series with the rotor windings since the short is gone and the current has to flow through the resistor and the rotor current goes down causing less flux in the coil.
Position 3: High rpm of the engine giving high voltage and the relay goes all the way to the lover postition, shorting the rotor, no current will flow in the rotor windings. There are still some flux in the iron core of the rotor and induction of voltage in the stator will still goes on. If the voltage get lower agian, the relay will go back to stage 2 or 1 and send current into the rotor windings (coils) and keeping a steady voltage to the battery.
3-phase rectifier: The rectifier turns the AC votage into a DC voltage by using 6 diodes to turn the minus part of the sine wave into a posive part. Since all three phases are appers at a different time (due to the phase shifting) the output from the rectifier will be a DC voltage with very little ripple voltages compare to a single face rectifier.
The alternator covered in oil and dirt, and the wiring was in a bad shape. When I was finished it looked like this:
Note: The blue wire is for the neutral indicator switch. When the switch ground the wire the indicator lamp will light.
Some of the steps:
Parts removed and the stator housing was cleaned and polished using a rotating brush.
All the terminals were polished too. Don’t forget to mount the isolation cap when assembling the brushes. If not you will short the positive pole down to ground.
The old sleeve was brittle and had to be replaced. I went for a more modern flexible type and wrapped around a textile type of tape able to withstand 150 deg C.
Bend down the locking lip of the terminal and pull out from the rear side. I did a lot of tips and tricks from YouTube how to remove the yellowing of the plastic. Used both ultrasonic cleaner and hydrogen peroxide + UV light. Not perfect, but it became better. Polished the terminals and mounted it all back in place.
Soldering:
You can replace all the wires, but I didn’t. Some of the wires were extended to make a proper wiring harness with correct lenght on all the wires. To make the soldering more easy I wrap a single wire around the wires to keep them both in place during the soldering. Added a double set of heat shrink tubings.
Mounted on the bike:
Measurements:
Stator windings:
Bike
L1-L2
L1-L3
L2-L3
GT750
0,7 ohm
0,7 ohm
0,7 ohm
GT380
0,7 ohm
0,7 ohm
0,7 ohm
Rotor windings:
Bike
Between green and black/white wire on GT380 and between green and ground (frame) on GT750
GT750
5,8 ohm
GT380
6,8 ohm
On the stator windings all measurements are done between the phases, the yellow wires.
Please notice the connectors. On the GT380 there is 6 pins and only 4 pins on the GT750. The ground (black/ white wire) and neutral indicator (blue wire) is missing. To measure the rotor on GT750 you therfore have to measure from the green wire to any GND points on the frame.
Note! If you measure rotor through the connector you will also measure through the brushes. If you get a very hight value you have bad connections in the brushes due to dirt and oxidation. Apply 12V DC for some seconds into the rotor and it will be all OK.
Numbers from the service manual:
I find those numbers hard to belive, can’t be right. The stator coil (windings) can’t vary from 0,26 ohm down to 0,1 and be valid.
Stator windings: The resistance in the stator windings don’t change over time. It’s not magic, it’s a wire winded around a some iron core and the length and thickness of the wire will define the resistance. If you get a short between some of the wires the resistance will go down and not increase. And you will see a difference between the three phases. You will never have three identical fault in the same stator. The measurements I did gave higher resistance and were identical on both the GT380 and the GT750.
Rotor windings:
The rotor resistance I measured on both GT750 and GT380 is much lower compare to the manual. OK, one reason can be a short between windings in the rotor. But i checked all 12 magnets and verified they all had the correct Norh and South poles with the same strenght. A short giving half the resitance compare to the number in the manual would give weak magnetic field on some of the magnets. Since I have about the same measuremetns on both rotors and the GT750 is running fine and gives the correct voltage to the battery I’m pretty sure it’s all OK. Why the numbers in the manual are so differnt, I really don’t know.
Sorry for a long time of doing nothing on my blog. Hope to do better in 2022….
I come to mind I did a upload some years ago on YouTube showing an alternative way of synchronizing the carburetors, but I never posted the link on my blog. So, here it is:
The service manual explain how to balance the carburetors, but it can be hard to get it right, and annoying surging will occur at low rpm. Some adjust the butterfly valve looking at the air/light gap holding the carbs up agains a light source, but again, not an easy task.
In the video I pour methylated spirit ( petrol smells too much) into the carburetors and get the time of the drainage. Adjusting the carbs to about the same speed of drainage and the surging is gone at low rpm. I learned this from Steve in Melbourne, Australia when I visited his sister Donna Ann 5 years ago.
I bought a new carburetor kit last summer and mounted it for a couple of weeks ago. Interesting to compare with the old one I had. The first test run was promising. After a longer ride I got more and more annoyed about the surging at low rpm. Pretty sure it’s more surging compare to the old one. The fuel consumption is better and the color on the spark plugs looks also better while using this kit. If I can get rid of the surging I will keep them on the bike. The new carb kit has no anti-surge jets mounted. The previous owner said they are all tapped and ready to be fitted with jets. I don’t have a new jet-kit, but I have a lathe and milling machine. Let’s make a set of three 0,8mm jets.
As usual, instant start on the GT. First ride this season. The spring has been horrible cold so far. Was snowing for a couple of days ago. Will enjoy the short MC season on the GT750 this year and hopefully the GT380 will be on the road the next year.
My GT750 A tank is back from repair and has got a new paint. My nephew Thomas has once again done a fabulous work on this. I’m so pleased with his work.
On my ride to the classic Suzuki rally in Denmark last summer i had an accident and got an annoying dent in the tank. Now it’s all gone 🙂
While doing the work on the carburetors I saw two of the butterfly shafts had bad threads for the 3mm screw holding the butterfly valve.
The one on the photo below looks like it has a previous repair using some sort of insert, not sure but it looks bad.
Click on the images for detailed view
The procedure below is a much better way to do the repair. Since the shaft is made of brass a new piece of brass can easily be soldered into the bad area.
