Browse Author: tore@grenasberg.no

Dismounting of carburetors

Before starting the disassembly process. 

 

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Disassembly:

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Important to organize the parts. Use boxes, plastic bags and label well to keep track of which parts belongs to the Left, Center and Right carburetor.

 

 

Splitting the carbs (pictures usorted at the moment )

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Test run

Have done some test runs  and it all looks good. The first one was not, had to change the points and adjust the timing once more. Now it works fine. I’m so pleased to have the bike on the road again 🙂

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First startup

At last, almost two years since I started the engine rebuild it was time to verify if all my posts on the blog were done correctly. So far so good. Final answer will come after the first test run. Getting the license plate next week. Most of the work on the motor was done after I completed the “Man Cave” and the Man Cave was essential for a successful ending. Old and blind I need light and space for such a job. Hopefully this blog can be at inspiration for others working with the same GT 750 issues and pleasures 🙂

Put the kettle on:

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Click on the images for detailed view

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No decals are used, all hand painted by my nephew Thomas Grenasberg.

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SRIS system

According to the information in fig 1 the SRIS tube from the front side should go from the left cylinder inlet ( see the label C in fig.2) to the center cylinder SRIS outlet port. The SRIS type 2 system. Center to Right and Right to Left, all according to fig.6-13 from the manual

 

SRIS

Fig.1

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Fig.2

From left cylinder inlet

 

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Fig.3

To the Center cylinder SRIS check valve outlet.

 

 

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Fig.4

Fig.4 shows the two remaining tubes. From Right to Left (red arrow) and Center to Right (yellow arrow ) .

Timing adjustments

Adjustment procedure for setting the correct ignition timing.

Please read previous post regarding the functional description about the GT750 ignition system. This post will cover the adjustment of the points to achieve the correct timing.

 

Timing crankchaft

Fig.1

In fig.1 you can see the Timing Plate showing the BTDC marker for each piston. When the marker is aligned with the black arrow at the bottom of the picture the piston is 24 deg befor the TDC (Top Dead Center ) or 3,64 mm below the top. 3,64mm for L and R and 3,42mm for C piston. This timing plate was the Suzuki original idea regarding the timing adjustments and no other equipments than a lamp was needed for doing the job. Quite simple. The points should open when the markers are aligned.

That’s basically all you need to know, the rest of it you can easy figure out by yourself…

Clamp on the Contact Point base plate with all its assembly and do the adjustment.

Some guidance can be needed. Suzuki understood after a while, the procedure using the timing plate was not accurate enough. Some extra tools can also be handy for an optimized adjustment.

 

Tools:

Tools2

Fig. 2

The Dial Gauge will be used to give a accurate measurement from the TDC down to the position where the points should open up (BTDC). I made my own instrument using a standard dial gauge and drilled a hole in a spark plug for mounting the gauge.  The lamp is also self made. A LED lamp with a magnet at the end for easy fastening on top of a screw or other parts which are magnetic.

Adjustments:

Timing Adjustment

Fig 3

By loosen up the three base plate screws (yellow marker) you can adjust the timing of all cylinders. This is to be done when you install the timing plate assembly for the first time after a rebuild and you know the timing was more or less correct  before the dismounting. Don’t loosen up any points individually before you have checked the timing using the following procedure:

 

Step 1, checking the timing the old way

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Fig.4

In fig.4 I do the timing adjustment using an external power supply and use a resistor instead of a coil and apply the + voltage to the Left point wire (white colour wire ) When the engine is mounted in the frame and the coils are connected the current will come from the battery trough the coils ( see previous posts and wiring diagram )

The lamp is connected to the yellow wire and ground on the left circuit breaker (left point ) If the points are closed the lamp is off. As soon as the points opens up the lamp will light. Remember, the points short the yellow wire down to ground when its closed. At open position the voltage at the yellow wire will be around 12V (depending of the type of lamp )

Step 2, restore previous timing settings

Move the crank counter clockwise until the lamp lights up.  If the timing is correct the timing marker in the timing plate should be aligned, see fig.5.  Red and yellow arrow on the picture shows the markers

Marker

Fig .5

This is the BTDC for the left piston and the piston is 3,64mm below the top position ( TDC , Top Dead Center )

If this is not correct, move the crank until the markers are aligned and loosen all of the three base plate screws and move the plate until the lamp goes on and off. Secure the screws.

Now you are back to the original state before dismounting the timing assembly. Center and right points should also be OK at this stage if they were correct before the dismounting.

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Fig. 6

In fig. 6 you can see the position of the piston at BTDC

Step 3, adjusting the points gap

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Rotate the crank until you have the maximum distance between the points. Measure and verify the gap. Should be between 0,3-0,4mm. If adjustment is needed, loosen the screw with a red label in fig.3. Check this for all of the three circuit breakers. Do this before the final timing adjustment. Any adjustment of the gap will also influence the timing and the timing has to be rechecked.

Step 4, accurate timing adjustment

As mentioned before, Suzuki recommend not to use the timing plate as the final adjustment. A dial gauge is needed to get this correct.

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There are different type of dial gauges on the market. Suzuki has a special one with a zero function. I use a standard type but capable to measure several mm. Whatever procedure you decide to use, this is important : Don’t count mm while rotating backwards. From the BTDC rotate the crank the normal direction (counter clockwise). The distance from BTDC to TDC is 3,64mm for the left and right pistons and 3,42mm for the center piston.

If the timing is wrong, rotate the crank until the distance is correct and loosen the points shifting plate ( two screw with green label in fig.3) Move the plate until the lamp goes on and off. Fasten the screws and repeat the measurements. Do this for all of the three circuit breakers (points)

 

IMPORTANT NOTE !

The procedure above is not compliant with the procedure in the Suzuki service manual, page 94.

Suzuki use a battery driven lamp / buzzer to measure the points. The leads out of such a device gives light / sound when you short the leads. The points does this short and the lamp will therefore go out when the points open up and will light when the points are closed, exact the oposite function of a test lamp using the voltage from the battery on the bike. One more time, opposite light function compared to my description above.

If you didn’t get this, read it one more time and give it a try on the bike.

Coils and capacitors

In this post I will try to give basic information about the electronics in the ignition system. The Suzuki service manual refer to two of the key components as Condenser  and Coil. In fact , that’s a bit wrong. The condenser is a capasitor and the coil is a transformer made of two coils, one for the primary side and one for the secondary side.

Condenser / capacitor

Capacitor

Fig.1

The word condenser is common to use on components able to store or convert energi (ex:converting gas into liquid)  but in electronics this device is more referred as a capacitor and I will use the word capacitor or CAP in the rest of my posts.

In fig 1 you can see three caps connected above each circuit breaker to protect the points in the breaker. One for each cylinder , Left, Centre and Right.

Ignite0

Fig.2,

In fig 2. you can see the electrical connections of the capacitor. There is only one wire connected, the yellow wire as shown in fig.1. Pin2 in the schematic symbol is internally connected to the body of the component and will therefore be grounded when mounted at the assembly plate. The schematics in fig.2 is only for one cyllinder. All together we have three coils, three circuit breakers and three capacitors, but one battery.

A capacitor consist of two metal plates (metal foil) with an electric insulated material in between. The Suzuki type is rolled and put in a can. That’s the reason for the shape of the cap.

If you apply voltage to a cap you will charge up the cap with current until it has the same voltage as you apply. Almost the same as a battery except from the speed. You can charge and discharge thousends of times within a second. The frequency can be many kHz. If we go above Mhz the performance will change and this type of cacitor will become a coil, but don’t worry, I will not discuss that now.

The units for measuring a cap is Farad. The value of the Suzuki cap should be in the range of 160-220 nF. That’s the same as 0,000000160-200 Farad.

How to measure the value :

If the capacitors are mounted and installed at the bike do the following:

Disconnect the wiring harness of the points from the connector, see fig.3

 

 

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Fig.3

 

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Fig.4.

Make sure the point is open and not closed. If the point is closed you have a short in paralell of the cap.

Use an instrument capable of measuring CAPACITOR. Connect the red test lead from the CAP output to the yellow wire on the capacitor. The black lead to any part at ground level. In fig. 4 the value is 192 nF and is a valid result.

 

COIL
As mentioned in the beginning of this post the Suzuki coil is actually an electromangnetic transformer and contains two coils. And three of those transformers is needed, one for each spark plug.

What is a the function of an electromagnetic transformer ?

 

transformer

Fig.5

Answer: On the primary side you apply a AC voltage and the windings around the iron core set up an alternating magnetic field. At the secondary side the magnetic fields are throwned across the windings. Whenever an inductor or a wire sees a varation in a magnetic field we have induction of voltage across the wires. And the bulb in fig. 5 can light up. The ratio on numbers for windings define the voltage. If you have 10 times more turns  on the primary side compare to the secondary side, the voltage will be 10 times lower on the lamp. And opposite, in the GT750 coil we have more windings on the secondary and transform the voltages up many times

 

And don’t be sad if you don’t understand why. Einstein didn’t understand it either. No one does, it’s only a fact of physics. And it only happen if we apply AC (alternated voltage) giving an alternated magnetic field. A static magnetic field does not induct any voltage at the secondary side.

In other words, a 12V DC battery gives no voltage at the secondary side. Stop !, that’s what we have in the GT750 electronics, a DC battery and a transformer. Yes, and you don’t get any voltage on the secondary side until you…..cut off the current with the circuit breakers. That’s what we use the points for, cutting a DC current into pieces , making a conversion from DC to AC.

 

Current into the GT750 coil:

Ignite1

Fig. 6

I measured the DC resistance in the primary side coil to about 4,7 ohm. 12V divided on 4,7 R gives about 2,5 Amps into the coil. This gives a solide magnetic field in the coil but no voltage at the secondary side. When the camshaft hit the circuit breaker and open up the points, the current are cutt off and the magnetic field collaps. This is the change in the field that gives the high voltage at the secondary side. The voltage across the opened point is also very hig but the capacitor in paralell charge up and protect the points from getting worn.

This sequence is also explained in the post “Ignition circuitry

This is meant to happen 24 deg before BTDC ( Before Top Dead Center ) A detailed explanation of the timing adjustments will come in a separate post

 

How to measure the coil :

Click on the image for details

Coil

Fig.7

An easy access to all of the three coils is to measure from the side panel and to the positive pole on the battery cable. Remember the signal from the meter has to go through the emergency switch and the ignition switch. I found 4,7 ohm on all of the secondary side coils.

See the wiring diagram and my previous post “Ignition circuitry

 

Figure 8. shows another access to the coils:

The orange wire is the common wire for all of the coils. Measure from the orange one and to the White (Left coil ) Black /yellow ( Center coil ) and Black wire for the Right coil.

connector coils

Fig.8

 

Ignition circuitry

The schematics shows the circuitry to ignite all of the spark plugs. This is a copy from the Suzuki service manual.

Ignition0

Fig1.

In the schematics all three points are drawn in a open position. This will never happen. The normal positon is closed and 24 deg before  pistons top position (BTDC)  the contact breaker (point) will open up if the adjustemen is done correct.

Step 1: Contact breakes are closed. 

Ignition1

Fig.2

Ig.swich and Egn.stop switch must of course be in closed position to allow current to the coils.

The red arrows in fig.2  shows how the current goes from the battery through the primary coil and down to ground through the contact breaker.The same will happen in the others coils as long as the corresponding contackt breaker is closed.

This is the reason why you drain the battery empty quite soon if you leave the ignition switch on while parking. A lot of current are going through the coils.

Step 2, Ignition

Ignition3

Fig.3

When the piston comes to the BTDC ( 24 deg before the top, 3,64mm on left and right cylinder and 3,42mm on the centre cylinder ) the contact braker open and cut off the current into the coil.

The next step is not easy to understand, but I will try to explain. Before the current was cut off it made a powerful magnetic field in the coil. When the field collapse (due to the opening of the contact breaker and cutting off the current) the change in the magnetic field will make a voltage induction many times higher than the 12V battery in the bike. This high voltage will also burn the points in the contact breaker if there where no capacitor connected in paralell. The capacitor will be recharged by this voltage and store the energy for a short perode of time. At the same time the primary side of the coil will also see the  switching of the magnetic field. The primary coil has more windings compare to the secondary and therefore transform the voltage to many thousand volts and we get the spark. See the blue arrows in fig. 3

The basic functions of coils and capacitors will be explained more in depth in a separete post.

The physical implementation and the adjustment procedure for the timing will also be explained in a separate post.

Wiring diagram

I have started the process of drawing a complete new schematics in colour for the GT750 wiring harness. In the new document there will be links to different parts with photos showing the real component and its location on the bike. Until then, here is a wiring diagram I “stole” from the web 🙂

As always on my blog, click on the images for a detailed view

GT750 wiring diagram in colour