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GT750 Oil Injector Pump

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Contributed by Richard Nowson


The GT750 oil pump is an interesting and strange device that appears to defy logic and yet is remarkably simple. The pump consists of three major assemblies seen in the picture below.

Top Cover

At the top is an aluminium cover which contains the bleed screw, oil intake and actuator arm. The actuator arm is moved either by a throttle cable (early models) or a by a linkage rod (CV carburettors). As the throttle is opened, the arm is "pulled" which makes it rotate in a counter clockwise direction. The arm rotates on a steel shaft which is ground into an eccentric in the centre.

The top cover, like the rest of the pump, is not designed to be serviced by the owner. In theory, that makes sense, but in the real world where some of use are trying to keep these ancient relics fully operational, there has to be a way to service the pump top cover. The top cover consists of a shaft with lever plus locating hardware (washers, springs and a loctited nut). The adjusting shaft is located by a small 1.5mm diameter steel pin. There is also a small oil seal set into the shaft to prevent leakage along the shaft.

Old pumps often leak a little along the shaft, and when they do, the only solution is to strip the top cover and replace the oil seal. Unfortunately Suzuki never supplied parts for the pump, but some items (gaskets for example) are available from third parties.

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Pump Body

The pump body contains the passageways through which oil is pumped in measured amounts through to six outlets - three for the intake ports and three to the crankshaft main bearings.

Pump Valve

The third sub assembly is the pump valve as it's called by Suzuki. It's a hardened ground steel cylinder containing six ports or passageways and looks much like the revolver of an old six gun. Into two of those chambers are pistons or plungers in Suzuki nomenclature. The pump valve is referred to in this document as the ROTOR, and the plungers as PISTONS.

The top of the rotor (pump valve) is ground into face cam with two flat lobes and in the centre at the top is a guide pin that locates the top of the rotor into the actuating shaft in the top cover.

Which pump do I have and does it matter?

There are 4 different pumps with 2 designs of lever arm, 2 designs of top covers and three Rotor designs:

J 1972 Pumps

Early model pumps contained three plungers rather than the two in later pump assemblies ( two long pistons and 1 short one). The additional plunger was for suction, to pull oil from the oil tank. That was found to be unnecessary and in fact all oil pumps are capable of collapsing the oil tank if the breather is blocked.

The actuating arm was also different to later models. It was operated by a 4th cable in the throttle cable assembly where later models with CV carbs used an actuating arm connected to the butterfly shaft. The actuating arm has teh number 310 stamped into it, indicating that it is a -130xx type pump..

Suzuki list only one pump for J and K models, part number 16100-31002. It is probably safe to assume that the change came relatively early in production and that Suzuki may not have offered a three plunger pump through the parts and service operation. Presumably the earlier pump would have carried an earlier number such as 16100-31001 or 16100-31000.

K 1973 Pumps

Suzuki amended the pump design and deleted the third plunger. That entailed a change to the main rotor. All other parts remained the same. That is the pump shown in parts lists as 16100-31002.

L-M 1974-5 Pumps

The lever arm was changed from cable actuated to one which was actuated by a pull rod from the new CV style BS40 mikuni carbs. All other parts including the rotor, main cast iron body and top cover were carryover parts. That design of pump is 16100-31201. Easily differentiated from the J-K pump by the lever arm which is stamped 312 to reflect the part number change to -312xx.

A-B 1976-7 Pumps

The A model of 1976 saw a new oil pump with part number 16100-31202. Externally very similar to the L-M -31201 pump but it had lost the hardened pin in the top cover and had a revised, taller rotor. The body, plungers and springs, actuating arm and shaft are all carryover parts. Why the design was changed is not clear. One less part and a couple of machining and assembly operations were eliminated, so maybe it was a cost reduction exercise or maybe they had durability issues, we have no way of knowing. If we find out, this will be updated.

Does it matter which pump I fit?

No. Any pump can be fitted to any year of engine simply by changing the lever arm to the appropriate type. It is probably better to avoid the very early 3 plunger pumps. The main cast iron pump body and top cover casting remained the same throughout, so all pumps are the same physical height and pump the same amount of oil.

The 'L' pump valve above left is visibly lower than the 76 on the right. The face can rubs on that hardened pin, whereas in the taller 76, the pump valve face cam rides directly on the actuator shaft.

The 1974 L model pump shown below left, uses a hardened pin in the top cover. The 1976 A model pump shown below right, dispenses with the pin and the valve body face cam acts upon the actuator shaft.

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How does it work ?

The pump is driven through a train of gears at 1 revolution per 62 engine revolutions. As it rotates the plungers on springs in the valve body cause the valve body to press upwards against the top cover. As the valve body continues to rotate, the face cam contacts the hardened pin in the top cover and that forces the valve body downwards expelling oil from a port in the lower face.

As throttle is opened, the actuating arm is rotated which allows the valve body to rise higher and creates a longer stroke and more oil is expelled.

What goes wrong ?

As a rule, very little. In normal operation all the pump components are fully immersed in oil. In addition, the housing is cast iron which is self lubricating up to a point. Run the pump dry though and the pump may be damaged before the pistons and crank.

One mode of failure seen from time to time are sticky pistons. If the pump has ingested foreign material or has passed a mix of say castor based oil and regular petroleum based oil, the pistons may be stuck in the rotor. This is most likely to occur on barn finds or on pumps removed and left on a shelf at a breakers yard. To fix sticky pistons, they must be removed and cleaned thoroughly or engine failure will quickly follow.

This pump was purchased recently from that well known on line auction site very cheaply, but if it had been fitted without a full checkout, it would have resulted in catastrophe.

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How can it be checked ?

In theory, the pump could be set up on a test jig and run at a fixed speed while output is checked. That would be a really good way to know if it's working to specifications, but a lengthy process for one pump.

A simpler way is to first place a pin through the hole in the lower part of the rotor shaft and rotate the rotor. With the actuating arm at in the IDLE position, resistance should be felt as the rotor is rotated. It should be possible to feel the cam effect every 180 degrees as it rises and falls slightly. Then move the actuating arm to the FULL THROTTLE position, and the rotor should now rise and fall considerably further. It's odd to see the whole shaft rise and fall, but that's why the drive shaft is slotted - to allow the drive pin to move in the slot. If that doesn't happen, the pump MUST be stripped and cleaned. If it passes that simple test, the next test is to see if it actually pumps oil. Before the pump is fitted, prime the oil lines with a syringe filled with oil a contrasting colour to the oil in the oil tank. Two stroke oils are often deep red or blue or purple, so use clear/gold 4 stroke oil to prime the lines. Fit the pump, taking care to avoid kinking the feed line and then bleed the pump. Start the engine and either hold the actuating arm wide open or take the bike for a short run.

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