GT750 oil pump question

[jabcb]

Per this reference, the GT750 oil pumps were effectively the same, except the actuator/lever arm had two different designs depending on the type of carbs.
http://www.pinkpossum.com/GT750/Oilpump.htm

The pump adjustment also depends on the type of carb.
The following are from the GT750 service manual.

GT750 oil pump output spec:


Early-model (72-73) GT750 oil pump adjustment with throttle open as shown:


Late-model (74-77) GT750 oil pump adjustment with throttle closed:


If you look closely at the two different actuator/lever arms designs, you can see the adjustment alignment marks are in different locations. The arms may also have different effective lengths.

Now for my issue:
I have a 75 GT750 with actuator/lever arm for the CV carbs but I have early-model carbs.
The arm has been modified to work with the early-model throttle cable.
I adjusted the arm per the late-model technique with the throttle closed.

Alternatively, I could switch out the arm but so far I haven’t found an early-model arm.

Question:
Am I done or do I need to find an early-model arm?

Also, it would be useful to see pics of the oil pump settings with the throttle closed & fully open for a stock early-model GT750.

[tz375]

Later carbs use a shorter arm to match the crank on the carb throttle shaft. I would expect that with a later short arm, that the pump would be less open because the shorter arm would not have rotated as far as a long (early) arm.

I'd check how open the pump is when the mark is showing on the carbs. Does it look as if the pump is open enough?

[Coyote]

I have a similar oil delivery chart in another manual. I was surprised to see that the flow to the crank is a constant. Once you're open 10 degrees, the oil going to the crank bearings is at max flow. Also surprising is the flow to the cylinders doesn't even start till you reach 10 degrees.
I know this doesn't answer your question, but my gut tells me you should be close enough. I never have figured what the dots are supposed to indicate.

[jabcb]

Later carbs use a shorter arm to match the crank on the carb throttle shaft. I would expect that with a later short arm, that the pump would be less open because the shorter arm would not have rotated as far as a long (early) arm.

I'd check how open the pump is when the mark is showing on the carbs. Does it look as if the pump is open enough?

Thanks, that’s what I needed to know.
If the arms are different lengths, then I can only get correct oil pump delivery at some throttle openings.

I bought a used early-model pump off of eBay today.
I’ll use the combo of my current pump + the arm from the early-model eBay pump.

[jabcb]

Now have an early-model arm.

Any tips for installing the arm without taking the pump out?

[tz375]

Undo nut. Remove washers. Slide actuating arm off. Slide new one on and fit washers and nut.

Seriously, that's all there is to it.

[rngdng]

I never noticed the difference in the arms.....I have slide-valve carbs, and used a stock 75 oil-pump operated by a cable, set up by the marks at zero throttle, and have run without issue for about 13 years.


Lane

[jabcb]

I never noticed the difference in the arms.....I have slide-valve carbs, and used a stock 75 oil-pump operated by a cable, set up by the marks at zero throttle, and have run without issue for about 13 years.


Lane

Thanks, that's good to know.

I'll measure the arm lengths to see how different they are.

A concern I had about switching out the arms was the possibility of getting the shaft 180 degrees out.
Don't know if Suzuki designed it so you can only assemble it correctly.

[Coyote]

Don't know if Suzuki designed it so you can only assemble it correctly.

I wish I knew the answer to that too. I have a pump with no arm and using a small crescent you can turn it 360 all day long. And what does it do if you're 180 out? My pump seems to be delivering a lot of oil. After only 20 minutes run time it was dripping out of the exhaust ports when I removed the mufflers. Makes me wonder if it's off a 750, though I am 99.99% sure I put the same one back on that came off the bike. Is there a way to tell those pumps apart? They look the same to me, but the part numbers are different.

[Coyote]

Don't know if Suzuki designed it so you can only assemble it correctly.

I wish I knew the answer to that too. I have a pump with no arm and using a small crescent you can turn it 360 all day long. And what does it do if you're 180 out? My pump seems to be delivering a lot of oil. After only 20 minutes run time it was dripping out of the exhaust ports when I removed the mufflers. Makes me wonder if it's off a 750, though I am 99.99% sure I put the same one back on that came off the bike. Is there a way to tell those pumps apart? They look the same to me, but the part numbers are different.

[tz375]

It's been a while since I had a pump apart but I am pretty sure that if you rotate the shaft "backwards with your fingers, it will be in the correct position.

Lane's point is that with the Rod link arm, the pump may not be 100% open at WOT but that would not make a huge difference and at small throttle openings it would be the same as a long arm for all intents and purposes. In fact the pump is at 100% output after about half throttle, so the only potential issue would be that at say 1/3 throttle it may not rotate quite as far as an old style arm and would pump slightly less oil. I'd still use the cable arm because it has the cable connection.

180 degrees out would supply too much oil at low revs and not enough at WOT

[jabcb]

The late-model rod-link arm has an alignment mark to adjust the pump when the throttle is closed.
So the throttle-closed pump setting will be correct when using either the CV or early-model carbs.

The early-model cable-link arm has an alignment mark to adjust the pump at part throttle.
You can only set this correctly with early-model carbs because its based on carb slide position.

So as I understand it, in Lane’s case the pump it correct at closed throttle.
The late-model rod-link arm is shorter & its rotation results in a shorter linear distance.
The throttle rotation conversion to linear distance is for the early-model cable-link arm.
Because of the short arm, Lane’s pump reaches 100% flow a bit earlier.

I currently have the same setup & adjustment as Lane.
Its working for Lane.
I don’t know if switching arms will make much of a difference in the real world.

[yeadon_m]

You definitely can have the shaft on which the arm fits 180 degrees out.
Then, the pump is delivering flat out at idle and lots at low throttle and only tails off it you keep the throttle pinned for a long time.
The bike will swig 2T oil.
Easy test: lift the pump, leaving the oil delivery line from the tank attached. Leave pump lever at idle and rotate the pump with a small screwdriver (shaft has a hole in it for the drive pin - don't lose that). Very little oil should be emitted if its the right way around. Now move the lever / arm half way and keep turning the pump. If the shaft is right way around it'll spit oil from 3 holes - those go to the upper cylinder.
I tested one and it was obvious it was 180 degrees out. Flipped the shaft, remounted the arm, all sorted.
Oil consumption dropped 3x.
Cheers,
Mike

[jabcb]

You definitely can have the shaft on which the arm fits 180 degrees out.
Then, the pump is delivering flat out at idle and lots at low throttle and only tails off it you keep the throttle pinned for a long time.
The bike will swig 2T oil.
Easy test: lift the pump, leaving the oil delivery line from the tank attached. Leave pump lever at idle and rotate the pump with a small screwdriver (shaft has a hole in it for the drive pin - don't lose that). Very little oil should be emitted if its the right way around. Now move the lever / arm half way and keep turning the pump. If the shaft is right way around it'll spit oil from 3 holes - those go to the upper cylinder.
I tested one and it was obvious it was 180 degrees out. Flipped the shaft, remounted the arm, all sorted.
Oil consumption dropped 3x.
Cheers,
Mike

That would suggest its easy to figure out.

Briefly try both positions. The one that smokes up a storm at idle is wrong.

[Coyote]

The one that smokes up a storm at idle is wrong.

I guess I'm OK then. Little to no smoke at idle. I think I know what it was now. I had removed the plugs and put a healthy squirt of CCI in all 3 holes. The oil accumulated in the exhaust ports was clean / unburned oil.