Older GT750 Carbs on Newer GT750

[Vintageman]

Has anyone ever put older (73) carbs on a newer (75) GT750
What's involved. For example can I use all 75 stock aribox and boots. But use 73 cables. Oil injection arm?
How about jetting 73 OK on a 75?

I hate CVs and want VMs

[Alan H]

Different porting etc on later bikes, so the jetting will be out.
Not sure that the rubbers are the same either.
They're different on the 550 early/late models.

[jabcb]

For the oil pump…

You can use the late-model actuator arm. Need to modify it to work with the throttle cable.
The late-model actuator arm is adjusted with the throttle closed.

Alternatively you can switch to an early-model actuator arm.
The early-model actuator arm is adjusted with the throttle partially open.

If you take the arm off, its easy to get the pump 180 degrees out.
For more info see: viewtopic.php?f=2&t=11662&start=15

[jabcb]

I forgot to mention the hardest part of the oil pump switchover.

Suzuki changed the crankcase.
For the early-model carbs, the oil pump cable adjuster screws into the crankcase.
That post was eliminated when they switched to CV carbs.

You have to figure out how to mount the cable adjuster when you do the carb switch.

[rngdng]

I wish I had my computer with me. Responding on phone. I can send a pic of how I attached
a cable on my 75. Triple Ed has 72 carbs on a stock 75 engine. This is on 72 buffalo. Power wheelie city. Great running bike.

Lane

[Vintageman]

Different porting etc on later bikes, so the jetting will be out.

I think it will very close if not spot on. I can tweak jet sizes up or down a little. The 1974 GT750 has the CV carbs and ported like the 72/73. Not much difference in jetting between 74 and 75.

I can send a pic of how I attached
a cable on my 75. Triple

rngdng,

I can wait for your pics please, not in rush just hate CVs on two stroke. I’ll keep them in case I sell bike.
The oil injection control seems like a tricky part of this to do well
Who is Triple ED? I would love to know more about this 72 carbs on 75

jabcb;
Thanks. I'll have to study your info a couple more times


All,

What is the nedl ejet and jet needle series # for 1973 carbs

188 -p3 = needle jet?
Jet needle?

[tz375]

Use 73 type intake rubbers to fit 73 carbs on a 74 on cylinder.

To make the oil cable work the best way is to buy a set of 72/73 crankcases, but the other way is to bend a piece of metal to do the same job.

And yes they are 188 P4-P3-P3 needle jets using Large Round main jets.

They don't make any more power than BS40 which flow enough to support about 100 rear wheel HP, according to an antipodean friend of mine. I use them on the Phat Trakka and they do 13.1 quarter mile times at 107, so they make enough power. On the street I wouldn't use anything else than BS40 CV carbs.

[jabcb]

My bike has the late-model crankcases.

The oil pump control rod boot (part 16765-31200 or 16765-31201) was replaced with a part that looks like a T250/T350/GT250 clutch cable adjuster holder (part 11371-10000).

Looks like the PO modified the holder a bit to make it fit. Also, he may have started with a part from some other bike.

Pic of the holder. Its flipped around on my bike so you the back side of the holder.

[Vintageman]

jabcb and Tz

thanks for tips

On the street I wouldn't use anything else than BS40 CV carbs.

Why?

For street riding I have this passion as follows ( you may know: coined “lazy aggressive”). I like to just open the throttle a very little beyond idle, just enough to let the bike climb up the rpms without out accelerating too hard. On the GT750 this is more important since the bike has more power than say the 350, 380, 500 or 550 Suz. The state of tune is not easy but once achieved it is absolutely amazing IMO.

I find as the gt750 climbs it may four stroke a couple times on the way up the rpms. It’s the same old give it a tad more throttle it won’t do that, but now you are accelerating harder than my goal.
With all new jet needle and needle jets I now get 37 mpg last check (not highway , just street) where it was 30-31 before. All stock jetting. The 4 stroking is too rich condition it seems and if you lean pilot jet better but it bucks if you let off throttle. And if you lean need its flat at larger throttle. Somewhere in between the two circuit.

I just noted that the 74 has 120 slide cuts versus 110 on my 75. I may try that. The other idea not tried is a size smaller pilot with air jet added to pilot circuit to stop bucking higher rpm. So far the best is all stock 3 rd clip needle.

But here is my concern with CVs: I wonder what happens to the round slide when the throttle flap is just barley opened and the rpms climbs above 5000 towards 6000. Do the round slides begin to lift due to increased vacuum? If so that may be the problem. Maybe I need a smaller air hole in round slide or stiffer spring?

With VM I control the slides. Any f’n way I want too !

[tz375]

The "problem" with slide carbs is that they are always opened more than necessary when accelerating and that drops gas velocity and jet signal as the throttle is opened slightly. With a CV carb, it's pressure differential across the bore that causes the slide to rise at a rate that is appropriate to the rise in engine load.

If yours 4 strokes it is too rich. Changing air jets will change the rate of change of fuel delivery which might help that situation. You could also try dropping the needles 1 notch to lean out that low throttle high rpm situation.

All street bikes I can think of changed from slide to CV carbs to improve manners and eventually they changed to EFI to improve emissions but in many cases they kept the diaphragm operated flat slide. On the street, CV carbs are smoother and more consistent - just harder to understand. Then again, very few people know how to jet carbs or when reading plugs makes any sense. But that's a whole other story.

What I can say is that with UNI filters and stock exhaust, stock CV 1974 jetting is lean at full throttle.

[Vintageman]

dropping the needles 1 notch to lean out that low throttle high rpm situation.

Yes. that helped, but too lean when there is a true added load on engine as one wants to open throttle to accel harder. I suspect the slide lifting as the rpm increases even though the load is constant.

The other idea stated was smaller pilot (42.5) with air jet in (solve high rpm bucking). Now its 45 without air jet added (minor bucking worse two-up)

CV tried to be smarter than people, but they only have pressure differential to work with. If your RPM was too low, gear too high, they help you from swapping out an engine not ready for WOT. I can do that too with my built in body sensors and right hand. Manual control-the experience if raw and feeling: how it should be when controlling a mechanism

Tell me about “110” versus “120” slide cut? Is one butterfly more egg shaped than the other?

I am afraid I am going to have those CV carbs apart many more times only to fail where I have been successful at this with VMs. The bike is not enjoyable for me like this on roads where speed limit is 40 - 50 mph with some stop and go.

Think I should pursue the VMs Keep pics and info coming

[rngdng]

I'll try this again...my pictures come out too large, and I don't know why...





Ahh, finally got it. I don't remember if I used an early pump lever or not, but probably.


Lane

[tz375]

Lane,

IIRC you fitted VM34s to Blueboy. were they regular large body 34s or snowmobile small body carbs? If they were regular carbs how did you deal with the float bowl resting on the clutch cover?

[Vintageman]

Thanks

I still don't understand 110 versus 120 on CVs butterflies. Oh well

I did note when it was about 70F out my bike started cold without need for choke. Just hit the E-starter for split second, no throttle ,and fired right up and idled all cyls firing. Hmm sounds rich?

The 1975 has All stock jet values and new OEM jets, needle, floats, gaskets etc. stock pilot fuel screw setting (3/4 turn). No air jet added. New OEM air filter pre-lubed

Does that sound like pilot jet too rich?

Maybe try a little longer on the CVs before I punt and use VMs?
The only part that sucks is the dang throttle stop cable. Need three hands.
I have fresh boots on both ends of carbs so that part not too bad. Just got to not over tighten bowl screws and strip threads in body. Someday I'll buy a torque screw driver

[rngdng]

Lane,

IIRC you fitted VM34s to Blueboy. were they regular large body 34s or snowmobile small body carbs? If they were regular carbs how did you deal with the float bowl resting on the clutch cover?

They were a bit tight, and I had to grind off part of the righthand drain plug to clear the clutch.


Lane