Suzuki 2 Strokes

Folks,
I have had fun adapting a set of VM28 carbs from a GT550 to fit my GT380B and been very pleased with the results. I don't know if 'performance' in any formal sense is better, but I do know that the bike is now just much nicer to ride, at all rpms and throttle openings.

So, as the lucky owner also of a GT550B, I am wondering if GT750 J/K carbs might likewise be adapted to fit on a 550?

I'm aware that UNlike the 550 to 380 swap where all the parts are the same except the carb bodies (making this a very easy, if time-consuming experiment) this isn't true for a possible 750 to 550 swap.

Has anyone ever done it and if so, what did it involve and what was learned? I have read that 'it works' but don't know anyone who has actually done it.

I'm in no rush to do it but fancy this as a future fettle. Nice to have new things to look forward to.

Cheers,
Mike

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Carburetor Change-GT380

After milling the head and possibly mild port modification, you could remove the GT380's 24mm carburetor assembly and replaced them with the 28mrn set of Mikunis used stock on the GT550. This modification is an easy bolt-on.

Rejetting is probably not necessary on a modified 380, since the 550's baseline jetting should be ideal on the modified smaller engine.

Main jet sizing on the GT550 carburetors should be 95 on the center cylinder and 97.5 on the outer carburetors.

Performance will remain stock on the low end, gradually improving through the midrange and the upper rpm range.

Carburetor Change-GT550

A mildly modified GT550 may also have its performance increased by replacing the stock carburetors with three 32mm slide-needle Mikunis.

....the stock throttle cable must be replaced with a (pre-CV carburetor model) cable from a GT750.

I believe the 28 mm helped the Gt380 as you say.

For GT550 28 mm to 32mm(?) You'll have to open Cyl inlets too. It may get thin too need to check.

They barley match the 28 id. ( I get 1.125" Carb ID for Gt550 = 28.575 mm) now.

I plan to run a 1 1/8" up to 1 3/16" drill into intake a ways and follow the angle the intake passage takes on the way down when I do: Not horizontal.

I plan to only go into the divider a bit at the bottom and little bit more at the top since drill will be angled versus end of divider plumb with carb stock.

This should better align the 28s and remove the kink the upper half of divider may cause to flow as it transitions from horizontal carbs down inlet angle.

The carbs would be better served if they could have been mounted on a slight angle like early T series.

You should paper trace the two ports to see how much area they have now versus area of GT750 carbs (square ports - round carbs so easy math). This may be the limit unless you open up.. suppose to be a little more area than carbs.

Thanks Vintageman,

Aside from opening it up, do you know how best to attach the VM32 carbs to the GT550 cylinder inlet? I expect the 550 rubbers will be too small for the carbs, but unsure if the 750 rubbers will be too big for the cylinders?

Then there is the little problem of three chokes with separate cables, but anything can be fixed in time.

Cheers,
Mike

I have drilled out existing ones in the past for different engines. If 4mm larger OD and ID that is only taking off < 0.080"

First drill with new ID all the way, Then use second drill with new OD part way for carb to go into only.

Now regular drills want to screw in fast when you drill into that soft grabby rubber (dried ones mod better). I take drill and run in reverse. You may have to sharpen it for that direction else it won't cut well.

The other Idea it to use a wooden dowel and place in center then use a wood drill (flat type with point) OD, The dowel will give the flat type wood drill a center. I never tried this one.


Some options to consider

yeadon_m wrote:
Then there is the little problem of three chokes with separate cables, but anything can be fixed in time.

Just use a 550J/K or 750 J choke lever and cable - there were 3 separate chokes on the early (separate) carbs and the cable was a 1 into 3 jobby.

I measured intake windows ( 72 04 73 GT550) and just under 21 mm wide and high each. If you assume square (there not all 4 corners have a significant radius) that would be 880 mm^2. If I factor in corners it is more like 19.5 mm ea. for 760 mm ^2.

28 mm Carb ID = 615 mm^2
30 mm Carb ID = 707 mm^2
32 mm Carb ID = 804 mm^2

Just a few random thoughts.

26mm carbs on an RZ350 (RD350YPVS) easily support 60 HP so if you think of a 550 as 1 1/2 350s (not mathematically accurate but close enough) then 26mm carbs would support around 90HP on a 550. Of course that's not the case because a 550 doesn't have sufficient port time area but the crabs are NOT a significant restriction.

When I ran a slew of MOTA simulations on the 550, 28mm was enough to support around 80 theoretical crank HP. 30mm added 3 at the peak and lost 2-3 HP up to 5,500. Going up to 32mm Lost a little more up to 6500 and added zero at the peak but added a scant 1hp or so after the peak.

That was a pretty basic simulation and perhaps more sophisticated inlet duct modeling might have generated a little more at the top end.

My conclusion was that 32mm was a waste of effort on even the most highly tuned motors. I would contemplate boring a set of 28s out to 29 or 29.5 if I had the time and cash, but for a street bike I'd use 550 28mm carbs and leave it at that.

What I would expect is for larger carbs to feel more powerful because the transition from less power at 5500 to more from 6000 upwards would be a little more pronounced. It's that rate of change that would make them feel more powerful but would actually be slower on the street.

That's how most aftermarket pipes on 4s used to work. They dropped into a midrange hole and the climb out of that hole made them feel much stronger than stock even when they made less peak HP. How they feel can be deceptive.

Thanks TZ, useful stuff. I wonder if your explanation of 'transition' is what is now making the VM28s from a 550 make my GT380, usually running VM24, feel 'much better'? I am making no claims for more power but riding the thing now with VM28s on its just so 'nice'. Any rpm, any throttle input, it feels smoother and, well, better. Some small but, to me, important changes include that now, when moving fast, if I'm riding with an almost rolled-off throttle, say leaning into a bend, then gentle roll-on again, there is no harshness and jerkiness that I used to experience on the VM24s.
After many carb tweaks, I find that 'nice' beats grunt anytime as its just very pleasurable to ride a smoothly responsive bike
In the fullness of time, if I struggle to make the 550 with its stock VM28s get to such a nirvana, I might try VM32s in the hope of a repeat. While I'm inexpert, I do like to play.
Cheers,
Mike

If your needle jets had any pitting inside that may be what you are explaining. I am 1000% sure pitted needle jets make for poor street riding for they are always in play. It does not take much roughness to mess that circuit up

But would larger carbs add power t GT380, one that one you could feel it almost everywhere. Sure do!

I think the reason larger carbs work on GT380 maybe be because the intake port has enough spare area to accommodate. There is some rule of thumb that port area at cyl is to be larger than carb area to be of value. It makes sense given restriction increases with path length.

I am not 100% sure but, I think the GT380 intake port area is the same as the GT/T250. And, the GT250 had 26 mm then the later had 28 mm carbs. The later GT250 bulletin said nothing about enlarging intake port (who knows what they really did). They added 28mm to get higher RPM response I assume

So Suz did all the work and proved bigger carbs add value for cc/HP the early GT/T250 is putting out more than GT380.

I have some 74 GT250 Cyls and can measure the area to see if this hypothesis holds up.

Anyone have GT380 cyl and can check intake window area?

Does anyone know the Needle Jet series in the later ganged GT380 carbs?

It looks to me like 32 mm are more than GT550 can chug down based on stock intake port area.

I believe they are 164 O-2 but best to check with a 380 owner

Just checked some late-380 carbs and yes, the NJ is a 164 O-2

JN is a 4DH7-2 and the slides are #3.0

Mike

looks like 164 series Needle Jet obsolete of course

Does anyone know if this 164 series crosses with and other series that maybe uses a Hex jet versus round for example?

You know like the 188 series does with the 159 for example.


Thanks

If someone has the dimensions, I can see if I can find a similar needle jet in my database.

Did OEM search and they're same as the 73-75 GT250 and a couple of the T250s. Have some of these T/GT250 carbs looses somewhere and will pull one a part. It would be good to see if there is a cross. Ivan's makes a kit too,

Also owe intake area ala paper trace for a GT250. Anyone have spare GT380 cyl to check?

yeadon_m got me motivated to up my carb size on my GT380 too.

I plan to bore a spare set of 24mm bodies and try that way. Won’t get to 28mm maybe near 26mm maybe. There is a lot of meat on the GT380 24mm to open up. Up main jet same percentage and needle clip richer a notch should do it

Does any have a loose set of GT380 cylinders and can measure the approximate inlet openings.
Don't count the crude chamfer they did just the true initial opening size at the start.

Back to this post
The 28 mm on GT550 as I mention above outlet were a little over 28mm (1.125"). And, the thinnest on OD is 1.3" these. Leaves 0.175" to work with. Not too much but, you may be able to make them 30mm!
32mm work well on the T305/350, It can be too much if you crack WOT from a lug (why would you) but, recall that intake is larger (don’t have one loose to paper trace anymore).
Think you would have to open up GT550 twin port windows. I am finding just a nice sharp bastard file works well when you can get straight access through port

Fun Stuff

Some data

164 Needle Jet dimension: Length : from top of shoulder to bottom where MJ attaches is 1.580" or maybe 40mm.
Shoulder thickness is 0.138" or maybe 3.5mm.

Each intake port window of a 74 gt250 is 18.5mm wide by 19.5 high. Corners are radius so effective area is more like17.5mm by 18.5mm

Area in intake port is 721.5 mm^2 (not counting corners)
647.5 mm^2 (if corner area excluded)

26mm carb area is 531 mm^2 so effective intake area is 20% more than carb


The cyl intake is 26.5 mm x 27. mm so nice if all aligns

Can anyone check a GT380 cylinder window and intake starting size. I have no spare myself