Anyone tried putting 550 carbs on a 380?

[Craig380]

Sounds really positive, Mike, hope it continues to go well! Keep us posted.

[Alan H]

And your next project can be to put GT750 carbs on a 550 Mike!!!

[tz375]

We have looked at that for a friend's GT550

[yeadon_m]

Guys,

That very thought did occur to me as I put the 380 away next to its bigger stablemate, the 550B!

I doubt its such a simple swap but maybe it would work.

I guess the choice of carb size is, like most things, a compromise - the manufacturer of a street bike has to balance usability with reliability, weight and power with gas mileage and so on, and also the overall fit within a model range. Sometimes they have nearly optimised things so that easy gains are not for the taking yet other times changes (which some owners will regard as gains, depending on how they want to use the machine) can be achieved. Maybe this is one such example.

The experiment has been fun, but as a general point, I'm a fan of the position that if you want more power, get a more powerful bike

Cheers,
Mike

[Craig380]

@Mike, the 550 airbox has a bigger intake snorkel, is that right? Might be worth removing the snorkel from the 380's airbox, just to see how it goes.

My own 380 (stock everything apart from J&R pipes) runs better without the snorkel than with.

[yeadon_m]

Craig, I've not looked believe you're right. Something to try for sure,
Cheers, Mike

[dyrberg123]

We have looked at that for a friend's GT550

Hi TZ375 - what was the outcome?

[tz375]

12" of snow on the ground. Will try when it's warmer

[markush]

...I am very interested in how it goes for you with those gt550 and Gt250A jetting

Hello!
Yesterday I had finally time and good weather (dry, 25 ° C), to test the GT550 carburettor on the GT380.
The entire jetting is, as already described above, as in the GT250A. Main jet 100. Needle clip pos. 3.
The engine started well and ran well at idle. For a better throttle response I had to turn the idle air screws something. They are now 1 turn out.
After a journey of 150km I have the impression of a somewhat elevated performance. Especially uphill on long slopes the engine pulls a longer time before you have to shift down.
Nevertheless, the carburetor seems to be tuned too lean at half to full throttle. When accelerating at full throttle, the engine sometimes pulls slightly better, if you take away a very little throttle. Also the spark plugs B9ES are very light brown.
I will test larger main jets. Normally, the motor should run with B8ES spark plugs. With the original 24mm-Carburetor/Setup the B9ES spark plugs were already very bright. This is probably because of the prolonged intake timing by the shorter X7-pistons and the air filter snorkel, which is replaced by a larger one.
Exhaust system is 3 into 3, with original exhausts left and right and a "bent" left exhaust for the middle cylinder, mounted on the right side.(Bent to fit with using the main stand.)
Pics, directories:
exhaust: http://suzuki-gt380.de/gt380_dateien/mo ... ff/3_in_3/
air filter snorkel: http://suzuki-gt380.de/gt380_dateien/mo ... terkasten/

[yeadon_m]

Markush,

What a coincidence! Today, I too had my first ride of length on my GT380B with VM28 carbs, stock jetting (pilots 30, mains 102.5, pilot screws 1.5 turns out; stock GT550 needle jets were superb 172 P4 and all have zero pitting which is great and now very rare. Not checked jet needles yet. Slides are #2.5 ). I am running B8 plugs, fine wire tips.

http://www.kettleclinick.com/viewtopic.php?f=16&t=5715" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

I considered increasing the jet size but, with my stock airbox and carbs, it seems not to be needed.

I had the same impression of improved drive-ability all round. It pulls well at any RPM and any throttle opening and better than on the VM24s. I will keep these carbs in place now.

Cheers!
Mike

[markush]

Hello!
Another test drive on Sunday, 100km, now with main jets #105. No noticeable change in driving behavior, but the B9ES spark plugs are now dark, the B8ES will be fine now.
There was an occasional judder with low or closed throttle at pushed engine (don't know the right term in english), which stopped almost entirely by setting the PilotAirScrew to about 3/4 turns out.
I will test larger pilot jet (#35).

[yeadon_m]

>> low or closed throttle at pushed engine (don't know the right term in english), <<

I might say 'over-run' or 'trailing throttle' or maybe simply 'coasting on a closed throttle'. The low throttle roughness, I know what you mean, and I call that 'riding on a cracked throttle'. Mine went totally once I had the 550 carbs fitted. It never showed jerkiness on a fully closed throttle with the 380 carbs anyway, but it could be rough on a cracked throttle at speed.

I have stayed with 102.5 main jets and 8 plugs. I have 30 pilots and pilot air screws at 1 1/2 turns out.

Now you have more Km on it, do you like it, prefer it like this, over the stock carbs?

Mike

[markush]

The carburetor will now remain on it, it works so well. But I am not yet through with the setting. The longer inlet time (X7-pistons) probably requires extra adjustment. I will install also new needle jets. The existing are used (unknown how much), only to test whether the whole thing works at all.

[Vintageman]

Yes I agree its works well

I did markush jetting approach. That is I used all parts for a 1976 GT250A. Why not use GT550 stuff as is? Well, I had three shiny new needle jets for Ealry GT250A. The 172-P0 in my GT550 carbs were less than ideal. I could have bought 169-p0 and raised clip notch (remember why?), but, that would have been another $60 and would have had to wait for them to come in.

I did compare jet needles dia and taper and the GT250 versus GT550 have same starting diameter for example. So maybe why GT250 calls a 30 pilot and GT550 has 25 (since PO-versus O-2). I expect the GT550 would work OK too, maybe better as stated in this post


I put the rejetted GT550 carbs on my stock GT380 (other than Jemco 3 in to 3s) just to make sure it worked well. It does! My plan is to take off GT550 carbs to put on a spare of GT380 cyls that have Ex height at 32mm versus 33.3mm and a little wider, tapered liner into transfer (like GT250) and shaved piston skirts 1.5mm and i'll use t350 gaskets to effectively shave head 0.020" .... someday soon I hope.

I am glad you guys did this post! It's a great little mod on its own

[Coyote]

I have no way to cast anything, but for the triples I would love to see someone make a real manifold that runs a single 2 barrel or small 4 barrel carburetor. A pipe dream of mine. Almost as trick as Velcro mounted carbs.