GT750 carbs on a GT550

[Vintageman]

yeadon_m,

GT550 carbs on GT380.

What did you use for air box and Y boot. All GT550 stuff ... fit GT380 like OEM?

What about boots engine to carbs

What jets/needles did the GT550 carbs have. All worked well small, mid, wot?

Thanks

[yeadon_m]

Its Mike, by the way.

The 550 carbs are using the stock 380 airbox complete. I'm told it may be restrictive for VM28 550 carbs but it sure doesn't feel like it. It is capable of pulling me fast up to speeds greater than the spindly chassis feels safe at anyway.

Boots to engine are also 380 stock. I appreciate that 'it cannot work well like that' because of smaller diameters but I am appreciative of the greater smoothness over what I was getting with VM24s. Every throttle position just feels better than before. It may be that all I've done is slightly richened up the whole curve and nothing more, with a result being 'nicer' fuelling all round (if so, far more by luck than insights). But, its not the same as a few years ago when I increased the pilots from #25 to #27.5 on the stock VM24 380 carbs, when I got distinctly rich off-idle feel all the time. Not nice.

The carb internals are all 550 stock, so needle jets are 172 P-4 (very nice condition), jet needles are I think 5DN21-4 (though my records are not clear at a glance - I've played with several sets). Mains are 102.5 and pilots are #30.

Cheers,
Mike

[Vintageman]

Mike,

my name is Mike as well. I'll use Mike S.

Thanks.

I would like to use GT380 Air box stuff too.

So the GT380 Y boot slipped over the 28mm carb mouths OK?

Mike S

[yeadon_m]

Mike,

Yes. The happy finding (which others knew I'm sure) is that the VM24 and VM28 carb bodies are identical except for the size of the venturi. In other words, the 380 carb has very thick walls!

As its also the same length carb, it slips in nicely at both ends and the stock airbox is in the right place.

Not going to be so easy with 750J/K VM32s on the 550 though!

Cheers,
Mike

[Alan H]

Stop messing about and just gerronwi'it man!!

[yeadon_m]

Pot, kettle, black!
Lets be hearing that 550!
>>ducks<<

Mike

[Alan H]

Send me some of your spare time and I will!!!
It's just the lack of it that's jiggering my bike up at the mo'.

[yeadon_m]

Ditto! wretched thing called 'work'. Can't wait to be able to afford to stop.
Mike

[Vintageman]

yeadon_m (Mike)

Where is you post where you say you put GT550 carbs on your GT380?

Well, my question is: Now that you had time with GT550 carbs on the GT380. Do you still believe it is jetted well all throttle positions or it needs/ed some fine tuning?

How is the gas mileage now?

Any side affects or trade offs in performance pors vs cons?

I think you used the 1977 version of carbs which upped the Needle Jet size from P0 to P4, all else jetting was the same for the ganged type later GT550s

Has others tried this?

So cool.

I am either going to run GT380 bored to 26mm or put GT550. GT550 need new needle jet and not sure if P0, P2, or P4 size. I also plan to do some mild porting and chambers to my gt380

Thanks

Mike S

[yeadon_m]

I'm away from home and don't recall where I posted my findings about late-550 VM28s on my 380B.

But yes, it fuels perfectly from idle to wide open and all points between, its the nicest-fueling bike I've ever owned now!
I don't know if it makes more power - I sense it does, just a little. But whats more impressive is just how great it feels to drive now, not a single glitch. Of course, others may get different results.

On how much fuel it uses, I have no idea nor do I care, sorry.

The needle jets are indeed P-4 in the VM28 carbs.

I like the set up so much I don't plan to return the VM24s to the bike (though I will keep them forever, as with the bike itself...owned it since 1978).

Cheers,
Mike

[markush]

Hello!

I use GT550-carbs on GT380, too.
Described in the searched thread here:
viewtopic.php?f=2&t=9585

Last Sunday it consumed about 7 liters for 70 miles.
(I put 10 ltrs into the empty tank and had to switch to REServe after 70 miles)

bye
Markus

[Alan H]

Last Sunday it consumed about 7 liters for 70 miles.
(I put 10 ltrs into the empty tank and had to switch to REServe after 70 miles)

It's better than that - the reserve is about 4.6 litres (1 gallon) normally, so you must be driving very smoothly to get that sort of fuel mileage.
I always zero my odometer when I fill up (brim the tank every time) which then makes it easy to work out.
The 550 averages 10 miles per litre or about 45mpg, unless I spank it and it drops to a shade under 8 miles per litre or about 35 miles per gallon.
From brimfull to reserve is about 90 miles, and by the time I fill up I get about 100 miles/10 litres - about a 50 mile reserve.
The 550 and 380 have the same tank, 15 litre, with 4.6 litre reserve.

[Vintageman]

markush (Markus) and Yeadon_m (Mike)

Markus, OK thanks for pointing out the GT380 rubber boots carb side may be too small for 28mm and need to drill it or get 550 boots

Markus and Mike I think you both use stock GT380 air box and both later model. Both left snorkel too?

Markus you are running X-7 pistons and we were thinking this raised intake timing 4-5 mm or so .... once you are ramming fuel into carb (momentum at higher rpm) I don't think jetting too different versus stock timing


Markus you used GT250A jetting as you starting point. Do you recall you final jet values?

Mike you stayed with GT550 (1977) jetting and you are very happy. It's like the old Haynes(?) article you noted predicted (guessed). Mike my challenge to you would be to check mpg some day. It can run very well even if the needle jet is size is 1 or 2 stoo big but, may vary like5-8 mpg(US) less.

I changed my GT380 intake timing by 1.5mm. Then I went to ~32-31.755mm (hands shake) on Ex. I plan to run T250 Aluminum metal head gaskets which are 1/2mm versus the 1mm stock ones. I plan to use stock GT380 (1974) air box and snorkel (hurts my ears otherwise). I run Jemcos 3 into 3.

Markus your OEM looing 3 into 3 exhaust is awesome (you Kawasaki lover!)

For GT550 carbs on 380 I think I’ll use P2 needle jet to start (unless Mike you tell me you are getting 40 mpg US). All else GT550.

For my 26mm GT380 carbs (2mm bored) I would keep all GT380 jets accept make needle one clip richer and up main jet a little based on area increase like -> 13^2/12^2 = 17% so I would go 15% larger. I have done the carb bore trick a few times in my life and that is kind of where I end up… low risk.

But it sounds like GT550 carbs should be close too.

Well my riding season is coming to an end.
I have no new bikes yet (Honda CB350/CB450 )but, plan to do this gt380 mod and mod my t500 with mild porting (1mm off intake, 1-1.5mm off exhaust and 1/2mm off head maybe).

I was happy with my mild porting on GT550 Ex now 38mm versus 39.5mm and intake 1.75mm off skirts. Shifted peak about >300-<400 rpm higher say 6800 versus 6500 what it was and over revs just past 8K quickly where before that was a struggle. You always wish you do more, but snap… you go too far for relaxed street riding. Bigger bores have the nice torque.
The T305 porting I recently did is not too far (I ride this a lot), but no more relaxed riding. I hope the GT380 ends up more in-between.

Fun stuff

[Vintageman]

markush (Markus) and Yeadon_m (Mike)

Markus, OK thanks for pointing out the GT380 rubber boots carb side may be too small for 28mm and need to drill it or get 550 boots

Markus and Mike I think you both use stock GT380 air box and both later model. Both left snorkel too?

Markus you are running X-7 pistons and we were thinking this raised intake timing 4-5 mm or so .... once you are ramming fuel into carb (momentum at higher rpm) I don't think jetting too different versus stock timing


Markus you used GT250A jetting as you starting point. Do you recall you final jet values?

Mike you stayed with GT550 (1977) jetting and you are very happy. It's like the old Haynes(?) article you noted predicted (guessed). Mike my challenge to you would be to check mpg some day. It can run very well even if the needle jet is size is 1 or 2 stoo big but, may vary like5-8 mpg(US) less.

I changed my GT380 intake timing by 1.5mm. Then I went to ~32-31.75 mm (hands shake) on Ex port height. I plan to run T250 Aluminum metal head gaskets which are 1/2mm versus the 1mm stock ones. I plan to use stock GT380 (1974) air box and snorkel (hurts my ears otherwise). I run Jemcos 3 into 3.

Markus your OEM looing 3 into 3 exhaust is awesome (you Kawasaki lover!)

For GT550 carbs on 380 I think I’ll use P2 needle jet to start (unless Mike you tell me you are getting 40 mpg US). All else GT550.

For my 26mm GT380 carbs (2mm bored) I would keep all GT380 jets accept make needle one clip richer and up main jet a little based on area increase like -> 13^2/12^2 = 17% so I would go 15% larger. I have done the carb bore trick a few times in my life and that is kind of where I end up… low risk.

But it sounds like GT550 carbs should be close too.

Well my riding season is coming to an end.
I have no new bikes yet (Honda CB350/CB450 )but, plan to do this gt380 mod and mod my t500 with mild porting (1mm off intake, 1-1.5mm off exhaust and 1/2mm off head maybe).

I was happy with my mild porting on GT550 Ex now 38mm versus 39.5mm and intake 1.75mm off skirts. Shifted peak about >300-<400 rpm higher say 6800 versus 6500 what it was and over revs just past 8K quickly where before that was a struggle. You always wish you do more, but snap… you go too far for relaxed street riding. Bigger bores have the nice torque.
The T305 porting I recently did is not too far (I ride this a lot), but no more relaxed riding. I hope the GT380 ends up more in-between.

Fun stuff

[markush]

Hello!

The 550 and 380 have the same tank, 15 litre, with 4.6 litre reserve.

I'm pretty sure that I have only about 3 liters reserve. Perhaps there are differences between the models or in different countries? I have fuel taps with different length tubes, both are in original condition. The left with the shorter tube has only a hose pushed over the pipe, because it has a longitudinal crack. I have installed the one with the shorter tube. (My GT380B owner's manual says 3 liters, too.)

Markus and Mike I think you both use stock GT380 air box and both later model. Both left snorkel too?

I use a modified, larger air filter inlet, it is no more a snorkel, which I have already used with the 24mm carburetors.
To be seen in this directory: http://suzuki-gt380.de/gt380_dateien/mo ... terkasten/

Markus you used GT250A jetting as you starting point. Do you recall you final jet values?

No changes since the last statement:
Needle Jet: Type 172, O-0
Needle: 5CN3 -3 (= clip third groove from above)
Main Jet: #105
Idle Jet: #30
It runs so very well.

Markus your OEM looing 3 into 3 exhaust is awesome (you Kawasaki lover!)

Thanks. I think the 2-stroke triple Kawasakis OK, but I'm not a real lover of them. You have to spend more time in working on it then in driving with it. The exhaust arrangement inevitably results. You could put the middle exhaust also to the left, to be distinguished from the Kawasakis, but then it's more difficult to work on the chain.

bye
Markus