Anyone tried putting 550 carbs on a 380?

[sportston]

GT550 Nozzles. GT250 needles (can't remember which groove I put clips on),
30 pilots and can't remember what mains I put in there now I think it might have been 92.5. Will check this weekend.
However, I think running problem may have been at least partly due to a faulty fuel filter allowing my carbs to fill up with crap from the fuel tank. I plan to swap the pilots to 27.5, clean the carbs, fit new filter and try again. If only my boss would stop giving me 12 hour days, I might have time to do this. Also found a pinprick hole in fuel tank causing my nice new paint to blister. Nggggh! Out comes the solder...

[sportston]

Finally found the time to have a tinker. After moving to Scotland, life got in the way of bike fettling for a few months. Found I had a problem with a float valve not sealing. I have rebuilt the GT550 carbs again with a second new float valve and double-checked each float for air leaks before double-checking the float height on all three carbs. Hope to refit carbs tomorrow. Sadly can't get bike out of garage for test ride until I have modified my garden path!

[sportston]

Damn! broke a float bowl gasket. Now I have to wait for some viton to arrive, so I can make some reusable ones.

[sportston]

Soaked a spare old wonky gasket in fuel for a few hours and made it swell enough to fit. Engine now running. Too late in the evening to tune and can't get it to the road yet, but that is a job for another day.
She now has 27.5 pilot jets, 100 main jet L+R and 102 Centre. Needle clips on 3rd groove.

[Alan H]

Nowadays, it always pays to fit an inline filter between tank and carb(s). It only takes a tiny piece of crap to block a pilot jet.

[sportston]

Nowadays, it always pays to fit an inline filter between tank and carb(s). It only takes a tiny piece of crap to block a pilot jet.

Absolutely! I do the same. Its daft not to.

[sportston]

Update.
Had some trouble with float valve not sealing when set to standard setting. After trial and error, finally set floats at about 27.5mm instead of 25.8. That problem now fixed. After changing to the 27.5 pilots it was possible to tune her to sensible CO% levels at idle. But she is still too rich during use. Judging by colour of plugs and how little choke she needs when just started and freezing cold. Needle was set one groove lower and still had trouble with over-fuelling and slightly poor light to mid throttle response. Open her up and she flies. Think I might try gt550 needles instead of gt250a ones.

[sportston]

GT550 needles tried today (groove below top)
98 mains (100 centre)
27.5 pilots
Very strong idle, poor low to mid throttle, awesome top end. Plugs far too dark.
NOS throttle cable fitted instead of cheap modern aftermarket. The difference is considerable. The cheapy plastic rubbish was far too stiff and made it difficult to twist the throttle. With NOS cable the throttle is light, controllable and returns immediately. Lesson learnt; don't use cheap cables.

I plan to retry with 25 pilots and drop the needle the last notch.

[sportston]

I have now tried various different size jets. Currently on:
25 pilots
needle on lowest setting (top groove)
mains 90

Think it is now too lean top end; having trouble with getting float valves to seal, which is throwing a spanner in the works. Have tried different seats (Mikuni) and needles, new floats, new fuel pipe, high-quality fuel filter, even swapped one of the carbs, and found that I am having to set the float level at 28mm! Sure this isn't right. So plan to use viton-tipped float needles.

[sportston]

Got better results using genuine Mikuni float needles in the end. Couldn’t find viton tipped ones that fitted.

[markush]

Hi...

Seems you are fiddling around a lot with different jets.

I made a spreadsheet for this, to compare and select jets, needles and nozzels before trying for real.
Maybe it can help you. Download here: http://suzuki-gt250.de/sonstiges/DuesenNadeln.xls

Which GT550-Needle? 5D1 or 5DH21,
they are quiet different and meant for the use with different needle-jet sizes P-0, P-4, O-8 etc...
e.g. 5D1 with O-0, O-2 (from GT250 28mm carburetor) may be too lean at larger throttle.

I installed #2.5 float valve seats instead of #2.0.
my other data here: viewtopic.php?f=2&t=9585&p=153121#p153121

Do you use stock pistons? Or any porting?

[sportston]

Currently on stock pistons with no major porting. But I found I previously had X7 pistons and didn’t realise it.
I put stock pistons in while I travel internationally. I didn’t fancy the old girl overheating whilst in Spain and France and thought keeping the porting stock would help. When I get back to Scotland I plan to go back to X7 pistons.
My current jet configuration for stock is:
30 pilots
GT250 needles (clip on middle groove (3rd))
GT250 needle jets
95 main jets
2.5 Mikuni Float jets
New floats from ebay.
This gives a slightly rich mixture but no sign of overheating. It gives me about 10mph lower top speed than with X7 pistons. She isn’t rapid but quick enough to have fun through the mountain roads and not use too much fuel. She is capable of 40mpg if ridden sensibly. If ridden very hard, that can drop to 30mpg.

[Alan H]

When I had my first 550J back in 1972, I got mine down to sub 20mpg with a bit of 21 year old exuberance, and when I try, I can beat that with the A which has skimmed head, porting mods etc. The J got 68.75 mpg last year going to Stranraer and the IOM, at a steady 4000 rpm (60mph aprox) after the rebuild. Lovely bike - currently for sale
Pics if you pm me.

[markush]

I just revisited the early posts of this thread from 10 years ago...

Did you ever verify the GT250A intake timing was not changed over KLM 250?

I was then wrong/unshure about this issue.
Some weeks later then i got some new measurements, but never corrected it here: also intake is lowered about 2 mm in A-model GT250. (to be exact, better late then never...
GT250 Timing Data: http://suzuki-gt250.de/allgemeinesnavi/ ... ?b=1&m=MT5
GT250lm vs. GT380 Timing Data: http://suzuki-gt380.de/gt380_dateien/mo ... 50_380.txt

Meanwhile i raised the exhaust ports of my GT380 (with X7-Pistons, 550 carbs etc...)
for about 3mm, from original 33,4 mm to ~30,5 mm from cyl.-top.
Full throttle in sixth gear from 3000rpm on isn't possible anymore, one has to go from about 4000.
From 6000rpm on you got an extra power boost which gets constantly stronger with raising revs. I haven't turned over 8000rpm in the sixth so far, (for health and legal reasons, country road etc...).
There is no performance gap and no sudden kick.The motorcycle is still very easy to drive, even with a relaxed riding style.
Fuel consumption is like mentioned above around 38...40mpg, not more than around 6L/100km, in any riding style.
This is significantly less than with the original 24mm carburettors in good condition (new jets, needles...)

[Vintageman]

wow 10 years ago. thanks for update markush.

first i wan to say gt550 carbs on a gt380 no way is it too much. it is lovely and you get direct increase in power in proprtion to added carb size. yup butt dyno. one way i knew for sure is when it gets up in rpm and horsepower, clutch would slip with this mod only. in my case stiffer clucth springs solved that pretty much. i was worried about running throttle position just off idle (cruising) that it would be rough, but not so. it is a really nice mod. an that should be a sticky reply for all that ask about it

i have changed port timing on 250, 305, 350 amd 380 in various ways over the years. of course the 250 has the early version then the 1976 and up with extra transfer ports and 29.5 mm exhaust height like rd350. that bike also from factory does not like to run below 4000 rpm. that engine is a rocket. put a set of DG chambers match edfor rd350 and it really fast for old aircooled 250. almost dangerous as it can't be ridden slow (be nme), it wants to simply go fast i took a 305 and raised ex port to about 30.5mm. it was even more peaky than the 250a. it would not run well below 4500 maybe 5000 and i had to change main jetting lean else it would drown with fuel when trying to give wot starting down low. once you were high in rpms stock jetting was ok, so going lean to steat ride made me nervous to run high rpms too long. running 8000 rpm and up the cranks will not last long on these twins as i have leared a few times. the big end bearing are too small in my opinion. i am looking at using the pe175/ts185 rod, the big end eye is 2 mm wider, their is are some silver cages bearing ou there that has roller bigger to make up this difference so you can use same skinny crank pin. if you look at bearing spec only it seems it will give 10% increase in strength, And the earlyy rod for 250, 305, 350 is obsolete. i am not sure if bigger rod will hit case, don't think so but should know soon an dmake a separate post. if would be nice to find a step crank pin, but for now i think the may be a good mod, we'll see

my gt380 ex is raised about 0.5mm and widened about 2mm. jemco chanber will allow another 1000 rpm over rev/more power and webt that route vs raising ex port higher. the gt380 is a very nice to streat ride, just little low in torque as it is little bike. it make sup for it once you get into higher rpms. it is avery nice bike for sure given what it is.