Anyone tried putting 550 carbs on a 380?

[markush]

Hi!

Another small change to the "Frankencarbs ".
I swapped the pilot jets from #30 to #35.
The Pilot-Air-Screw can now be set to 1+1/2 turns out.

The bike RUNS.
I am thinking of a second front brake disc.

Markus

[yeadon_m]

Markus,

Interesting! is it 'better' off idle and part throttle than on 30 pilots?
I have 30s and its as close to perfect fuelling on my 380B as I could wish for.
What prompted your experiment? expansion chambers? I have stock exhausts.

Cheers,
Mike

[Vintageman]

When I go WOT and then pass through 6000 RPM the clutch slips (until gear case warms up). Didn't do that with stock gt380 carbs

I am running 30 pilots, but thinking of trying 27.5... for crackling at very very small throttle light loading. I bench sync carbs. This crackling seems like it’s a tad rich... but, I never get that right (32.5?). Well, if you are too lean typically it will buck a little closed throttle and not the case. I have this fetish about running my bikes small throttle... this is at just about closed, maybe not fixable and it’s that classic crackling.

Maybe what I should do first is compare the size of the pilot hole in the GT380 versus GT550 carb bodies… If GT550 larger maybe why carbs on GT550 used 25 pilot?

Markrush is running X7 pistons? A lot more intake timing for what every that does

I run JEMCO chambers and you can hear everything the engine does. Jetting is really very dang near perfect as is and should open pilot air screw a little (that never does poop for me). Or light city traffic let it crackle... it does so with a very regular 2 stroke rhythm

[yeadon_m]

Hah! I'm not the only obsessive then.
I too like 'cracked throttle' feel to be as good as can be. 550B is a little woolly (might need to play with pilot air or jet needle clip?), the 750A is nasty (surge) but the 380B running VM28s is puurfect!
Also, I too can just induce clutch slip at max power on my 380 with frankencarbs, but I have to try very hard to do it, and I don't usually.
I have not measured the 380 carbs, but my notes on my VM28 550 carbs (which are from UK A or B model) show these rough estimates used gauged wires, carefully used:
-pilot outlet (vertical hole) allowed a 1mm to pass but not a 1.5mm, guess they are 1.25mm-ish.
-pilot bypass (forward facing hole) just passed 0.5mm, slightly tight but not forced fit.
I'd be interested to learn if the VM24 380 carb pilot holes are same or different, for you make a very good point.
Cheers,
Mike

[markush]

Hi!

... is it 'better' off idle and part throttle than on 30 pilots?
I have 30s and its as close to perfect fuelling on my 380B as I could wish for.
What prompted your experiment? expansion chambers? I have stock exhausts...

The engine was running too lean at low throttle. See also my posts here from last June.
I think it's not ideal if you have to turn in the pilot air screw so far. With the #35 pilot jet the air screw is set more in the middle of its adjustment range.

Markus

[Vintageman]

markus,

refresh me what else about your bike is not stock. Note I used the early (O-2 needle jet not O-0) GT250A jetting similar to what you did. I had shiny new Needle Jets for it and that's why


yeadon_m "woolly" love that description

Funny my GT550K seams perfect small throttle now, the GT380 with 550 carbs is just a hint off as stated, I am not sure I would use woolly (maybe in denial), as much as regular crackling sound (I should take a video). It sounds good - even but, I don’t like crackling and to me it should never.

There is nothing more enjoyable for a street rider than good off idle to about 1/4 throttle. I like jetting well center so you can climb a wide rpm range holding a fix throttle position … kind of a lazy aggressive riding style. If you have to work throttle even a bit to find it smooth it’s not as relaxing or as enjoyable as it can be.

The carb information is in the "Suzuki Document" I think you posted it once to, has all the info... I have not tried to look for it again and probably at OJB site.

My GT750M does not buck anymore. I did the air jet mod and that didn't take it all away. I just changed the pilot jets to 47.5 from 45s. Two were stuck, what a @#$%!. I did replace the Needle Jets with NOS … a couple of mine were little rough inside but, seen worse... it doesn’t take much to make things woolly.

My GT750 has needle on 4 th clip down from top. I tried 3 clip per Suz specifications but. it wants 4 th for too lean small throttle (funny they say needle does cover down that low, but it sure does). My understanding that may be how it came from factory for I am 2nd owner and said he never did anything to them. That bike is smooth as silk and no more bucking. I have not recheckd my MPG yet to see if it’s still only 30-31 MPG or got better.

[markush]

Hi!

...refresh me what else about your bike is not stock...

OK:
"Frankencarb" with
Needle Jet: O-0
Jet Needle: 5CN3 (-3)
Main Jet: 105
Pilot jet / Pilot Air Screw: *)
#30 / 1/2 to 3/4 turns out (was)
or
#35 / 1,5 turns out (now is)
fuel/float level: stock

modified bigger air filter box inlet

X7 pistons (prolonged inlet time)

3 in 3 exhaust, made from stock exhausts

electronic ignition

*) I once read in a Mikuni release, if you have to turn the pilot air screw very far in or out for a good setup, it would be better to go to the next pilot jet size, which I find plausible. That's why I made the change.

[Vintageman]

I am running 30 pilots, but thinking of trying 27.5... for crackling at very very small throttle light loading. I bench sync carbs. This crackling seems like it’s a tad rich...

Just an update on this. I had a very small hole in one of my carb's float,left side. The other day I noticed gas coming out of overflows when idling at a stop.

Tore it down and indeeed one float was partially filled with gas. I fixed that and thought about changing pilots from 30 to 27.5 as well but resisted The float leaking must have been my very small throttle crakling issue all a long and just got worse over time as that float sank a little further.

I will now say that the later GT250A jetting values I used (accept I use early GT250A needle jet) as noted somewhere here is perfect for a stock GT380L with Jemcos. Also the 28mm are not too big in the sense even if you are too high of gear and give it WOT it swallows it no problem and when you back off throttle partially there is no signs of too rich or too lean leaving WOT position. As far as small -to near mid throttle the bike will climb from low rpm past peak RPMs never moving the throttle even a bit: even at the smallest opening possible.

I think Mike also states the same for the stock GT550 jetting he noted.

Did it add more power? Hmmm I think it does respond beter if you are passing 6000 rpm and using ~4/5 throttle then go 5/5... you feel extra pull from the extra 1/5.

This was too easy.

[markush]

Hello again...

Did it add more power? ...

Yes, it did.
I now made an indirect power measurement, using a digital speedometer which also shows the acceleration time between 0 and a chosen speed.
With the weight of the bike+driver one can calculate approximately the power (1.014PS =1HP).
If I use kg-weight and the acceleration time in seconds between speeds 0 and 100km/h , (as is usual here in germany), the approximate formula becomes very simple.

power(PS) = Weight(kg) / acc.Time (sec)

Time measurement shows 6.2 sec
Weight: 265 kg
So I get 265/6.2 = 42.74(PS), about 42 HP,
some extra HP compared to the original state

[Vintageman]

One thing I just discovered about the "GT550 carbs on GT380 mod". The air box is the same except the snorkel letting air into the air box is larger on GT550 vs GT380. The GT380 has 2 air inlet tunnels where as the GT550 has 3 similar sized ones for about 50% more area. Just swapped mine for GT550 ... should try soon on my 74 gt380. I don't know is was mentioned on this post or not but you need to use the GT550 rubber boots that mount carb to engine for GT550 is larger diameter than gt380 boots.

[markush]

Hi!
You're right!
Both was mentioned in this posts:
snorkel
rubber boots

[Vintageman]

Yes, Thanks for link to pics. It shows how small the gt380 snorkel is vs gt550. I missed that first time around. I am awaiting a new rear sprocket and should try my 1974 GT380 bike again soon. My gt380 has been down for more than a year due to crankshaft failure. Now has a full rebuilt crank, 1.5 OS Wiseco pistons, Mild porting (i.e. wider on Ex and Intake ports but, same heights)... I put new clutch springs this time as well, since it slipped clutch once I put the GT550 Carbs.

It is a nice mod and this post should cover all points

[sportston]

I've got the GT550 carbs on mine but forgot to change the snorkel. Just seen how expensive it is to get a gt550 airbox with snorkel. Holy crap! Think I might modify mine with a piece of stainless and rivets too. That will cost me nowt since I have both.

[sportston]

So I modded my airbox by cutting off the bottom of the snorkel. Took bike out for first ride. Ran crap and progressively got worse. Turned out battery wasn't being charged due to broken wire to reg/rectifier. Fixed that and bike runs better. Problem I am having is poor light to mid throttle and invasive crackle and hesitation just before the powerband kicks in. Mixture is rich at idle (4.5% CO) and won't lean off much. I'm beginning to think the needles may need to be set at a different height and perhaps different pilot jets.
Was surprised to see how high the gearing is on this bike. 6th gear was barely used even on dual carriageway. Perhaps I need a smaller front sprocket.

[markush]

Would you tell us the setup of your GT380?

Carburetor nozzles, needles, sizes, setting?
Tuning / porting?
Secondary transmission?
exhaust system?
etc.