Jet sizes for GT380 with GT550 carbs

[sportston]

Would anyone have an idea as to what is a good starting point for jet sizes when fitting a set of GT550M carbs to a 0.5mm overbored GT380M?
I've bought the larger carbs to upgrade my GT380 but still keep the standard look, but obviously the GT550 jets will be the wrong sizes for the smaller engine. I know this is not an uncommon mod, so wondered if anyone had any idea as to what size jets to put in them.

[Vintageman]

a couple years back there was a post on this I think one person is yeadon and myself and others too

I run GT250A jetting specs in my later style GT550 carbs on my 1974 gt380. all new brass - needle jet, jet needle, pilot, mains etc, I use the GT550 intake rubbers since larger.

The other gentleman stayed with later stock GT550 jetting. Mine works wonderful and so did his. This is a nice performance boost. I have no dyno, but the gt380 will now slip the clutch unless bike fully warmed,. And, it only does it when I pass into and over 6KRPM and use > 3/4 throttle out of those 28mm GT550 carbs. The gt380 cylinder intake port area is greater that the area of 28mm carb so IMO why it works well. Still run well small throttle too. Stock air box and chambers

[sportston]

I run GT250A jetting specs in my later style GT550 carbs on my 1974 gt380. all new brass - needle jet, jet needle, pilot, mains etc, I use the GT550 intake rubbers since larger.

The other gentleman stayed with later stock GT550 jetting. Mine works wonderful and so did his.

I've looked up those settings in the book.
The stock GT550M has 95 (with 97.6 in the centre carb) for main jets. And 27.5 for pilot jets with a 5DH21.3 needle jet.
The GT250A has 92.5 for main jets and 30 for pilot jets with a 5CN3-3 needle jet.
So they are pretty close; only one step different from each other.
I'm guessing the greater difference in the overlap from pilot to main jet is to account for the larger capacity of the GT550 cylinder having a little extra torque and not needing such a rich mixture at lighter throttle openings?
The needle jets are different serial numbers but i couldn't tell you what they mean. I'm guessing they are the same size (denoted by the 5?) but different profile. But I'd be happy for anyone with superior knowledge to correct me on that.

[Vintageman]

don't forget Needle jet. The early GT250A versus later tweaked some brass sizes (needle jet, main etc.). I don't remember anymore which I run or combination thereof.

[sportston]

After further investigation I have found that the jetting for the GT250A is listed differently according to where you read from. In some manual it says the left carb has a different main jet than the right carb. i guess this is to compensate for the lack of choke on the right carb. So I will have to play around with jetting. I am thinking of starting with 97.5 on the centre cylinder and 95 on the outer two for main jets and 30 pilot jet with GT250 needle.
Incidentally, iI think the rubber inlet for the gt550 carbs is the same as on the gt380. The outside diameter is the same on the flange of the carb. Only the inside diameter of the carb is wider on the GT550. All external measurements are identical.
So I am going to use the same rubber inlets as I got for the GT380 carbs

[Vintageman]

. All external measurements are identical.
So I am going to use the same rubber inlets as I got for the GT380 carbs

Internals are different so if you don't swap flow will be bottle necked

After further investigation I have found that the jetting for the GT250A is listed differently according to where you read from.

I think there both the same L&R per suz manuals I have seen. The GT250A later (see service bulletin) made some jetting changes versus early A to help with warranty failures... when you ride the GT250A/B its hard not to find you self hold WOT all the time and she puts out alot of hp for air cooled engine IMO.

I think the reason the GT550 stock has a smaller main jet center is because is because of air inlet design (just my reasoning no basis) For example on the 72/73 the center boot has a longer horn. Longer horn give a little more air flow restriction so a smaller main.. seams like suz detuned the center a bit to ensure it ran a little cooler... agin not sure but what it looks like to me

[sportston]

Ah I didn't realise the internals where different and could create a bottle neck. I will check if the aftermarket ones I have are suitable tomorrow.
Yes I thought it was a bit strange having different jet sizes in left and right carbs on a twin. I only found that differentiation in one manual. All the others listed them as the same. Go figure!
I can understand having a different jet size in the middle pot on a triple since there is a temperature differential to compensate for. Make it run slightly richer and you have a cooler cylinder. But on a parallel twin.... I found it hard to understand why too. Perhaps it was an error in the manual.
However....I notice you mention a smaller jet in the middle pot. I notice this is specified in the Suzuki and Haynes manuals for the GT550 but on the GT380 it is not.
I have come to the conclusion that you may have got it right about air flow. The middle pot has a shorter distance between filter and carb than the other two. This distance is exaggerated on the wider GT550 engine, but on the smaller GT380 it is less important. Or perhaps...Suzuki realised that you needed a richer jet on the middle of the higher tuned GT380 to stop it from getting so hot.
That is my opinion anyway. I still plan to put a richer jet in the middle. Maybe I'll be proved wrong. If it runs too rich on that pot I will put in one the same size as its neighbours. But I'd rather risk sooting up a plug than seize a piston.

[tz375]

It used to be expected that the center cylinder would get hotter that the outer pair and so would need a larger main jet to keep it cool. That was the case on Triumph and BSA triples IIRC.

On GT550 and 750 the center cylinder has smaller main jets and that is believed to be because the split exhaust pipes create a stronger pulse which in turns pulls in more fuel. Hence the slightly smaller main jet on the center cylinder. It appears to be true that the center cylinder runs cooler with stock pipes but whether that is from the airbox or exhaust, I don't know that anyone has tested those theories.

Looking at the intake plumbing I can see no real flow restrictions but I haven't put on on a flow bench either.

When we fit different carbs and triple chambers, it is normal to jet the same across all three cylinders and oddly enough my bike still runs cooler on the center pot without an airbox, so perhaps it's a function of coolant flow and effectiveness.

A 380 is basically 1.5 250's so 250 jetting is a useful baseline as long as the carbs are more or less the same size as a 550 carb. On a 380 with chambers and 550 carbs it is likely that the bike will make as much power as a stock 550 and will flow as much air which suggests that 550 jetting would be another good point to start from.

[Vintageman]

Looking at the intake plumbing I can see no real flow restrictions but I haven't put on on a flow bench either.

When I look at the early GT550 intake 1-3 boot, more so than the later gt550 design, the center carb has a few if not more inches of rubber tube extension where the others do not. Typically, this added length of tube creates a restriction intake side( and other affects.) allowing the main jet to contribute more fuel ration wise than without extension. It looks very weird to me to see how much they added (almost hits the back wall or early bike). And, since I run stock air box I continue to respect this main jet scaling versus the other two carbs as Suz did. No data to back just reasoning/inspection and decision I made back when. I run chambers and have for a while. Right or wrong that's my setup.


There are audio tapes for GT750 sometimes set sold on eBay where parts you can listen to on YouTube. Women with British accent. She says the reason the center pot has smaller main for it has less volume. How is that true? How does she know this.

I suppose the exhaust could pull harder too, but not sure why that would change inlet ratio characteristics unless the jet was not well centered within the expect min to max CFM for the inlet air path. Usually changing intake (like pods) can be significant.

Would be fun trivial if someone had a relative in Japan that knew the engineering team who designed the bike and why the did this.

[tz375]

Those DVD's from the UK I believe were made by Baz Wilkes and the voice over is a local newscaster - or that's what I think I recall of that story.

Center cylinder and crankcase are the same volume as the outer pair, and so is the inlet tract, so I am not sure what they intended to convey with the comment. Maybe a reference to the exhaust volume?