GS550E full throttle stutter at 8000rpm

[Vloukole]

Hi guys

My 1980 GS550E has a stutter at 8k rpm. I can't seem to get rid of it. This is what I have done regarding maintenance:

Disassembled and cleaned carbs by dipping and using ultrasonic cleaner
New O rings for carbs and intake manifolds
Fuel level set at 4mm with clear fuel hose
Valve clearance are all in range
New points and condensers
Timing was set with a timing light
Coils are still good when tested
Airbox is sealed and air filter oiled
No air leaks that I can find
Plug chop on idle circuit and mid range produced "tanned" plugs

That is all I can remember for now.

The bike starts and idles great. Mid range/half throttle pulls strong, but when going full throttle it always stutters at 8000rpm.

I did a plug chop on the main circuit
92.5 (stock) came back white and so did 95 and 100.
I then decided to use 110 and sure enough the bike was running really rich.

So here is my question: How is it possible that a stock (intake and exhuast) GS550 need main jets between 100 and 110 to not run lean?

[Rob S.]

So here is my question: How is it possible that a stock (intake and exhuast) GS550 need main jets between 100 and 110 to not run lean?

I believe they came from the factory lean, for emissions purposes.

[Cipher]

My Canadian spec 83 is jetted richer than its US counterpart though I forget the specs.
Its not an EPA thing though I do believe they increased restrictions in 1980 or therabouts.

https://www.cmsnl.com/products/jet-main925_0949192004/ a European site I think has the stock jet as 92.5 also.

[steveb922]

92,5 main was standard for all CV 550s, regardless of market.

You state that your float height is 4mm - can you give a bit more detail on that? I'm guessing maybe a typo. And do you have a fuel filter fitted? Is the filter on the fuel tap clear?

[TxGSrider]

Around here (in the US) gas has been reformulated for the past several years and it doesn't color the plugs like it used to. Tan used to be spot on, but now they will look nearly white when they are correct.

[Nessism]

Could the air filter be over oiled?

And a 1980 550 should have electronic ignition, not points. Something wrong there.
Edit: Just saw that you are in S. Africa. My comment above is for most parts of the world but not sure about there.

[Vloukole]

Steveb922 the fuel level is set at 4mm and float height is set at 22mm.
No in-line filter and tap filter is clear. Ran the tap on prime with gas cap removed but made no difference.

Ive read most “surging/stutter at full throttle” threads I could find and applied all the fixes with no change.

Nessism I think SA was a year behind with regards to models.(What I heard)
So my 550 would actually be a 1979 with CV carbs, points and no kick start.

[Vloukole]

Steveb922 will you please post a picture of your GS550 needle jet?
Mine is a X-6

https://www.ebay.ie/itm/1980-1981-1982-Suzuki-GS550-L-T-M-carburetor-needle-jet-MIKUNI-266-X-6-/124092729417

[steveb922]

I'm afraid I don't have a photo, but I can confirm that I also have x-6 needle jets, the same as yours.

Regarding points, i also have them. UK market changed to electronic ignition in 1981.

Float height and fuel level on yours are fractionally low - should be 22.4mm and 5mm. Is that enough to make a difference at 8k rpm? I'm not sure.

[Vloukole]

It is set a little bit on the richer side of the in spec range. Hoping that it would help but full throttle is really lean.

[Vloukole]

image.jpg
both are X-6 but so very different

[steveb922]

They are very different - when I get a minute I'll open a spare set of carbs and try to get a photo. But I'm about as sure as I can be that mine are the same as the bottom one in your second photo.

[steveb922]

Ok, I have an x-6 in my hand now - I've just taken it out a spare set of 550 carbs. It is the same as the bottom one in your photo of two.

[Vloukole]

So I went with 115 Main jets and it pulls way better with little to no stutter.
Plug chop indicates that it is still lean but very close. Plugs are like a dirty white colour.
Prefer something a little darker. I think 117.5 is going to work well. I dont have 117.5 Main jets so now I have to wait for the Lockdown to end so that I can order.

Where can I find original Mikuni jets?

[steveb922]

I've never bought from these people, but they seem to have what you want: https://www.motorcycleproducts.co.uk...-jets/1/016772

It looks like they are genuine Mikuni, but it might be worth asking for clarification. They also say to contact them for overseas shipping rates.