1982 GS570T - 1/4 Throttle Onwards Flat Spot

[SamLack99]

Up to quarter throttle, Suzy is an absolute dream to ride.

But above that?

Nightmare.

Pretty much exactly on quarter (I’ve marked the throttle grip with tippex) she bogs. She doesn’t really lose power, she just kinda flattens out and makes a loud ‘BURRRR’ sound.

If I keep turning the throttle, she will try to punch through the flat spot, and when she does, it’s like she’s been kicked up the arse and jerks forward. By this time, I’m either running out of road or fast approaching the speed limit.

I took her on a 70mph duel carriage way recently to see if she just needed the cobwebs blowing out. Managed to get through the flat spot. All seemed well, but then she started losing speed and power. I had to nurse her back to below quarter throttle until I could get off the dual carriage. Not fun having an 18-wheel truck up your exhaust at 45mph.

Haven’t done that since.

But I have:

• Replaced the Delkevic 4 into 1 exhaust with the original stock exhaust.
• Replaced exhaust seals.
• Checked for air leaks on the intake boots - I bought OEM new last year. No leaks.
• Cleaned the K&N pods and re-oiled.

What about the Carbs? I hear you say.

Well, previous to all of this, I created this topic.

And I:

• Used Ed Ness’s excellent CV Carburetor Rebuild Tutorial to Strip the carbs down and clean them.
• Replaced all O-Rings using the kit from Nessim.
• Sent Choke plungers to Zed1015 for his Viton replacement seat treatment
• Fitted Zed1015’s air correctors in the 4 O’clock position on each carb

Oh, and followed everything in Ed Ness’s most excellent Mikuni BS (CV) Carburetor Rebuild Tutorial (Float heights, measuring float levels, Bench Sync etc).

Along the way I discovered a tear in carb diaphram #4 - Replaced.

I also discovered Carb #3 had a 45 pilot jet in it whilst the others had 42.5 - Replaced.

Main jets were 145, factory is 112.5. Was advised to go one size over so have fitted 115's.

Pilot Air jets are all 170 - Factory spec.

I even had a go at Zinc Plating the brackets and fixtures.

(That's not rust, it's my attempt at Yellow Passivate!)



Result...

...Still the same.

Dream from idle to 1/4 throttle. Nightmare onwards.

Today I've taken 10% off the original 2,50mm thickness of the plastic spacer on top of the needle clip.

And will see how that goes.

Test ride tomorrow.

I'll feed back.

[salty_monk]

Valves been checked?

Does it have the electronic ignition or is there a mechanical advancer? I'd look at the springs etc if mechanical & might also suspect the electronics. Just a couple of other places to look... It's not always fuelling.

Did it ever run right? If so in what configuration? In my experience stock exhaust vs a Delkevic or similar has very little effect on the fuelling, 95% is what you do with the intake.

[SamLack99]

Did it ever run right?

Now that's the question, isn't it?

I bought it off a guy who was diagnosed with terminal cancer and who was busy trying to sell everything of any value before he died, so he could save his wife the hassle.

He'd bought it as a project, and he'd only ever started it. Never ridden it. And he had no information on the previous owner before him.

So I guess it has never run right.

If I were starting this journey again - knowing what I know now - I'd focus all my effort on sourcing the original air cleaner and box. If I never come across another set of pods in my life it'll be too soon =)

Yesterday I removed the 0,66mm 'tab' protruding from the bottom of the needle holder guides. And - long story short - ended up taking the original 2,50 needle ring sat on top of the clip down to 0.50 (in 10 and 20 percent increments).

Suzy now runs 95% percent better than she did.

She will transition smoothly from idle --> 1/4 --> 1/2 and onwards. In 5th I never got to WOT as the speedo was already way above the speed limit.

I might try with no needle ring and see if that makes things any better. But I might just enjoy her as she is now. She's so much fun to ride without a 1/4 throttle bog.

[rphillips]

Congrats, Glad you got it this close, wondering now that you found the right place to tinker with, would a little more tinkering get it completely as it should be?

[SamLack99]

Congrats, Glad you got it this close, wondering now that you found the right place to tinker with, would a little more tinkering get it completely as it should be?

Exactly.

I'm taking a break from Suzy for a little while (I've two other bikes) and then I'll see if a little bit more will get it completely as it should be.

I guess I need to find out what happens if you 'de-shim' needles too much =)