Flat Spot and Hanging Idle - Another CV Question (with my apologies)

[DevilsAdvoduck]

I know everyone is absolutely thrilled to see another pod/pipe/cv carb thread, so you have both my apologies and an open invitation for a free beer if you find yourself in my neck of the woods.

Here's where I'm at:

Bike: 1980 GS1000GL

Symptoms:
- When warm, idle climbs to ~2500-3000, blipping throttle does not return to normal idle, adjusting idle knob will bring down the idle but dies off after a few seconds
- Flat spot at 5500 rpm to 6000 rpm. Worst at WOT, if I roll back to 1/2 throttle it's still flat but comes through the flat spot easier. Pulls HARD once it gets through
- Plugs show medium carbon, not wet

Mods:
- APE pods (properly oiled)
- Djelkevic 4-1 exhaust
- Dynojet Stage 3 kit
- Current jetting: DJ155 mains (down from the 170s in the kit), Mikuni 42.5 pilots (up one size from stock), needle clip on third notch
- Mixture screws set at 2 turns out, except cylinder 4 set at 3 turns out (this cylinder was running hot, appeared lean, extra turn resolved heating issue)

Maintenance:
- New plugs and wires
- valves adjusted and within spec
- Carbs stripped, dipped, new o-rings, all passages verified open, float height confirmed good
- New carb boot o-rings, boots still supple and no cracks
- Throttle cable lubed and properly adjusted with appropriate slack
- Verified petcock flow
- New vacuum line for petcock
- Vacuum sync

Background:

With the DJ170s, the bike was running ridiculously rich. Moved down to the 165s in the kit, but still too rich, and was bogging considerably high. I ordered 160s and 155s and put in the 155s. The bog was gone and ran much better. Still had the hanging idle at around 3500, but overall power was much more where it should be with the exception of the 5500-6000 rpm flat spot. The bike was popping like crazy on deceleration and some at idle, so I upped the pilots by one size. After changing the pilots, the idle now sits closer to 2500 which is an improvement, but still dies off if I try to dial back the idle adjuster. I've confirmed the choke is not hanging up. I've checked for vacuum leaks with both carb cleaner and an unlit propane torch, but no changes to idle during testing from either.

I'm thinking I have two different issues: the idle circuit is screwy, and the flat spot.

Regarding the idle, if going up one size on the pilot jet helped reduce the high idle, should I be going up one more size? the black plugs tell me I'm running rich, but may be worth a shot.

As to the flat spot, are my mains still too large? If the flat spot is worse at WOT, am I correct in thinking my mains are too large? At 1/2 throttle it still feels the flat spot, but it passes through it easier.

Anyway, that's where I'm at. Thanks in advance for any thoughts you may have, and if you think I'm bluffing on the beer, drop on by and find out.

[Rob S.]

It needs to be jetted on a dynomometer.

[Suzukian]

Yup, Pods=Jetting. If your bike has a 4 into 2 exhaust, on a lot of Suzuki's number 2 and 3 cylinder scavenge off each other, they should be a half a ball lower than 1 and 4 of suction, of course, nobody has that weird Suzuki contraption, but you can extrapolate from the pic in the Manual and adjust the center 2 carbs accordingly.

Suzuki Sync.jpg

[Spiff]

If your running one cylinder with more fuel than the others then that might be why it's running high on idle.
From my experiments with the mixture screw one full turn can be the difference between idling and not idling or stuttering and stopping. So I'm thinking if one cylinder is at peak torque perhaps and the three others aren't it might be running high and dragging the three others with it and when you turn down the idle screw far enough the three others are "dying" and then nr4 eventually goes to and the engine stops. Idk, just a thought. Did you set mix screws for highest idle?

On my engine(a gsx1100) I have now upped the pilot jet two sizes from stock (47.5). Full exhaust and pods on that one too.
Initially it ran very lean on low throttle, mix screws 4 turns out, stuttering, decel popping and hard to ride at low throttle/low speed.
Needle was raised two notches, made it go great "on the needle" and full throttle but still the same symptoms on low throttle/low speed.
Went up to a 50 pilot jet, mix screws about 3.5 turns out, made it better, almost perfect, but hit a wall of richness at medium throttle, so I lowered the needles to standard setting and the symptoms on low throttle came back.
So with the dynojet needles they clearly come into play a lot sooner than what does carb tuning circuit schemes seem to portray.
I Upped it one more time to 52.5 to bring mix screws back into the 1-3 turns out range, it's now sitting at 2¾ out, still a bit of stuttering and decel popping. But then I raised the needles one notch and it cured it. I can now trot along at 2000rpm and not have the bike feel like it wants to buck me off.

If I where you I would try to set mix screws for highest idle first. Pilot jets like I said previously have a small impact quite a bit up in the higher throttle openings as well.
Reading plugs aren't a good way to view mixtures unless you can start with brand new plugs every time and do a plug chop.
If it feels flat at 1/2 throttle it might be too lean and you could perhaps try raising the needles one notch.
Your asking for power but it can't deliver. If it delivered to much it would bog or misfire. When you get through it and apply full throttle the needle is out of the equation and your on the main jet, and it's getting what it wants.

Diagnosing carb problems is easier if you speek in terms of throttle openings, not rpms or speed, makes it easier to define which circuit is having an issue

[Rob S.]

You're asking for power but it can't deliver. If it delivered too much it would bog or misfire. When you get through it and apply full throttle the needle is out of the equation and you're on the main jet, and it's getting what it wants.

Solution: always use full throttle!

[spchips]

Check the airbox - try spraying ether around all the joints and boots (if the RPM spikes, you hit a leak) . I forget where and why, but I remember somewhere between the filter gaskets and the airbox fitment, the 1980 1000 GL specifically these tends to leak a lot of extra air and run lean as a result. I had the same problem.

Don't try tuning it until the air leaks are resolved. If in doubt, consider replacing the upstream and downstream carb boots and air filter frame, if you can get them.

[Rob S.]

Check the airbox
Don't try tuning it until the air leaks are resolved. If in doubt, consider replacing the upstream and downstream carb boots and air filter frame, if you can get them.

Airbox? He uses APE pods. A LOT simpler to get to battery on 82-83 11E.

[DevilsAdvoduck]

Thanks for the suggestions! Had to go out of town, but I'll be tinkering with it again this week and will post an update.

Solution: always use full throttle!

I like your style. My motorcycle insurance carrier probably does not.

If I where you I would try to set mix screws for highest idle first. Pilot jets like I said previously have a small impact quite a bit up in the higher throttle openings as well.
Reading plugs aren't a good way to view mixtures unless you can start with brand new plugs every time and do a plug chop.
If it feels flat at 1/2 throttle it might be too lean and you could perhaps try raising the needles one notch.
Your asking for power but it can't deliver. If it delivered to much it would bog or misfire. When you get through it and apply full throttle the needle is out of the equation and your on the main jet, and it's getting what it wants.

I'll try resetting the highest idle and see where that gets me, and try the next size up on pilots after depending on how its doing then.