GS550 won't rev past 6000 RPM

[JohnnyL]

I just rebuilt my 1980 Suzuki GS550L as a cafe racer. You can see the build here: https://www.thegsresources.com/_foru...50L-Cafe-Build

Anyway...the motor has been completely rebuilt, carbs have been gone through, bike was re-wired with a moto-gadget blue unit, Dyna 3 ohm coils, Dyna ignition, stock air box removed, very expensive pod air filters installed, and a Mac 4 into 1 header into a reverse cone quieter core muffler. Bike fires up and runs great up to 5000 RPM and then starts stuttering. Timing is right on the money(checked dynamically with a timing light). All carbs are synced just about perfect. In neutral, it will rev up to 10-11,000 easy. When running in gear down the road, the bike struggles to get past 5500 RPM. It will get up to 6000 RPM but barely. It's like it's starving for gas or like there's a rev limiter. I put new plugs in today and it still does the same thing. I took it around the block and before turning into my driveway, I ran the bike in third gear for about 30 seconds at the maximum RPM I could get(which was 6000). Right before turning into my driveway, I pulled in the clutch and killed the bike. I coasted up my driveway and then pulled the plugs. I didn't expect to see what I did. Everyone of them is telling a different story. Please someone help me figure this out. Here are the plugs:

Cylinder One plug: (Looks almost perfect)


Cylinder Two plug: (Detonation?)


Cylinder Three plug: (Extremely rich)


Cylinder Four plug: (Almost perfect)

[bwringer]

Hmmm... the tale of the plugs is a new wrinkle.


A couple of things come to mind.

1) The one thing 2 & 3 have in common is the ignition circuit.

2) Incomplete combustion will leave soft carbon deposits like you see on #3. You see some hints of similar deposits on #2.

3) It's harder to make spark under load; once in a while you do find a system that tests OK but gets intermittent under load.


Previously, I thought the ability to rev at idle pointed to carburetor issues. Now I'm not so sure.

So now I'm back to thinking there's some weakness in the ignition on 2 & 3 under load.


A compression test could be worthwhile, too, just to make sure they're all in the same rough ballpark.

[JohnnyL]

Hmmm... the tale of the plugs is a new wrinkle.


A couple of things come to mind.

1) The one thing 2 & 3 have in common is the ignition circuit.

2) Incomplete combustion will leave soft carbon deposits like you see on #3. You see some hints of similar deposits on #2.

3) It's harder to make spark under load; once in a while you do find a system that tests OK but gets intermittent under load.


Previously, I thought the ability to rev at idle pointed to carburetor issues. Now I'm not so sure.

So now I'm back to thinking there's some weakness in the ignition on 2 & 3 under load.


A compression test could be worthwhile, too, just to make sure they're all in the same rough ballpark.

I will test compression today. I will also do a leak down test with the new leak down tester I purchased(been dying to try it out anyway).

What has me so confused is 1 and 4 look almost identical. 2 and 3 look vastly different! Since they're on the same ignition circuit, they should look the same....shouldn't they? 2 looks like detonation to me while 3 looks extremely rich. This is what has me scratching my head.

Will supply compression results later today along with the leak down results.

[bwringer]

This is only the barest guess, but I was thinking #2 and 3 might have both been building up soft deposits about the same and they happened to fall off on #2 first.

I'm not sure that's a thing that actually happens, but it was my theory...

[Gorminrider]

...or just too lean? "ok neutral revs but no power" usually means not enough fuel to me... float-level below the jets? the jets themselves? even a lagging petcock, if the bike can go along ok at low speeds....the vacuum petcock needs vacuum. Mine will sometimes settle back a bit coasting...is the dark plug cylinder where the vacuum line is?
or maybe though unlikely on these bikes: timing advanced....the weights would be stuck or not settling back would be easy to check...maybe hanging up on a wire ...

MY plugs are never that white unless I burn Chevron 94octane for a long trip. The dark plug is an outlier but shares a coil so I still would suspect carburetion. It's not that bad-a bit of soot will not affect power but the white ones-they seem more suspicious to me.

Pods (which I know little of) seem to tend to rejetting issues on the forum too...aren't they intended to deliver more air? and want more gas for more "power"?

[Steve]

The only one of those plugs that I would consider to be anywhere near "acceptable" would be #4.

#1 might as well be a brand new plug that you accidently took a picture of, instead of one that was just run.
WAAYY too lean for my tastes.

#3 is way too rich or not firing. That is almost shiny enough to be oil deposits, though. Either way, something is wrong with #3.

#2 is puzzling. The porcelain looks too lean, I don't know how to interpret the deposits.

You mentioned most of the things that you have done, but did not mention any jetting changes. With 3 out of 4 plugs showing signs of leanness, I am wondering what jets you have in there. That would not explain the rich-appearing #3, which might have its own unique problem.

.

[JohnnyL]

Those plugs were brand new and ridden about two miles. I have never been able to get the bike to rev over 6000 RPM. I’ve had 112.5 mains, 107.5 mains and now it has 100 mains.

[Rich82GS750TZ]

It seems to me that you keep going down in jet sizes? Smaller openings/less gas/making an already lean condition even leaner. #3’s issues not withstanding. Am I not understanding this? That’s entirely possible/even likely.

[JohnnyL]

It seems to me that you keep going down in jet sizes? Smaller openings/less gas/making an already lean condition even leaner. #3’s issues not withstanding. Am I not understanding this? That’s entirely possible/even likely.

Yes. I went smaller in jet sizes and the bike never changed the way it performed. I can try some larger jets but I think I found the problem. Besides, at 1/2 throttle, I don’t think I’ve even hit the main jet yet.

Here are my results of the compression tests:

Cylinder One: 120 psi
Cylinder Two: 145 psi
Cylinder Three: 145 psi
Cylinder Four: 145 psi

Not sure why cylinder one is so low but then I did a leak down test:

Cylinder One: No leaks
Cylinder Two: No leaks
Cylinder Three: At 90 psi in I have 3% leakage. I can hear air coming out on the intake side. Bad valve?
Cylinder Four: No leaks

Brian Wringer and I checked the valve spacing and #3 was good on intake and exhaust. I guess I'll have to pull the cover and check again.

[Big T]

One of the issues is that the main jet does little to nothing at the rpm where you're having issues

It's the needle that's the issue.

[Steve]

Which jet is in use is more related to throttle opening, not engine speed.

He said he ran it at the maximum he could get, but did not mention throttle position.

<My> guess is that if he was going for maximum, the throttle would be wide open.

What was the throttle position, Johnny?

.

[JohnnyL]

Which jet is in use is more related to throttle opening, not engine speed.

He said he ran it at the maximum he could get, but did not mention throttle position.

<My> guess is that if he was going for maximum, the throttle would be wide open.

What was the throttle position, Johnny?

.

Half throttle gets me to 6000 RPM and no further. It starts stuttering at 1/2 throttle. No matter where I put it after that(3/4 or full) it still won’t go over 6000 Rpm.

Did you see the compression and leak down results I posted?

[Spyder]

I think you have 2 separate problems going on here. First, It looks like you are too lean on the main jet. Second, I would make sure your 2-3 coil is properly grounded. May as well be sure the other one is too while your under there. Plug 2 looking ok, while 3 looks like it isn't firing sounds like a ground issue to me. Try that first and see if it improves the rpm issue, then address the jetting.

[JohnnyL]

I'm confused why no one is commenting on my leak down test. #3 intake valve was leaking. Isn't this a BIG problem? I have removed the valve cover, and the head this morning. I removed #3 intake valve and found an area that was suspect(see photos). I lapped the valve again and got most of this area cleaned up about the best I could. I will reassemble everything and check valve clearance and double check my coils again for a good ground.

Again....I don't think I have even hit the main jet yet at 6000 RPM(1/2 throttle). Running a 112.5 main ran the exact same as running a 100 main. I'm hoping this leaky intake valve is the culprit. I guess we'll see.

Valves after head removed:


Valve not seating here:


Lapping valve with drill, valve grinding compound and piece of tubing


Suspect area after first round of lapping(see pitting?)


About the best I could do after 3 rounds of lapping

[Gorminrider]

I'm confused why no one is commenting on my leak down test. #3 intake valve was leaking. Isn't this a BIG problem?

well, I've had a leaky valve on a twin... compression test and leakdown quite a difference between cylinders and I was still able to wind the bike up. But that's good what you're doing. I'm just not sure it's the answer though...