GS650GLZ -- Won't rev over 2,000rpms

[joegs]

Hello all. I am new to the forum, somewhat new to street motorcycles, and relatively new to engines. I wanted to pick up a project, and got a good deal on an '82 GS650GLZ. Previous owner had a lot of the service records, it only has about 4,000 miles on it.

When I got it, it wouldn't start. It would turn over, but not even close to grabbing. It is now idling fine, but under load it bogs down fast when the throttle is opened up. Won't rev past 2,000-2,500 RPMs. Feels like I'm shifting from 1st to 2nd and back over and over. (Accelerate, slow down, accelerate, slow down, repeat...) It does this at pretty much anything past 1/4 throttle. If I hold it at about 1/3 throttle, it will s l o w l y climb up to 3,000 RPMs.

If I run it on choke, it seems to improve vastly. I only attempted this once, however.

So far I have done (in no particular order):

1. FULLY cleaned carbs. Dipped in Berryman's. Replaced o-rings. No damage, but jets don't appear to be original Mikunis. The size on each one appear to be correct, however.
2. Replaced fuel filter.
3. Checked petcock for filth, was very clean.
4. Checked valve clearance, replaced two shims.
5. Replaced intake boot o-rings.
6. Checked airbox for leaks/cracks.
7. Replaced spark plugs.

... as well as other things like brakes and such, but don't relate to this problem.

I feel like it's running lean because it runs better on choke, but when I took the plugs out they were black with carbon, not white like I would have expected.

A probably-unrelated problem that just happened today was that my starter button popped off. It looks like it melted a bit on the inside. (???) It won't try to start at all now, but I may be missing a piece for the button. Probably unrelated, but thought I would mention that anyway...

Not sure where to turn now. Any help would be appreciated. Thanks!

 

[joegs]

I forgot to mention... it only does this under load. In neutral, it revs through the whole range with no issues.

 

[jsandidge]

Your problem may be the fuel filter if you're talking about an in-line filter. These bikes didn't come stock with them. Most in-line filters don't work well with a gravity fed fuel system like these bikes have. They don't let enough fuel through. There are some filters that will work, I've seen them in the auto parts store near the lawnmower stuff. Best thing to do though, is to take it off and replace your fuel line unless you have enough length with the current one. The filter that is built into the petcock is enough.

 

[tom203]

"Previous owner had a lot of the service records, it only has about 4,000 miles on it."

not much service to be in 4k miles, so be leery.
Do you have an air filter element installed?? When you cleaned carbs, did you check vacuum diaphragms for pinholes? These need to commence liftoff or it will fall flat- using the "choke" supplies additional mixture and at idle,it should rev easily to 4000revs with choke applied.
If you suspect non stock main jets, get suspicious and wonder about needles- maybe someone got creative.ll

Make sure your main fuse is 15 amp, and all others 10 amp. Melted starter button might mean a short from that switch,thru clutch safety switch,and down to solenoid itself. Here's a color wiring diagram to trace stuff..

 

[Nessism]

One other thing to check is the mechanical spark advance unit. You should be able to grab the rotor and turn it against the spring and when released, it should snap back.

 

[joegs]

Your problem may be the fuel filter if you're talking about an in-line filter...

Good to know, I didn't know the inline filter wasn't a stock piece. I'll replace the fuel line with a longer one with no filter.

Do you have an air filter element installed?? When you cleaned carbs, did you check vacuum diaphragms for pinholes? These need to commence liftoff or it will fall flat- using the "choke" supplies additional mixture and at idle,it should rev easily to 4000revs with choke applied.
If you suspect non stock main jets, get suspicious and wonder about needles- maybe someone got creative.ll

Make sure your main fuse is 15 amp, and all others 10 amp. Melted starter button might mean a short from that switch,thru clutch safety switch,and down to solenoid itself. Here's a color wiring diagram to trace stuff..

Yes, air filter element/box/cleaner appear to all be original. Haven't done anything to it.

No holes at all in diaphragms, I inspected them very closely.

What would I be looking for in regards to needles if someone messed with them?

One other thing to check is the mechanical spark advance unit. You should be able to grab the rotor and turn it against the spring and when released, it should snap back.

Good to know, thanks. I'll check it out.

 

[Steve]

You mentioned that you checked the airbox for leaks/cracks. Is it safe to assume that it is installed?

I agree with jsandidge, ditch the extra fuel filter. Too often, they are too restrictive, but there should be enough fuel in the bowls to get past 2000 RPM in at least one gear before running dry.

When you rebuilt the carbs, did you install the rubber plugs over the pilot jets? Missing rubber plugs will cause it to run poorly above the pilot circuit (idle speeds), but won't necessarily improve when applying the "choke".

.

 

[Nessism]

BTW, did you hold up the pilot jets to a light after cleaning to make sure the orifice is open?

 

[JTGS850GL]

If removing the filter doesn't fix it the it could also be electrical. Verify voltage feed at the coils. Should be near battery voltage (12V+) with the ignition on. Poor spark can show up as incomplete combustion and breakup under load.

 

[tom203]

"No holes at all in diaphragms, I inspected them very closely."

very good and of course you reinstalled them oriented the right way?

 

[joegs]

Update: I removed the inline fuel filter, oiled the air filter (didn't previously realize I needed to do this...), and gave the carbs another cleaning. Now she's running great, but only on prime. Quickly starves on "On". Vacuum line is attached, no cracks visible. When I hold my thumb over it, there is a definite vacuum, but I have no way of measuring how much it needs. Does this point to an issue with the petcock?

You mentioned that you checked the airbox for leaks/cracks. Is it safe to assume that it is installed?

I agree with jsandidge, ditch the extra fuel filter. Too often, they are too restrictive, but there should be enough fuel in the bowls to get past 2000 RPM in at least one gear before running dry.

When you rebuilt the carbs, did you install the rubber plugs over the pilot jets? Missing rubber plugs will cause it to run poorly above the pilot circuit (idle speeds), but won't necessarily improve when applying the "choke"..

Yes, airbox is installed, as well as rubber plugs. One of the plugs seems slightly petrified, so I probably should replace at some point.

BTW, did you hold up the pilot jets to a light after cleaning to make sure the orifice is open?

Yes, I did. They are definitely clean.

If removing the filter doesn't fix it the it could also be electrical. Verify voltage feed at the coils. Should be near battery voltage (12V+) with the ignition on. Poor spark can show up as incomplete combustion and breakup under load.

Voltage feed at coils is good, near battery voltage.

"No holes at all in diaphragms, I inspected them very closely."

very good and of course you reinstalled them oriented the right way?

Yes, with tabs in the correct place.

 

[Nessism]

So your "clean" carbs weren't so clean after all eh? At any rate, sounds like it's running well now. Regarding the petcock, best thing to do is just replace it. Suzuki doesn't sell petcock parts and aftermarket rebuild kits suck.

 

[joegs]

Good to know, thanks. I actually think the carbs were clean as a whistle, I just wanted to spray 'em down one more time just in case. Seems to me the combo of oiling the air filter (less air) and removing inline fuel filter (more fuel) was the combo it needed. The jets were all perfectly clear when I took them out.

I *think* it's running well now. I haven't had a chance to run it through anything past ride it down the street and back, and I'm out of town now. Didn't have any problems with accelerating. Hope it's good when I get back. Is it harmful to run it on prime or does it just run the risk of overflowing into the engine when it's not running?