Is this a sync or a fuel problem?

[joeyplumley]

Starts fine, idles fine, revs to redline in neutral; but under power it won't rev past 5000 - like it's being starved of fuel or something.

Background:
1980 GS550L. Carbs real gummy from the last owner (I mean I couldn't move the slides at all). Anyhow - disassembled them, dunked 'em, blew out all passages w/ compressed air, reassembled (and hopefully didn't touch any adjustments).

Gas stains were all over crankcase, so figured petcock was faulty. Disassembled it, and there was no spring in it (to counteract the vacuum on the diaphragm from the engine). So, I put a spring in there from another petcock ('81 Yamaha).

Before I spend $50 on a new petcock, or $100 on a carb synchronizer, I'd like some input on where my money would best be spent. The whole bike was $100 to begin with. ANy ideas?

[tconroy]

Starts fine, idles fine, revs to redline in neutral; but under power it won't rev past 5000 - like it's being starved of fuel or something.

Background:
1980 GS550L. Carbs real gummy from the last owner (I mean I couldn't move the slides at all). Anyhow - disassembled them, dunked 'em, blew out all passages w/ compressed air, reassembled (and hopefully didn't touch any adjustments).

Gas stains were all over crankcase, so figured petcock was faulty. Disassembled it, and there was no spring in it (to counteract the vacuum on the diaphragm from the engine). So, I put a spring in there from another petcock ('81 Yamaha).

Before I spend $50 on a new petcock, or $100 on a carb synchronizer, I'd like some input on where my money would best be spent. The whole bike was $100 to begin with. ANy ideas?

bench sync your carbs before putting them back on and you will be close enough to start.The carb synchronizer is on sale at www.kneedraggers.com. Check it out.Im going to order one myself. if your petcock doesnt work you can get a pingle to replace it.They are the ones most used by people on the forum.

[Nessism]

It's very unlikely that carb sync would cause a problem that that. Fuel flow maybe, carb problem, or even an ignition problem. Keep at it.

[49er]

Your petcock diaphragm spring may be too strong. The Yamaha one may not work correctly for your setup. You can find out whether there this is causing your fuel supply problem under load by turning the petcock to prime and seeing how it revs out. If the situation doesn't change remove the fuel line from the carb tee with the petcock lever turned on. Check that there is no split in the pipe as that would cause the stains on the crankcase that you mentioned.
Place a jar under the fuel line and turn the lever to prime. You should see a steady flow of fuel. If not, check your fuel level in the tank and try it with the cap still off the tank. The cap breather may be partically blocked. Failing that, you may have your carb float levels set too low.
If they are too low the engine should run lean jerking and misfiring at that 5000rpm under load.

[joeyplumley]

Just took her out for a spin around the neighborhood with the petcock on "PRI". No change.

Rode around with no gas cap on, no difference there either - still goes flat at 5500, like it's hitting a rev limiter or something. When I let it sink a few hundred RPM lower, it pulls real hard back to 5500. I held it at full throttle at 5500, and no more acceleration, it just sits there cruising along. Very odd.

It does jerk a little though right there - I hate to pull the carbs again, but looks like i have no choice. Looks like I'll need to check the float levels, although I did my best not to mess with them when I disassembled everything last time.

I'm going to replace the spark plugs today - hopefully they'll have them at the auto parts store down the street. Maybe it's running on 3 cylinders? Remember I bought this thing non-running, so don't have a reference point.

Oh, and I heard there's a mechanical advance on my electronic ignition? Is there? Where's it at?

[Juob]

- still goes flat at 5500, like it's hitting a rev limiter or something. When I let it sink a few hundred RPM lower, it pulls real hard back to 5500. I held it at full throttle at 5500, and no more acceleration, it just sits there cruising along. Very odd.

I rode an 80 or 81 650 once that ran as you described. It turned out that the rubber plugs that fit over the pilot jets were missing. I'm not sure if your model has the plugs, maybe someone else here can confirm it or check a manual if you have one. If so, check if they are present and fit well so they don't leak.

[joeyplumley]

When I "rebuilt" the carbs, I replaced all 4 of those.

I'll double-check them when take the carbs back off.

[49er]

While you have the carbs off, check the size of the of the main jets and the float needle jets against standard specs. The PO may have changed the size of either which may also cause a lean condition under load. A chop and plug read at 5500rpm, would have confirmed this condition.

[t3rmin]

Also make sure you haven't accidentally saddled up a sawhorse you've got stored in the garage. Happens to me all the time and I gotta say the performance at all RPMs leaves something to be desired.*

[Dave8338]

Also make sure you haven't accidentally saddled up a sawhorse you've got stored in the garage. Happens to me all the time and I gotta say the performance at all RPMs leaves something to be desired.*

Now that's good humor.... :-D

[bwringer]

Did you put the airbox back on?

Unless it's been rejetted to suit, a GS runs like crap without an airbox and filter mounted.

[joeyplumley]

Yes, I put the airbox back on immediately. Dang thing would barely move w/o it. It took all it had to make it back up the driveway. Everything appears to be stock on the bike, and the mixture plugs are still there, so I'm hoping the jets are stock.

Sawhorse

I don't have a set of calipers to check the float level - I'll eyeball it tomorrow with a ruler. I noticed last night a screw holding the rod to the choke plunger was missing from carb #3, so that may have something to do with it. (?) Anyhow, going to make a new one out of a hex head screw tomorrow, since the OEM part isn't available anymore (at least where I've looked).

[joeyplumley]

A chop and plug read at 5500rpm, would have confirmed this condition.

I've read that this is a great tool for diagnosis, and then I've heard that people swear by disregarding anything they get from a plug read.

I myself would appeal to the former, but now that the carbs are off, we'll have to wait till next time. Besides, I'd have to find a way to lug the beast back to my garage to get the plugs out.

Maybe the wife can push me back while I steer and make engine noises.... hmmmm....

[bwringer]

Oh, and I heard there's a mechanical advance on my electronic ignition? Is there? Where's it at?

The mechanical advance is the doohickey just behind the two hexes on the end of the crankshaft. The trigger blade is pointing to about 10:00 in this photo.


The advancer is spring loaded -- you should be able to move the blade clockwise a bit with your finger. Play around with it a bit, and you'll get the idea. When the engine is running, there are weights that move outward at a certain RPM to advance the ignition timing a bit. If it's rusted so that it won't move, that's a problem...

If you want to remove it for cleaning and lubing, remove the small center bolt and the pickup mounting plate, and the advancer unit should slide off the crankshaft. You may need to use a simple puller if it's really grungy.

This is from a GS850 engine I happen to have lying around, but yours will look pretty much the same.

From this page -- the top photo is the solid trigger plate found on '83 and up (and a few '82) all-electronic ignitions.

http://bwringer.com/gs/tdc.html

[49er]

Sawhorses and steers. What next, a Rocking Horse? If youre fit enough, you should get good rpms out of one. Don't know about it's centrifugal advance though.