Started up the bike yesterday for the first time in about 4 1/2 months, had to jump start it with my buddies Supra. After starting it I let it run for about 15 minutes and then proceeded to start slightly reving the engine, upon doing this I instantly noticed the Tach was jumping all over the place. It seemed to idle correclty but as soon as I started giving it some power the tach would jump up a couple thousand RPM's and then right back down to 0.  I took it for a quick ride around the block and continued to have the same problem. At one point in first gear it felt like it was about to stall out then backfired and seemed to ride ok again. I checked the cables and nothing seems to be lose. I guess i didn't let it run nearly long enough to charge the battery either because as soon as I turned it off the battery depleted again. I have off work all week so I'll have some time to mess around with it, if anyone has any suggestions what else to check please let me know.

QuoteAfter starting it I let it run for about 15 minutes and then proceeded to start slightly reving the engine, upon doing this I instantly noticed the Tach was jumping all over the place. It seemed to idle correclty but as soon as I started giving it some power the tach would jump up a couple thousand RPM's and then right back down to 0.

Does the engine sound like the tachometer indicates?

Put another way, if you closed your eyes, would the engine sound like it was doing what the tach indicated?

If the engine sounds OK but the tach reading is wacky, suspect the tach connections and/or linkages to the engine. It is just a mechanical linkage tach and not an electric meter type tach. The 2003+ GS has an electric meter type.

You most definitely would have to run it a while to charge up after sitting for so long. Do the obvious things first and check the battery fluid level and tighten the connections.

Was the fuel sitting in it all this time? If the tank had a quart in it, chances are the quality has fallen off substantially and it needed stabilizer. If mostly full it may be OK.

Have you inspected the insides of the tank for rust and gunk or crap? If you see rust in the tank, it is probably in the carbs or petcocks or lines.

This sounds like a similar post from a week or two ago.

Disconnect the tach cable and check to see if it's broken inside the housing.
If it's ok, it just needs some lube.

Riding on the freeway for a half hour or longer should get the battery back up if it is still good. Low speed doesn't put out as much charge.
Check the battery with a volt meter first. Then start the bike and check it while it's running. Raise the rpms and watch if the volts go up, it should read between 13.5 and 15.5 volts.

Since I just fixed my tact I can comment on this one....  starwalt's question was a great one.... did the engine sound like the tach indicated?  If not, then you probably have some kind of problem in the connections between the engine and the indicator.  Check the following:

1. Check the connections and ensure they're not loose
2. Disconnect the cable from the engine.  Use a flat head screw driver and slot it into the receiver in the tach cable and give it a twist, see if the needle jumps.  
3. If the needle didnt jumped look inside the engine and ensure the little rod looking thing looks good.  Turn on the bike and see if the thing is rotating.  
4. If the needle didnt jump, disconnect cable from the tach and see if you can see the cable turning on the inside while doing the screwdriver thing again.

You should pretty much be able to diagnose which part is broken from there (gear driven tach assembly, cable, or tach indicator)

QuoteDoes the engine sound like the tachometer indicates?

It sounds like the tach is reading correctly until I twist the throttle, at this point the RPM meter starts twitching back & forth from 0 and what sounds to me like the indicated RPM's should be reading.. I will try disconnecting the cable below as JCH suggested as it seems to be running correctly just not the correct indication on the tach. By the way I forgot to mention before this is a 2004 GS500F.

Quote
Was the fuel sitting in it all this time? If the tank had a quart in it, chances are the quality has fallen off substantially and it needed stabilizer. If mostly full it may be OK.

Tank was full and I added fuel stabilizer before I stored it for the winter. Also the battery was disconnected and stored in the basement but not charged up at all while it sat for 4 months.

Thanks for the suggestions and I'll be sure to let you guys know as soon as I get it corrected.

Oy.  Everything I posted applied to my E version, but I imagine it would remain the same or similar