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1980 GS750 won't rev over 5 grand.

  • Thread starter Thread starter eighty6gt
  • Start date Start date
E

eighty6gt

Guest
Bike starts and idles ok. The battery is shot so I have it hooked up @ 2 amps on the battery charger, and then click it up when I have the starter going. Once the bike is running I can take the charger off; but the symptoms remain the same. I didn't have my multimeter so I don't know what the system voltage is, since that might be an issue. I'll find out later.

Anyway, throttle response off the bottom is OK, the bike just refuses to revv over 5000-6000 rpm with no load (haven't road tested it yet). The bike will sometimes backfire and respond poorly to throttle after revving.

Floats much too low? I checked them they were around 24-25 mm.

Ignition?

Thanks for any advice!

James
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If you are measuring the correct part of the float, they are too high. There is a step on top of the float where it mounts to the pivot, you need to measure at the bottom of this step. If you measured to the top of the float, then they are too low.

Do you have an air filter attached? Does not matter if it's the stock air box or individual pods, but you have to have something there for a little restriction.
 
Get a multi-meter. What you describe can be a bucket load of things. Start with a good battery and move to carb cleaning.
 
Carbs have been dissasembled, dipped, cleaned, swabbed... The diaphrams are good, the boots are all new, the airbox is in place w/K&N (bike was jetted for a wolf header/exhaust and a open airbox w/K&N in place of stock)

I'll take a multimeter over there thursday night, and a new battery.

I guess I'd be checking system voltage, voltage @ coils, wire resistance (?) what else?

The floats in this bike are round.

Thanks!


James
 
Put the new battery in a week ago. Much nicer to diagnose this thing w/o a battery charger or booster cables around.

I made major headway when I taped the airbox lid hole shut. Now the bike revv's to the redline. Response, however, is still terrible, the bike will not revv cleanly in the midrange, and does not respond to throttle very well at all. Spits and hesistates, you can open it up WOT at low speeds in 1st and it bogs badly.

The carbs are NOT balanced in any way. I'm going to do this first.

I'll have to get the jet numbers off this thing when I pull the carbs off again. The jets are in bad shape, chips from the wrong screwdrivers used in the past.
 
eighty6gt said:
Put the new battery in a week ago. Much nicer to diagnose this thing w/o a battery charger or booster cables around.

I made major headway when I taped the airbox lid hole shut. Now the bike revv's to the redline. Response, however, is still terrible, the bike will not revv cleanly in the midrange, and does not respond to throttle very well at all. Spits and hesistates, you can open it up WOT at low speeds in 1st and it bogs badly.

The carbs are NOT balanced in any way. I'm going to do this first.

I'll have to get the jet numbers off this thing when I pull the carbs off again. The jets are in bad shape, chips from the wrong screwdrivers used in the past.
Click to expand...


Have you done this yet? If so, what were the results?
 
Yep, Just finished up yesterday.

Turns out a main jet had turned itself out and fallen into one bowl. Either I didn't crank it down far enough or I was interrupted in the process of tightening them down! It was on the end.

Bike runs fantastic now. Pulls hard to redline. Sounds incredible.

Now, the only issue is the good deal of copper that I saw in the pan when I changed the oil (not my bike, first change I've done on it), a vibration around 5k that buzzes the instruments, and a constant knocking at hot idle. Bye bye bearings? Who knows.
 
Not sure where the copper is coming from, but the vibration at 5k could easily be a carb sync problem. The smaller bikes are not as susceptible as the 1000s and 1100s, but it can still happen. Before you sync the carbs, check the valve adjustment, and that has to be done with the engine cold.

.
 
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