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81 gs750e wont run good till 5k rpm

  • Thread starter Thread starter beachbum3478
  • Start date Start date
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beachbum3478

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I picked up a really nice 81 gs750e. It had been sitting for a few years, but in incredible condition. It has a vance hines 4-1, k&n drop in filter, and the it was re-jetted. I cleaned the carbs and got it running. It runs alright, but it feels like one cylinder isnt firing until around 5k rpm. The coolest exhaust pipe is #4. If i spray carb cleaner in the #4 vac port for the carb sync it will start to fire at idle. The #4 idle mixture screw has no effect when i try to tune it. Once i get up to around 5k rpm, it seems to fire and really takes off all a sudden. When i first started riding it, it had a dead spot around 5k as if the spark cut out too. That hasnt happened in a while though. Im very familiar with tuning these old GS's, but never one that has performance parts on it.

I noticed that the previous owner plugged the vaccum line going to the fuel petcock. There was also a restrictor in the petcock vac line too, but obv deemed useless since the line was plugged anyway. Why was it plugged?

Any ideas where to start?
 
If you are getting no response when trying to adjust the mixture screw for #4, the tip of the screw might be broken and wedged in the hole

Loosen the screw and see if you can still feel the tip poking through the little hole on the top inside of the passage.

Nic
 
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beachbum3478 said:
I noticed that the previous owner plugged the vaccum line going to the fuel petcock. There was also a restrictor in the petcock vac line too, but obv deemed useless since the line was plugged anyway. Why was it plugged?
Click to expand...

Hi,

Is this a stock petcock or aftermarket?

Have you tried pulling the plug wires one at a time to see how it affects an idleing engine? It could be a bad plug, bad plug cap, plugged emulsion tube in the carb(s), or a broken idle mixture screw tip as mentioned above. Did you clean the carbs using the proper GSR procedure? Did you go over all of the information in your "mega-welcome"?

Thank you for your indulgence,

BassCliff
 
I think im going to go through the carbs again and check everything the entire bike with the multimeter.

Can anyone tell me why someone would plug the fuel petcock vac line? Its the stock petcock. Im assuming its something to do with having bigger jets for the pipe.
 
beachbum3478 said:
Can anyone tell me why someone would plug the fuel petcock vac line? Its the stock petcock. Im assuming its something to do with having bigger jets for the pipe.
Click to expand...

Hi,

One reason I can think of is if the petcock stopped working properly in the "ON" and/or "RES"erve positions. The PO put it on the "PRI"me position to let fuel flow via gravity, capped the vacuum, and called it done.

These vacuum operated petcocks are not supposed to let fuel flow in the "ON" or "RES" position unless a vacuum is applied.


Thank you for your indulgence,

BassCliff
 
I tested the fuel petcock and it works fine. Doesnt leak. I hooked it up in stock form, and it pumped fine. I put the restricter in vac line, and it pumped a little less fuel out of the fuel line. Is is possible it was something to do with the bigger pilot jets and maybe having too much fuel at idle. And not being able to compesate enough with the needle adjusters?
 
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Hi,

"Restricter" in the vacuum line? I've never heard of such a thing.

Or are you dealing with an inline fuel filter?

Sorry for my confusion. These bikes do not "pump" the fuel, there's no fuel pump, it's all gravity fed.

Thank you for your indulgence,

BassCliff
 
Yea there was actual air restrictor (smaller hole) inline on the vac line, but also totally plugged off at the top. So I unplugged it and left the restrictor in line. And when i say it pumps the fuel, i mean the pulse from the vac line was weaker and caused the pulse of fuel to have a little less volume then it did without the restrictor all together. It must have something to do with the bigger jets. Would the bike run the same with the petcock on prime and the vac line plugged off?
 
Hi,

Why not hook everything up like is supposed to be? Connect the vacuum port on the petcock to the vacuum nipple on the #2 carb. Connect the fuel line to the "T" between #2 and #3 carb. No restricters, no inline fuel filters, just the way the it left the factory.

See: Where Do These Hoses Go?



Thank you for your indulgence,

BassCliff
 
I actually tried to tune it when I hooked it up in stock form, but i couldnt get it to idle right or rev correctly. With the restrictor back in, i got it to run alot better. weird right? I thought id have a quick fix with this bike, but it just looks like im going to have to go through "everything". thanks for the help
 
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