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Runs better w/ Full Tank?

  • Thread starter Thread starter wrgum
  • Start date Start date
W

wrgum

Guest
GS Guru's,

Here's one for you:

1981 GS 650G

Valves adjusted, carbs cleaned (to the exact specs from BassCliff's site), carbs tuned with ColorTune, carbs synchronized, new spark plugs.

Just after fine-tuning the idle and colortune/sync the other day, I took the bike out for a ride. Boy did it snap! Great throttle response throughout the entire spectrum. It was like a new bike!

Today, I rode it up to Fresno (35 miles) and the tank was near empty when I got to town (110 miles on the clock). I noticed that the throttle response was not what I expected, but not so bad as to be troubling. I filled the tank up and headed over to the Iron Bird Cafe for a cup of joe. Holy crap! The power is back!! Immediately, I'm thinking this is a petcock issue. Once I get the tank near empty again, I'll try and notice the diminished power and then switch to prime. This should tell me if my vacuum actuated petcock is jacked. Any other thoughts?

Check six,
WG
 
wrgum said:
GS Guru's,

Here's one for you:

1981 GS 650G

Valves adjusted, carbs cleaned (to the exact specs from BassCliff's site), carbs tuned with ColorTune, carbs synchronized, new spark plugs.

Just after fine-tuning the idle and colortune/sync the other day, I took the bike out for a ride. Boy did it snap! Great throttle response throughout the entire spectrum. It was like a new bike!

Today, I rode it up to Fresno (35 miles) and the tank was near empty when I got to town (110 miles on the clock). I noticed that the throttle response was not what I expected, but not so bad as to be troubling. I filled the tank up and headed over to the Iron Bird Cafe for a cup of joe. Holy crap! The power is back!! Immediately, I'm thinking this is a petcock issue. Once I get the tank near empty again, I'll try and notice the diminished power and then switch to prime. This should tell me if my vacuum actuated petcock is jacked. Any other thoughts?

Check six,
WG
Click to expand...

Sounds like you're on the money there. Switching to PRIME should tell you if it's the petcock or not. Hopefully the power will come back when switched to PRIme. Then all you will need to do is switch to a new petcock.

Good Luck.
 
Vent in the gas cap clogged?

Fuel line pinched? (Low fuel level means less weight of fuel pushing it to the carbs)

Aftermarket filter too small or melted?

Petcock screen blocked?
 
What about Reserve?

When the power diminishes, that's the sign you need to switch to reserve
 
Thanks to all for the input. I'll check the lines, filter, cap vent, etc. My money's still on the petcock itself. When it gets low again in a day or two, I'll be able to really get to the bottom of it!
 
If PRIME and RESERVE don't seem to help, how about this?

You have an in-line fuel filter in there? :-k

Get it out and try it again. :o

.
 
Hi,

Big T said:
What about Reserve?

When the power diminishes, that's the sign you need to switch to reserve
Click to expand...

Yes. This is a common scenario when you get close to the edge of your range. I know, to within about 10 miles (depending on how much I'm twisting it), how far I can go before hitting the reserve. When I feel the engine start to peter out I know it's time to flip it over to RES.

I agree with the above also in that you should check to make sure your gas cap is venting properly. It can be disassembled and cleaned.

Make sure the vent hole in the filler tube on the top of the tank is clear.

fillerneckhole.jpg


I don't use or recommend an inline fuel filter. It's not necessary if your tank and carbs are clean.

If you consistently get to 110 miles when this occurs then you have figured out your maximum range before switching to reserve. :)


Thank you for your indulgence,

BassCliff
 
i seem to get MORE power when im on reserve or when im low on gas.. any thoughts to that? sorry for threadjack lol
 
Steve said:
If PRIME and RESERVE don't seem to help, how about this?

You have an in-line fuel filter in there? :-k

Get it out and try it again. :o

.
Click to expand...



Bingo. Fuel filters create a restriction you don't need. Your petcock already includes a very good filter, assuming someone hasn't broken it off or something.
 
William,
When you start to lose power, try opening and closing the gas cap. If that resolves it, you've confirmed a venting issue.

Also, you might try installing clear fuel line. Makes it incredibly easy to check the fuel flow in cases like this. Should be available in the lawnmower section of your nearest chain parts store.
Caleb
 
wrgum said:
GS Guru's,

Here's one for you:


Today, I rode it up to Fresno (35 miles) and the tank was near empty when I got to town (110 miles on the clock). I noticed that the throttle response was not what I expected, but not so bad as to be troubling. I filled the tank up and headed over to the Iron Bird Cafe for a cup of joe. Holy crap! The power is back!! Immediately, I'm thinking this is a petcock issue. Once I get the tank near empty again, I'll try and notice the diminished power and then switch to prime. This should tell me if my vacuum actuated petcock is jacked. Any other thoughts?

Check six,
WG
Click to expand...
Personally, I think the pickup tube running on "ON" position is too high in this model's tank. I run in very hilly terrain and if my fuel level gets low , I get fuel starved running downhill. I tend to run in " RES" to compensate. Check for proper petcock operation- check that fuel flows decently when fuel gauge reads < 1/4 and you apply vacuum to petcock.
 
Tank is starting to get low again, and power is missing. Also, when I brake to a stop at a light/stop sign, the idle drops considerably and nearly stalls. I'm thinking that the gas is sloshing forward in the tank and starving the petcock. However, shouldn't there be enough juice in the float bowls to have this not matter? Confused.
 
PS. I'm taking the line filter out today and running her on reserve on a trip up to Fresno. We'll see.
 
Figured it out. Your advice was all well-warranted and will certainly help those using the search function with issues in the future. My problem ended up being a stuck needle valve in the #4 (farthest starboard) carb. I noticed that my overflow tube was leaking a little gas when I started up the other day. I immediately thought that the needle valve was stuck. Hoping that it was the #4 and not #3, I took off the float bowl with the carbs still on the bike. Pulled out the float pin, float, and needle valve assembly. Sure enough, the brass sheath had been bent a little when I was taking out/reinstalling it causing the valve to stick a bit. I ordered a new one, installed it, and we're back to full performance! Thanks for all the input.

wrgum
 
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