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Bike runs great for 5 minutes, then sputters before eventually dying.

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18 Apr 2022 16:19 - 18 Apr 2022 16:37 #865704 by Buffalo515
I have a 1981 Gpz 550. The bikes idles fine and rides fine for about five minutes. It starts with light sputter after that first five minutes of riding fine, then progressively gets worse until it dies out completely. 

Things I have tried:
-Initially I thought that maybe my carbs were clogging, so I cleaned the bowls and jets out with carb cleaner. I also measured the fuel level via clear tube method to make sure they were getting enough. 
-I tried running the bike with the gas cap open.
-I unscrewed the bowl drain screws to see if they were each getting fuel (they were)
-I measured the resistance of the two ignition coils. They both test around 2.7ohms. I have order two new ones anyhow from Z1 just in case.

There are two things that I am suspicious of but unsure if they would result in the symptoms my bike displays. First, the boots from my airbox to my carbs are a bit beat up. They are still soft, but two of they have tiny gauges in them from using a flat head screw driving to get they back on. They also do not have the retaining bands on them. I am not sure if a air leak from the air intake side of the carbs could cause this problem. Second, I just had to replace an exhaust manifold bolt, so while I was working in the head I re-shimmed a few of the valves to get to within tolerance. 

I have included a picture of one of the plugs in case it might help, it appears to be slightly wet. 

 So, I am wondering if you all have any advice. Thank you in advance.
 

Travis
Maryland
1981 GPZ 550 -gone
1984 GPZ 550
1985 GPZ 550
Last edit: 18 Apr 2022 16:37 by Street Fighter LTD.

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18 Apr 2022 20:54 #865728 by F64
You may be running too rich and fouling out the plugs.

Cover half of your airbox with tape.(this will cause a rich condition)
RIde it.
Better, worse, or the same?

Take off the tape and crack open your airbox.(this will cause a lean condition)
Ride it.
Better, worse, or the same?

81-KZ440-D2.
Louis Dudzik's GM HEI ignitor conversion installed 2015 s3.amazonaws.com/gpzweb/Ignition/GPZgmHEImod.html
Motogadget m-unit blue installed 2017.
LIC, NY

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  • ThatGPzGuy
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  • Sustaining Member
  • More Sparky than Speed Racer
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20 Apr 2022 06:00 #865812 by ThatGPzGuy
Petcocks not flowing enough fuel? Try to run it with an auxilary tank of some sort. I use a bottle on an IV stand but you can get away with an old lawn mower tank held above the bike. 

Jim
North GA
2016 Yamaha FJR1300ES
1982 GPz750 R1
1974 Kawasaki H1
1976 Kawasaki KZ400
1979 Yamaha XS650 cafe'
2001 KZ1000P
2001 Yamaha YZ426
1981 Honda XR200 stroked in an '89 CR125 chassis
1965 Mustang
1967 Triumph GT6
1976 Bronco
"If you didn't build it, it's not really yours"

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20 Apr 2022 07:26 #865813 by ckahleer
Rule out fuel starvation by attaching clear tubes to the carburetor drains, When the bike starts sputtering, check fuel level.
However, the plug indicates rich. Turn mixture screws almost or all the way in. And/or go for a ride with the airbox cover removed.
 

94 KE100
81 CM200t
82 KZ305
85 VF1100c

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20 Apr 2022 16:08 - 20 Apr 2022 16:10 #865852 by Buffalo515
Thank you for all the replies. Sorry for taking up your alls time. I am just an idiot lol. I did not perform a fuel level measurement correctly the first time and after rereading the procedure in the manual and redoing the measurement I found that 3 out of 4 of my carb bowls were holding too much fuel. Runs better now that they are all in spec. 

Travis
Maryland
1981 GPZ 550 -gone
1984 GPZ 550
1985 GPZ 550
Last edit: 20 Apr 2022 16:10 by Buffalo515.
The following user(s) said Thank You: slayer61

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  • ThatGPzGuy
  • Offline
  • Sustaining Member
  • More Sparky than Speed Racer
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21 Apr 2022 09:06 #865892 by ThatGPzGuy
No worries. We have all done something similar. If someone else is having the same problem, then hopefully your thread will be of use to them. 

Jim
North GA
2016 Yamaha FJR1300ES
1982 GPz750 R1
1974 Kawasaki H1
1976 Kawasaki KZ400
1979 Yamaha XS650 cafe'
2001 KZ1000P
2001 Yamaha YZ426
1981 Honda XR200 stroked in an '89 CR125 chassis
1965 Mustang
1967 Triumph GT6
1976 Bronco
"If you didn't build it, it's not really yours"

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