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Engine dies when throttle opened quickly 08 Feb 2024 10:58 #895003

  • Andyp62
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Hi all
i have sorted some problems with the help of you guys here, I am at the stage now where my KZ 650B2 1978 will start easily now and seems to run ok but if you snap the throttle back quickly it hesitates and dies, it has a new Dyna ignition and coils and the timing is spot on at idle and full advance, compression is 155 on all 4 cylinders so I think it must be carburation, it has stock air box and jets, needle is on middle position but the plugs are quite black 
thanks 
1978 Z650 B2, US import to the UK
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Engine dies when throttle opened quickly 08 Feb 2024 11:05 #895004

  • !Seymore
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How many turns out on the air screws?
You're running really rich...
C.
79' KZ650-D2 [fsm] (Max)
83' KZ750-F1 LTD [clymers] (Kay)
82' KZ1100-D1 Specter (another project)
78' KZ650-B2a (J&H, A Project)
91' KZ1000-P (P = parts)

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Engine dies when throttle opened quickly 08 Feb 2024 12:28 #895007

  • Warren3200gt
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Hi all
i have sorted some problems with the help of you guys here, I am at the stage now where my KZ 650B2 1978 will start easily now and seems to run ok but if you snap the throttle back quickly it hesitates and dies, it has a new Dyna ignition and coils and the timing is spot on at idle and full advance, compression is 155 on all 4 cylinders so I think it must be carburation, it has stock air box and jets, needle is on middle position but the plugs are quite black 
thanks 
Totally normal for a normal carburated engine.
If you wack open the throttle very very quickly, the available air flow through the slide is big. As its big there is little vacuum to draw fuel through the pilot jets. 
Result is loads of air with very little fuel ultra lean mixture and there is no combustion to keep the engine running. 
This situation will only apply if in neutral and not evident If the bike is being ridden then the engine will continue being turned over via forward motion through the gearbox and everything will catch up with itself. 
This is one of the reasons most modern vehicles use fuel injection as both fuel and air can be increased simultaneously.

Get your fuel mixture correct and open the throttle as though you were riding and it won't cut out. 

Z1000J2 somewhat modified!

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Last edit: by Warren3200gt.

Engine dies when throttle opened quickly 09 Feb 2024 01:06 #895016

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I have had the air screw from one turn to 3 out with little effect on tickover 
1978 Z650 B2, US import to the UK

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Engine dies when throttle opened quickly 09 Feb 2024 01:09 #895017

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Thanks guys
1978 Z650 B2, US import to the UK

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Engine dies when throttle opened quickly 09 Feb 2024 07:27 #895026

  • TexasKZ
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I have had the air screw from one turn to 3 out with little effect on tickover 
This makes me suspect that the carbs are not clean. The pilot mixture passageways are really small and can clog easily. If the air screw passageways are clogged, the idle mixture will be very rich.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough
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Engine dies when throttle opened quickly 09 Feb 2024 09:12 #895031

  • gd4now
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I know you have been through several things with your machine,, but do not recall all of them off the top of my head and have not gone back and looked at your other posts. But I might ask if the choke/enricher is leaking?
1977 KZ650 B1
Pods and Denco header


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Engine dies when throttle opened quickly 09 Feb 2024 13:33 #895039

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Thanks guys
OK Andy... I suspect a good carb cleaning is in order. Put the rack and go to town...

It's too bad that the .INFO site is no longer, I had a great write-up there on carb cleaning...
C.
79' KZ650-D2 [fsm] (Max)
83' KZ750-F1 LTD [clymers] (Kay)
82' KZ1100-D1 Specter (another project)
78' KZ650-B2a (J&H, A Project)
91' KZ1000-P (P = parts)
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Engine dies when throttle opened quickly 11 Feb 2024 03:57 #895117

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Not sure about the choke leaking but I will check it out 
1978 Z650 B2, US import to the UK

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Engine dies when throttle opened quickly 12 Feb 2024 11:33 #895165

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Not sure about the choke leaking but I will check it out



 
The rubber seals go hard, that lets extra gas into the throats at all speeds, black spark plugs are a clue. This is typical on +45 year old carbs.

Small circuits in the carb bodies (pilot jets) can also close up with gas tarnish over time especially if they were stored without flushing correctly.

You can spray all the passages in the carbs without splitting them, just remove from the rubbers and hang rack off the side of the engine by the cables (support with bungee) Take the bowls off then spray with carb cleaner, remove all jets and spray with carb cleaner, blow out with compressed air, reassemble.
 
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Last edit: by Injected. Reason: info

Engine dies when throttle opened quickly 13 Feb 2024 07:51 #895214

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Thanks I will give that a try 
1978 Z650 B2, US import to the UK

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Engine dies when throttle opened quickly 13 Feb 2024 08:14 #895215

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Thanks I will give that a try 
 
Just noticed your in the UK.

You should get a hold of Zed1015 as he rebuilds choke plunger seals and is in the UK.

He is also on the Z1OC forum and the Z650 forum both run out of the UK so its not that hard to track him down.

 

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