Engine dies when throttle opened quickly

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 

How many turns out on the air screws?
You're running really rich...

Andyp62 wrote:

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. 

I have had the air screw from one turn to 3 out with little effect on tickover 

Thanks guys

Andyp62 wrote:

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.

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?

Andyp62 wrote:

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...

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

Andyp62 wrote:

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.
 

Thanks I will give that a try 

Andyp62 wrote:

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.