Ok, this is doing my head in.
One of my old ZX10 (88-90, very similar to ZZR1100 with carbs) engines has lost it's idle, didn't want to mess with the balance screws
without the right gear so bought a set of 4 vac gauges & being a sensible sort of chap thought I'd set up the "good" engine first
so's I'd know what to expect. First engine has setup perfectly with about 5 ins mg on all cylinders & perfect idle. Gone to set the
misperforming one up & I have really low readings (<1) on all cylinders & basically 0 or close to it on no.4, as soon as I screw the
adjusters in the direction to improve the vac balance it kills the engine completely. If I screw in the main throttle idle screw it will idle, but
then climbs to about 5,000rpm, as soon as you start to reduce the idle speed it comes down fairly steadily & then just dies away completely &
wants a dab on the choke to get a restart.
It sounds like it's running really weak (can't get it on a 4 gas at the mo) & if it was a car engine I'd suspect an air leak, but
there's no evidence of an air leak & can't see anywhere it could leak that would affect all 4 cyls (it appears to be running on all 4)
I'd wondered about fuel starvation, but as soon as you get it on the road it goes like a rocket & pulls right round past the redline.
The low vac would tend to suggest that the throttle is open further than it should be for a normal idle - surely I couldn't have 4 blocked idle
jets!!!
Any suggestions would be appreciated (yes, I've already thought of a gallon of petrol & a match)
? idle speed too low 
You need airflow to create a vacuum.
Normally you'd set the idle speed then sync the carbs then re-adjust the idle speed. I'd leave the main throttle idle jet completely.
Is it definately firing on all four cylinders? I'd get it running then pull off some spark leads to check that first.
Tight valve clearance will cause the symptoms you have.
quote:
Originally posted by rusty nuts
Tight valve clearance will cause the symptoms you have.
quote:
Originally posted by MikeRJ
quote:
Originally posted by rusty nuts
Tight valve clearance will cause the symptoms you have.
That would be my guess as well.
Unbalanced carbs would be my guess. Mine was running lumpy under around 1500rpm, coughing and banging in the exhaust then above that cleared its head
and ran smooth.
Balancing the carbs got rid of the lumpy running and it settles down to a very smooth 900rpm when warm. It idles at 500rpm when cold although a little
bottom end jerky, anything over 600rpm is nice and smooth.
quote:
Originally posted by rusty nuts
Great minds think alike!
Thanx for the suggestions - had wondered about tappet clearances/burnt valve but didn't seem to fit with it running on all 4. Have ordered a cam cover gasket so I'll take a look at valve clearances Monday.