Driving me mad, give me some ideas please!

[thewormman]

I have a 1976 Kawasaki KZ900. I has had the top end of the engine stripped down. Cylinder head and top of barrels skimmed flat. Standard bores honed and new piston rings fitted. 2 new exhaust valve guides fitted as out of spec. 2 new exhaust valves fitted. All parts genuine kawasaki. all valve clearances re-shimmed, cam timing set up correctly. Dana electronic ignition fitted. New NGK plugs and correct HT leads fitted. New loom fitted. carbs have been all ultrasonically cleaned and balanced using vacuum gauges. Choke plungers have new seals. Mixture screws are 21/4 turns out which is a lot more than standard spec but plugs still get a bit sooty. It will not run if they are screwed in further, I used a colour tune to get them to this point.

What it is doing:

It starts from cold straight away on full choke then runs ok on reduced choke. As it warms up it gets to the point where it still needs a little choke but will not idle, it needs revving off choke or it splutters and dies. If it is then ridden to heat it fully the choke can be turned off but if you give it a little throttle it will splutter again and want to stall, you need a bit more throttle and revs. At this point it will idle at about 1600 revs steadily but after a minute or so of idling the revs will suddenly start to rapidly drop and if it isn’t revved it will stall. If you rev it at about 3000 revs for a minute or so then let it idle it will repeat this, it will run steadily at idle for a minute or so then the revs will suddenly drop and if not revved it will stall.

I have rechecked valve clearances and had to re-shim a couple of valves that had settled, but this made no difference.

If I take it out for a test ride after a couple of miles and up to full temp it starts to run rougher and gets progressively worse till it will not idle at all.

Help! Ideas I am pulling my hair out???

[zed9]

Could be ignition timing, have you checked it using strobe at full advance when revved if it wont idle at low revs.

Did it idle and rev ok before rebuild?

[Ultim8pc]

It can be one or many issues, please take a look at this article I wrote which outlines all the things you should check that can contribute to incorrect running:
https://airevalleyclassics.co.uk/badrunning.html

[warren3200gt]

Plug caps and plugs both have resistors resulting in weak spark sooting the plugs at low revs.
At higher revs your getting a better cleaner spark due to increased charge voltage.......
Maybe? One or no resistor/s per ignition line only.

[gray]

What are the compressions like now? Is the airbox inlet free of obstructions? Is the exhaust coked up? Are 1 of the cams 1 tooth out?

[gray]

if it revved ok before the work, its not likely to be the advance/ retard unit

[thewormman]

Could be ignition timing, have you checked it using strobe at full advance when revved if it wont idle at low revs.
Did it idle and rev ok before rebuild?

Hadn’t checked with a strobe, I have found my strobe now so will check. Yes it revved ok before the work even though it had two bent valves!!! Frustratingly it also idled ok before the work as well!

Plug caps and plugs both have resistors resulting in weak spark sooting the plugs at low revs.
At higher revs your getting a better cleaner spark due to increased charge voltage.......
Maybe? One or no resistor/s per ignition line only.

I have Dynatek leads as well as the coils, plugs are new NGK B8ES, but have had NGK plugs that have failed before. These have done 19 miles but will check for consistent spark.

What are the compressions like now? Is the airbox inlet free of obstructions? Is the exhaust coked up? Are 1 of the cams 1 tooth out?

Will warm it up and check compression haven’t done that yet. Airbox is original and all good, new air cleaner, exhaust assumed to be clear? Its a Kerker 4 into 1. Cams definitely correct checked and double checked.

if it revved ok before the work, its not likely to be the advance/ retard unit

Yes it revved ok before the work even though it had two bent valves!!! Frustratingly it also idled ok. Advance retard was cleaned checked and lightly oiled on re-assembly.

It can be one or many issues, please take a look at this article I wrote which outlines all the things you should check that can contribute to incorrect running:
https://airevalleyclassics.co.uk/badrunning.html

Will work my way through these.




Thank you all for the replies I will report back when I have investigated more