Won't start without throttle

My 1981 KZ750 will not start without giving it some throttle. It has Keihin CV34s on it. The background information is that I recently removed the carbs and cleaned them up and replaced seals, float needles, and mixture screws. I visually synced them but have not vacuum synced yet. Spark plugs are new as of last month, properly gapped using a gapping tool.

The problem only occurs if the engine is cold. If it has recently (last hour or so) has been running it will start up right away again. Lifting the choke lever does not help at all. After the bike has warmed up, using the choke will raise the idle RPM. The idle RPM after being warmed up is about 1100rpm (hard to tell on the gauge).

I also hear a bit of a tapping sound (thinking it is valves) when the engine is cold but mostly clears up when it is warm.

Final symptom is new since reinstalling the carbs. I get a popping in the exhaust (afterfire?) out of the right exhaust pipe.

Something that may or may not be a symptom is the gas mileage. The last tank of gas I got about 77 miles out of 2.28 gallons or about 34 MPG. Is this about right for the bike?

I checked the float levels by connecting a clear tube to the drain/overflow hoses and all four carbs are about 6-7mm below the float bowl O-ring. Obviously this is a problem.

My troubleshooting plan is do a compression test on all four cylinders to get an idea of what is going on there. Then check the clearances on the valves. Next would be to adjust the floats. After that, sync the carbs with vacuum gauges. Finally, idle mixture adjustments.

Does this seem like a reasonable sequence of things to check? Any ideas of things to watch for? Also in the back of my mind is the timing advancer. If none of the above seem to clear things up and make it start easier I will look at cleaning and lubricating the advancer.

Is the best way to adjust the float levels to remove the carbs from the bike and adjust? Seems like it would be very time consuming to get them set right. as it would be lots of on and off the bike.

Maybe I'll narrow my question down.

Is it possible to adjust the float level with the carbs on the bike or do I need to take them off?

they will have to come off. the tapping noise could be comin from a few places maybe valves but i doubt it most likely cam chain related loose or worn tensioner blades. the after fire is caused by a lean pilot screw setting richen it up by a quarter of a turn and it should clear up. to set the floats the carbs should only have to come off once.

Can you explain the process of adjusting the levels then? There would be lots of removing the float bowls as I fill them and check the level then adjust and check again, etc, right?

lots of people like to do it that way but you can also do it by pulling the bowls and inverting the carbs and measuring the height of the float where it is just starting to make contact with the valve there is a spec in your manual for this you have to make sure that they are just making contact not putting pressure on the plunger so you will have to tip the carb body's at an angle the key is dont bend the float assembly it self just the small tab that makes contact with the float valve also when you get the right measurement you must also make sure the floats are even with each other if then when you think you have it right hook a fuel source to them to check for leakage. if they leak and your sure they are right you will probably have to get new needles and seat assemblies. check the valve for pitting or grooving . if you find any you are better off just forgetting messing with those and buy new assemblies you can buy them from z1 or if you have decent bike shop around your place that uses motovan distributuing they have them also.

Assure carbs are level on bench when checking levels by clear tube test.

Only one float bowl screw is needed during the several float bowl removals and replacements which may be necessary. A little fuel seepage doesn't invalidate the reading.

If the float height measurement is used, always verify actual level via clear tube test.

Good Fortune!