87 GPX750 coldstart/running problems

My 87 GPX750 is giving me a weird problem. Very difficult to cold start. Drains the battery trying to start it, then connect to a car battery and drains half of that before it will start. Starter motor is turning fast enough. Battery has plenty of juice. Carbys have had new float valves and pilot screws put in. Carby fuel levels are spot on. Put new ignition coils, spark plugs, and leads on. Spark coming out of plugs OK. Choke's working. I tried a different set of carbys, no change. Tried starting without the airbox, no change. Engine is 70,000 km's old and was rebuilt 20,000 km's ago, with valve clearance and carby's tuned 7,500 km's ago. Bike is standard except a 4-into-1 exhaust.
When it finally does start, there is a slight flutter when it's running, and sometimes it will run on (from what it sounds like) 2 pistons (sounds great but performance is terrible.) Engine braking seems weaker. Hot start is still instant (one turn of the engine.)
I don't understand how this happened. Been working fine for couple of years, started cold after just 2 turns of the engine. Then one morning it wouldn't start. Recent thoughts are it might be an IC igniter problem, but I can't find any similar problems posted anywhere regarding a faulty IC unit. Igniter resistance testing following a Hayne's manual test table seemed inconclusive.
Any help/suggestions will be greatly appreciated.

What is your spark strength like? A weak spark can be supressed by high compression. Might not hurt to check the compression too, just as a precaution.

Thanks for the suggestion screamin TE. The bike is at a mechanics at the moment. I am going to call him tomorrow to ask to check spark strength and compression.

hhhmmm
cold no start..???
hot easy start?

the fuel tank vac valve is stuffed
when its all hot no problem..the valve works (heat loosens stuff remember)
when cold..?? well it just wont let enough/any fuel thru
lots of choke wont help either..generally fuel flow will fluctuate
(or it did on my old GeePeeX7fiddy until the valve got replaced)

get your mech to check the fuel flow and the tank valve
also make SURE the vac tube that controls it is connected, not twisted and in good nick!

Yoda speaks wise words......

Hi Smitty,
thanks for the reply. Fuel tank is not the problem as I have tried starting it without the tank on and just using a reservoir of fuel connected to the tube. I rang the mechanic who is currently looking over the bike and he found low engine compression. He and I are hoping it's just valve clearance, but he will fill me in when he's confident what is causing it. I still don't understand how it happened overnight, unless something just came loose. I'll complete this topic when I'm fully briefed.

Turns out the mechanic managed to get 180 (psi I think) from cylinders 1,2, and 4. 3 is running at 150. I think he did this by readjusting the valve clearance. So it seems my valves and/or valve seats are worn. I suspect the engine rebuild that happened a while ago only covered the pistons, rings, cams, and lifters. The head may have been overlooked. There are also blocked galleys in carbs 1 and 3, so the carbs are going to be stripped down and given a nice sonic bath.

poor starting from cold, and good starting when hot - are typical symptoms of valve-clearances that have closed up. Unless it was specified during the engine re-build that the valves were to be replaced, or valve-seats re-cut - then you can *guarantee* that the head was not touched during the rebuild....

Had the exact same issue on my GPX and yes was related to the valve receeding into the head.