1986 gpz 1000 rx...engine knocks at start up...only in idle...give it gas it goes away until the the engine goes back to idle...even after about a half hour of running...knocking is still there..nut only at idle...help what could this be?

I have one that was doing the same thing. adjust the carburator throttle body screws you have a carburator thats out. thats the phillips head screw in the front of the carb. it adjust's the butterfly needs to be balanced. or take carbs off look inside at the butterflies can use a verry small drill bit to adjust make them all the same. or get carb sticks. and adjust the carbs running. while you have them off blow out the idle ports. just remember how many turns the screws were set at dont loose the little o rings that are down in the holes take then out first then air. when you get it running leave air cleaner off start bike put your hand over one carb opening at a time will **** out any crud in the bowl. it's gonna flod but it works good luck

RXs haven't got hydraulic tappets tho' Squim.

If it's a heavy knock you might have a big-end bearing gone sadly.

When you say running for half an hour do you mean at a standstill or have you ridden it? If it rides well then ignore the above.

Is the engine in need of a service? Older engines particularly can sound horrendous low-down if they are running uneven, valve-clearance and carb-balance and new plugs might just put it right.

86gpz, hmmmm probably has hydraulic valve lifters or tappets. probably one is not working, maybe a piece of dirt in the release valve. Probably not serious. May even go away suddenly.
Repair is extensive. Run it till it dies.