5 second rev to 4000 only at start up after sitting overnight?

[AndrewinVictoria]

I am having a hard time finding this problem on this site.
I have been slowly doing every step to ensure a good running bike. My bike in question is a 1979 GS850 with 74000 kms on it. My dad said it has been bored out and ported and polished, that was 20 years ago. I have soaked and rebuilt the carbs, new air filter and and airbox boots, sealed airbox, new intake orings, new points and condensors, adjusted timing, new throttle cables, checked and cleaned all electrical connections, new spark plugs and caps, checked petcock(working properly), pretty much everything but valve adjustments(I will be doing this next).
So the problem is, when I leave the bike overnight the next morning it revs up to 4000 rpms but only for 5 seconds and then drops down to 1100 rpms and runs and rides better than I have ever had it running. Anybody have this problem or know how to fix it? Is it because the valves need to be adjusted?
Thank in advance for your help and sorry if this is posted somewhere hidden that i could not find.

[Brendan W]

How much choke do you use for a cold start ? Mine would run probably all the way to 5000 on certain days if I let it. Do you park on the main stand.? What setting is the petcock at when this happens?

[AndrewinVictoria]

That is with no choke. The choke out increases the rpm. I do park it on its side stand not the center stand, and the petcock is in the on position.

[Brendan W]

Maybe that petcock is not as tight as you think ?

[AndrewinVictoria]

its worth a double check I am sure. I just have to ride some of the gas out. When i pull off the hoses from the petcock there is no gas coming out, just a few drops and then nothing. Could those few drops cause this problem or is it just whats left in the line from the last time it was running?

[Brendan W]

The delivery line will have some in there. The vacuum line should be as dry as a bone. If it is leaking very slowly you might not notice on casual inspection. You could try disconnecting both lines one night.

[AndrewinVictoria]

Could it be the float heights? I only took them off to clean and replace parts but didn't bend or adjust them at all. Plus my bike is equipped with an overflow which would spill the excess gas onto the ground instead of into the carbs or cylinders, or am I wrong about that? Thanks for your interest in my problem. I just got back from a 75km ride with high and low speeds and everything seems perfect. Other than startup.

[AndrewinVictoria]

i will try that tonight

[Brendan W]

8:35 pm ..................have to ride some of the gas out
8:43 pm ..................just got back......75 km

Nothing wrong with that bike

[AndrewinVictoria]

its rideable thats for sure, maybe it has to do with my carb repair and settings that i posted a while ago with no replies.

[AndrewinVictoria]

http://www.thegsresources.com/_forum/showthread.php?239808-I-drilled-out-my-broken-fuel-screws-1979-GS850