RPMs with choke on?

Hello everyone,
About 3 weeks ago i just got my uncle's 1982 xj750J maxim. After the carbs were cleaned and the spark plugs were changed she has been running great. However, there is one thing that disturbs me about this bike, the choke. As far as i know, its used to warm up the motorcycle before riding (this is my first motorcycle BTW). So when i start the motorcycle with the choke on full, the rpms are about 1800 and then it starts to fluctuate about +/- 300 rpms. After a minute or so the rpms will increase to around 2200 rpm with the same amount of fluctuation. Is this normal? I have not syncronized the carbs yet because i dont have the tool... so im guessing this might be a reason why?

Peter

 

A hunting idle would indicate to me that you have a vaccum leak somewhere. Break out the WD-40 and start hunting for that leak. RPM increase is not uncommon nor unexpected. Good luck!

 

Yep could be vacuum but you no mention what its like after a ride ect at idle , any ways a sync might be in order ??

 

As the bikes warms up the RPM may rise because the bike is burning the extra fuel given to it by the choke. This is normal with the choke on. If it is still fluctuating after the choke is off and the bike has been run around the block then like the guys said look for an air leak. Fluctuating at idle with the choke on is also normal. The bike is burning a rich mixture and may slightly foul a plug or flood a cylinder. As the cylinders warm up they better vaporize the fuel due to the heat produced and a variance of 300 RPM can be produced by just one cylinder cutting in and out.

 

Thanks for the input guys. I'm glad i found out about this forum since not many people around me knows anything about motorcycles.

After i turn off the choke, the idle is pretty steady (tho sometimes it drops and eventually stalls. I'm told that this is because the Maxim is "cold blooded" and needs a bit of warming up before it can idle properly). I was concerned that the bike could have a lot more problems with the fluctuating idle while on choke, but as you gentlemen pointed out, it is normal so i guess i have nothing to worry about for now.

Robert, in case i do have a vacuum leak, what do i spray with WD40?

 

Typically you would use the little red straw and poke it around the intake boots and manifolds around the carburetors. Get next to the intake manifolds adjacent to the head. Listen for the idle to drop slightly and smooth out as the liquid is sucked into, and temporarily blocking, the leak. Clear as mud right?

 

Thanks again Robert. Helpful as always.