runnaway rpms

[New2DOlds]

alright i know this is almost exactly the same problem that someelse has had which is why i thought someone might have an idea before i take it to the shop as my last ditch effort, also im still new to the bike repair area.

when i start the bike it starts as normal then after a second or two it takes off the bike revs up to about 4500 rpm then stays there spewing black smoke from the exhaust pipes, i already know i need new jets cause, obviously, the thick black powdery smoke from the exhaust and the fact that i get roughly 55 miles to the TANK on a good day, besides that, it usually revs high naturally until i get her warmed up then itll idle at 1500 to 1800 rpm it will stay there until i give it just a little throttle then it, once reaching 2000 rpm, will take off toward 4500 again if i choke it will drop down then go back up, i dont understand, this also leads me to the fact that when im running out of gas the rpm sky rocket as well, im lost can someone help?

[mitch]

carbs

Start by cleaning the carbs and bench sync them also check when u have the carbs off that the throttle cable works free. All the info u need is in the maintence ,repair , or modification files. Good luck and welcome to the site .....MITCH

[mitch]

Jets

Jets are a piece of brass with a hole in them, they dont wear out, so unless u had the cabs apart and put the wrong ones in they should be fine.Most times its just we get playing with the adjustment screws not knowing what were doing. Guilty of that myself, but the info here will show u how to do it right..........MITCH

[scalded dog]

Check for cracks in your intake boots, check operation of throttle cable, check for continued gas flow when the petcock is in off position, bench synch, as already suggested, clean carbs as already suggested, if you are only getting 55 miles to the tank, your bike is running very poorly, if you get it running right, be ready for extreme power and speed. Later 'Dog

[skids]

Re: runnaway rpms

If you are VERY new to this machine, be aware of the "choke" settings.

I once had a "run-away" rpm condition caused by a broken piece of hardware at the throttle sleeve near the right hand grip. Also, if the throttle cable routing is binding the cable, this can happen. On the other end, make sure that the carb throttle linkages are returning to the stops because it is possible (especially if you soaked the carbs in cleaning solution) that the butterfly axles are sticky.

I suspect that you may need to equalize vacuum at idle speed ("synch the carbs"). As mentioned, you can get them in the ball park by adjusting the buterfly plates until they are all about the same by using a paper clip as a feeler gage (bench synch). Then, turn down the idle setting as needed and use a set of mercury sticks (manometers) to equalize vacuum at idle speed. Of course none of this will be worthwhile if the manifold boots (outlet end of the carbs) are leaking.

You can use RTV Ultra Black to seal any cracks in the boots. Make sure it sets well before starting the engine, and do not coat the insides of the boots!

On final thing. Make sure that the "choke" linkages have not come loose. It is possible for a set screw to come loose, leaving a starter jet to do its own thing.

[srq68]

I had the same problem after rebuilding my XS850, upper engine and carbs. RPM were fine, a little high, but would rev up as I sat at a light or stop sign, only a choke action would bring it down. Well, after synchronizing the carbs and ajusting mixtures, I realized that the breather hose going from the crankcase to the air intake was sending exhaust fumes through the carb, making the whole ratio leaner, which in turn increased the RPM, which sent more exhaust in the intake and so forth, hence enriching the misture by pulling the choke lever brought things back to mormal.
I disconnected the breather hose from the intake, plugges the hole on the intake box with a nice big SS bolt, and put a little chrome air filter from the local auto part store at the end of the breather hose. Problem is gone!
The engine had 35000 mi. and one piston was shot, I replaced it with new rings, but like an idiot and cheap bastard I didn't do the other ones, which now I believe leak some compression into the crankcase...
Try it....

[New2DOlds]

busy busy

thanx guys, looks like im gonna be spendin a lot of time with my baby on the bench thanx for the great ideas, ill work on it and get back to you...thanx again.