Idling Problem

[al_ngl]

I think my bike's carbs may be out of tune because The bike idles slow (<800rpms) cold, but once the bike is warmed up it idles at 3k+ rpms. The throttle isn't stuck and there isn't an air leak, so what would be causing the problem? I probably just need to adjust the jets or something, but I don't want to change anything until someone knows what causes the problem. The idle is perfectly smooth, just wrong. I have never worked on anything with multiple carbs before either, so I may need help keeping them close in adjustment to each other. Has anyone else had this problem?

[madmax-im]

well assuming that you have taken the choke off...if you look on the bottom side of the carb bank you will see in the middle a adjustment screw.This is the idle adjustment knob.Turn it clockwise to raise the RPM's and counter-clockwise to lower the RPMs'.You have to do this once the bike is warmed up and it is "idling".You will need to wear gloves to get your hand near the hot engine...but its doable.Your idle should be 1100 RPMs.Try that first..if you still have a problem come back and the real carb gurus will step up to the plate.

[Tusler]

Like he said

Hi,
Perhaps you are not familiar with the operation of carb's of the old days.

When a carb'd motor is first started the carbs are very inefficient at providing atomized (vaporized) fuel for combustion then as the engine heats up the carb's and the intake begin to provide a correct atomized fuel air mixture to the cyl for combustion and things start working correctly.

You have a 2 stage choke, all the way out is like full rich for starting cold then in 1 notch to keep it running till it warms up.

I got my bike good and hot then set the idle. When I get it running cold I hop on and gingerly get things moving and just close the choke to the normal running position.
Hope this helps

[barberad]

when I first got my XJ, it was set to about 1500. The PO was a young guy and didn't seem to care to much about an old heavy bike. After I changed the oil and plugs and cleaned the air filters x3 K&N's (# 3 of 4 had been missing for longer than the PO knew about) the idle jumped quite a bit, like 2200. After riding around a little while I set back to 1200. Never hurts to do a little tune up. Air, plugs and oil. I'd knock out the easy stuff first.
One more thought: I thought I read a post about a plugged crank case breather doing something odd like that. I'll have to look it up.

[al_ngl]

I know how carbs work, but I've never had carbs that acted like this. I've had a '73 VW sandrail and my 75 Yamaha DT400, and neither of those had any change in their idle, but then again I had those both tuned right from the start, and the VW had an electric choke. I guess the idle screw must be the one screw near the throttle cable. I have a "genuine yamaha" service manual, so I'll see the whole process. I'll probably just adjust the idle for now, and this fall/winter after I store the bike I'll rebuild the carbs.

[ClarkGriswald]

Mine does it too but Im not sure if it could be the same. I keep gettin little bits of compression back here and there (hope it keeps goin) but, ive put the sync guages on a bunch of times and fine tuned it when it starts sounding rough at the low end.. today was another time "startin fresh" and I had to adjust it once it warmed up as it was gettin up there.. Now Ive ridden a few more small rides and its pretty good. Guess thats just the way to set em hot. I cant vouch for anything my bikes doin though cause its in a weird state.

[tarzan]

You should list your model (year ,special or standard)in your avatar or signature so everyone knows what bike you have so they can help you better.You didnt say if the bike was fine then suddenly started doing this or if you bought it that way.If it idles at 800 then goes to 3000rpms.If you turn that idle screw down it will die at idle.I would think you either have carbs out of sync or maybe a vacuum leak.Are your carb boots ok(not cracked )?,Do the vacuum plugs on the carb boots seal tight?There could be a
number of things that could be wrongWe will probably need a little more info.

[oseaghdha]

Your carbs are dirty.

[barberad]

I put "crancase breather" in the search and see a lot of references to oil getting past it. I thought I read one before about it being plugged but cannot find it.
So what would happen if the breather was plugged?

[TADracer]

Your carbs might be out of sync. If that is the case then one carb will actually drag the others along and raise the idle RPM,s up. Because you have four, you need to sync up these carbs regularly and that is part of the tuneup routine.

[al_ngl]

I'm pretty sure the carbs are fairly clean because I run fuel system cleaner in my gas (and yes I run premium).

Is it possible to adjust all four carbs without removing them (or the gas tank) from the bike? I have heard that the fuel system on this bike is a real pain in the arse.

I am positive that the carbs are out of tune and probably out of synch because the PO never tuned them in the few years he owned the bike. Everything else with the bike seems fine, compression is about 130 in all cylinders, just replaced plugs wires and coils, oil and filter. So there are just a few little tune up things I need to still do, and then I need replace mid and final gear oil and put new seals on my forks.

Either this weekend or next I'll tune the carbs and try to find an air filter. If the bike still doesn't idle right, I'll take the carbs apart.

[egsols]

You sync the carbs on the bike but the gas tank has to be removed to gain access to the adjusting screws.

I believe their is a step by step in the Tech tips.

Make sure that when you sync the bike is fully warmed up. I synced mine and the bike wasn't a 100 percent warm, had to go back and redo.

[Ken Talbot]

On a Standard, like your F, you can run longer fuel lines so your right petcock feeds the left carbs and the left petcock feeds the right carbs. Routed properly, and at just the right length, you will be able to lift the rear end of the tank, slip in a 2 or 3 inch block of wood, and have enough room to access the three screws for adjusting sync, as well as the four pilot adjustment screws. Set the petcocks to prime while you've got the vacuum nipples connected to the sync tool and you're ready to go.

This way, you don't have to worry about making up an auxiliary tank, or having your tank fall off the seat while turned backwards and balanced on the frame under the seat.

Running a bit of fuel system cleaner once in a while is a good idea. Premium gas, IMHO, costs more and probably doesn't help keep things clean.

You may be "pretty sure the carbs are fairly clean", but there is a way to be absolutely sure they are spotlessly clean. Want to guess what that might be?

[bikerphil]

tool

This carb adjusting tool is great to have ($25 JCW), the idle mixtures and sync can be set without even taking off the tank or the seat.

[al_ngl]

I said I was going to rebuild the carbs sometime, I just don't want to do it while there's still good riding to be had. So I need a carb synchronizer? Aren't those like really expensive? And is it very safe to keep that long fuel line on the bike after the carb tuning is complete?