Topic: Cb650 high idle

[Davecb650c]

Hey all I just picked up my first bike, a 1981 cb650c, not running condition, but I went though the carbs and put new plugs an was able to get it running today, but it idles at like 5k rpm off choke, any ideas on what could be causing that? I sprayed around the carb boots with no change, and the idle speed screw is backed off so that it's not even contacting the throttle linkage so I'm kind of running out of ideas as to what is making it run so fast. any help would be greatly appreciated!

[70CB750]

Did you check the timing?

[Davecb650c]

No didnt check that, how do you go about doing that on one of these?

[70CB750]

With a timing light. Not sure how much you know, here are some basics.

http://m.youtube.com/watch?v=MX3vsRDfKJg

On motorcycle the points plate has line F and line T where F means fire and on idle it should line up with a mark on the engine.  I use sharpie to make a line on the mark and line on the F line. Under the stroboscopic ie flashing  light of the timing gun, those two lines should line up on idle. To change timing you need to rotate the points plate as necessary.

Typically start with cylinder 1-4 and if that matches move to 2-3 and check the second pair - thats for CB750 but timing is very similar process no matter what vehicle you are dealing with.   

I believe manual shows how to do it too.

[Davecb650c]

I know a decent bit about automotive stuff but this is the first time working on a bike, ill check that and see what I get

[cb650]

With the 650's cdi timing wont be much of a issue.  Check the facts idle cam or you idle jets are pluged.

[DaveBarbier]

In case you're looking around for a facts idle cam with no luck, try looking for the fast idle cam instead.

From the little I know of carbs I'd ask how crapped up were they to begin with? You said that the idle stop screw is out all the way, but is the sprung arm thing that gets pushed by the idle stop screw returning all the way to it's resting position? I'm basically asking if the slides could be stuck or not closing all the way.

Also, I hear that carbs that are very out of sync could also cause a high idle.

Any modifications to the bike? Do you have a stock airbox with filter and exhaust? An open air box or free flowing exhaust without rejetting could cause a high idle. (Someone who's smart please speak up if this is wrong.)


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1978 Honda CB550K

[scunny]

check the throttle cable isn't adjusted up to tight

[Davecb650c]

Wel I took the carbs off again, double checked the idle jets an they were clean. it was out of sync decently far tho so I reset that. but if I start it up from cold it idles lower like 1500 then slowly creeps up to about 2500 over the course of a couple minutes and then it takes off after that and spins up to 5k again. And as of now it doesn't have an air filter on it but it is stock exhaust. It has the cv carbs but the butterfly's appear to be closing all the way

[Davecb650c]

Just put the air box back on and got nothin still so I'm out of ideas, seems like an air leak kind of thing almost but no change from spraying carb cleaner around anything

[DaveBarbier]

Is there a filter in the air box? Do the carbs have factory jets installed? How are you syncing the carbs? Do you have a manometer or vacuum gauge?

I honestly don't know much about cv carbs or that type of ignition so I can't comment on that. Hopefully someone else will chime in. If you look through my posts you'll see that I had a stubborn high idle on my 550 which was never really solved. But people think my carbs were just really bad (cracks, pitting, etc.) I'm currently in the process of getting another set ready. They are different bikes, yours and mine, but it might help to skim through my post for ideas.


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1978 Honda CB550K

[Davecb650c]

It's stock jets as far as I know I've only owned the bike a week, but it has pressed in idle jets and 120 mains. Air box has a filter but it doesn't seem to matter if I have the air oz on or not. I only bench synced them because I don't have a vacuum setup. I forgot to mention it has aftermarket accel coils and wires but I don't think they are resistance wires or plugs, could that make it run funky?

[DaveBarbier]

Yeah, it seems the 120 is right. But I don't know about the pressed in idle jets. I think it's a non-removable part. Even though my '78 550K also has pressed in idle jets and they're actually quite easy to replace.
http://www.cmsnl.com/honda-cb650-1981-usa_model7224/partslist/E++1901.html#results

Here's a site I found that might help you along your journey. Doesn't say anything about high idles though.
http://www.myhonda650.com/cb650problems.htm

Because I know really nothing about CV carbs and if a leaking diaphragm could cause a high idle, I can only assume you should check the condition of them to rule it out.


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1978 Honda CB550K

[curemode2002]

The idle jets can be changed even if you have the press in type but you have to be very gentle if you tweak anything while pulling the old ones out or pressing the new ones in the seat can crack. At that point you have to repair the crack and use something to maintaint the pressed fit (just be really careful this is easier  ).

Sirius Consolidated Inc. in Canada actually carries the press in idle jets (this link should give you an idea this is for the #35 slow jet they have other sizes available http://siriusconinc.com/pro-detail.php?pid=&product_id=3788). Looks like the reason they are press in is due to the same reason your air mixture screws are capped emmisions, that way you can't mess with your idle settings too much.

[Davecb650c]

Alright well I took the carbs off again, they were clean blew out the idle jets yet again still seemed just fine. I don't see any diaphragms in the carbs other than the accelerator pump, in the top is just like a metal piston thing, is there something missing? Also is the 3rd tube thing next to the idle jet and main jet supposed to an open passage? because I can't make air go through that.  Anyway it still will start and idle from cold at like 1500 then slowly climbs to 2500 over a minute or 2 then once it hits 2500 it shoots to 5k in a matter of seconds. But now it's popping and backfiring from number 1 and 4 cyl seems fishy as they are on the same coil but could the non resistance plugs and wires have something to do with it? This thing really has me stumped

[ewgoforth]

High idle and popping sounds like an air leak to me, but you said you've sprayed around the carbs with carb cleaner and it didn't change. 

[Davecb650c]

Yeah that's what I was thinkin too but no sign of change with carb cleaner. The carbs weren't too bad to start with and I've cleaned them 3 times since then so I'm leaning towards something else, ignition perhaps? I haven't checked the timing, is it gonna be worth checkin while it's running that fast?

[70CB750]

CB650 says it has a cdi timing - means it is most likely not the problem.

Sorry if I mislead you assuming it has points.

[trueblue]

Just for #$%*s and giggles pull the pulser cover off the right side of the engine. When the F mark is lined up with the mark in the little window, the little metal tit on the rotor should line up with the little metal tit on the pickup. This is true for both 1/4 and 2/3. It is possible your advancer is gummy and stuck in the advanced position, also while in there pull the advancer apart and lube it even if it isn't the problem. While it is apart make sure you don't have a broken spring or something silly like that. Advanced timing will cause a high idle.

The next thing I would check is that your slides move freely. You should be able to raise them with your finger and they should then fall all the way down smoothly with a dampened effect but not catch up anywhere, not even slightly. These carbs don't have diaphragms they just have the alloy piston which you have already found. They should also have a spring on top of the piston, if this is missing you will have a high idle, but this is normally accompanied by erratic running or hunting.

The other thing is make sure your throttle cables have a little slack in them. It takes very little throttle movement for these engines to free rev to 5000 rpm. Also if your push cable is fighting your pull cable it can cause it to not return to idle. 

[iron_worker]

The diaphragms are under the big round covers on top and control the slide movement by the amount of vacuum in the intake vs. the atmospheric pressure. If your diaphragms or slides are sticking, torn, or somehow bound up the carbs will never work right. There are also little ports inside your carb bodies that allow the vacuum inside the intake to reach the diaphragms chambers. These must be free and clear as well. Same goes for the atmospheric vent ports.

One possibility may be that you're running on 2 or 3 cylinders and then the other cylinders are starting to fire again and speeding up your idle. My bike used to do something like that when it had plugged slow jets ... it would start up on 3 cylinders and you'd have to crank the idle speed up to get it to idle  ... then once it warmed up enough that cylinder would come back to life and you'd have to crank the idle adjuster way down again.


IW

[Davecb650c]

Yeah all the slides are nice and return on their own, the pulse rotor lines up on f I did notice that when I checked the gap on that but I didn't check the advance mech. Is the 3rd tube(next to the main and idle jet tubes) supposed to be open? Cause that is plugged on all 4 carbs

[trueblue]

Yeah all the slides are nice and return on their own, the pulse rotor lines up on f I did notice that when I checked the gap on that but I didn't check the advance mech. Is the 3rd tube(next to the main and idle jet tubes) supposed to be open? Cause that is plugged on all 4 carbs

It has been a while since I looked inside the CV carbs, do you have a picture of the one you are talking about?  There is a fair chance that if it is plugged, it isn't right. 

Have you checked your throttle cables are fully returning?

[Davecb650c]

Yeah throttle is slack, and the one I'm talking about is the top one in the picture

[Davecb650c]

Is it worth tryin to pull out these jets to clean them or will it just ruin them? I may just get a set of 82 carbs and go from there instead

[trueblue]

I have the 82 with the removable jets, I haven't had to try to remove the jets of the 81's.  As far as I'm aware they aren't removeable.  If I get a chance tomorrow, I'll have a look at the carbs I have in the shed and see what that 3rd tube does.