Just trying to figure out which are the problems I have to solve and which are normal older CBR 600 behavior(s). After a carb rebuild, I have the '87 running so-so. One thing that really bugs me is the inconsistent idle speed. After it is completely warmed up and I have the idle speed adjusted to ~1200RPM, it will increase the idle speed to about 2000 for a number of seconds after I run the bike up into the 3000-4000 range. Sometimes the idle will actually stay higher for a bit. It comes back down to earth after 5-15 seconds, As a Harley guy where you the throttle is VERY much exactly what you expect (twist get power, drop and it immediately drops to Harley rumbling idle), I find this behavior very disturbing while riding, largely because I don't feel like I am going to get the expected engine braking I want.

So is this normal? Or do I still have gremlins in the bike?

Another piece of data also is that the choke seems VERY touchy. Move it a millimeter and the idle goes from struggling to 2000 RPM. So it is a bit difficult to warm up if you don't like idling at 2000 RPM during that period (or stalling, your choice).

The carbs were completely disassembled, parts soaked in cleaner, jets replaced, needle replaced, gaskets and orings replaced, all passages blown out with air coming thru confirmed as best I could and I found no obvious defects in the carbs while doing this.

Hamlin6 has been helping on another thread with the basic debugging, but here I just really want to understand if this increase in idle for short periods is normal. I don't want to spend time trying to make this bike into something I would like as a Harley guy vs. what is expected from this old sport bike.

In this video, I bleep the throttle once, let it fall, then I hold the throttle open to > 3000 once and let it fall and the rest the bike is doing on its own, including the drop and then increase back to 2000 RPM and then the eventual drop back to idle speed.
https://www.youtube.com/watch?v=dMYP...ature=youtu.be

 

I'm not sure if it's normal, but mine does the same thing during warm up. Once the choke is off, mine runs pretty consistently

You should probably check out the choke and throttle cables and make sure they are properly operating/adjusted. The sensitive choke can be caused by the cable being stretched or having too much free play. Could even be binding, but you'd probably feel resistance if so. Either way it could do funny things like not fully engage/disengage the choke which would lead to all sorts of bad behavior

Likewise for the throttle, it should be snappy, so if you watch the cable end (on the carb) while blipping the throttle you should see the cables snap right back to place, with only a bit of free play at zero throttle. Adjust if necessary. If they lag or bind up it's a sign of trouble. Aftermarket cables are cheap and easy

1200 is a bit high. I'd probably recommend setting idle when warm at 1000-1100 if it'll take it without stalling. Has the bike been modified at all? After the carb rebuild did it have a final tuneup, or just a bench tune?

 

The carb has 112 main jet, which is modified from 105 stock jets. Everything else looks stock. I have the 105 jets but decided to stay with the 112 due to modified exhaust (just assumed someone at some point made an attempt to match the jetting to the exhaust). I did a bench sync and I do have the meter for a dynamic sync; but really don't see this having any affect on the idle-up thing I see. I did try the meters and it is pretty close but has a lot of flutter making it hard to determine exactly where I am at. I also think I want to adjust the valves before I do a serious dynamic sync, but note that the compression is pretty good and consistent (150-ish on 1-3 and a little bit higher on 4). There is a lot of slack in choke cable. A little more slack in the throttle than I would normally do on purpose to avoid the throttle cable pulling the carbs. I don't think the throttle cable is binding at all; but who knows. If it were, it doesn't seem like it would ever come back to idle, which it does after the 5-15 second delay, where it sometimes goes up and down a few times. Another thing of note, when we got the bike, the spring was missing between carb 1& 2 and 3&4. So it was basically a two cylinder. I don't recall it having the very high idle issue with the choke on at that point; in fact I thought it idled 1/2 decent but ran like total garbage. Kinda been chasing it ever since with a more thorough rebuild and even buying a second used carb rack that looks pretty clean, but leaked some fuel from fuel rail on my first attempt (so didn't get far enough along to see about idle up thing). I also found a plug wire that came apart inside of the plug tube (that was fun getting out) and I had to repair that cable. I decided to separate all of the cables, measure the resister and put some electric spray on all contracts. I also soaked some parts in navy jelly for a short bit to get the large amount of rust off. Three of the wires now read in the 4.5k to 5.0k range; one is closer to 2.0k. I would re-read them across the coil packs since the manual does have specs for that; but that obviously won't show a deviation in one wire or the other; just the overall high voltage side resistance.

 

I just installed a second used carb rack I ordered for this thing to diagnose. Yesterday the original carb rack that I rebuilt had the bike idling at 4000 RPM while riding it; as well as missing, backfiring and hesitating on acceleration. Note that this suddenly came on after it was starting to run a little better. I shut if off while idling near 4000 RPM, disassembled while carefully looking at choke position of all 4 carbs and also looking at throttle position. Everything was in normal idle position. The used carb rack I installed was only tested around the block and I didn't get this behavior; although I still feel the hesitation when getting on the throttle. Would not be surprised if the hesitation and the idle issue are completely separate; although the backfiring and missing came along with the high idle; but the other stuff has been there even with this other carb (which by the way I haven't touched other than to fix a fuel leak in the fuel rail; so its conditions is TBD).

 

I'll say off the bat that I'm not a carb expert. They are a bit of a rare breed these days, and I grew up in the age where fancy computers were already figuring this stuff out for us, so take my advice with a grain of salt...

From your posts it sounds like there's a few things going on. Probably a carb issue, but you've got a few of the basics to cover off before troubleshooting them further

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...I also think I want to adjust the valves before I do a serious dynamic sync,

Yes, you definitely want to confirm valve adjustment. While you're at it, check spark plug electrodes for color. This can help tell you if the bike is lean/rich. You also get to do a visual of the valvetrain which can tell you a lot about the engine

There are many possible causes for this. Could be a bad air-fuel mixture. It's less likely to be a bad spark, but it's easier to check. After the valve adjustment, confirm spark using an inline tester (or the old fashioned method where you ground the electrode to the cylinder head while cranking it over). All four should be a strong spark. Once spark is ruled out, we can go back to the fuel system. I'd recommend draining the old gas and using fresh gas. May as well get a visual of the petcock strainer screen as it's common for those to get clogged up. Throw a fresh air filter at it if it needs one, and it's always a good idea to put a fuel filter on it while the system's drained.

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Another thing of note, when we got the bike, the spring was missing between carb 1& 2 and 3&4. So it was basically a two cylinder

Eek. Missing parts from the carburetors is not a good sign... Could be dragons

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There is a lot of slack in choke cable

You should fix this. It's as easy as propping the tank up and making sure the choke opens/closes fully. There should be a screw-collar on the cable end holding it in place. Loosen the screw to adjust cable end position, then tighten back down. Easy.

Makes sense. Did the previous owner put a lot of miles on it, or was it sitting for a long time? Was the registration current? It's pretty common to come across a deal on a bike only to find out that the previous owner couldn't get it running right so sold it instead

Anyway basically I'd do:
-valve adjustment
-visual on spark plugs
-confirm spark
-confirm voltage
-drain gas or use a temporary tank (recommend temp tank to save hassle)
-fuel and air filter just cause you're in there anyway
-adjust choke cable
-confirm idle adjustment screws are set to factory settings (these are those terrible little screws on the side of each carburetor. Should be 2 & 1/2 turns from seat as I recall)

If it runs at that point, warm up and set idle using the little thumbscrew thing. As it's running, blip the throttle and watch to confirm that the cable snaps back, and that the rpms also drop down. If the cable snaps back but the rpms stay high, you've got a bad air-fuel mixture. Likewise if you can't get it to idle right, you're probably going back into the carbs. At least if you can isolate the problem you'll have a fightin' chance

Hope that helps

 

Idle fuel mixture screw is at 2 1/2 turns out. Before rebuild it was 3 turns out. Was going to fine tune this when it was running ok; which hasn't happened yet. That of course doesn't mean that the carbs I cleaned out do not still have a problem on at least one of the four carbs causing this behavior. My guess would be something is loose in there causing the intermittent behavior. But I am running a different carb rack at the moment that I haven't really touched yet from an '89 CBR 600 with lower miles. I plan to refresh these carbs at some point; but I am trying to limit the variables. I looked at the JETS and they are a 35 pilot jet and a 102 main jet (small). I haven't touch the mixture screw on this setup or the idle screw (again, to limit the changes).

The only thing on your basic things to do list that I have not yet done is ...
- valve adjustment
- confirm the spark

I tried to pull out the petcock with the fuel strainer but something is preventing it from coming out. I didn't want to break it and I was just getting started on the bike. At this point, I would break it and roll it off a cliff

Also note that when I changed the plugs, one of the wires was in pieces inside of the spark plug area and was a real fun time getting back out. I pulled the wire apart, cleaned it all with electrical lube and ohm tested it at around 4.7k ohms.

I did have the radiator dropped to do the compression test, so this pretty easy. The valve adjustment is on my list but figured I would get the thing running consistent first so I could see the difference (if any).

The spark plugs were very black before I changed them and now they seem to be a little white; but not enough miles with a consistent carb setup to really have a clue.

As for the history of the bike; no clue. My son bought it for a few hundred $ from his future brother in law in what I assume was a garage clearing exercise. He has owned this bike more than once; so even the history from him is pretty dodgey. The fast that it was missing a carb spring between 2 and 3 leaves me to believe it was not really running in any real sense.

 

One more interesting observation from just running the bike at idle again. Seems to start and idle pretty good for about 2 minutes. Seems like as it warms up it starts to 'lurch' in the idle. Along with that, I start hearing a lot of ticking that I assume is valve clatter. So I gotta wonder (and looking for an opinion), if that could be the valve's are 'loose' and get worse the minute the engine start to get warm. The compression test I did was cold. So perhaps I am being mislead by my reasonable compression numbers because the valves get significantly worse the minute the engine warms up. Just a thought.

 

Sounds like you cam chain tensioner,..its most noticable at start up ,..then kinda goes away once it reaches operating temperature,unless its gotten too bad then you hear it lightly..sounds like bb's in a can

 

This is a bike at idle.

 

The rattle should get less noticeable at warm up - if it gets worse you could have a problem. I'd get in and do the valve adjustment, and give the area a visual

The cam chain rattle will be noticeable at idle. Virtually any bike of this vintage will have some rattle. This isn't the same as valve rattle, which would be caused by having the valves too loose, or in cases of premature wear - hence why I say it could be a problem, but most likely is not