Topic: Idle Mixture Screws - 6 turns out

[Hodgeman82]

Hi,

I own a '82 CB650SC (yes, it's a SOHC - one of the last).  Bought it a couple of years ago and immediatley noticed it was running too lean (owner sold it to me cheap because he could never get the carbs adjusted right).  So, I synced the carbs and went through the Clymer manual steps on tuning them by starting at 1.5 turns out from seated.  Waaay too lean.  To get the right air/fuel mix, I had to file off those little fins on the bottom of the carbs to allow me to turn the idle mixture screws out more than one turn past the factory settings.  Finally, at 6 turns out, she purred and had all the signs of a well tuned bike.   But the screws are turned so far out that I actually had one migrate all of the way out on me while on a ride.  If I set them even 1/2 turn leaner than 6 turns out, the carbs cough and the engine searches while riding at lower RPM's (2000-3000) which tells me it's starving for fuel.

What's going on???  The guy at the local Honda shop said it may be due to oxygenated fuel which is all they sell in this state (MN).  Does that make sense?  Sounds logical - more oxygen in the fuel would mean the need to run it richer - maybe.  His other suggestion is to replace the pilot jet with larger ones.  If it was just one carb causing the problem, I'd be looking into other causes but each one identical to 6 turns out and the fact that it runs good tells me that this is where they should be.

Any ideas???

[Aaron J Williams]

Possible plugged/restricted idle jets and/or passages. It sounds like a good cleaning is in order. Could also be an intake leak causing a lean condition. Spray some carb cleaner around the rubber intake boots while the bike is running. If it revs up you have a vacuum leak.

[OakBehringer]

Yea, that is not normal. Also, keep in mind that the mixture screw only affects your idle speed.

You should do a complete tuneup (point gaps, timing, valve clearances, new plugs, intense carb cleaning). You can't get the carbs tuned right if the first four things listed are way off. Also, if you're going to old this old of a bike, you should know how to do a complete tuneup.

If you've never done this stuff: It sounds intimidating, but it will all seem pretty easy in hindsight.

[MJL]

Possible plugged/restricted idle jets and/or passages. It sounds like a good cleaning is in order. Could also be an intake leak causing a lean condition. Spray some carb cleaner around the rubber intake boots while the bike is running. If it revs up you have a vacuum leak.

  Add to this list a sync, and not just on the bench.   Look up a thread called homemade manometer and build one with the gauges, not liquid style. That kind will drive you nuts.

[Hodgeman82]

I went through everything on this bike shortly after I bought it:  Completely dismantled and cleaned the carbs, new air filter, carb sync, valve clearance adjustment, timing, etc. Everything is right on.  Again, if it was just one carb behaving this way, I would suspect something going on internally - dirt, poor vacuum, etc.  However, all 4 carbs need the mixture adjustments turned way out to the exact number of turns on each - makes me think otherwise.  I think the answer is in the fuel, check the 3rd paragraph of this out, near the bottom:  http://en.wikipedia.org/wiki/Air-fuel_ratio

I think a re-jet may be in order.

[BobbyR]

You have not mentioned if you are using a stock airbox and filter, Pods, open pipes etc. All of this can effect the mixture. You mention rejetting, however the mains have littile effect at idle. Unless you bought the bike new, it is hard to know what has been done to it. 

[goon 1492]

I'm suprised TT didn't ask yet, do you have pods or factory air box? Do you have stock exhaust or aftermarket 4-1, these two things alone will affect/cause your lean condition.  Look up what size your jets are too and make sure one of the P.O's didn't try to work their magic on it and leave you with a mystery... Good luck

[lrutt]

Usually the rule of thumb is that if you have to go more than 3 to 4 turns out you should go up on the low speed jet a notch. You might consider that. It could bring you down 2 turns. BTW, it does affect more than just idle, it can affect the off idle transition and very lowest speed operation. At that point slide and needle take over.

As others have said, if you have modded anything on intake or exhaust you definately are looking at changes to the low speed jet size. Not sure on those carbs if they are easily replacable or even available.

[PJ]

If like you say it runs great at 6 turns out and you have no other drivability issues it may be that in it's 30yr life that maybe carbs, cam, etc was swapped from another bike needing a different tune. I would look for larger pilots and not stress over it.

[Triffecpa]

The carbs on those particular models were way lean from the factory.  Go with a larger pilot jet and you'll be fine.  If you can't find a larger jet, it is possible to drill out the stock ones, but that's a story for a different post...

Tracy

[MJL]

Can you get jets for those carbs? (CV) All I remember about them is that the needles are fixed and I had to get a jet kit with new adjustable needles for mine. Maybe the PO put in adjustable needles and stuck the clip on the wrong notch.

[Hodgeman82]

Everything is stock on the bike.  It had less than 10,000 miles on it when I bought it - near mint.  The PO let it go for $800    because he could not solve the carb problem.  I forgot to meantion that I did replace the air filter (stock) as routine maintenance so a dirty filter can be ruled out too.  I still want to try non-oxygenated gas to see if that makes a difference.  Here in Minnesota, it's illegal to sell non-oxy gas unless the station fills out some paperwork to exempt them from the ban.  The result is that very few places carry it.  However, there happens to be one nearby that can sell it so it is worth a try in my bike.  Thank you all so much for your suggestions.  I'll keep you informed on the results.

[Hodgeman82]

I experimented on the fuel this weekend:  Completely drained the tank and carbs of oxygenated fuel and filled up with with non-oxy.  I was able to turn the idle mixture screws back in from 6 turns out to 4.5 turns out from seated.  So, fuel does make a difference.  However, this is not a fix since few stations carry the non-oxy in MN and it is expensive: $4.14/gallon .  It only comes in premium 92 octane.  So, I think a re-jet is in order.  I looked in the Clymer manual and it looks like the '82 model CB650 does allow removal of the jets.  Anybody have an idea of what each jet will cost?  Anybody had any experience with swapping them out?  Just wondering how many turns back in on the idle mixture will result.

Thanks!

[Soos]

Jets for your '82 cb650 cabrs should be about 10-20 a set.


Changing jets is pretty straightforward.
Drop the floatbowl, hold the emulsion tube with a wrench, and use a flathead screwdriver to remove the jet.
screw the new one in, and button up the float bowl, and you're done.


The (??) easy way is to remove the carbs first.

Now changing jets those carbs on your bike....
Thats another level of difficulty.
Unless you like contorting your hands.
When changing jets on the bike I prefer to change jets with the emulsion tube out of the carbs....less hand time under the carbs.



When you said you went through the carbs, how far did you tear them down?
Did you remove the slow jet and clean the emulsion holes?



l8r

[Hodgeman82]

Soos - thanks for your info.  I think I'll pull the whole carb set off before getting into this.  I've tried to drop a bowl from the installed position once (because I had a stuck float) and it was a real strain in the hands and patience.

I had everything apart on those carbs when I first bought the machine.  But, I'll check the emulsion tubes again just to make sure they are clean.

On jet sizes, the Clymer says to go only one size up at a time.  Is this what you recommend?  I just don't want to install one size and have it only gain me one turn in on the idle mixture screws - I'd like to achieve at least a couple turns, if not more.

 

[mystic_1]

OK, based on your statement that turning the Idle Mixture Screw out results in a richer mixture, that means that the IMS on your carb is metering gas, not air.  So, if your carbs are getting too much air at idle, and everything is stock, have you checked for air leaks in the intake system?

Something is definitely awry, with everything in stock configuration and operating properly, you should not need to re-jet.

mystic_1

[Hodgeman82]

mystic-1, Thanks for your suggestion.  I sprayed starting fluid all around the carbs at idle last night.  No difference in rpms so I don't think there is an air leak.

You are right, something is awry.  I spoke with a Honda service dept. guy this morning and he also said I should not need to re-jet the carbs if everything else is operating properly (unless a PO re-jetted before I owned the bike).  He suggested I tear them down and re-clean them.  He said he's had customers swear they've cleaned their carbs good but a more thorough cleaning was all that was necessary to correct the problem.  So, I guess this is what I am going to try first.

However, I'm still puzzled; if it is dirty carbs then why would all be behaving exactly the same?  Since all IMS's are set exactly the same, wouldn't each carb have to be exactly as dirty as the next to achieve this?  Otherwise you'd have, say one clean IMS turned out to 2.5 turns while a dirtier one is at, say 5 turns out.  Or, does one dirty carb effect them all?

Thanks

[mystic_1]

That is puzzling.  I agree that if dirt was the problem than you'd see differences between the carbs.

SO, let's take a step back and consider the big picture.  The only thing that all four carbs share is the airbox.  Check that for leaks, gaps, etc. 

Also double-check your float height, don't know if the float tangs are adjustable on that bike or not but too low of a fuel level will cause lean operation.  Wouldn't necessarily expect that to manifest at idle though.

Next thing I would do is go over the rest of the bike, timing, spark plugs etc.  Using stock plugs?  Gapped correctly?  Rule out other things before returning to the carbs.

mystic_1

[WFO]

That is puzzling.  I agree that if dirt was the problem than you'd see differences between the carbs.

SO, let's take a step back and consider the big picture.  The only thing that all four carbs share is the airbox.  Check that for leaks, gaps, etc. 

Also double-check your float height, don't know if the float tangs are adjustable on that bike or not but too low of a fuel level will cause lean operation.  Wouldn't necessarily expect that to manifest at idle though.

Next thing I would do is go over the rest of the bike, timing, spark plugs etc.  Using stock plugs?  Gapped correctly?  Rule out other things before returning to the carbs.

mystic_1

 I got the shop manual and it appears the float is not adjustable ( float level: 15.1 +/- 1mm ) it says replace the float if the float is not within specs.

[WFO]

And damn reading a little more the initial setting for the pilot screw is no where near 6 turns out???

[eurban]

IMSs will not properly meter fuel when turned more than 3-4 turns out.  6 turns out is way beyond their design range.  What are you using as a basis to adjust the screws?  The factory procedure for setting them will probably require an accurate tachometer and involve adjusting the screws to achieve highest idle speed at a given throttle setting and then turning a set amount further in one direction or another.  Keep in mind that the mix screws have a very limited range of significant control in the mixture, only up to about 1/8 throttle openings.  Is this where you are experiencing symptoms of leaness?  If you accelerator pump is not functioning, I can guarantee you that your bike will act lean and perform poorly when you crank open the throttle to accelerate.   Richening the mix screws might help to blend this transition but the carbs will not function properly.  Perhaps this is your issue? Have you visually confirmed that the pump is squirting fuel out of each brass nozzle located at the bottom of the carb bore on the air fliter side? . . .Bottom line is that if your bike is stock and all its systems are functioning properly, then you most certainly shouldn't need to turn your mix screws 6 turns out or need to rejet.  Figure out what is wrong! 

[Hodgeman82]

Yes, Clymer says no adjustment on the floats.  Maybe the way I adjust my IMS's will shed some light on the matter:  If I follow the manual's direction, the IMS's are more or less unresponsive once turned out just past the seated position.  I always get too lean of a mix and the bike coughs at idle and searches and surges around 2000-3000 rpms.  So, by trial and error, I've found the best way to adjust the IMS's is to warm the bike up, set the throttle lock at 4,000 rpm's and turn each IMS out until each cylinder sounds good.  The coughing and surging at 2000-3000 rpms goes away but I don't achieve this until 6 turns out from seated on each.  Then the bike runs good but I risk losing an IMS because they are in there so loose.

I have stock plugs and timing/valve clearances are right on.  No air leaks from carb into head but now you have me suspicious about the air box.  I sprayed starting fluid around those fittings the other night with no results.  But would this even cause the engine to speed up if sucked in from the intake side of the carbs?  Any other suggestions on how to check the air box for leaks?   

Regarding the accelerator pump, does that even contribute at idle?  I thought it only kicked in at acceleration.  I'm wondering if we can rule this out since I'm getting no response out of my IMS's when I turn them at idle.  All indications are a dirty carb but, again, all are exact on number of turns out from seated.  So, like mystic_1 says, it should be something that all carbs have in common.

Thanks

[WFO]

Yes, Clymer says no adjustment on the floats.  Maybe the way I adjust my IMS's will shed some light on the matter:  If I follow the manual's direction, the IMS's are more or less unresponsive once turned out just past the seated position.  I always get too lean of a mix and the bike coughs at idle and searches and surges around 2000-3000 rpms.  So, by trial and error, I've found the best way to adjust the IMS's is to warm the bike up, set the throttle lock at 4,000 rpm's and turn each IMS out until each cylinder sounds good.  The coughing and surging at 2000-3000 rpms goes away but I don't achieve this until 6 turns out from seated on each.  Then the bike runs good but I risk losing an IMS because they are in there so loose.

I have stock plugs and timing/valve clearances are right on.  No air leaks from carb into head but now you have me suspicious about the air box.  I sprayed starting fluid around those fittings the other night with no results.  But would this even cause the engine to speed up if sucked in from the intake side of the carbs?  Any other suggestions on how to check the air box for leaks?   

Regarding the accelerator pump, does that even contribute at idle?  I thought it only kicked in at acceleration.  I'm wondering if we can rule this out since I'm getting no response out of my IMS's when I turn them at idle.  All indications are a dirty carb but, again, all are exact on number of turns out from seated.  So, like mystic_1 says, it should be something that all carbs have in common.

Thanks

I had a ATV that the pilot screw would work it's way back out after riding so i put a small strip of teflon tape on the threads and it worked maybe it will help you keep those suckers in there.


ps: I wonder what size jets are in your carbs now?

[eurban]

The accel pump squirts fuel when you quickly whack open the throttle.  It allows for good performance in spite of a relatively lean idle mix (which helps with emissions).  These carbs need the accel pump functioning to operate smoothly. The pump has no effect at idle (other than allowing for leaner idle mixture).  I suggested the pump as a potential problem based on your vague statements that your bike is running lean and my feeling that you are incorrectly trying use your IMSs to fix performance problems for throttle openings larger than their effective range. Now you say that you are running the engine at 4000 rpm to adjust your idle mixture screws??  This is way out of the range where these screws can be effectively used to alter the mix. In addition, when you turn them out more than four turns you might still be richening the mixture but it likely won't be in a controlled manner nor will it be in the design range of the screw.  You find it odd that the carbs all seem to be "malfunctioning" the same way but (please don't take this the wrong way) the real constant here seems to be you, and the procedure that you are using. Time to rethink your procedure .  Assuming engine, ignition is healthy, you will need start with a clean, properly functioning set of carbs, that are bench synchronized and have the mix screws set to the factory baseline (1.5 turns out??)  If turning the IMSs in and out make no difference in idle performance then most likely your idle circuits are still dirty (don't forget that crud from a cruddy gas tank can reclog the circuits in no time) and you are wasting your time trying to fine tune malfunctioning carbs. . . .I would also still suggest removing the air box and peaking at the accelerator pump nozzles while you whack open the throttle.  Best to make sure. . . .

[Hodgeman82]

eurban - thanks for your advice.  No offense taken.  You are right about my unconventional way of adjusting those carbs.  Seems I found a way to make the bike run better by doing that but didn't consider that I'm only covering up the problem by forcing it to perform out of spec.  I'm going to pull the carbs and go through them again.  When I did this last (a couple of years ago), I followed the Clymer manual and noticed that there are very few parts than can be physically withdrawn from each carb body for soaking (e.g.- slow jet is not removable).  What's your advice on cleaning all of the little ports in the body?  The manual advises against using a wire but I've also seen posts from guys using a copper wire on this forum.  One guy even boiled each carb in water to loosen up the gunk.  I'll check that accelerator pump too.  What I do know now is that the bike is very responsive when I whack the throttle open at idle - even when set way too lean.  I'll check the existing jet size too.  Who knows, maybe the PO switched those out.

Thanks