Topic: cb650 carb pilot screw adjustment HELP

[Thor0812]

I rode my bike to work for the first time today (26 miles). Checked the plugs at lunchtime and they looked a touch lean so I turned all 4 pilot screws out 3/4 turn. I rode it home this evening and as soon as I got home I checked the plugs again. They still look a touch lean? When I reassembled the carbs I set them to 1 1/2 turns out like the manual says. I guess my question is, is turning the screws out the proper way to richen the mix? Also, after I turned the screws out I had to idle it down about 500 rpm. Is this normal. Bike runs great and I dont want to burn a piston so please, any help will be greatly appreciated.
Thanks in advance

[TwoTired]

Another name for the Pilot screws is Idle Mixture Screws...because they mainly effect idle mixtures.

I don't know which model 650 you have of if it has Mecanical slide carbs and CV carbs, but the slide needle is what adjust mixture from 1/4 to 3/4 throttle, which is what your using cruising on the street. 

Another effector on mixture is the air filter.  Carbs were tuned for the stock one.  So, if you've changed those, probably more carb adjustment than the pilot screws is in order.  Exhaust changes can require a mixture change, too.

Another possibility is the carb fuel level height, whether it has air cutt-offs or other failing features.  But, since you weren't specific about what your bike is or has, we can't really address those items.

Anyway the book adjustments are for a bike in stock configuration.  If that is changed, then the book number are a starting point toward final settings tailored to your bike's current set up.

Cheers,

[Thor0812]

Sorry, it is a 79. The bike has stock filter and there is what looks like stock 4 to 2 exhaust. The only adjustments on the carbs I can find are the pilot screws and the idle adjustment. Like I said the plugs dont look overly lean but in this high Texas heat I dont want to take any chances

[tonycb650]

79 cb650 is also 1.5 turns out

[mystic_1]

How about posting pics of your plugs?


TT's point about the needle jets is a good one.  The idle mixture screws will only affect the mixture at idle.  Since you were checking after having ridden a longish way, it's probably not your idle mixture that's off.  I'm not sure if the needle jets on a 79 650 are adjustable, on a 750 you can move the retaining clip up or down to adjust the position of the needle.

Having to readjust idle speed after adjusting the idle mixture screws is normal.


Have you checked for induction system leaks?  That would cause a slightly lean condition and no amount of fiddling with the carbs will make it right.  Hose down the areas where the carbs meet the rubber boots (both on the engine side and the airbox side) with carb cleaner and listen for minute changes in idle.  If the idle changes when you spray a particular area, you've found a leak.

cheers
mystic_1

[Thor0812]

I checked for leaks when I installed the carbs and didnt have anything then. Here is the best pic I could get. They all look pretty much the same
http://img.photobucket.com/albums/v73/rachelandglenn/plugs011.jpg

[tonycb650]

79 and 80 cb650 carbs are mechanical slide type , 81 and 82 are cv type. the needle jet is non-adjustable on all cb650s.

[Thor0812]

It runs fine, I dont know, maybe Im just paranoid lol. Like I said I just want to be sure I dont burn a piston

[mystic_1]

I agree that plug doesn't look all that bad.  Admittedly it's hard to read plugs from a photo Any other symptoms to indicate a lean condition (pinging under load etc)?    If not, you're probably OK.

TwoTired posted a link to the following chard a while back, where do your plugs fall on this?

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

cheers
mystic_1

[Hush]

I've got the same bike (79 CB650) and your plugs look like mine, I can't see you running a hole in your piston as the light black fluffy stuff on the outer ring of the plug is minor carbon fouling so ergo you are getting heaps of gas to your engine.
If she's running nice and there are no other problems I'd ride her to get used to the feel of the bike, the more you ride the more you will know your bike.
There is no better way to analyze bike probs than being the one who rides it daily.
Happy riding..............Hush.

[eurban]

One point here.  If when you turned out your idle screws, your idle speed jumped up by 500 then your initial IMS setting was probably too lean.  The stock baseline of 1.5 turns out is just a baseline and typically you adjust the screws, one at time to acheive max idle speed (at a give throttle setting).  You then readjust idle speed and proceed to the next cylinder. A 3/4 turn out is a huge jump however and you could have very likely overshot the the sweet spots.

[Frankenkit]

just... to clarify because I'm a pilot screw idiot,
for '79 and '80 650 mechanical carbs, 'out' is richer and 'in' is leaner, right? 

My bike was having a rough time getting started, idled rough until it was *really* warm, wouldn't take gas at all 'til very warm.  I attempted to adjust the pilot screws, but then had to take off the carbs and clean them anyway, replace torn up air cut-off valves, etc... so I went back to the pilot screws that were at 'close' to the same settings, but doing it upside down and sideways, they were all more 'arbitrary' ... so I set them to 2 1/4 out, which is what 3 of the 4 were at.   

Plugs get fouled relatively easily right now, which says 'rich' but def. won't start without choke, which says 'lean'.  Suggestions?

[TwoTired]

just... to clarify because I'm a pilot screw idiot,
for '79 and '80 650 mechanical carbs, 'out' is richer and 'in' is leaner, right?

yes   

Plugs get fouled relatively easily right now, which says 'rich' but def. won't start without choke, which says 'lean'.  Suggestions?

Clarify your choke application habits.  It is normal to need choke to start when the engine is cold.  Shouldn't need it to start when the engine is at operating temp.

Operating with choke often can foul plugs.  Normally, you start the bike on choke, then ride it around without it on, where the higher engine temps and leaner mixtures burn off the soot.

Have you been abusing plugs?   Do you need to join yet another support group? 

I thought you had an accelerator pump?  If that is working right, it ought to take throttle pretty well.  If too rich, the engine will "burble" as the plugs clear while it is regaining power.

Does the problem remain for clean plugs?

Cheers,

[Hush]

Kit, which adjustment are you playing with? The 650 mechanical carbs have hardly any adjustments on the outside to fiddle with.
The wee screw right up behind the carb bowl is the "air screw" which should be (according to my old Honda mechanic) initially set by turning in until it lightly seats (touches something solid  ) then turn it out 1and 1/4 turns, run the bike and if it runs too rich turn out "up to" 1 and a 1/2 turns.
There are hardly any other adjustments we can make to our old mech carbs (thank God).
If you want to dig inside of course you can play with needle height and main jet size, personally with my bike being stock I don't need the headache.

P.S. how come you ressurected this thread after nearly a year?

[Frankenkit]

TT- I only use the choke when starting from cold, never ride the bike with choke on.
I'm going to change the plugs tomorrow to see if it persists with clean plugs.

Hush- I resurrected this thread after a year because my search-fu brought this one up, and it had an apt title, so I didn't see the use of making a new one. 
I'm playing with the air screw you're talking about, which was originally set by my mechanic back in nov. when it was really cold and since has gotten to act weird at idle when in warmer weather

[Hush]

I see, but just remember you only have a 1/4 turn to play with, anything more and you will end up well out of adjustment.   
Refresh my memory, did Delilah stab her lover in the back or just give him a good haircut?

[TwoTired]

Kit, which adjustment are you playing with? The 650 mechanical carbs have hardly any adjustments on the outside to fiddle with.
The wee screw right up behind the carb bowl is the "air screw" which should be (according to my old Honda mechanic) initially set by turning in until it lightly seats (touches something solid  ) then turn it out 1and 1/4 turns, run the bike and if it runs too rich turn out "up to" 1 and a 1/2 turns.

I don't think so Hush.  Pretty sure you have Idle Mixture Screws between carb bowl and engine.  PD Carbs beginning in 77 all had IMS, AFAIK.  Turning them in makes leaner, out makes richer.  Next time you have them apart, notice the adjust screw tip blocks or opens the hole entering the carb throat.  Closing that hole shuts off any fuel from the slow jet making the mixture leaner.

Cheers,

[Frankenkit]

ah.  TT is right, between the carb bowl and engine.  The fine point fits into a hole in the carb body, closing off that hole, so yes, in = leaner, out = richer, which would also make sense as to why the mechanic would say that the pilot screws were set for cold weather starting/idling, but might be too rich when it warms up. How much adjustment is the norm when compensating for differences in outdoor temperatures?

[Frankenkit]

bump!

just wondering if there's a standard amount of turns out from the 'starting point' I should consider 'normal'? I don't want to set it way out of 'spec' and not know it.

[cb650PK]

Setting the idle mixture screws can be done with accurate tachometer, to richen up the mid range use shims under the non adjustable needles.

[TwoTired]

The "spec" setting applies to an all stock bike, exhaust and air filter.  I'd guess around 2 turns out from lightly seated out to get you going.  After it is running you turn them in to get peak RPM, assuming you have an accell pump.  You adjust back and forth between the IMS and idle knob. Leaning the IMS make RPM higher, which counteract with the idle knob.  At some pont, the IMS leaning stop making a speed increase, that is the point where it is adjusted properly.  An accurate tach sure helps.

Cheers,

[Hush]

Nuts, now I have to find that adjustment!
Good job I have that spare set to play with.
Thanks TT.

[Hush]

OK have looked really hard at that spare carb rack I have and can only find one screw that is adjustable.
The one I found is what I would call the "air screw" and it is tucked up under the back of the carb bowl.
The only other screw is a blank of some sort that is on the neck of the carb just behind the rack that holds all the carbs together.
Now I'm confused?

[TwoTired]

The one in front of your carb bowl is the IMS.  The other one is for the vacuum tap for carb sync.

True air screws regulate air going into the pilot/slow/idle jet system.  Out-more air, in - less air or; out - leaner, In - Richer

The screw you have there (IMS) regulates the combined fuel air mix from the pilot/slow/idle jet system.  Out - more air/fuel, In - less air/fuel. Or; Out - richer, In - leaner.

IMS adjusters often have less of an adjustment range than air screws.

Cheers,

[GOSH]

I have rcently stripped and re-built the Carb on a CB550 as after leaving bike idle too long it ran like hell.

I am ready to re-install-balance and adjust can anyone give me advice on step by step instructions as it is not really in manaul.
I have a carbun tune meter.
Thanks