Topic: Idle won't go below 1250RPM

[Johnny340]

My idle won't go low enough on my '81 CB650.
I synced the carbs to the #2 carb (which is not adjustable) but the idle is too high even after backing out the adjuster to the point that it stops making contact.
Any ideas?

[flybox1]

Verify your idle mixture is correct.
My 750 idles best at that rpm.
I don't see an issue with it.

[Johnny340]

I'm trying to set the mixture screws as per the manual and it says to get it to 1050 RPM and adjust the screws to see 50RPM increase, etc.  It seems it idles higher the more I back out the adjusters (till they fall out)..

[flybox1]

Set your mixture screws based on plug color (light tank insulator tip) not on how low you can get it to idle.
This was a new plug after a 5min idle 1200-1250. 
Once mixture is correct, adjust your idle set screw to a comfortable idle AFTER the bike is fully warmed.  There will be no need to adjust it again.
On starts, use decreasing choke until the bike is up to op temps.

[70CB750]

Throttle cable stuck?

[MCRider]

Throttle cable stuck?

Or the "A" cable may be too tight. It should be adjusted at the bellcrank so that there is about an 1/8 turn slack in the twist grip. Once you're sure of that, you can take a little of the slack out of the cable at the twist grip.

[Duanob]

All of my old Hondas have idled best at 1200+ RPMs. I don't think a couple of hundred RPMs is a big deal for an engine that can cruise at 9000+. If it were over 1500 idle speed then I'd probably look into it.

[Johnny340]

The cable is not stuck nor too tight.  I'll do a plug check with new plugs because I do need to sort out the mixture screws.  It just doesn't have good acceleration right off the bottom or good response when blipping the throttle for a downshift.. 

[flybox1]

It just doesn't have good acceleration right off the bottom or good response when blipping the throttle for a downshift..

Air/Fuel mixture issue. 
Mixture should always be checked/adjusted from largest to smallest fuel contributor (main jet->needle clip pos->pilot jet->mixture screw)
If done in the reverse, the correction introduced can effect mixture/throttle response in the higher throttle positions.
Fine tuning (mixture screw) always comes last.

[Deltarider]

Do yo know how accurate your tachometer is?

[Johnny340]

The tach I used is accurate to a a tenth of an RPM.  The engine runs perfect on the top end and middle but seems lean on the bottom.  The pilot jets are pressed in so I didn't change them.  Can anyone tell from the picture if the needle height is adjustable?  They're just visible in the bottom of the pic and I remember not seeing slots to change a clip position..

[flybox1]

looks like you could shim those needles, if warranted.

[Johnny340]

Does shimming seem warranted?  I know it overlaps the low RPM range but is there anything else I should consider first? 

I cleaned the carbs with an ultrasonic cleaner because that was the only way a couple stubborn pilot circuits would clear.  Afterwards, I could spray carb cleaner through all passages.   The only mod is a 4 into 1 pipe. 

Shouldn't the mixture screws be somewhere in the neighborhood of 2T out?

[flybox1]

if you complete an accurate plug chop at 2/3 throttle position, and your plugs are near white, then yes, a shim is warranted.

The BOOK setting for your mixture screw is a baseline setting for a bike with a stock intake and exhaust.  Fine tuning is always suggested.
If your intake or exhaust is different, then mixture screws should be adjusted based on your PLUG COLOR after an idle chop.

[Johnny340]

OK, I'll get new plugs and do that.  Thank you. 
If I do need to richen up the needle after, I assume I would simply put a small washer over the needles before putting them in?

[flybox1]

Correct. 
IIRC they were M2.5 x 6mm (.5mm thick)
I can measure one after work.

[Duanob]

Do your CVs have the air cutoff valve? Those can cause trouble if not clean and the diaphram isn't in good shape. I have a CX500 with those simlar carbs and a total PITA to clean and rebuild compared to older type carbs. The press in pilot jets were not a good idea but they can be removed. My guess is the pilots need cleaning both for gas and air orifices. Also the passageways that feed the pilot jets should be cleaned too. The CX500FORUM.com has a lot of info on taking these carbs apart.

[grcamna2]

I know you're adjusting/fine tuning the carb. jetting but just wanted to ask if you're sure that you're mechanical ign. advance assm. is working well? and if you lightly lubed the moving parts & checked the 2) springs for proper tension before you set the timing w/ a stroboscopic timing light.
I think that needs to be done first before synching or fine tuning the jetting.

[Johnny340]

These carbs do not have the air cut off valve but were a total PITA to clean anyway.  With non removable pilots and passages that bend 90° I could not use a wire or anything to ensure everything was clear.  The intake to the pilot jets (arrow in pic) are clear but passage seems to split into several smaller holes that enter the carb under where the star is in the pic.  I can spray carb cleaner through here but maybe it is still partially plugged up??

Thank you for the ignition suggestion because that makes sense too.  I'll check that before pulling these carbs...again.

BTW, has anyone simply put a 4 into 1 exhaust on a 650 and had to make the needles richer or needed any other of jetting changes?

[flybox1]

only a few plug chops, or time on a (dyno) gas analyzer can tell you if jetting is necessary after an exhaust swap.
IMO, its usually under 1/4 throttle and over 2/3 throttle needing (slight) adjustment. 

[harisuluv]

What jumps out to me is a common mistake.  The carbs can all be synced to each other, however, they can be UNsynced with the idle adjust screw.  To be very clear, each carb can be synced perfectly to each other and still botched because they are not synced in relation to the idle adjust screw. 

With the DOHC carbs, they are EXTREMELY sensitive.  They can be very tricky to bench sync well and often times need a vacuum sync.  Did you use the drill bit method?  On these I just sync based on light I can see from the other side (flashlight).  If you used a drill bit, you will run out of idle screw before you get them closed enough.  And almost always, you will at least need to loosen the screws to finely adjust the placement/seating of the flaps so that they can seal correctly for syncing.

You should be able to basically turn the idle down til it dies, if you can't do that, you might need to resync them.

[Johnny340]

What jumps out to me is a common mistake.  The carbs can all be synced to each other, however, they can be UNsynced with the idle adjust screw.  To be very clear, each carb can be synced perfectly to each other and still botched because they are not synced in relation to the idle adjust screw. 

I vacuum synced the carbs at around 2000RPM and they were very sensitive but I eventually got them exact.  Are you saying that at a lower idle they could be out of sync?

[harisuluv]

No.

Let's say you have all four exactly right.  Ok.  Got it.  Irrelevant. 

The sync of all four has to be synced IN RELATION TO THE IDLE SCREW.  You do not have adjustment lower. 

[grcamna2]

No.

Let's say you have all four exactly right.  Ok.  Got it.  Irrelevant. 

The sync of all four has to be synced IN RELATION TO THE IDLE SCREW.  You do not have adjustment lower.

ok,
Are you saying to Johnny340 that he needs to establish a type of 'base idle' w/ #2 carb. along w/ all the rest..,possibly a lower rpm to synch ?