Carb idle circuit

Hello all, I'm fixing up my uncles '82 XJ750, all done and bolted everything back together and it runs super except it wont idle with the choke off. I had cleaned the carbs but obviously missed the idle circuit. I'm going to take them back off and try again but where should I be focusing my attention? The pilot jets I know for a fact were perfectly clean, as were the main jets and the emulsion tubes, are there passage ways in the carb body somewhere that need to be cleaned?

 

May be sucking air. Spray WD-40, water or brake cleaner, etc around the boots where they go to the carbs and cylinder head. Idle will react when it gets sucked into the motor.

Massive air leak will make it want enrichment to idle.

See if you have a cold exhaust pipe while idling. If so, you know which cylinder(s) are having problems.

 

While you have the carbs off you might use the clear tube method to confirm your float levels.

 

Two Primary Passageways:

1) & 1a: Passageway from Pilot FUEL Jet -to- Pilot Mixture Screw. Passage supplies FUEL from Pilot Jet to the connecting AIR Passage ... from the Pilot AIR Jet -to- Pilot Mixture Screw.

Flush from Upper AIR Jet down through Pilot FUEL Jet with FUEL Jet extracted.
Flush from Upper AIR Jet over through Pilot MIXTURE Jet through AIR Jet with orifice for FUEL Jet BLOCKED with a finger tip.

2) Main AIR Passage: Passageway from Main AIR Jet to Outlet in Carb Body feeding Void surrounding Emulsion Tube.
Flush through Main AIR Jet through lower orifice which discharges in Center Tube near Main FUEL Jet with Emulsion Tube extracted.

 

Thank you!

To the other posters, thanks but I've confirmed my float levels already and already checked for vacuum leaks (there were none), so it's definitely something in the idle circuit.

I didn't remove the mixture screws when I cleaned the carbs but I guess I probably should have.

 

Holy crap this is driving me nuts!

I cleaned it out very carefully (again), fluid seemed to be coming out in all the right places so the passages must be clear, I even fed some wire through the passages ways that I could. Double checked the pilot jets were clear, and gave everything else a quick once over while I was in there. Once again verified float levels (clear tube method), and bolted it all back together.

Again, it doesn't idle without the choke on, even with the idle speed turned way up it will barely idle and dies easily if you attempt to give it any gas.

When I cleaned it this time I removed the mixture screws and cleaned the passage ways out, I put the mixture screws back in 2.5 turns back from full in, they had been set at 3.5 turns out. (I was hoping that was the problem ).

Any other suggestions? My last though is tomorrow play around with different settings for the mixture screws, other than that I'm out of ideas, everything looks good, and it runs good, it just wont idle! (no vacuum leaks either)

 

You gotta do a vacuum synch of the engine before even thinking about twidling with the mixture screws. Out-of-synch engines can exhibit all sorts of strange symptoms that can easily be attributed to other causes, and thus "incorrect solutions" are the norm........

 

The carbs were bench sync'd they ought to be close enough for the engine to remain running I would have thought.

 

Bench synch is "just enough"---to get the engine started and running, with the idea that now you're going to then immediately do a vac synch. The two different types of synch procedures....bench vs. running (vac) synch.....have two completely different goals and outcomes!

 

Life was easier with my moped, only one carb to deal with

 

Bench Sync the Carbs using Strips of 3X5 Card.
Begin KNOWING the Throttles are CLOSED.

Make the Bike IDLE by Adjusting the PILOT (AIR) Mixture Screws.
A COLORTUNE Plug will allow this process to be achieved in no time flat.

Absent a COLORTUNE Plug, ... the IDLE can be set "BY EAR"
<><><><><><><><><><><><>

Be a Tweaker.
This is “Real Old School Tuning”
By “Ear”

Start 2.5 ~ 3 Turns OUT from bottom on all 4 Carbs
For 750 Bikes I’d set them at just between 2-3/4 & 3 … closer to 3
Put two drops of light Oil in each Pilot Mixture Hole'

Important:
Make yourself a screwdriver that fits the slot on the Pilot Mixture Screw with Precision.
Even if your Mixture Screws loose and won’t bind … you want to be able to “Feel” the Screw moving. If they aren’t loose an ill-fitting screwdriver will destroy the slot, causing a big problem!

Above the Starter Motor ... Under Carbs 2 & 3 is the Idle Adjustment Rod.
It has a Thumb Wheel on the end. Oil it.
You’ll need to be able to make highly precise adjustments without difficulty!

Turn the Idle Rod Clockwise a tiny bit at a time until the engine is running steadily.
Don't exceed 1,200 rpm’s (1,050 if you can get it. 950 if the Tuning permits)

While the Engine’s running.
Begin the Tweaking.

Turn the Pilot Mixture Screw for #-1 OUT ... real S-L-O-W-L-Y.
Listen for the RPM's to INCREASE.
The revs should rise as you give the Mixture Screw SLOW Turning out for more Fuel and Air at the Pilot Screw.
This is Pilot Mixture for Idling.

Listen to the Engine very intently. The rpm’s are about to change.

As the revs rise here’s what to do...
MAINTAIN the RPM's at 1,100 by reaching under the Carbs and lowering the Idle Adjustment Knob as needed.
Stay close to 1,000 rpm’s at all times.

"Tweak" OUT ... the #-1 Pilot Mixture Screw listening to the Engine and Exhaust note VERY Carefully.

Keep bringing the SCREW ~> OUT IF the RPM's continue to rise.

At some point ... The RPM's ~~> will NOT continue to rise any further.
Remember THIS Point.
You have to come back to it.
At THIS point ... MICROSCOPICALLY continue the Turning OUT...

UNTIL <~~

The Mixture becomes TOO RICH and the Engine:
Coughs
Misfires
Burps
Runs rough
Etc.

>> STOP <<

THEN <~~ Get ready to make a CRITICAL Adjustment.
Pretend you are about to move THE SECOND HAND on a STOPWATCH!
One FULL Turn would be 60 Seconds ... right?
OK.


Turn that Pilot Screw ~~> IN <~~

Two (2) Seconds ... maybe Three (3) (Degrees, small, a (my-nute amount)
Don't go too far.

Run fans to keep the bike cool.
Throttle it up and let it idle.

Adjust the IDLE rpm's with the Idle Adjustment ROD if needed.
Maintain 1,100 rpm's Max.

Tweak #-2
Just exactly like #-1

(If the rev's exceed 2,000 ... back-off the IDLE SPEED ROD some more)

Then finish-up doing #-3 -&- #-4.

When you are done ... TWEAKING ... you will need to Road Test for THROTTLE RESPONSE.

IF...

You are Idling, say, at a Red Light and when the light turns green and you take-off ... (Getting “Out-of-the-hole”) … and, …

The Bike ~> BACKFIRES a little bit before the Power comes on:
Your Mixture is too LEAN.

The Pilot Screws need to be:
Tweaked OUT
Two (2) Seconds.
Begin Charting where your Mixture Screws are at from this moment on …
+2 Seconds

IF...

You are at the light and take-off...
The Bike ~> BOGGS for the slightest bit before the Power comes on:
Your too RICH

The Pilot Screws need to be:
Tweaked IN
Two (2) Seconds.
(Almost not turning the screw at all. A nudge!)

Fine-tuning will be complete when you have Tweaked-Out the Backfire's, "Poppity~poppities" or the Hesitation during Further Load Testing.

All further Tweaking is to be done in increments of:

Just two (2) Seconds at a time.
Go find a closed Industrial Park and do it all in one sitting.
Have somebody put the tools you need in a sack and follow you over there in the car.
Don’t forget a Flashlight.

Having a 2nd Set of Spark Plugs so you don't have to handle HOT Spark Plugs will make the Plug Chopping exercise much easier!

Backfires = Lean = OUT
Hesitation = Lean = OUT

Bogging = Rich = IN

Go for it!

Get Tweaked.
Watch your Plugs and treat each Cylinder INDIVIDUALLY.
Read the COLOR of the Plug on the Center Electrode’s Ceramic Collar.

Too Dark (Beyond chocolate brown)
Tweak IN: 2-Seconds

Too Clean: (Clean or no coloration)
Tweak OUT: 2-Seconds
As you Tweak INDIDIDUAL Carbs for the right Mixture …
Watch the Plugs coloration at the Center Electrode Ceramic Collar.

Clean:
BAD. Lean.
Tweak Screw OUT for MORE Fuel.
+++++++++++++++++++++
Although the Bike will run superbly Lean like that … it’s a dangerous condition that can HARM the Bike permanently.
+++++++++++++++++++++

Light Tan:
Lean but OK.
This is a Power setting.
The Mixture is burning Fast making big power. You’ll have fast acceleration with the Plugs a light tan.

You must monitor the situation and keep an eye on the Oil Level. If your Oil starts disappearing and you don’t have a leak … you are still too Lean.

Light Brown:
(Brown Paper Lunch-Bag): Normal

Dark Brown:
Rich.
Most people like to have them at the Dark Brown Mixture setting.
The Engine makes good smooth power and cruises really nicely, staying cooler than the Lean settings.

Copyright ©
RE Massey
9.28.2010

 

Man, I'd be super happy if you posted a vid of the engine during the tweaking process so we have something to listen too...

 

Also , may have read too quickly, but did you clear the hole in the carb bowl that feeds the enrichment tube?
Clogs easily and should be double checked even if cleaned....

 

Ok got it sorted out today!

Two problems, first, after removing the mixture screws again I noticed the hole going before the throttle plate in one of them was plugged still, i could have sworn it was clean but it wasn't. So I cleaned that out.

Next problem, I needed to play with the mixture screws, but they were seized up. I freed 3 of them, but the 4th stripped and had to be extracted. I ordered a replacement. I set the idle very high to get it to run without the enrichment circuit. Then started adjusting and worked my way to where it would idle at the correct speed, and then went through again fine tuning the mixtures as rick described. After that it purred like a kitten and runs great.

Thanks for your help Rick

 

Thanks for asking a great question eEjeremy cause Rick's tuning post is gold!
RickCoMatic you should cut & past that answer into the FAQ forum.