Carb operation at idle

[pullshocks]

Let me see if I have this right.  I know the answer is "dunk your carb again," but I'd really like to understand what's going on.

At an idle of 1300 rpm you have 325 intake strokes per minute @ 275 ccs per stroke, so about 90 liters per minute going through each carb.

At idle the butterfly is closed, so most or all of the 90 liters per minute is coming through through the pilot air jet, mixing with fuel from the pilot jet, and the mixture enters the carb throat through the group of small holes below the butterfly.

(I think this is slightly over simplified, because idle speed is well below 1300 with the butterflies fully closed.  The idle is raised to 1300 and synch adjusted by slightly opening the butterflies, and they probably don't seal perfectly when closed anyway)

When the "choke" plunger is pulled out, additional mixture is admitted through the larger hole below the butterfly valve.  Presumably this is a richer mix.  When the plunger is pushed back in, it goes back to most mixture coming in through the pilot circuit.

So here's my first question:  how does the pilot screw/ needle system regulate the idle mixture?  I can see that the needle is in line with the single small hole near the bottom of the throat, and the tapered needle is regulating flow.  Does it control the flow of extra fuel into the pilot mixture, or does it control mixture from somewhere else?  Where does this fuel or mixture enter the pilot screw area?

In my case I know the passage from the single small hole up through the hole the pilot needle sits in is open because I can see light and blow air through it.  So I have to think my blockage is upstream from there

Second question:  It looks like the "choke" system receives air from near the top of the carb and combines it with a presumably very rich mixture or pure vapor from the "accelerator control valve".   It appears the accelerator control valve draws fuel from below and vapor from the head space above the fuel in the float bowl.  How does this work?  What creates the upward movement of fuel through the accelerator control valve into this system?  And why is it called an accelerator control valve if it is part of the starter enrichment circuit?

Third question:  what is the effect of the fuel level being too low or too high?

[Brian Moffet]

At an idle of 1300 rpm you have 325 intake strokes per minute @ 275 ccs per stroke, so about 90 liters per minute going through each carb.

A brief clarification on this one point:

at idle, your Manifold intake pressure is fairly low (large vacuum), so while your displacement of the per stroke might be correct, the amount of air (once translated to atmospheric pressure) will be a lot less than you mention.  I don't know the manifold pressure at idle, but I suspect it is closer to 1/3 of standard atmospheric pressure, so the actual amount of air being pulled into the cylinders is more like 30 liters per minute going through each carb.

Back to the actual carb works, and if anyone does know the manifold pressures (which is actually a 2-d chart) I would be interested in seeing it.

Brian

[Lucky]

forget the accelerator controll valve, it does nothing, it's an incomplete curcit, believe it or not..  even the manual is wrong.  the only thing it can do is cause an internal vacuume leak if the 'clear' valve fails, or the flat rubber o-ring is misadjusted.

your butterflies are never fully closed, so your always drawing mixture thru the idle curcit.

the pilot screws regulate mixture, not just fuel or air.

what EXctly ARE YOUR SYMPTOMS? (oops.. )

[pullshocks]

Let's start with

1  RPM does not change no matter how you turn the rear pilot screw -- fully in or turned out 5 turns   (need to verify I have spark on that cylinder)

2  Runaway throttle response.  Slight roll on the throttle drives it up to 4k RPM and it stays there even when the throttle is pushed back to the stop.

[Tiger]

 :) You may still have a carb issue, however, here are a couple of other things to check... ;) Many times one problem is masked by one or two other problems... :o

??? A leaking YIC's unit/vacuum leak will give similar symptoms. Disconnect the YIC's unit from the front and back cylinders and cap each one off...air tight!! Does this help ??

I would advise that you change all of your vacuum hoses...25 year old rubber will degrade, crack, split, etc and cause a problem.

??? Do you have the throttle cable routed/adjusted right... ???
If the throttle cable is routed incorrectly it can lead to high idle and/or an increase in revs as you turn the handle bars :o.
Reroute if routed wrong. There is an adjuster toward the upper end of the cable just before it enters the throttle twist grip...run the lock disc back toward the twist grip and ease the adjuster back...blip the throttle after each adjustment... :-\

??? Is the throttle stop at the carb end of the throttle cable to far in...?? Adjust as required... :-\

I'm now onto my 8th Vision and have had, among many other things, ALL of the above problems at some point... :o ::) :D :D :D :D :D :D... ;)

                 8).......TIGER....... 8)

[pullshocks]

Thanks Tiger.  YICS is off, so that's not it.  I had the same throttle cable problems previously, but that is not the problem this time.  I will try replacing the vac hoses.

[Lucky]

#2 SOUNDS LIKE WILDLY OUT OF SYNQ CARBS oops again..
they can be far enough out that one carb is closed, the other is open enough to keep the bike running.  when the throttle is cracked, the 'open' carb surges ahead.

pull the carbs, set the synq visually so a paperclip width is showing thru both butterflies from the bottom.  then install & set the synq from this baseline.

[pullshocks]

OK, I checked the sync and it was off.  Got synch adjusted pretty well at low RPM idle.  Fairly smooth at about 500 RPM.  It dies (sometimes with a backfire) when you roll the throttle at all.

When I adjust the idle thumbwheel upward, the idle speed becomes very jumpy and unstable.


It still surges up to 3 or 4K.  As the RPM goes up, synch goes way out of whack.  Stronger vacuum on the front cylinder.

The situation with the pilot screws is about the same as ever.  The engines slows down slightly when the front is screwed all the way in  Turning the rear screw has no noticeable effect .

I rode it around the block and idle jumped up and down either 500  or 3000.  Nothing in between.  When the idle was low, it was very hard to get going from stop
The other thing I notice is it needed full choke until really warmed up.  And the choke is all or nothing.

When I pull out the carbs I'll try your synch pre-adjust trick.

[h2olawyer]

I know you've cleaned them well, but it still sounds like dirty carbs to me.  Did you take the accel pump nozzles out & clean them?  There's a check ball & weight inside that system.  Low idle is 1300 RPM for factory specs - but many set it to 1500.  I can set mine to about 750 and keep it idling for a little while when warm, but it eventually dies turning that slow.

The fact you need to have the choke on full is an indication of something still gumming up the carbs.  I fully dipped mine several times & used air to clean out all the passages, but I never could eliminate the Vision stumble entirely.  I swapped out the carbs for another set I knew worked well & it was like riding a new bike.  I'll be going through the carbs I removed sometime this winter to see what is keeping them from flowing properly.  I'll also be getting the 83 fuel system fully installed as time permits.  I want the fuel gauge operational and I hear the 83s tend to get a little better fuel economy - not that I complain about the 43 MPG average I get now.

Setting the synch as Lucky described above is only the starting point.  You need to make sure both carbs are pulling equal vacuum.  On Lucky's website is a diagram of an inexpensive manometer made with clear tubing and ATF.  I use one on my V and it works great.

If I was going to be home longer while you're out in this part of the world, I'd tell you to bring your carbs & I would see if I could get them to work on my bike, then have you take them home to try out again.  Maybe one of the other CoROVers can do this for you?  Or, we might have time on Wednesday if I can get up to Estes early enough with the necessary tools.  Just a thought.

H₂O

[YellowJacket!]

What condition are your carb boots in? The solid rubber boots between the carbs and the intake manifold.  Check the brass tubes that go into the boots to make sure they are sealed and you are not getting a vacuum leak.  Also check to make sure they, particularly the rear cyl one is seated tightly.

What type of air filter are you using too?  Stock? or K&N?

David

[h2olawyer]

What condition are your carb boots in? The solid rubber boots between the carbs and the intake manifold.  Check the brass tubes that go into the boots to make sure they are sealed and you are not getting a vacuum leak.  Also check to make sure they, particularly the rear cyl one is seated tightly.

What type of air filter are you using too?  Stock? or K&N?

David

Good point on the carb boots - but the air filters won't make much difference - even if K&N was available for the Vision.  I think you meant UniFilter.   ;)

H₂O

[pullshocks]

Stock air filter, very recent.

Last time I looked at the carb booots they had some cracks on the surface but nothing going through.  I looked them over as well as I could just now and nothing obvious, but I'll give them a more thorough look when I have time to pull things apart again.

I did notice that the rear clamp was not tightened up all the way, which could  have the same effect.  So I'll be keeping my fingers crossed when I fire it up again.

One other thing I notice is a bit of seepage at the petcock, so I may have problems there, but I'm not sure how that would fit my symptoms.

[Lucky]

do tiny shots of carb cleaner with the red straw tube around the carbs.  any leak will cause idle to change.  pay attention to the throttle shaft seal area's i'm doing mine now...

--Lucky

[pullshocks]

Have you ever known the choke plunger seals to cause problems?

[Lucky]

mine turned out to be bad throttle shaft seals. on the right side (83's)  i poped some #6 o rings over them & voila`!

runs like a champ...

[jasonm.]

I think many of you have missed the most important thing. This is NOT manifold pressure. it's VACUUM. Meaning it will draw fluid or gases from whatever where-ever it can. If it's plugged it can't draw. Also the vacuum is highest on any sync tool when the throttle is closed because the measurment is done below the throttle plates. When opening the throttle you are not increasing vacuum(cm/merc) as much as you are increasing volume available , of gases(air/fuel) drawn into the system. So either a vacuum leak which is easiest to find using a spray fluid on suspected areas or a plugged jet is 90% the issue. Don't over think it. Start by checking for vacuum leaks. No leaks it's a plugged jet. Plugged jets result in less power than a small vacuum leak....my .02...Glad Lucky went for the o-rings. Simplest solution is usually it. But how hard was it to do? Did you remove the throttle plates?...you did say right side. Which is in back of the springs.

[Brian Moffet]

This is NOT manifold pressure. it's VACUUM.

There is no such thing as vacuum.  There is only lack of pressure  ;D

Brian (only semi-joking.  Looking at the manifold pressure gauge in engines it measures absolute pressure, as the throttle is closed, pressure drops below atmospheric pressure.)

[Rick G]

In the HVAC trade its called negative pressure.

[Lucky]

left side. on an 83 it has the nuts, seems to have finally done the trick.

i'm prepaired to pull the butterflies & do the other side if need be, i have good replacement screw, red locktite & the ability to peen without bending.

not afrade of my engine eating a screw  ;D

[Rick G]

I've tried O rings on some bikes , but it usually resulted in a sticking  throttle, or excessive throttle pull resistance.