Learn me carb nonsense, specifically flat spots

On Susie I've got a Carter YFA, which is a carb I'm convinced was someone's deal with a devil.  Apparently it's a great carb for any number of vehicles, which means when it isnt set up right it's a horrible carb for all vehicles.

I currently have a massive flat spot just off idle.  I know the problem is the jet and metering rod being mismatched for my engine as I have cleaned and cleared everything else in the system that could cause a flat spot.

I cannot find any metering rods to swap out, so I'm stuck with trying to swap the jet and make it work. Problem is I dont know which way to go.  I currently have a .107" jet in there, do I go bigger or smaller to clear a flat spot?

I can post the metering rod numbers and some more info if necessary. 

Carter

Remove the carburetor and set fire to it in a ritual sacrifice.  Then endure a lifetime of bad luck because the gods and devils want GOOD E36 M3 to be sacrificed to them, not garbage.  That's what sacrifice means after all.  They don't want the horns and tails, they want the fatted lamb.

Make sure your ignition timing is correct, and that your vacuum advance works, and is connected to ported vacuum on the carb.  90% of carb problems are ignition problems.

You have to go stupid rich (like puff of black smoke) to lose power. 99% of all bogs or soft spots will be lean.  Accelerator pump working ok?

Came here to say "Check the accelerator pump".

Make sure the flyweights in the distributor move smoothly and the bushings aren't worn out.

In reply to Everyone. :

I haven't played with the ignition timing recently, but I also haven't loosened the distributor since I last set it.  I'll double check but it doesn't behave like a timing issue.  Top end, when I get there, is good under load.  Problem is solely when I lead foot it from idle, doesn't matter if it's in the garage in neutral or out on the street. 

Accelerator pump is working well, especially since I went to nothing but ethanol free gas.  Tested it last night as I was resetting the adjustment screw on the metering rod (inside the blasted carb, you have to take the top half off)

Video of the bog, at about 0:18

Vigo, if I do have to richen it up do you have any input on how big of a step up I should go?  Carter uses Edelbrock's jet numbering system, so this jet is 120-407 I believe. 

Ignore the ethanol propaganda.  Carburetors would develop problems long before ethanol was added to fuel.

Just because you never loosened the distributor does not mean there are no problems!  Get out a timing light, unplug the vacuum advance, verify the timing starts advancing shortly after idle.  Then hook up a vacuum source to the vacuum can and verify that it advances.  

How old are the plugs?  Gap is correct, tips are nice and sharp edged and not all worn rounded?  Don't guess, verify.

Only after all that do you dig into the carb.

If you move the throttle lever and look down the carb throat.... is there a lag between the lever moving and the accelerator pump kicking in a squirt of fuel?

In reply to Knurled. :

Ethanol comment was more because the ethanol was destroying the diaphragm of the accelerator pump.  Literally rebuilt the carb every other month for a while there, every time the diaphragm was damn near gone.  Since switching to ethanol free its lasted with no issues.

Plugs and wires are roughly 5-6 years, or under 1000 miles old.  I'll pull them and check them as well as the rest.

In reply to NOHOME :

I dont think so. I'll try and get a video tonight. 

Sorry if I seemed a bit short, I was starting to enter the post and my appointment finally showed up, so I had to be brief.

There is the possibility that passages are gunked up if the diaphragms were failing.  I'd not just pay attention to the accelerator pump circuit but also the main emulsion wells.  It's been a long time since I dug into one of those carbs, but they all basically work similar.  The accel pump is partially there to cover the time from going from the idle circuit to the main circuit, if there is anything delaying the mains from coming on then you get a flat spot.

Also, are you using a distributor setup for ported vacuum advance or manifold vacuum advance?

If your distributor does not match the vacuum source, it will always have that goofy tip in/off idle stumble. 

Manifold vacuum is high at idle and goes down with throttle. Ported vacuum is lower at idle and then comes up a bit and then goes down with throttle.

I believe that a: ported vacuum is the devil (but if that is what your distributor needs you better use it), and b: that ported vacuum was created in the early seventies to try and combat idle emissions.

This is a good-ol opportunity for general checks on the tune.

This goes without saying, but sometimes new parts suck worse than the old ones.  The 1-year-old plug wires on my SS lasted about 9 months before a few of them went super-high resistance and one of the plug gaps went way too wide.

That type of bogging would take me straight to two things:  Ignition and throttle shaft.

To check the ignition, grab the dizzy and advance it a small handful.  Re-test.  Don't go WOT since you'll ping like crazy, just do some gentle runs from idle to 1500 to see if the bog has changed.  If it has changed, figure out what' going on.  Is it a stock dizzy? Vac advance canister could have a hole.  Springs in the advance weights can rust/fatigue.  The other thing is that the dizzy might be just fine, but there are different setups for manual and automatic.  I skimmed the other thread and saw T5.  Was it automatic to start with?  If so, the dizzy in that car was not set up to ever see that load at that RPM.  It had TC stall.  It never lugged, so the timing curve was tuned accordingly.

Checking the carb throttle shaft can be a tough one.  I had a 2-jet that refused to show symptoms except when you use the foot throttle.  I couldn't reproduce the issue with any other technique; manual throttle input, starting fluid, stethescope, nothing.  Once I fixed the throttle shaft, it worked great.  Jet Performance has a bronze bushing kit that comes with reamer.  It's a life saver.

I would look to ignition first.  It seems to happen (in the video) when it's lugging at low RPMs under load.  That is almost always ignition.

oooo.... Also.... 

Getting idle/timing set up is trickier than most think.  Too often people go with too much idle throttle because the timing is too retarded.  The throttle should be darn near closed at idle.  You can test this by advancing a couple degrees (which should raise idle) and backing off the idle screw.  It's possible that you have too much throttle opening when setting the timing.

If it's ported vacuum, what happens is you're already getting vacuum signal to the canister and have to set the dizzy too retarded.  What is supposed to happen in this case is base timing is set with no vac signal and then kicks up as soon as you touch the throttle.  Since the port would be uncovered, it's already getting signal, so you set the dizzy back to get your correct readings.  When you go for throttle, there is no additional advance and you get a bog.

If it's manifold vacuum, you have the opposite problem.  The dizzy is getting LESS vacuum than it expects if the idle is set too high and you aren't getting proper idle advance, so you don't get the expected curve when you call for throttle.

It's important to mention that I'm speaking of throttle position, not RPMs of idle speed.  You might have a proper 600 rpm idle, but the throttle is too far open because the timing is retarded.  The current 2-jet on the LeMans, I can advance the dizzy to the point where I can take the idle adjustment screw completely off the throttle and it will idle just fine.

Many times people start the tune with too much idle and not enough advance to be "safe."  Then they work backwards focusing on the throttle.  I start WAY advanced and work by backing off advance first, then adjust idle when it starts to get too low.  Don't fret about idling a car with too much advance for the sake of tuning.  Anywhere from no advance up to where it kicks back on the starter (probably about 25 degrees) is fine for starting a tune.  Driving it with too much advance is bad.  Tuning/idling with too much advance won't hurt a thing.

I also didn't see if you have a different cam.  If you do, throw your initial timing specs out the window.  Initial timing is ONLY a means of setting a baseline to get the correct curve in the rest of the load/throttle/rpm range.  I had a BBC with a little mismatch in cam and compression (too much cam) and cylinder pressures were E36 M3ty.  I ended up locking mechanical and setting it at 34 degrees initial and using ported vacuum to mask the mismatch.  The actual initial number is useless except for setting the baseline for the original application.  Once you change something, that number is useless.

It's just that it's easy to set 12 degrees at idle, but it's hard to set total timing at 36 degrees when you're at redline.  Idle advance is just a baseline number based on what the dizzy is engineered to do everywhere else, but it's not true that "12 degrees is the correct advance for idle" or whatever.

Problems just off idle won't be caused by metering rods or main jets. As said before: make sure your distributor is seeing ported vacuum and not manifold vacuum. And make sure your accelerator pump is giving a nice strong shot as soon as you touch the throttle. 
Again, if you are fine at higher rpm under load it is not a main jet issue. This is screaming timing at me. Also check your cap and rotor. 

Lot of thoughts there.  Gonna have to go through em when I get to a full screen. Thanks guys, I'll go through later tonight with replies.

Knurled - no worries, didn't come off like that to me.

Ported vacuum is nice because there is zero vacuum at idle.  This is nice and stable, you won't get a situation where high vacuum adds a bunch of timing which holds the idle speed high, or vice versa.  And then you crack the throttle open and get a nice shot of vacuum advance on tip in, which is usually a good time for it.

Curtis73 said:

Jet Performance has a bronze bushing kit that comes with reamer.  It's a life saver.

THANK YOU!!!  I have a 4150 that has badly egged out throttle shaft bores in the baseplate, and had been coming up blank for where to source a repair kit.

If you have a carter with off idle stumble and you the usual things are not the suspects its almost always the metering rod springs.  I have taken freshly built motors that idle like crap to rusted out motors to almost perfect off idle with just this change. 

Take a reading of the motor at idle for vacuum, pull divide by 2 and that puts you right on the money for what metering springs to use. Chances are you will need pink or blue. unless you have one serious cam 

https://www.carburetor-parts.com/Carter-AFB-Metering-Rod-Springs_p_315.html

Knurled. said:

Ported vacuum is nice because there is zero vacuum at idle.  This is nice and stable, you won't get a situation where high vacuum adds a bunch of timing which holds the idle speed high, or vice versa.  And then you crack the throttle open and get a nice shot of vacuum advance on tip in, which is usually a good time for it.

While I agree, it has a lot to do with the rest of the tuning.  Ported vacuum for ignition was more of an emissions thing back in the 70s.  Old school stuff was often manifold (hence why you have to pull the vacuum line to set timing) and the canister and mechanical advance were set up to know what to do with a manifold vacuum source.  Typically canisters for a manifold source had stronger springs but not always.

The nice thing is, manifold and ported are going to have the same signal above about 1/8 throttle, so it really doesn't matter much in the grand scheme.  For stock cams, I usually go ported.  There is enough cylinder pressure at idle to not need the extra help from vacuum advance, but in cases of lower compression or bigger cam, manifold can help with idle quality.

I would suggest he use the one that works best for his application.  If he can get the throttles closed enough at idle with ported vacuum, it will be fine.  If he can't, I would suggest an adjustable vac advance can and go manifold.

The secret to setting timing and idle is using a vacuum gauge on a ported source.  Keep advancing timing and backing off the throttle until you have less than 1" vacuum reading.  That will ensure that the throttles are closed enough to not uncover the port.

Knurled. said:

Curtis73 said:

Jet Performance has a bronze bushing kit that comes with reamer.  It's a life saver.

THANK YOU!!!  I have a 4150 that has badly egged out throttle shaft bores in the baseplate, and had been coming up blank for where to source a repair kit.

Sure.  The 2-jet I was talking about above... I went to install the bushing, and before I even reamed the hole, the bushing just fell through.  It was so worn that I couldn't use it.  I tried some epoxy putty and re-drilling, but it didn't last.  Instead I had to find a junkyard baseplate that didn't suck as much as the one I had.

So, I checked everything I could.  Plugs are good, wires ohm out good, timing was dead on from the last time I set it.  No issues I could find.

Ported vacuum

Car was an auto before, I need to dig into the differences between the auto and manual Duraspark II distributor to see how far off that might be shifting things.  I know there is a spring set that gets rid of some of the emissions setup Ford did

Accelerator pump squirts well and has no lag I can see

What else...

Curtis73 said:

The secret to setting timing and idle is using a vacuum gauge on a ported source.  Keep advancing timing and backing off the throttle until you have less than 1" vacuum reading.  That will ensure that the throttles are closed enough to not uncover the port.

For this one, I started from scratch and reset the idle screw, then backed the throttle and choke screws out enough so they weren't touching. Couldn't get below 8-9" of steady vacuum.  Advanced past 20 degrees with virtually no change.

For this one, I started from scratch and reset the idle screw, then backed the throttle and choke screws out enough so they weren't touching. Couldn't get below 8-9" of steady vacuum.  Advanced past 20 degrees with virtually no change.

I'm reading this as, you can't ever get below 8" vac on a ported vac port? Is that correct? 

I think Curtis is suggesting that you muck around with idle screw and mixture and timing such that you can have the throttle plate very closed, and no vacuum is being applied to the vacuum port.  That is, less than 1" of vacuum as measured from a PORTED source.

If you're measuring vacuum from a MANIFOLD source, 8" is pretty low vacuum for an idle and you might be doing it wrong.  Or you have a sweet and choppy cam.