carbs 101 - pooling gas?

[dlb]

since i recently asked the stupid question about oil change dry starts and got some good info, i'll keep the ball rolling and ask a few stupid Q's about carbs. i've got just a tad above zero experience doing anything with them, besides putting them on and taking them off.

the reason i ask is that today when i was messing around with the weber i noticed that when i gave it throttle, i could see gas pool in the bottom of the passenger side barrel. first off, i don't know if that's normal or not but having never seen it before, i wondered if that might be the cause of my poor mileage. i know that the gas is supposed to be atomized, or made into a fine mist, by the jets and the thin, small pool of gas down there was definitely not a fine mist. secondly, which barrel is the primary and which is the secondary? how do they work in conjunction with each other? which one did i see the gas pooling in?

i've googled how carbs work and have a very basic understanding of the vacuum sucking air and fuel, and how the choke and throttle are just plates that block off the air so that the carb sucks more fuel and less air but that's about it. if someone is able to walk me through this in baby steps or direct me to a good guide or youtube clip, i'd really appreciate it.

[4wdchico]

Check these books out:

John Deere. ... Fundamentals of Service (FOS) Manuals

Really well illustrated and written. So what if it is intended to teach folks the basics so they could fix tractors? Fundamentals are what we are talking about and these books get it done.

If you search I'll bet that you can get the whole set for pretty cheap used. Or try your local library.

[arbskynxnex]

What you want to do (with the car off) is pull the air cleaner off, and hold open the choke and manually trigger the throttle. It is a progressive carb so as you open the throttle the primary then the secondary butterfly valves will open at the bottom of the carb. So if you have gas pooling up it is either on the butterfly valve when it is closed so that should be the secondary or it is pooling in the intake manifold and could be either. It could be several things from a bad gasket or valve letting fuel in when it shouldn't be to jets that are too large.

Jets basically control how much fuel is sucked into the air coming through the carb. Large jets allow more fuel in, but can cause the engine to run rich if there isn't enough air to mix with and properly atomize the fuel. There are other things that control it too, but the jets are what people primarily seem to mess with on the Webers. Hopefully that is general enough info to sucker Petros into giving us the professional lecture.

I ordered a set of jets from racetep.com that they calibrate based on my engine size and vehicle. My Weber came off of a dodge d50 and burns way too rich and the jets are the base ones that it ships with, so the default might be too rich for you too, depends though. I've been getting mileage that is in the low 20s and you can smell the gas over 75mph. I'll report back on the results with the company and the jetting.

There several books on weber carbs, including our 32 36, just do a search on Amazon. I'm going to have to not be cheap and buy one soon.

[Petros]

Sounds like you got some bad info. The Primary barrel is the one that opens first. In ancient times cars had only one barrel and one throttle plate. Simple but to get good fuel metering you need a smaller barrel so you get you can keep it closer to the ideal fuel/air ratio over more of the driving range. But that would limit the total amount of air you can let in (the amount of air going through an opening is limited by "choked flow" when it reaches sonic speed-no matter how much you suck on it no more air will get through-hence you need to add more barrels the more air you need to suck through it). So you add a second barrel and throttle plate only for the times when you are more than about half throttle.

The air speeds up as it passes through the venturi, and the low pressure area in the venturi allows it to draw in fuel from the bowl through the metering jets. This you likely know. But what the carb is supposed to accomplish is keep the air/fuel ratio at all throttle conditions and all temperatures within a fairly narrow range of air/fuel ratio. When you add in emissions requirements than the acceptable range of air/fuel ratio is even smaller. Hence all the gadgets and sub systems on the carb, and eventually the auto makers having to abandon the carb all together and go to electronically controlled fuel injection.

What the throttle actually does is reduce the density of the air in the intake manifold. the air/fuel ratio should always stay fairly close no matter the throttle position. When the air density is low (at idle or part throttle), the amount of oxygen and fuel will also be proportionately lower, and power output is lower. At wide open throttle you will have close to atmosphere pressure in the intake manifold, and the maximum amount of oxygen will be available and the corresponding amount of fuel. This btw is how a turbo charger produces more power, it makes the intake air density higher than atmospheric, allowing more oxygen and fuel into the engine than can be achieved with naturally aspirated engines (and they do not increase efficiency at all, they usually lower it slightly, but they make a small engine act like a larger one).

The object is actually not to atomize the fuel perfectly into a pure vapor. this is a myth and a common object of the back yard tinker's claim of inventing a carburetor that will deliver 100 mpg on a Cadillac (these silly claims are as old as the automobile). It has been determined by experimentation that you have to deliver the fuel in a mixture of about 40/60 vapor to small droplets for best efficiency and combustion. Once the valve closes and the piston compresses the mixture the rest of the droplets break up into vapor. The problem is fuel vapor is too low density and will not deliver optimum pressures in the combustion chamber. Both carburetors and fuel injection can deliver this mixture, but it is far easier to control the vapor/droplet ratio (and the fuel/air ratio) over a much bigger range of operating conditions with EFI.

These droplets can pool in the manifold in certain cold air conditions, but only momentarily. The heated part of the manifold is supposed to quickly evaporate the liquid fuel back into the air stream. If it forms large pools than the mixture is way too rich, or the droplets are too large to stay in the airstream as it makes the bend from the barrel to the intake manifold runners. And yes, if you getting noticeable pooling for very long it is likely due to something wrong in the system. BTW, this is why intake air temperature is controlled with the heat riser, to prevent the pooling in the manifold until after the engine warms up. Not sure that taking in heated air can be easily accomplished with the weber set up, but it should not really be a problem after it warms up. Is your heat valve in the exhaust manifold working properly?

The function of the idle jet, mid range jet, and main jet, emulsion tubes, metering rods, by pass vents, etc. is to maintain the proper air/fuel ratio over the full range of throttle positions and air temperatures. IF you have one of these systems plugged or not working or damaged, it could cause an excessive rich contrition. IF your engine and fuel system was running good at one time and it has suddenly changed, you will just have to work through the system until you find out why. There is no magic that makes it run good sometimes, and suddenly runs bad with nothing changing. YOu have to find out what changed.

Do you possibly have too much fuel pressure, or a bad float or needle valve. IF fuel keeps coming into the bowl when it is not needed it will spill out the vent into the barrels. Sounds like a possible source of your problem. Too bad the weber does not have a sight window, they had not been invented yet when the weber was designed.

[dlb]

thanks guys. i took another look at this today and found that the 'pooling' isn't actually pooling. it's just the butterfly valve on the primary barrel getting momentarily wet with fuel, and it appears normal according to all the info you guys just shared.

what does not appear normal is that the secondary jet does not appear to be spitting fuel at all. with the engine off, when i give it throttle the primary jet spits as the primary butterfly is opened, but when i give it more throttle and engage the secondary butterfly, the secondary jet does not spit. it seems like the secondary jet is clogged or something, am i right?

[danzo]

Just to clarify dlb - when you say the the butterfly valve is wet with fuel are we correct in ass-uming this happens when the engine is off, right? Can you see fuel dripping from the nozzle in the venturi? This is not supposed to happen and can be the result of an improperly set float. This is the first item Pierce Manifolds (whom I bought my weber from) mentioned to me when I said I can smell fuel after shutoff so badly that nobody wants to park next me. Costly way to avoid door dings!

Pierce sells a tool to set the float, which I got and used along with a few new jets. Now I don't smell gas so much, but it could be due to much colder outside temps right now. They did mention my poor 25 mpg and hard-to-start-when-warm problems could be from a weak ignition. Former listmember gatemaster did an ignition upgrade which I plan to do soon, sorry I don't have the link handy. It involves an earlier dizzy and aftermarket high output external coil. Upgrading my ignition worked wonders on my old Triumph, both in terms of starting and power/mpg, so I plan to do the mod asap.

Not sure if the secondary is supposed to spurt fuel when goosed, but this doesn't come into play unless you go more than 2/3 throttle. Did you buy your carb new? If not there could be a clog, unlikely if it's new.

David why did you start this thread? Not trying to give you guff. Are you getting poor gas mileage? As per splatter all of us weber peeps should be getting in the low 30s unless you drive like you were picked on as a kid and are trying to wage revenge on society. But it could be running too rich. One trick is you can make an 'econo air/fuel ratio gauge'. Just hook a voltmeter to the O2 sensor and read it while driving - a proper 14.7 a/f ratio reading should give .5 volts (there are charts online everywhere). Mine is closer to 1 volt so it's too rich - could be overly large jets but it also could be unburnt fuel from a weak ignition.

[rer233]

with the engine off, opening the throttle actuates the accelerator pump on the primary side (causing the squirt of fuel you see.) don't think the secondary side has an accelerator pump, hence no squirt.

[dlb]

danzo, you're right that the engine was off when i performed the test in question but it looked about the same when it was running.

for the secondary, i made sure that the butterfly was opened all the way but still no fuel. i guess if rer is right, that makes sense.

i started this thread because in trying to track down the reason for my poor mileage, i'm giving the carb a good hard look but i don't have enough understanding of the carb as is to come to any conclusions so i'm just kinda asking random questions and piecing it together. i did buy the weber new though so i don't think it's like it would be clogged either but i had to ask.

[Lil Beast]

with the engine off, opening the throttle actuates the accelerator pump on the primary side (causing the squirt of fuel you see.) don't think the secondary side has an accelerator pump, hence no squirt.

I was about to write some like that and my computer froze on me!
And as for with the engine running it might be kind of hard to see it flow the gas is been atomized. sucked in real fast.

[dlb]

dnazo, in answer to the running rich question, i don't think so because my plugs are a nice medium brown colour and the idle mixture adjustment screw is within spec (i think spec is between 2-2.5 turns out). if you have to adjust it more or less than that, you need bigger or smaller jets. so it appears the stock weber jets are the right size.

[Petros]

as noted the squirt is the accelerator pump that squirts liquid fuel down the primaries. You can see the fuel coming through the venturi, but it is not easy. You have to use a strong flash light, with choke open and engine running look down the barrel and open the throttle slowly, you will see a white vapor trail coming out of the axillary venturie. You should see it in the secondaries too but that means you have to hold the throttle wide open without any load on it, it will almost certainly over rev. We used to do it on the dynamometer so there was load on the engine, allowing full throttle without over reving it.

If the color of the spark plugs is correct, and it does not have any flat spots or bad behavior, there is nothing to learn by seeing the fuel stream into the barrels. You will either need a dyno or at least a CO meter to get all the jets and other adjustments correct for this engine. Or you could trial and error it, but that takes a long time to get correct that way.