Is this normal carb behavior?

[ConchZ]

I get a slight fuel smell coming into the cabin from the engine compartment when I gun it hard, and then release the throttle. I've made sure the floats are right, the float gaskets are new, and the carbs are adjusted perfectly, at least in my mind. The car runs great, idles smooth, no problems.

To simulate what happens, I took off the air cleaners, reved the motor up, then covered a carb mouth with my hand. I was rewarded with a splash of fuel into my hand. Its like the needle doesn't drop down fast enough to shut off the fuel when the air is throttled. I took off the domes and cleaned the insides, to make sure they slide smoothly.

This might just be how these things work, but if not, I'd like to get it to stop. Anything I'm missing? Oh yea, I'm reasonably sure the smell is not coming from the back (gas tank and lines).

Thanks,

Matt

[Bruce Palmer]

Matt,

The needle will never shut off the flow of fuel as even at the base of the needle it's smaller than the orificie in the nozzle.

From what you are describing I can't even venture a guess as toi whether your carbs are way rich or way lean. Take the mixture nuts from where they are to the full up position and count the number of turns. You can always put them back to where they are (as they are perfect) later. I'm just curious as to how far down they are.

What are you trying to show/duplicate by putting your hand over the mouth of the carb?

[ConchZ]

The mixture nuts are at 2.5 turns. What I'm trying to duplicate is the rapid cut off of air flow that the carb experiences when going from wide open throttle to nearly closed throttle. I suppose I'm actually overdoing it (vs just using the throttle itself), but its one way of being able to see and feel what's happening fuel wise. The reason I'm pretty sure the fuel smell comes from the engine is that I smell much less of it now that I've installed and/or repaired the various firewall grommets and the air intake elbows. I also went through all the hoses, connections and grommets on the fuel lines and vent lines while I had the car apart for refresh. No problems were found, as those lines had been replaced a few years back.

As for mixture, I think its right. I'm using the following instructions from http://www2.zhome.com:81/ZCMnL/tech/su.html plus notes I took from your video.

4. Raise the lifting pin (or use a screwdriver if you don't have the pins) so that the piston rises no more than 1/16". Listen to the engine's exhaust note and compare it to the following conditions:

If the exhaust note rises and stays high till you drop the piston, this carburetor is adjusted too rich. Turn the mixture nut one flat (one-sixth of a turn) up, moving the jet toward the bridge, then repeat Step 4.

If the exhaust note falls and the car sounds as though it is going to stall, this carburetor is adjusted too lean. Turn the mixture nut one flat (one-sixth of a turn) down, moving the jet away from the bridge, then repeat Step 4.

If the exhaust note rises briefly and then settles back down to something like the original RPM level, this carburetor is set correctly. When you have achieved this setting for both carburetors, continue with Step 5.

I'm wondering if I'm just too picky in my attempt to de-stinkify the car.

Edited May 31, 2009 by ConchZ

[Arne]

FWIW, I have noticed this exact symptom on mine a while back, after I raised the float level a bit during some diagnosis. Dropping the float level back down corrected it.

[ConchZ]

FWIW, I have noticed this exact symptom on mine a while back, after I raised the float level a bit during some diagnosis. Dropping the float level back down corrected it.

I have a suspicion that its related to the float, too. The rear float stuck open once, causing gas to squirt out the overflow tube and onto the exhaust manifold. No fire, luckily. I could smell it immediately and shut it down. A whack on top of the float bowl temporarily freed the float, but it just did it again. Turned out there was something stuck in the valve. I cleaned it up and re-installed it, and moved the vent lines so they won't pee on the manifold. While this solved the obvious fuel problem, I still have the slight fuel smell when going from WOT to idle. I've set the float correctly, or so I think. I could try dropping it down, I suppose.

Now that I'm thinking about it, this could all be because my float bowl overflow lines are not connected to the air cleaners. The PO installed the little duel K&N round air cleaners, which don't have a connection port for the float vent lines. Maybe I should drill a hole in the back of one of them and plumb the vent lines to it? While I'm at it, I should probably plumb the valve cover breather to one of the air cleaners. I've heard that you can get some stink from that line, too.

Edited June 1, 2009 by ConchZ

[ConchZ]

Ok, before going after the fuel overflow and valve cover vent plumbing, I decided to tinker more with the carbs, as I really think they are set too rich. The exhaust stinks and it puts out a bit of whiteish/grayish smoke, and I only got 13 MPG on my last tank.

I've never noticed the mixture nuts doing anything, and to prove it, I turned them all the way up with the car running, and it does not affect the idle at all. I ran them out 3 turns, with no change in the idle.

Taking a tip from this page, I decided to look at the needles, to see if they were seating into the nozzles. I also wanted to see if the nozzles were awash in fuel. I cranked the nozzles all the way up and took off the dashpots. There was a bit of fuel on top of the nozzles, like there were moist, but not wet. I've never seen them on another car, so I hope this is correct. I did check the float level, and it seems to be according to specs, so I think my fuel level in the bowls is ok. By specs, I mean that the float valve shuts off when the top of the float is level with the float bowl lid.

I cleaned up the dashpots and reseated the needles, using the method in the link above. The steps were:

1) Loosen the set-screw that holds the needle so that the needle can move a bit

2 )Turn the adjusting nuts all the way in (located underneath the carb throat, where the fuel line enters from the float bowl)

3)Pull the needles out about 1/8" or so, you should see the shoulder of the needle sticking out past the dashpot body

4)CAREFULLY reinstall the dashpot, and push down ever-so-gently to seat the needle fully in the nozzle (that's the hole the needle sits in)

5) Pull the dashpot back out, snug down the needle set screws, and put them back together.

I think the needle shoulder is sticking out farther than before, which is probably good. Previously it was even, or a little below the dashpot. After reinstallation, I can crank the mixture screws all the way in without the car dying, but the idle does get slightly slower. The best idle seems to be around 1.5 turns out. The exhaust doesn't stink much with this setting, and it's clean looking. (It drives good, also, but then it always seemed to run good.) Raising the dashpots a little will result in a slight decrease in idle.

Here's my questions:

1) Since raising the dashpot slightly will decrease idle a bit, am I running too lean?

2) Should I be concerned that I can turn the mixture screws all the way in with only a small decrease in idle? I was expecting the car to die when I did this.

Idle is set at ~720, and the valves were recently adjusted. Initial timing is at 17, and the vacuum advance doesn't seem to work, as there's no difference in timing with vacuum connected or not connected to it. I read somewhere that if you don't have vacuum advance that you should set your initial timing this high. I get no pinging, at least. The PO had set it higher, but that made it harder to start. Mechanical advance does work, from watching the timing increase to about 30 when revving it up.

Thanks,

Matt

[ConchZ]

Thought I'd clean up some hanging chad posts, starting with this one. The solution was to adjust the choke cable so that it lets the choke completely off when the lever is in the off position. This is tricky, because my 73 has 72 carbs, but retained the 73 choke cables. The 73 cables come out on the right side of the fire wall, vs the left for the 72. I think the 73 must have had more travel in the choke mechanism than the earlier carbs, as with the cables adjusted so that when the level is fully forward, the chokes are fully off, you can only pull the lever back halfway before the chokes are fully engaged.

Anyway, it's all better now. Doesn't run rich and stinky all the time. Thanks.

[Tony D]

The 73 had a real choke---it did the same thing as what your hand did: choke the airflow off through the entire carburettor, exposing cranking or running vacuum directly on the jets sucking fuel through them to enrich the mix for starting and cold starting.

The 72 Carbs lower the jet to accomplish the same thing. The starter system will be engaged more than enough at the lever 1/2 way back--the 'light is on' in either event.

When you drop-throttle the vacuum is dropped on the head side of the carb---it will not 'suck fuel out the jet' like when you stick your hand over it. This is what should happen.

When the engine is at high speed, the pressure delivered to the carbs is up there around 5-7 psi to compensate for the small float bowl and the fuel being sucked out the jet so fast. This increase in pressure keeps the fuel level constant so your AFR stays the same.

On drop throttle, it will take a fraction of a second to refill the float bowl and raise the float to shut the fuel off. The needles may have trouble shutting it off, it may be overwhelmed, but unless you get more bouyancy or a better needle design (grose-jet) that shutoff during that time may be sketchy.

As for 'two turns down' on the fuel nuts. That is a starting point, actual proper fuel mix is best set using 'rich best' or 'lean best' techniques where you adjust the idle speed to give the best idle. Rich Best is turning the needle nuts down till it starts to stumble, and then turning it back up 1/2 turn. Lean Best is the opposite, raising the nuts till it starts to stumble and then turning them back down 1/2 turn. Lean Best is your 'emissions' setting, Rich Best is your 'performance' setting.

Unfortunately with the Round Top SU's what you set at one point is what determines the mix all along the rpm/load range.

The carbs you had on the car originally (the flat tops) had an independent idle circuit, meaning you could have a preset jet for off-idle, and adjust the idle independently very lean with precision for best emissions. The SU HIF6 is a similar carb to the Flat Top, with the independent idle circuitry, and a jet for the main that is externally adjustable.

I digress...