Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
STOP!! Never post your email address in open forums. Bots can "harvest" your email! If you must share your email use a Private Message or use the 
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
This is probably one of my dumber questions, anyways in trying to understand the float and needle set up on an SU carb better, I can see how it would be necessary to have the needle shut the fuel off from running over and out the over flow, and I can see how getting enough fuel into the bowl is important also. But how is it when they give problems like running rich like when an improper ball jet is used? I guess they make the system run rich by letting little more fuel in? once, I had a float assembly make the engine run rough, and there I think the seat might have been partially closed and the engine ran lean. I guess my question is how critical is the needle and seat for optimal running. Does the fuel flow out of the bowls need to be super actuate?
-
Hi Guest!
You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
There are some perks with a member upgrade!**Upgrade Now**
(PS: Subscribers don't see this gawd-aweful banner
TR2/3/3A understanding the float and needle set up better
- Thread starter sp53
- Start date
Share this page
TR3driver
Great Pumpkin - R.I.P
Offline
The problem is that the flow into the bowl needs to exactly match the flow out of the bowl. The flow out of the bowl varies widely under different conditions, and if the flow into the bowl doesn't match exactly, then the level in the bowl will creep up or creep down. If the level is too low, the engine runs lean, which in turn causes lack of power, misfiring and potentially even engine damage. Too high makes it run rich, which causes similar problems.
Yes that is a good point. Long before I joined this forum, I remember once I ran a tr3 with an electric fuel pump and regulator. If the regulator was turned down to 1 psi the car would start to miss going up a hill and I think if I turned to 10 it would pump fuel out the bowls. I guess that is my question; how exactly is that exact flow achieved? There are all these variables like hills, temperature, downhill, idle and more. To me it seems some of fuel can be allowed for by setting the needle adjustment at the piston, I guess that setting is really at idle. Perhaps one way to see is wide open throttle on a flat track which I did once and stopped at 110mph.
Anyways, I am basically asking because I get this weird miss that comes and goes. My carbs probably need a total rebuild on this car and sure a tune up is often a good thing, but again in my head I see the fuel coming and going and wonder just how critical the needle and float is; can that be this miss.
Anyways, I am basically asking because I get this weird miss that comes and goes. My carbs probably need a total rebuild on this car and sure a tune up is often a good thing, but again in my head I see the fuel coming and going and wonder just how critical the needle and float is; can that be this miss.
TR3driver
Great Pumpkin - R.I.P
Offline
Maybe I'm not understanding the question. The float is supposed to ride on top of the fuel level in the bowl, if the level starts to sink then the float sinks as well and lets the float valve open a bit more to flow more fuel. If the level starts to rise, the float should also rise and close the valve. Of course, there has to be enough fuel pressure that enough fuel flows with the valve fully open (1 psi probably isn't enough, spec is 1.25 to 2.5 psi), but not so much that the float can't press hard enough to shut off the flow.
The flow out of the bowl is created by the venturi effect (google it). These are what's called "variable venturi" or "constant depression" carburetors, meaning they adjust the size of the venturi in real time to keep the air velocity through the venturi (and hence the depression applied to the metering jet) roughly constant over varying conditions. Steady state fuel flow is controlled by means of the tapered mixture needle being moved up and down by the action of the air piston (that forms the adjustable part of the venturi). (This same write-up in different words is in the workshop manual, page P15.)
So, for example, when you start up a hill, you open the throttle some more, which causes extra air flow through the venturi. The piston doesn't move immediately, so the extra air flow creates a stronger vacuum signal (aka depression), which pulls the piston up (against the damper action) and also pulls more fuel through the metering jet. It takes a little bit of time for the fuel to move through the jet faster, so the piston also impedes air movement just a bit, creating a momentarily richer mixture. Then the piston eventually moves and things return to normal.
Thus, the viscosity of the oil in the damper actually tunes what happens when you first open the throttle. If the oil is too thin, you may get a "lean bog" or with no oil at all, the engine may actually stop running.
My experience has been that these carbs actually work fairly well even in advanced states of wear. The main wear area is where the throttle shaft comes out of the front of the front carb; excess wear at that point can result in erratic idle rpm (have to blip the throttle to get idle rpm down).
But problems with the float valve can certainly cause issues. Crud in the fuel (eg from deteriorating hoses) can get caught and block the valve open, causing rough idle and fuel coming out the bowl vents. High fuel pressure (or using Grose Jets) can lead to a situation where atmospheric pressure actually plays a role in fuel level (and hence mixture). Lotta years ago, I was driving a friend's beautifully restored TR3A when a storm front blew through and it started dumping fuel out the bowl vents. I could actually see the slick on the road behind me! (after smelling the raw fuel) Fortunately, he had left the old float valves in the tool box in the trunk, so it only took a few minutes to swap the Grose Jets for the stock valves, and it ran great for the rest of my visit.
That was 2003 IIRC, Fred Thomas let me drive his car during the combined TRF/VTR Summer party. RIP Fred. You were a class act and a true gentleman!
The flow out of the bowl is created by the venturi effect (google it). These are what's called "variable venturi" or "constant depression" carburetors, meaning they adjust the size of the venturi in real time to keep the air velocity through the venturi (and hence the depression applied to the metering jet) roughly constant over varying conditions. Steady state fuel flow is controlled by means of the tapered mixture needle being moved up and down by the action of the air piston (that forms the adjustable part of the venturi). (This same write-up in different words is in the workshop manual, page P15.)
So, for example, when you start up a hill, you open the throttle some more, which causes extra air flow through the venturi. The piston doesn't move immediately, so the extra air flow creates a stronger vacuum signal (aka depression), which pulls the piston up (against the damper action) and also pulls more fuel through the metering jet. It takes a little bit of time for the fuel to move through the jet faster, so the piston also impedes air movement just a bit, creating a momentarily richer mixture. Then the piston eventually moves and things return to normal.
Thus, the viscosity of the oil in the damper actually tunes what happens when you first open the throttle. If the oil is too thin, you may get a "lean bog" or with no oil at all, the engine may actually stop running.
My experience has been that these carbs actually work fairly well even in advanced states of wear. The main wear area is where the throttle shaft comes out of the front of the front carb; excess wear at that point can result in erratic idle rpm (have to blip the throttle to get idle rpm down).
But problems with the float valve can certainly cause issues. Crud in the fuel (eg from deteriorating hoses) can get caught and block the valve open, causing rough idle and fuel coming out the bowl vents. High fuel pressure (or using Grose Jets) can lead to a situation where atmospheric pressure actually plays a role in fuel level (and hence mixture). Lotta years ago, I was driving a friend's beautifully restored TR3A when a storm front blew through and it started dumping fuel out the bowl vents. I could actually see the slick on the road behind me! (after smelling the raw fuel) Fortunately, he had left the old float valves in the tool box in the trunk, so it only took a few minutes to swap the Grose Jets for the stock valves, and it ran great for the rest of my visit.
That was 2003 IIRC, Fred Thomas let me drive his car during the combined TRF/VTR Summer party. RIP Fred. You were a class act and a true gentleman!
CJD
Yoda
Offline
A carburetor is a balancing act...and pretty amazing for what it does, all mechanically. To go straight to your questions...
The float sets the fuel level, so the balancing act has a starting point...a base line. As you saw with your pressure experiment, the float is very sensitive to the pressure coming in, so that makes a difference in fuel level. The goal in an SU, though, so to end up with the fuel about 1/16 to 1/8"" below the top of the jet with the pressure your have coming in. The level must be constant or you start chasing your tail in the balancing act.
So, now the fuel is sitting at the opening of the airway in the carb. The size of the jet (the opening) is the next thing that controls how much can be sucked into the engine. Naturally, the larger the jet opening, the more fuel will be sucked in...so the jet size is selected for a particular engine. But the problem now is, the fuel flow is only controlled by the suction, with no adjustment for engine speed or load. An engine running slow at wide open throttle will produce the same suction as an engine running fast with very little throttle. As you can imagine, the fuel required for those 2 extremes is vastly different!
That's where the needle comes in. The needle fits into the jet, and can move up and down to fine tune the fuel that can be sucked out of the jet. If the needle is down, then the fat portion of the needle blocks most of the fuel from sucking out. As it moves upward, the needle is thinner, so more fuel can pass through the jet. For a particular engine, you could even have an adjustment that is not linear, by having the some parts of the needle thin faster, or even flatten out for much finer adjustment. Pretty cool idea!
The vacuum chamber is the key to needle placement. The chamber balances between a spring and the weight of the piston trying to push the needle down, and vacuum pulling the needle up. Here are some obvious conditions the needle can see:
At idle there is little air flow to generate a vacuum, so the piston falls to the bottom and the needle is in the jet all the way. So little fuel can be sucked out of the jet...perfect for an idle condition.
Now you floor it. All of a sudden you have lots of suction...especially since the piston partially blocks the air flow when fully down. More fuel is sucked out, and the piston starts lifting from the suction to let more fuel through the needle. The cool thing is the piston raises the needle at just the right rate to control the increased fuel as the engine speed increases.
Now you coast at cruise speed. The vacuum is medium, so the piston floats up and down to a balanced position for the particular engine speed and throttle opening. The needle sits near the perfect spot for correct fuel flow for that condition.
Way lengthy. The bottom line is the jet is:
Float sets the base line for the balancing act.
Jet controls the coarse fuel supply for the class of engine, say, cubic inches and number of carbs used.
The needle fine tunes the fuel flow, for say, cam or compression differences.
The piston dome spring fine tunes that needle position for best cruise mixture.
The dashpot slows the piston for rapid acceleration enrichment.
Don't get me started on the idle mixture and cold enrichener circuits...
The float sets the fuel level, so the balancing act has a starting point...a base line. As you saw with your pressure experiment, the float is very sensitive to the pressure coming in, so that makes a difference in fuel level. The goal in an SU, though, so to end up with the fuel about 1/16 to 1/8"" below the top of the jet with the pressure your have coming in. The level must be constant or you start chasing your tail in the balancing act.
So, now the fuel is sitting at the opening of the airway in the carb. The size of the jet (the opening) is the next thing that controls how much can be sucked into the engine. Naturally, the larger the jet opening, the more fuel will be sucked in...so the jet size is selected for a particular engine. But the problem now is, the fuel flow is only controlled by the suction, with no adjustment for engine speed or load. An engine running slow at wide open throttle will produce the same suction as an engine running fast with very little throttle. As you can imagine, the fuel required for those 2 extremes is vastly different!
That's where the needle comes in. The needle fits into the jet, and can move up and down to fine tune the fuel that can be sucked out of the jet. If the needle is down, then the fat portion of the needle blocks most of the fuel from sucking out. As it moves upward, the needle is thinner, so more fuel can pass through the jet. For a particular engine, you could even have an adjustment that is not linear, by having the some parts of the needle thin faster, or even flatten out for much finer adjustment. Pretty cool idea!
The vacuum chamber is the key to needle placement. The chamber balances between a spring and the weight of the piston trying to push the needle down, and vacuum pulling the needle up. Here are some obvious conditions the needle can see:
At idle there is little air flow to generate a vacuum, so the piston falls to the bottom and the needle is in the jet all the way. So little fuel can be sucked out of the jet...perfect for an idle condition.
Now you floor it. All of a sudden you have lots of suction...especially since the piston partially blocks the air flow when fully down. More fuel is sucked out, and the piston starts lifting from the suction to let more fuel through the needle. The cool thing is the piston raises the needle at just the right rate to control the increased fuel as the engine speed increases.
Now you coast at cruise speed. The vacuum is medium, so the piston floats up and down to a balanced position for the particular engine speed and throttle opening. The needle sits near the perfect spot for correct fuel flow for that condition.
Way lengthy. The bottom line is the jet is:
Float sets the base line for the balancing act.
Jet controls the coarse fuel supply for the class of engine, say, cubic inches and number of carbs used.
The needle fine tunes the fuel flow, for say, cam or compression differences.
The piston dome spring fine tunes that needle position for best cruise mixture.
The dashpot slows the piston for rapid acceleration enrichment.
Don't get me started on the idle mixture and cold enrichener circuits...
Last edited:
