Emergency help needed. Anybody willing to lend me SU's?

[Jeff G 78]

I don't have any pics handy of the fuel site hose, but here are a few pics of my visible bowl. You can see the low level compared to the sharpie line at the right height. The level is the same whether I use the carb bowl with sight hose or plastic test bowl. The float itself is higher than the FSM says it should be when the valve shuts off, but the height is still low.

One more bit of info is that the float rises smoothly and then "pops up" when it closes. Similar to how a toilet valve works, I guess. The float doesn't simply rise and stop with a smooth finish. I have no idea if that's normal or not.

Edited July 10, 2013 by Jeff G 78

[cbuczesk]

I wish I could check out your float problem...and drive your car! I love a challenge. But I'll be racing at Summit Point this weekend. BTW, I'm also running the 1972 carbs and found them to be a PITA to adjust. I'm going to work on a solution for that. :-)

I'm more of a FI guy than a carb guy so I'm going to ask a silly question. How much does it matter if the float is 5mm low? Shouldn't an electric fuel pump that puts out a lot more volume than the stock mechanical pump be able to overcome that? I'm running this 96gph/7psi pump. I think it's a re-branded Holley pump.

http://www.summitracing.com/parts/SUM-G3136-1

Chuck

[Jeff G 78]

Chuck, my pump is a Carter from Summit. It's the one that Bryan Little's website recommends and several other Z racers use the same pump with great results.

The carbs I'm borrowing are 4 screws, so at least they won't have the float height differences.

I don't know for sure how much the height matters, but from what I read, a low level will cause a lean condition at high RPM like Blue suggested. And that is exactly what I have had happen in previous races. The car runs great in the driveway with no load.

Edited July 10, 2013 by Jeff G 78

[Captain Obvious]

I've had similar troubles getting the float levels correct on round tops, and I don't have an answer either. I understand how the float and the valve work. There's no rocket science there and the geometry is simple. And that simplicity is what makes it so frustrating. Too low, adjust. Still too low, adjust more, and now overflowing out the vent nipple. After much fiddling, I got my fuel levels within a mm or two (low) and called it a day. I have since switched to a FI car so I haven't looked back to analyze the situation more to figure out what was really really going on.

I'm not sure how much of a difference 5mm would make, but there are some people who say CV carbs are very sensitive to fuel level in the nozzle area and others who say they are very fault tolerant to float levels. I'm no expert on the topic, but physics dictates that it would take more energy to lift the fuel up out of the bowl and into the stream the farther it had to lift it. Everyone seems to agree that you would run leaner the lower the level, but the magnitude of the impact is up for debate.

If you're running so low that you suck air at high G turns, then that's a different issue.

The flat tops have about 50% more bowl volume than the round tops. Coincidence?

[madkaw]

Do the needle assemblies on the SUs have a packing washer that can be removed to gain height?

[Jeff G 78]

Do the needle assemblies on the SUs have a packing washer that can be removed to gain height?

Yes Steve. I could try that.

[madkaw]

Make sure you don't have 2 washers in there- been there done that!

[madkaw]

You could even go crazy and redrill the posts for the float lower if there was enough there to do that. You would be lowering the Fulcrum point to gain height on the float

[zKars]

We are forgetting a couple of simple things. Check the level with the car running, ie fuel coming in and going out in the balanced dynamic state. This means having a site tube either in the drain (if there) or T'ed into the hose. Static settings of the float with the methods you're using are a good starting point but may not reflect the real time operating condition.

You can also take the bowl vent hose off and stick a Q-tip (no fuzz on the end in the bowl) in the nipple to probe the float to see a, if its free to bounce and staying steady when running, and b, what the level is, of the float, not the fuel level. Compare front to rear at the very least. A standard Q-tip (if there is such a thing) will have about 3mm of clean stick showing below the top fuzzy part with it touching the float top.

When the engine is running, the fuel is constantly flowing out of the jet tube as long as the venturi pressure is less than the pressure in the bowl above the fuel, the static fuel level is no longer "important". This only has a minor minor effect of the extra pressure drop in the venturi that has to overcome that extra 5mm of fuel rise in the jet tube during run-time.

If your running level is 5mm low in the float bowl, then Blue's ideas of increasing the storage capacity in the bowls will compensate well. Like most things, you can't really tell how the car will run until you do that long full throttle pull and see if the float bowl stays full.

[Jeff G 78]

Great ideas Jim. I will add a T to the bowl-to-nozzle hose and check the dynamic level as well as try the Q-Tip test. My front and rear float heights are different because it's a '72, but I can still tell what they are doing. The tops of the floats *should* be at the FSM specified heights of 16mm and 12mm from the lid while running, right? I do have bowl drains, but I can't think of a good way to add a fitting without leaking. It's a small metric threaded bolt in the drain so other than drilling the center bore of a stud, I can't think of another good way. That might be tricky to get right in my drill press. T-ing the supply line will work as a quick test.

If I am still 5mm low and I can't fix the condition, should I try to compensate by giving the mixture knobs an extra few turns out, or will that just cause other issues?

[Jeff G 78]

Yep. I checked.

[zKars]

Don't play with the mix knobs to compensate. Mix adjusts AFR, not the supply volume. The running condition doesn't care about the static level. Low level means less in reserve when you need it during high demand periods, that's all. Do what you can to get the dynamic running fuel bowl levels close first. T into that bottom little hose with a vacuum T, zip tie the clear hose to the side of the bowl, leave it there!