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Adventures in carburetion...

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I hate to be the one to dig up old threads, but...

I recently acquired a spare ZS (got it for $30 on eBay /ubbthreads/images/graemlins/laugh.gif) that I've scalped for several parts. I've also installed a new fuel pump that seems to be delivering much better fuel pressure than the one that was already in the car.

The Air piston needle was the first part I nabbed out of the spare carb, because when I compared the "spare" to the "original" I found the "spare" actually came to a point where the "original" was clearly blunted flat where the tip should have been.

The other parts that I scalped out of the spare were the float and float-bowl needle valve. I nabbed these because I remembered someone mentioning the float drop may not be enough(?). I've gotten them installed but the problem is still persisting (poor/hesitant acceloration with flat spots at higher RPMs). I used the washer (a relatively thick one) that was installed with the valve in the spare carb and set the float height to about 16mm. Despite the use of the different valve, washer, and float, the fuel filter is still completely emptying and not refilling while the engine is running.

I have to ask, how do you make the float drop the right amount to allow the fuel to flow enough into the bowl and keep the filter at a relatively constant level on these ZS carbs? The design places the valve very close to the pivot axis of the float which in turn would require the float to move more to allow the valve to open than if the valve were placed in a position further from the pivot axis.

Finally, since I can't physically alter the float travel (relative to the valve), does that mean that my only option is to keep setting the float height to a higher level (smaller measurement since the ZS's float height is measured upside down)? And keep doing so until the fuel filter no longer completely drains itself while the engine is running?

As always, thanks again for any words of wisdom that you guys might impart upon me. /ubbthreads/images/graemlins/smile.gif
 
If it was the float that was causing the fuel filter to never fill up(indicating the valve is stuck open or that the float is adjusted way to high so the valve never closes) then it would be flooding the engine somthing terrible. I would still suspect the fuel pump, or it is possible that this is the way that fuel filter normally operates.Some filters look empty but it's just that the fuel is running through and not collecting in them like it does when the engine is stopped.
Setting the float higher would actually let more fuel into the bowl and the needle valve would open sooner asking for more fuel, drianing your filter faster.
The fuel pump should be able to supply far more fuel than the engine is ever going to require, never allowing the filter to "run dry".
Regardless of the float level, as long as it's in the proper range, the enging is calling for roughly the same amount of fuel, and this shoulden't have a drastic effect on fuel delivery.
Just some "food for thought" Guv'na Good luck
 
Well, maybe I need to re-describe what happens when I drive my car. The fuel filter starts full, or at least very close to full with the exception of a small air bubble. I start the engine let it warm up for a few minutes, and then go for a drive. In less than an hour of driving the fuel filter will go from nearly full to completely empty. Turn the car off and let the car sit for a while and the filter seems to magically refill over a period of about 30 to 60 minutes.

The fuel pump seems to be ok. It will fill a 1 pint mason jar in a matter of seconds. If I attempt to plug the line by placing my thumb over the end the pressure causes a spritzer/sprayer effect, similar to holding your thumb over the end of a garden hose. So, I'm about 90% sure that the fuel pump is good. I'm also about 90% sure there isn't a blockage problem in the lines because of how fast it fills a mason jar.

The filter itself is one of those "universal" clear plastic body with a corrugated paper looking filter inside type units made by Fram. I replaced it when I replaced the pump. The filter is in the engine bay and installed at the end of the "hard" line before the rubber fuel line that connects to the carb.

If the lines and pump are good, then that only leaves the tank and the carb as possible faults. But if the fuel flow is good (which it seems to be) then there shouldn't be any problems with getting fuel from the tank. Which finally leaves the carburetor settings, and that's primarily in the float and mixture settings.

I suppose, as a long shot, I could be experiencing mild "vapor lock"? But that shouldn't happen since my pump is near the tank and is "pressurizing" the lines all the way to the carb. However, since the filter is under the hood and the filter has an air bubble in it, could it be possible that the heat radiating from the engine is causing the air "bubble" to grow as the heat intensifies under the bonnet? I've noticed it takes longer for the filter to "run dry" if I'm working on the car with the bonnet up, and it takes considerably less time for the filter to "run dry" if the bonnet is down and I'm driving around. Should I try moving the filter to a different location, like between the tank and the pump to see if it still happens there?
 
Just a late night thought from left field. Could it be possible after an hour of driving or so the pump it self could be overheating and reducing its flow thus draining the filter? Have you checked the flow rate after an hour of driving before the pump has had a chance to cool down? Also could it be possible since after shutdown the carb will increase in temp before it starts to cool down that the higher temp is expanding the fuel in the float bowl and sending it back down the fuel line to the filter? Ok its 3am and I'm going to bed now. I'll re read this in the morning to see if its some what coherent.
 
Ignore the visuals in the fuel filter. There is either an air leak on the suction side of the pump or the float valve is not letting enough fuel in. If a thick gasket is used on the float needle seat it will reduce the amount of available float drop (opening). The needle needs about 1/8" of opening travel (drop) or more. If the float needle has a small plunger & compression spring at it's float contact point, set the level with the spring not compressed.
D
 
I have one more possibility that seems reasonable to me...what is the condidtion of your tank. It MAY be that your fuel line or pick up in the tank is getting clogged as the engine runs and sucks more gunk into the pickup screen in the tank. If you have a drain plug, I would run the tank low and open her up and find out what is in the bottom of your tank.

Another possibility is that there is another inline filter...I found one betweem my pump and line...that is clogged or pinched.

Still, it sounds like a delivery problem to me.

Bruce /ubbthreads/images/graemlins/cheers.gif
 
I really don't think that the filter appearing to drain while the car is running is a problem (see if you can find another on in use to watch, I'm pretty sure that's just how they work). Sounds like you have plenty of fuel pressure. And you have replaced all of those parts involved without changing the problem.
other things to think about that can cause your symptoms are;
1.Ignition timing, Too much base timing advance, or a faulty vacuume advance, or a seized mechanical advance.
2.The secondary ignition system, Faulty wires,cap,rotor, plugs. or a coil that is weak and can't keep up at speed.
3. a vacuume leak or a plugged air filter.
try to take a step back, wipe the chalkboard clean, grab a pint, and be open to all possibilities. I've gotten myself locked in on certian things countless times, and missed the problem staring me in the face. When you finally find that gremlin be sure to smash him flat in your shop manual.then take a victory lap. /ubbthreads/images/graemlins/cheers.gif
 
question for you California smogger types... using the Gunson Gastester, what CO reading do you shoot for? My Haynes manual says 5.5% (at idle), but when I look at my emissions test sheet the CO limits are 1.46% (at 15mph) and 1.26% (at 25mph). What should I shoot for when I set the fuel and air mixtures?
 
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