• 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
Tips
Tips

Adventures in carburetion...

Share this page

Nunyas

Nunyas

Yoda
Country flag
Offline
Hi,

I just rebuilt my carb (ZS 175 CD-2S). Installed it on the car, turned on the ignition and waited for the pump to stop, but it never stopped. So, I looked under the hood and saw fuel flowing out of the jet (the air filter was off). Ok, so I removed the carb went back to the bench and consulted my manual. I readjusted the float arm, refitted the carb to the manifold, and turned the ignition on again. This time the pump clicks a few times then stops. So, I figured I got the adjustments correct this time. I tighten everything down, and start the car. She runs for a little, but I can't get her to idle under 1500 RPMs, because below that she stalls. Also, while pondering what may be causing that issue, I found that the line that goes from the top of the carb to the charcoal canister had some new moisture on it. So I check it out and find that it's gas. So, I disconnect the line and turn the ignition back on. Now, the pump doesn't fully stop any more and is slowly pumping fuel. If I'm not mistaken, the float should have closed the Grose Jet long before the fuel reached the height in the carb that it did. However, what truely boggles me is the fact that this didn't happen until I had run the engine and gotten it warmed up. I suppose I can try readjusting the float arm or trying a different shim under the Grose Jet, but I'm uncertian that this will actually resolve the issue.

Does anyone know what might cause the bowl to operate initially, but then flood after a few minutes of operation?
 
Hi Nunyas,Yes I think you are on the right track, try a thicker washer under the Grose Jet,as I remember they are a little bit shorter than the old needle type and be sure you are using the correct spacing for the float arm.--FWIW---Keoke PS while the bowl is open shake the float and make sure there is no fuel inside!
 
thanks Keoke. I think i fixed the flooding issue now. I went with a spacer that's about double the thickness of the first one, and remeasured the float height. I got the car to run for a bit, but she still wants to stall after a few minutes. I think i'm going to call it quits for the day, and pick back up tomorrow. Starting with the valves, I think i might still have those a touch too tight. Then I'll see if I can caress the ignition timing a bit too. and finally 'tuning' the Zenith...

Quick question... I've recently started to use medium grade octane fuel, and I read somewhere that it burns slower than lower grades. Is that a mistake to use the higher grade fuels in B's? I didn't notice any "knocking" prior to switching to it, but thought that maybe it'd run a bit cleaner if I did... Is my assumption a misconception?
 
By any chance did you remove the float from the carb? When I rebuilt mine I ran into a similar problem and it turned out that the fit between the shaft the floats ride on and the 2 prongs that hold it in place became rather loose and would pop loose allowing the carb to flood. I tightened up the fit and reset the float level to spec and that problem was solved.
 
yes, I did remove the float. Although, in hind sight I think that was a step in the rebuild that I didn't need to take. The spring clips that hold the float pivot shaft on my carb were still good and required me to use a flat blade screw driver to get the float back into position. I think my problem was that the spacer that I used with the new Grose Jet was just too thin, because I put a thicker one in and the flooding seems to have gone away now.
 
Quick answer /ubbthreads/images/graemlins/smile.gif Given the poor quality of fuel we have available now, the higher octane fuel is preferable.--FWIW---Keoke
 
One more thing concerning my carburetor. While I had it disassembled, I looked at the metering needle that goes into the air piston again. I noticed that the tip of the needle seemed to look as though it were blunted and certianly did not come to a fine 'sharp' point at its tip. I believe its size is 45H. So from this information I have 2 questions. 1) Is the needle damaged and should be replaced? and 2) For those of you in Cali, is this the correct size needle?

I wonder about the needle because I recently tried to get smogged, and I had the carburetor tuned really lean but still failed the CO emissions (6.66% /ubbthreads/images/graemlins/eek.gif ).
 
Nunyas;
Most Needles do not come to a sharp point on the end rather they are some what rounded or moderately tapered off.I could not find that needle in my available references ,Perhaps some of the other MG owners can come to our rescue.--FWIW---Keoke
 
wow... had a learning enlightenment into the behavior of my '76 B today. After reading the "Why Dual Carbs?" thread, particularly areog's comments on how aircraft engines with a single carb run as the mixture is leaned out, I couldn't help but notice his descriptions of the idle quality described how my B has been running 'to the T' lately. So, I revisited my mixture settings and enriched it as much as I could. The result was after initial warm up I can now set my idle lower than I've ever had it set in the past (around 600 to 750 RPMs) and the engine idle stays as smooth as silk... well as smooth as silk as B engines get anyways. The entire past year I had pretty much given in and resigned to the engine not wanting to idle under 1K, because it would always start getting extremely lumpy (shake the car violently lumpy) down there.

She's still not perfect yet, but this was a huge step in the right direction for me. I think I may need to revisit the float bowl again, because when I took her out for a 20 minute test spin she started to hesitate ALOT under acceleration about half way through the drive, and sometimes even stall when she came back down to idle. The hesitation was also still there once we were "at speed". This hesitation under acceleration and at speed (after a few minutes of driving) is the only thing holding her back from "perfection".

I may look into the flow from my fuel pump too, because after every drive (like tonight's) the under bonnet fuel filter is always 'dry' when I check it with the engine still idling. When I say 'dry', I mean you cannot see a single drop of liquid in it. However, when I switch the engine off but leave the ignition on (and therefore the fuel pump) the filter will fill back up slowly (over a period of a couple of minutes). I realize it won't always stay "full", but it should never appear to be completely empty, should it?
 
I would say that checking the floats is definitely a good idea. If they are sticking closed then no matter how much your fuel pump is pumping, you will not be getting enough gas to the engine nor see gas in the fuel filter. This would also cause the car to run ultra-lean which might have been off-set by going to a 'full-rich' setting.

Unhook the fuel line and stick the end in a pickle jar or similar. Turn on the ignition. If the pump starts filling up the filter and jar, you know it is float bowl adjustment. If there is no or very little flow, it may be the pump afterall, but my money is on the float adjustment.

If the grose jet is not opening, then there will be pressure in the line and the gas won't advance even to fill up the fuel filter. If the jet allows gas to just trickle by, the car would get just enough to run, but have flat spots when you're consuming more gas than is being delivered and, when shut off, would allow enough of a trickle to fill the fuel filter. If the jet is opening (and closing) properly, gas should fill up the fuel filter, and bowls and then stop moving. Then you will be able to "lean" the car back out ("you've got to be lean to be mean") and still get that low idle. Hope this helps!!
 
I was just rereading several message threads, and seem to recall someone giving out a figure for an adequate flow rate. However, I cannot find the post that had the flow figure. The closest I can find is a vague statement of "plenty of flow". So, does anyone have a good flow rate ball park number that I should expect to be near when I go to pull the line of my carb? Are we talking free flow rates of Gallons Per Minute?

I'm not 100% sure it's going to be the float, because the car ran better than it ever has in the past for several minutes before it started acting lean again...

lesingepsycho - If I correctly understand what you're saying, then the in-line filter will drain as the float bowls drain, but it won't refill unless the bowl refills? I'm not too sure I follow. The in-line filter is before the grose jet, so shouldn't it retain some visible amount of fuel in it at all times (assuming the pump works as it should)?

One last question... If the flow rate turns out good, and I have to readjust my floats again, is there a truly reliable method for measuring the float height on a ZS carb? When I rebuilt it I ended up using a metric ruler and a straight edge. I layed the straight edge across the floats and held the ruler on the carb's body (on a part where the float bowl screws into the body) to measure the float height. However, I found this to be tricky to get an accurate measure, because the weight of the straight edge would push the floats down a small amount. Anyone have a more reliable method for measuring the float height?
 
I just remembered something about the way my car is currently behaving that may have some impact on my fuel supply/carburation trouble shooting... Ever since I rebuilt my carb, the choke has been acting 'odd'. The PO changed the stock choke out for a manual choke... actually I think it's a conversion kit that used most of the original choke assembly. When I pull the handle out part way the idle goes up very little. However, when I pull the choke handle out all the way the engine runs extremely lumpy (shake the whole car kind of lumpy) and doesn't want to keep running. While I was examining the functioning of the choke, I noticed that nothing that I could see visibly happened for the first half of the choke's movement. The last quater to half of the movement engaged the butterfly. I didn't go any further than removing the cover to the choke to see what happens when I use it, and made sure everything was as it was before when I put the cover back on. Anyone know what this behavior might mean? Could a low float bowl cause this to happen?
 
Perhaps a small analogy can help to understand why no fuel will arrive at the fuel filter if the jet is closed.

Think of a brake line with an air bubble in it. When the bleeder valve is shut, the line is under pressure and the air bubble can't escape. When you pump the brakes, it just compresses the air in the line, but the bubble doesn't go anywhere. To get rid of the air bubble you have to open the bleeder valve to allow the liquids (air is a liquid too) to flow. Now when you pump the brakes the fluid moves forward until the air bubble has been pushed out.

So, the air bubble is like the fuel filter, the bleeder valve is like the grose jet, and your pedal pumping action is like the pumping action of the fuel pump. Until the grose jet is open and allows the fluid, in this case gasoline, to move forward, everything stays put including a fuel filter full of air.

I think this answers what you are asking and I hope the analogy helps it makes a little more sense. You could also make the same analogy with a garden hose, household plumbing, electricity, all sorts of things. Good luck!
 
[ QUOTE ]
I was just rereading several message threads, and seem to recall someone giving out a figure for an adequate flow rate. However, I cannot find the post that had the flow figure. The closest I can find is a vague statement of "plenty of flow". So, does anyone have a good flow rate ball park number that I should expect to be near when I go to pull the line of my carb? Are we talking free flow rates of Gallons Per Minute?

[/ QUOTE ]
The free flow rate should be a minimum of seven gallons per hour. This is a little less than one pint per minute.
D
 
well, I've just spent several hours tinkering with this stuff. First I checked into the "free flow rate". The free flowing fuel pump filled a "wide mouth" mason jar (1 pint i think) in about 17 seconds. Seems to be flowing ok... i think.

My best eyeball measurements for the float height looked to be 19mm when I opened the float bowl. So I readjusted it, and remeasured the float height around 16 to 17mm. I measured, remeasured and remeasured the float height several times, and cross referenced it with my shop manuals more times than I care to count this morning in an attempt to ensure it's at the proper height. Haynes carburetor says 16 to 17mm; Haynes MGB manual says 15.5 to 17mm ... I got it set to 16.5 to the best of my abilities and tools.

Reinstalled the carb, but left the fuel line off. Switched on the ignition and held the fuel line in a vertical position and un-plugged it. As soon as the fuel started to come out of the top of the line I replugged the line and reconnected it to the carb. As the float bowl refilled i kept an eye on the filter. The filter was about 80% full when the float bowl finally cut the flow.

I took her out for a spin and she seemed ok at first (a little lean but still ok). As I continued driving I noticed that the longer I drove the more and more lean she acted. After 20 or 30 minutes I got her back home and checked under the hood and sure enough the fuel filter was completely empty again.

Assuming this is still a float bowl problem, I just noticed that the float heights in the manuals are for needle valves, and not Grose Jets. The heights should still be the same right?
 
Ok, I just went back out and checked my inline filter. After just sitting there with nothing on the filter seems to have 'magically' filled itself back up to what it was before i started my test driving earlier today (80% full)... i'm not sure why it would fill back up without the pump pushing fuel to it... anyone have some insight into this phenomenon?
 
There are two issues with floats & needles. The first, is that the needle has to shut the fuel off when it reaches the correct level IN THE BOWL. The float to body measurment "level setting" is an indirect specification which is intended to give the correct level in the bowl, provided that the inlet valve & seat are of the "original intended design" The second, is that the needle has to open wide enough for sufficient fuel to flow to meet the engine's requirements. "Float drop", which may not meet specs.

I think one or the other or both of these items may be a problem. The fact that you have had to experiment with washers under the jet seat indicates that the needle - seat combination is not ideal for your carb. There are a number of Grosse needle & seat combinations thay may fit into your carb, but only one of them "might" be correct. Grosse jets, though popular with some people, are not exactly noted for giving the correct float geometry without a lot of fiddling around.

I personally, would replace the needle & seat with known correct new original design parts, set them per the shop manual, & see what happens.
D
 
Nunyas, try readjusting the float level to 15 mm and see what happens. Also if you have a fuel pump pressure guage I would install a "T" in the fuel line and monitor fuel pump pressure while driving it should have a nominal value of 3 PSI. ---Keoke
 
I'll try readjusting the float again when I get home tonight. However, I do not have a fuel pressure guage in my tool box. Are these available at local parts store chains? And approximately how much do they run?
 
Yes Nunyas, they are usually available at most discount Auto stores. I think one good enough for this type test should be obtainable for under 20 bucks.---Keoke
 
Back
Top