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MG MGA - Carb float chamber overflow
| I've been battling with an irritating, intermittent problem with an overflowing float chamber on my 1600 Mk II that's been dragging on for ages. It only happens on the front carb, and for a long time I thought it was just the needle valve getting blocked by grit from the fuel tank. I've fitted an inline filter just ahead of the carbs, but it's still happening, and that still wouldn't explain why it only happens on the front carb. It tends to happen more often in slow or stop/start urban traffic. I've just been reading some articles about carb heatshields, vapor locks, and airflow onto the front carb, and I'm now wondering if it's possible that my front float chamber is getting too hot either through lack of air flow (I don't have an air hose fitted from radiator to carb) or insufficient heatshielding. Could the fuel be partially vaporising, or at least expanding, and causing the overflow, rather than the needle being stuck open? I'd be interested to know if this is possible in principle. Thanks for the help. |
| Richard Ross |
| You may have a worn needle and seat. On the otherhand you may have a partly sunk float - holed. Take the float out and give it a shake, you will soon know. Mine was holed (very fine pin prick). I held the float submerged in hot water to locate the hole, made the hole larger with a drill. Drilled a second hole to allow the fuel to drain, then soldered both holes. Worked fine ever since. Steve |
| Steve Gyles |
| Richard, Mine embarrassed me by gushing fuel three weeks ago but I discovered that the front float had gone porous and was filling and sinking. Luckily Burlen Fuel Services are 10 minutes away, so I replaced both floats and all is now well. Sorry, doesn't answer your question... Shane |
| shane rj |
| Sorry ... I should have said ... I have also done a complete rebuild of the carbs - the needles are new, and the floats are fine - I even swapped them from one carb to the other, and no change. Richard. |
| Richard Ross |
| Good to hear from you Shane nonetheless ... seems a long time since Silverstone last summer. Richard. |
| Richard Ross |
| And all those jobs still waiting to be done. Last week changing engine & box in the BGT. Must take the A out on Sunday... Shane |
| shane rj |
| Richard Think a lot of us have been here. You say you fitted an in-line filter. Is this one of the pretty chromed ones with a glass centre section with a mesh inside? I got one of these and it was useless I still had the rear carbie overflowing on occasions. I was convinced it was fine dirt jamming the needle. The fine magnetic dirt was passing straight through all the strainers. Note I said rear carb. This made sense to me as this one gets the lot, the front one kind of gets a cleaner overflow. I found I could get all the dirt out of the float chambers with one of those telescopic magnetic things. So I purchased a £1.50 standard clear plastic filter with a paper element. This solved the problem in 1998 and I haven't cleaned or replaced it since. I would still give the float a good shake. I bought a faulty one of those too once. Assume you have checked the height of the float arm for a 7/16" gap as per manual... Let's know what it was. Pete |
| Pete Tipping |
| Hi Richard. I had a similar problem with my MGA many years ago. Likely, grit is jambing your needle valve open, or your needle valve and seat are worn and sticking. I recommend installing a set of grose jets in both carbs. Installing a set of grose jets and a good fuel filter cured my float bowl overflow problem permanently! Well for about 20 years anyway. Best of luck! Glenn |
| Glenn |
| Grose Jets are good if you can find, like Glenn's and my own, a set that was made at least ten years ago. There has been some controversy about those made and sold more recently and not everyone buying the new ones is happy with them. But they are worth a try, and who knows? You might get lucky. Altho it seems a bit of a stretch to some, you could have a crack in the float bowl lid at the threads where the float valve seat screws in. I have seen this once, and the consensus among the other mechanics was that it must have been a poor casting (not unusual during the period these carbs were made) and/or some hamfisted DPO/DPM wrenched a valve seat in way too tight and cracked it. Normally, the seat should seal at least decently without a gasket. |
| Bob Muenchausen |
| Take a look at the forked lever that interfaces between the float and the needle valve. I think that if these are not shaped properley that the set up can jam when the float rises. Maybe compare the front to the raer. |
| J.H. Hall |
| what fuel pump are you using? |
| mog |
| Richard, after reading all of the above, I would try swaping the float bowl tops and see if the problem migrates. That would at least narrow it down. If the problem does not migrate, you are probably looking at heat as you suggest. You said you switched the floats and the problem stayed so the floats are most likely ruled out. I have gross jets in my A, but they are more than 10 years old. I think the above question may be looking for too much fuel pressure. You have probably already checked that. Again, I would try switching the tops of the float bowls. 5 minute job. I assume that they are identical on the 1600. Dennis, Az |
| Dennis |
| Thanks for all the suggestions ... been away for a couple of days. Pete ... you're right about the filter - it's one of the pretty ones - and there is still some grit coming through - the magnetic stuff as you describe. Maybe I will have to downsize to one of the cheap ones. Good to know. Glenn - I fitted Grose jets when I rebuilt the carbs. Do you know if the angle of the 'fork arm' is meant to be different when you're using Grose jets? Bob - no evident crack in float bowl lid. Mog - standard SU fuel pump so shouldn't be a problem Dennis - I'll try swapping the lids ... but are they not 'handed' - not sure if they can swap and still attach the fuel line properly. The likeliest problem still seems to be grit getting through the shiny filter ... but still can't work out why that might cause a problem only for the front pot. Richard. |
| Richard Ross |
| Richard Looking back at your opening comments, I can't see why localised heating around the front carb should make your car any different than the rest of us. In fact with my arrangement I have an MGB expansion tank fitted between the radiator and the front carb and I have no problems whatsoever. I also struggle with the grit theory. What is the state of your fuel tank? My money goes simply on a sticky/poorly seated fuel shut-off. Steve |
| Steve Gyles |
| Steve, I've seen the expansion tank in the pictures on your site ... so I take your point. There is still some very residual grit coming through, as I'm seeing it at the bottom of my float chamber. What I don't understand, and I'm struggling to work out, is why this should only affect the front carb ... ... your theoriy of a poor fuel shut-off is one I hadn't thought of ... the tank may well be in fairly poor condition. Richard. |
| Richard Ross |
| How old are your fuel lines? And where is your filter in relation to the carbs? If close to the carbs, then perhaps you are getting crud past either the filter (which may not be filtering) from the tank, or from the fuel lines themselves. I had a problem with the metal line that runs from my fuel pump to the hose connection flaking off some sort of scale-like deposits which found their way into the float bowls. Where it seems to be only the front carb in your case, could it be some sort of contamination from the hose that passes fuel on from the float fitting on the rear carb? Many british cars which have the SUs above the exhaust manifold have a problem in hot weather with fuel boiling in the float chambers once the cars have shut down and heat rises. Where it seems to be only the front carb, I would wonder if your heat shielding is complete on the back side of the shield itself? The asbestos insulating material on the backsides often does disintegrate or disappear in time and this might explain a hot spot there if you have one. A good way to check the heat patterns in your engine compartment is with one of those new infrared thermometers which can take spot surface readings. If you can get access to one (or buy one from Harbor Freight) you might be better able to isolate the heat source if that is your problem and not the valves or floats. |
| Bob Muenchausen |
| I would go with the theory that the fuel line between the two carbs is deteriorating. The crude you are finding in the front card is actually the interior of the fuel line. This would suggest why it is only in the front carb that is affected. Try changing the hose between the two carbs and your problem should disappear. |
| Gordon Harrison |
| Richard, Sorry for giving you a bum steer. I was looking thru my Moss Catalog last night and the front and rear tops are NOT supposed to be identical. It seems PO must have lost or broken one and replaced with whatever was lying around. If you try swapping the tops, you will probably have to twist the tops 45 to 90 degrees so that the fuel line is on the outside so as not to interfere with the dash pots. Obviously, at 90 degrees, you probably will not have the air filters on. I always wondered why one pointed in and one pointed out on mine. Dennis, Az |
| Dennis |
| As part of my attempts to get to the bottom of this, I've now changed the fuel pipe between front and back carb. Chopped open the old one and it seemed to be in pretty good shape ... and the crud in the float chamber does seem to be more metallic than rubbery in character. As it's always been an intermittent problem, I'm still waiting to see if it goes away completely. Bob ... I'll try the heat shield temperature option ... worth a try, and I think you might be right. |
| Richard Ross |
| remember the su carb has a filter (if I remember correctly) on the bottom, also the su carbs had filters inside the top covers of the float chambers, the latter are long gone if yours are like mine. Best to fit a nice new filter just before the carbs, anyway good luck. |
| mog |
| Mog, The little thimble-shaped filters inside the lids are still there, although I don't think they'd even catch a fish - they're not very fine meshed. The grit is a lot smaller. |
| Richard Ross |
| thats your problem then, you need bigger grit ;) |
| mog |
| Richard If that fine muck is magnetic, that's exactly how mine was. It's definitely coming from the tank and the standard wire mesh strainers are useless for removal of this stuff. I fitted a cheap standard paper filter in a plastic housing instead of the useless "strainer" and as I say, have had no problems since. Somehow the odd particle must get clamped between the needle and its seat, stopping it from sealing properly and the fuel just goes on flooding through. You may find that giving the float chamber a smart tap when its flooding next time, will temporarily clear the muck as the needle jumps off the seat. Would that convince you? Please try a paper filter and lets hear what happened. Pete |
| Pete Tipping |
| Personally, if it is from the tank I would treat the cause rather than the effect. There are some good tank sealants on the market. Steve |
| Steve Gyles |
| OK Pete, it's done. I changed the shiny filter for a paper one today, and cleaned out any remaining magnetic grit from the float chamber - I was amazed how much had gathered in the short time since I last cleaned it out (less than five miles on the road since I did it last). In the process of removing the glass/chrome filter, I discovered that the inner part had loosened, so the cylindrical gauze filter was effectively open at one end, which would explain how the muck was getting through. Anyway - too many moving parts, too much possibility for failure, so it's gone. No problems so far, but I'll need to give it time to be sure the leak is cured. However, I'm pretty sure you're right about the cause being the tiny bits of grit getting caught in the needle. Thanks for everyone's help on this one along the way. Richard. |
| Richard Ross |
| Well done Richard! Pretty easy job having already fitted that useless strainer wasn't it. As I say, I fitted the paper filter around 1998 and haven't touched it since or had a problem. When the tank leaks. I'll change it then! Pete |
| Pete Tipping |
| Painfully simple - no tank change required - yet! ... all it needed to get to this stage was a complete carb rebuild, a search for a 'pretty' filter, a new fuel pipe from front carb to back carb, and an awful lot of time. All that to discover that all I needed was a £2.20 filter from Halfords. You live and learn. |
| Richard Ross |
| Richard I go back to what I said earlier, you are treating the symptons rather than the source of the trouble. Don't forget that all this stuff is passing through a filter in the fuel pump and some 12 feet of pipe. You may be just storing up trouble for yourself. What if your motor suddenly stops on the motorway? The £50 tow away charge is going to be expensive compared to a few hours work of removing the tank and cleansing/sealing. My opinion for what its worth. Steve |
| Steve Gyles |
| Steve, You are right - and I take your point. This can't be doing the fuel pump any good either. I will think about sorting the tank but in the meantime I'm just relieved not to have a flood of fuel uncomfortably close to the manifold. Richard. |
| Richard Ross |
This thread was discussed between 21/04/2006 and 03/05/2006
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