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MG Midget and Sprite Technical Flooding carbs BBS discussion at MG-Cars.net
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MG Midget and Sprite Technical - Flooding carbs
| Hi folks I've been doing the 36000 mile service this weekend, not much more to do. However, I've just run into a major problem. I took the car out on a test run, only about a mile. It smelt very stongly of petrol. On stopping I saw that there was petrol pouring off the tops of both the float chambers.(HS2 types) Now, I have made an amendment, in that I have done away with the fragile looking plastic inline filter, replacing it with a straight through length of fuel pipe. I haven't disturbed any other parts of the carb except to oil the linkages and to top up the dampers with engine oil. It has a known recent history of tending to backfire when trying to accelerate under load, but only when the engine is cold. It needed choke to start as usual, too little for the first mile causes it to lose power and backfire a lot. I noticed when checking it that not only was the petrol pouring off the top of the float chambers, but it was smoking slightly from where the spindle with the butterfly valve protrudes on the front carb. Please could someone advise me what to check for? Thanks, Dom Picture 1 - showing the inline filter.  |
| Dominic Excell |
Picture 2 - showing the new fuel pipe
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| Dominic Excell |
Picture 3 - showing where the fuel was collecting prior to washing down over the heatshield, perilously close to the exhaust! Just between the 2 uppermost screws and below the vacuum pipe shows where the smoke was seeping out.
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| Dominic Excell |
| first thing you could try is once it's running hit the fuel bowls hard with the handle of a screw driver in case the needle and seats are stuck or have crud or have been 'improved' to those Grose ones if that don't work you need to take the needles out and clean as required bowls too, check adjustment of float bowl as you know personally I put back a new fresh fuel filter and you can save money by putting in the smaller size, I've got one on mine and it doesn't hold back for high speed use (Mann 32/1 - and that's off the top of my head, I must be fixated) very unlikely I know but also just check the connections and fuel lines to the carbs for cracking and leaking - I had so much rubbish rubber fuel line on my previous classic from different suppliers and batches you wouldn't believe it and get them foam air filters cleaned (if they don't fall apart) :) |
| Nigel Atkins |
| Dominic, Check for bits under float needles valves /seats Check needle valves for wear / ridges and replace if worn. Check float levels and clean out chambers - all generally Nigel has said. Also worth checking the pump is an SU version or the same low pressure type as originally fitted if an alternative. Did the filter have much crud in it ? I.e rubbish from tank now getting to carbs ? R. |
| richard boobier |
| Dominic, ISTR that when you bought this car the seller warned you that it occasionally suffered from a sticking needle. I suspect you thought he meant a carb fuel jet needle but it is more likely that he meant the float chamber needle. And if both fuel chambers are overflowing it is likely that they have been fitted at some time with grose needle valves, as Nigel mentions. They are notorious for sticking. I would replace them with standard needle valves in both carbs, and check/ adjust the float level settings at the same time. It may well be that in removing the fuel filter, you have dislodged some dirt in the pipe and this has triggered the problem with the jets. However, unlike Nigel, I never use a fuel filter. I don't think they are really necessary with the quality of fuel in the UK. I have never had any problem, and it is one less thing to go wrong. |
| Guy Weller |
| I can't disagree with Guy (on this occasion at least :) ) about adding in another thing to go wrong with a fuel filter but what you can get in with fuel from a garage pump can be surprising and if you're changing the plastic filter every 12 months/12k-mile then you're unlikely to get trouble from it (the look of the one that was fitted was that it probably went back to when the engine was rebuilt), plus with a car you don't know that had little use who knows the state of the fuel tank and petrol pump unlike your saloon car Minor with a single carb you're now into a sports car with (potentially) troublesome twins, the SU carb is supposed to be very simple but what isn't in doubt is it's lack of fuel efficiency by modern terms so don't expect the carbs exterior to be forever totally bone dry even allowing for the excesses at the moment |
| Nigel Atkins |
| I think the arguments for and against having a fuel filter are probably pretty evenly balanced. I suspect that the less a car is used, the more the benefit is in having an in line filter. Fuel left standing in the tank is likely to pick up more sediment. And it also seems likely to me that a part-filled tank that doesn't benefit from a daily "swishing" with fuel, will deteriorate, releasing dirt into the fuel supply. |
| Guy Weller |
| Thanks for the input chaps, I'll have a tinker when I get home from work tomorrow. Though the carbs were bone dry when photographed, they were awash with petrol overflowing the lip on the float chamber lid, and trickling in a steady stream down by the exhaust. Fun indeed... The filters are coming off anyway at the end of the week Nigel, servo is coming out and new filters in the original housings are waiting to go in. Cheers for now! Dom |
| Dominic Excell |
| Dom, you only want to worry when there so much petrol dripping that the exhaust can't burn it all off :D if you're going back to the old saucepan filters, which are OK, then have a look for Mann filters for them from local motor factors as they'll probably be less expensive than the specialist suppliers and more than adequate to do the job for the 12 months/12k-mile life between services which also reminds me if you're going back to the ancient tap valve (as I have on mine) then the one from Ashley Hinton (on eBay) is probably by far the least likely to leak but you'll also need to add a spacer copper washer, not supplied, as you have the later heater box and the angle that the tap points towards it is different to the earlier models just like the Minor there are lots of differences between the models years despite them all looking similar, hence the need for reliable sources of parts info (factory parts catalogues) |
| Nigel Atkins |
| Thanks for that Nigel. Bought 2 pairs of new old stock unipart filters on ebay for just over £5 the pair plus postage, so they should keep me going a couple of years. I'll be getting the Ashley Hinton tap, thanks for the tip re copper washer. |
| Dominic Excell |
| As Richard said, worth checking the type of fuel pump fitted. My last midget had a non-functioning pump when I bought it. It had been off the road for five years. I fitted a spare pump that I had - one of the electronic pumps with a blue end cover. It suffered from poor running and flooding carbs. I replaced the needle valves, but no difference. I rebuilt the original fuel pump and it solved the problem. I concluded that the aftermarket pump had too much pressure for the needle valves to seal properly. |
| Dave O'Neill2 |
| Yes Dave, blue covered electronic pump recently fitted to this car. I'm therefore now on the lookout for an original SU. Bought some new old stock SU float chamber needles and valves along with the gaskets today. Still some on ebay if anyone is interested! I'm wondering if this problem is in part self-inflicted. When I replaced the flexible fuel pipe I had to dress the end of the pipe as it had been fairly well butchered in the past. I filed and then sanded it smooth, and (so I thought at least) cleaned away any traces of filings. Maybe a few got through.... I suppose there could also have been some crud in the old stock fabric covered pipe. A mate used to have a seagull outboard motor - he found eventually that the cause of lack of fuel flow was a small dead spider stuck in the carb... |
| Dominic Excell |
| >>blue covered electronic pump recently fitted to this car. I'm therefore now on the lookout for an original SU<< why do that until you've checked the needles and seats, if it's a case or originality you'll have a list of things to alter that are fine as they are, if it's for the 'fun'(?) of messing about with the points on it then surely that wont be for many years if the pump is new I've had a QH QFP171E on for 5 years now with no problems that I'm aware of but I've had previously problems with Grose needles and other new needles, one out of a set of two new needles I replaced with a s/h spare a mate had and that stopped the overflows I'm lost as to which pipe it is you had to file and sand (copper) but next time just cut off the ragged end |
| Nigel Atkins |
| Nigel, it was the copper line from tank to engine compartment. I couldn't be a*sed undoing the clip holding it in place so as I could trim it back with a mini pipe cutter. << one of the electronic pumps with a blue end cover. It suffered from poor running and flooding carbs. I replaced the needle valves, but no difference. I rebuilt the original fuel pump and it solved the problem. I concluded that the aftermarket pump had too much pressure >> Just wondered if that might be part of the problem. |
| Dominic Excell |
| >>couldn't be a*sed<< yeap we've all been there and often regret it and often end up having to do more work than if we'd done it properly first time or worry about what problems the short cut might cause as for the pump, it's best to change one item (or set) at a time to see what resolves the problem and if you have a new and working pump why not leave it for the time being, lot easier and cheaper to start with the needles especially if you're using the car my Q&H blue ended electronic pump doesn't seem to be a problem in my car set that I've noticed carbs are (to others) simple and stable items but they're not very efficient with the overall petrol comsumption |
| Nigel Atkins |
| I have one of the blue ended electronic pumps too, and it gives no trouble. I think that with the correct float chamber needle valves (i.e. NOT Grose jets) the SU carbs are reasonably tolerant of different fuel pressures. If /when changing the valves in the carb float chamber lids I would also follow the SU advice about setting the float height accurately. After which you will probably need to readjust the fuel jet setting in the venturi. Check out the guidance notes for SU carbs on the Burlen.co.uk website. |
| Guy Weller |
| Unfortunately Moss advertise the Grose valve as "superior design" whereas all the feedback on them suggests otherwise. I bought one to stop flooding based on the Moss recommendation and removed it half an hour later! It was flooding more than the original. I think it was too sensitive to engine vibration. Standard "cone" needle and seat is much superior! |
| G Williams (Graeme) |
| Hi Lads FWIW More info on Grose needle and seats: Were originally made here in Aus (S.U Midel)and supplied to the usual suspects Usual suspects have now sourced cheaper oriental supplies but still cxalled "Grose" My queries to SU midel were answered by saying that indeed the fuel pump is generally over-achieving SU Midel can supply better ones (their claim) They can also supply SU parts e.g. butterfly shaft bushes @ AU$ 7-00 each Hope this helps https://sumidel.site-trust.net/index.html Cheers Rod |
| R W Bowers |
| Carbs are now off the car - ready for careful stripping and checking tomorrow. Looks like the nrrdles and jets do need centreing - will have to see if there's any wear. |
| Dominic Excell |
| Dominic, If you haven't yet come across them, the SU technical articles from Burlen are well worth looking through: http://sucarb.co.uk/technical/ Guy |
| Guy Weller |
| Thanks Guy, yes, I have a copy of both the Burlen SU manual, and also the Haynes SU manual. I stripped my HS2 Morris carb some years ago, so have a vague idea what needs doing... I'm sure the memory will kick in as I proceed! :-) |
| Dominic Excell |
| Well folks I've partially dismantled the rear carb. The manifold was showing signs of blacking inside, presumably from the backfiring when cold. Some mid-brown sediment in the float chamber, the float needle cuts off beautifully if blowing down the inlet tube. Two potential problems found. 1) Upon release of the choke lever, the jet doesn't always fully spring back into the fully raised position. PO mentioned it stuck occasionally. 2) A small amout of wear at the top end of the needle. See photos. Reckon it's needle replacement time? Apart from that thickest 1/4", it looks fine. Cheers Chaps, Dom.  |
| Dominic Excell |
and here's the other side of the needle
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| Dominic Excell |
| sticking choke on the return.... I had that... to be honest im not sure as to the factory set up... but I did what everyone else does... add a spring to keep the pressure on the choke linkage...it works I see cars on.ebay and many other photo sources and they dont have the so called choke spring in place...so im not sure if it came from the fzctory with a spring or not... I put the spring on the front carb linkage like all the others ive seen FYI prop |
| Prop and the Blackhole Midget |
| Yeah, there was an added spring - but still hanging up. Just checked the float clearance measurements between top of float and u/s of lid, should be between 3.2 & 4.8 mm - actual measurement is about 1.5mm. Also the pin holding the float is badly worn. I guess new parts needed... |
| Dominic Excell |
| wow...dom, 1.5mm isnt exactly chicken feed my guess is there is SO MUCH play fir the float needle to move up.and down that the needle maybe falling out of the seat housing and getting caught up do you have the adjustable floats or the HS2 solid plastic floats I to had worn out float pins... but moss and victoria british wanted to.rape me without any lube for the shipping cost. I took the pins to the local hardware and craft store and was able to match up 18 inches of brass rod in.the same.dia...I cut them to length and bashed one end with a hammer to prevent it from sliding out... installed and bob became my uncle but I think if you can increase that 1.5mm gap... you will be much happerier hmmm... a last thought... at 1.5 mm im betting the angle of the float has changed and may even be riding the walls inside.the float chamber also prop |
| Prop and the Blackhole Midget |
| if the float is riding the inner walls of the float chamber that could create the same effect as a sticking float needle and if the float needle isnt sealing that would explain the float chamber as being empty bottom line dom, dont let these carbs scare you... once you play with them a bit, they really are a cool piece of simple enginerring... there like smartphones... the more you play with them the easier it is to love the little guys for what they can do prop |
| Prop and the Blackhole Midget |
| Plastic floats here Prop. I've almost decided to bite the bullet and get replacement floats, pins needles etc and get them working well again. I suppose improved fuel consumption will pay for the parts within a year! (Petrol here is about $8 per US gallon) The only scary part is tuning and synchronising them! Dom |
| Dominic Excell |
| These John twist videos are excellent at giving information, some may not be exactly for the model of carbs in your car but the basic principles apply and all full of useful information 44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog&feature=plcp 43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc&feature=plcp 29 SU H-Type Carburetters - http://www.youtube.com/watch?v=GHNlT5yHDxk&feature=plcp 41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA 147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0&feature=plcp 35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&feature=plcp |
| Nigel Atkins |
| ETA: - and of course there's information in the Driver's Handbook |
| Nigel Atkins |
| There's not a lot that can go wrong with plastic floats, unless they split. Otherwise there isn't much point in changing for change sake. Similarly with the pivoting pin. A bit of wear there doesn't matter because the slack is automatically taken up as it floats. So unless you are a psychotic perfectionist, then just get new needle valves and reset the float clearance using fibre washers to shim the valves out, giving the required 1/8" to 3/16" clearance. Probably also worth getting a new gasket for the float chamber lid as they do tear so easily. |
| Guy Weller |
| Dom, Before you put in a parts order, check your throttle shafts and shaft bushings. I think that is where you said you were seeing wisps of smoke coming from. The tune and sync will be a far more pleasant job if the carbs are in good proper condition. And, if air is leaking at the throttle shafts, setting the mixture is just a guessing game. Charley |
| C R Huff |
| hey dom check the plastic float that the self that the needle rides agianst isnt broken...ive got the plastic float as well and that self broke right out...it would sure account for the 1.5 mm clearance or lack of that you have...I think the plastic gets brittle over time. ill see.if I can find a pic or the old broken all plastic float so you can see what im referancing...no there not repairable that im aware of. I replaced my solid hs2 plastic floats with the plastic bodied but metal clipped /shelved HS 4 floats with good sucess and they are adjustable just a thought prop |
| Prop and the Blackhole Midget |
| unfortantly...my old.broken plastic floats are at the shop or the land fill... and my older photos are on.the laptop buried in.the closet... but if you.got only 1.5 mm of clearance on unadjustable white plastic floats...im strongly thinking that little shelf is broken off that the end of the needle rides on.... if its not... then those float pins are super seriously warped, bent or damaged to pull that much gap out prop |
| Prop and the Blackhole Midget |
| Carbs are now cleaned and reassembled, with new needle valve kits for the float chambers. The main needles and jets are now properly aligned, as far as moderate wear will allow. The throttle spindle on the front carb has a fair amount of slop, so that was where the smoke was coming from (probably from the oil that I had just lubricated it with!) I guess a new pair of carbs will be needed one day... but they ran well enough apart from causing backfiring / misfiring when the engine was stone cold. I think the sticking jet problem (it was hanging up sometimes from when the choke was operated)was simply misalignment in the main jets. Fingers crossed that it'll work better now! Thanks again folks for all your advice and comments Dom. |
| Dominic Excell |
| Dominic, Very often most of the throttle spindle wear seems to be on the brass shafts - the alloy seems to wear much less. Therefore just replacing the shafts with new can (and did for me) take up most of the wear and give a good very servicable result. R. |
| richard boobier |
| richard is dead on... if your butter fly shafts are worn... just a new shaft is 89% of the time is all thats needed...if you dont.... these worn out shafts will suck air and make tuning a real hair puller...btw, do you have pop it valves on you butterflys ?? if so now is definatly the time to replace the butterflys with non-popit valve butterflys as your replacing the spindles anywy...besides there not that expensive...make sure they are from bulen and not china... we saw a rash of butterflys that were not acouple years ago that where little more then round brass slugs btw.... what float gap did you end up with on the underside of the floats?? prop |
| Prop and the Blackhole Midget |
| That's useful to know Richard, thanks. I'll get on to Burlen on Monday. Just plain butterflies Prop. Regards, Dom. |
| Dominic Excell |
| Float gap I managed to increase to just over 4mm - about halfway between max and min tolerances. I used thin gasket material to make some washers. Fibre washers I had were far too thick giving a gap of nearly double the maximum! |
| Dominic Excell |
| Hi Dominic, a small closed rubber grommet just fits over the worn outside ends of the butterfly shafts and seals very well, a tiny dob of superglue ensures they do not drop off. Keep a spare in the boot if one does! This worked a treat on one of my previous midgets. Makes mixture adjustment easier! Regards Tony |
| Tony G |
| hmmm... is there any way we can see some photos of the modified floats...esp the area you worked on. prop |
| Prop and the Blackhole Midget |
| ding ding ding... we have a winner!!!! congratulations tony... your the new hillbilly redneck achiver of the week award....hahaha I wish I had thought of an O ring to seal the shaft..thats a good tip prop |
| Prop and the Blackhole Midget |
| It's all reassembled now Prop, sorry!. I simply removed the brass valve unit, made a gasket paper washer to fit, and then fitted a new valve in place. It was a 0.6mm thick gasket paper which when compressed seemed about right. I like the idea of the rubber bung as a temporary fix Tony. I might use silicone sealant to fix it in place rather than superglue - as that can wick into places you do not want it by capillary action! |
| Dominic Excell |
| Hi dominic, silicone makes the seal too slippy and takes too long to set. A tiny ring of superglue (the gel type) on the outer edge, away from the shaft, with the closed grommet held in place for a few seconds, works very well. Lasted for 6 - 8 years on one of my previous midgets, before I sold it - they are probably still there! The grommets do stay in place without the glue if you find the right diameter. The area must be clean though. Regards Tony |
| Tony G |
| Up and running again! I was very careful not to disturb any of the carb settings, measured the exact gap between adjusting nut and jet fixing nut so I could replicate it. All seems ok for now, no doubt there will be adjustments needed in due course. Use of the choke doesn't cause jet-jamming any more, nor are there any floods of petrol!Also seemed to pull away from cold better than before without backfiring. Nigel, I've also re-positioned the bonnet release to it's rightful place now! New static seatbelts fitted today also, the original ones were extremely frayed around the edges, much as I am a lot of the time! :-) Servo removal tomorrow, then I can refit the proper air filters, though I need to refit the radiator expansion bottle further to the front, as it's on the inner wing right now. |
| Dominic Excell |
| Dom, better for shelf access now you've moved the bonnet release but fitting those nasty spider's nest static seat belts, you are a one for the old stuff so, as such, and as you moved it - I think I did put before something like 'not that it matters but it's wrong type of bonnet pull', you've got a t-bar handle when it should be a round handle - now it doesn't matter at all both function the same but now you know, and liking the old ways, and originality - it will be on your mind - I'm evvvil, hoo, hoo, hoo, haaa ;) |
| Nigel Atkins |
| Dom What sort of belts did you fit? Sourced from where? |
| Dave O'Neill2 |
| Dave, I used MGOC Spares - securon static belts. Reasonable value at approx £50 the pair, and unobtrusive black which goes well with the black interior. I may well sell the originals, as they are no use to me. On my first Morris Minor, the original belts were in a poor state, so I replaced them and put the originals on ebay, all defects shown and noted, and they went for an astonishing £55. |
| Dominic Excell |
This thread was discussed between 24/02/2013 and 04/03/2013
MG Midget and Sprite Technical index
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