MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuel system... little help

I have the dual SU carbs. And the outside floater for the left one (looking at them from drivers side) just drains fuel out of the bottom. The engine turns over, but wont ignite... I know its not getting fuel because I see it all drain out the bottom. I was wondering if anyone had a diagram of the fuel lines and such so I can see why its draining... Heres a little info to help out. I sprayed carb cleaner in the hole in the bottom of the car and it shot out into the floaterbowl... so I assume the line isnt clogged, no carb cleaner was lost either out the bottom. Why would fuel be draining if the Carb cleaner doesnt? Could I just be flooding it and the gas is bad? IT just drains out like the line isnt even attached... Any help is appreciated.
Bryan Corey

Ok Bryan: We're here to help: what year is your B?
Do you have a Moss or Victoria British catalog handy? There are illustrations of the various parts in these that will help explain things. Parts of the Moss Motors catalog is online, and if you go to their site you can also request a free one.
From your descriptions so far we need to clarify what you mean by "bottom". I think you mean the top of the jet into which the needle goes up and down to meter fuel. It sounds like the float needle is sticking open either because it is bad or the float is bad. Floats can split and fill with fuel, then they dont float anymore and fuel will run out of the overflow pipe.
Andrew Blackley

no no, the jet isnt getting ANY fuel. The floater OUTSIDE the carb has a line that runs out the bottom and then up into the jet which shoots it into the carb.. right? The fuel drains out before traveling that line between the floater chamber and the jet. Its a 1970 mgb roadster. The fuel just drains off the bottom of the floater chamber down onto the subframe and ground... outside of any engine areas. Its not getting inside the jet at all... but when I shot carb cleaner down the hole in the jet it came bubbling out the floater... so I would assume the line isnt clogged... right? Maybe the jet isnt opening at all, and thats why its clogging and forcing itself out the edge of the fuel line?
Bryan Corey

Bryan, the jet assemblies have a once flexible ( when new- they harden from the fuel/age) plastic supply tube attached to them, the other end of which is secured into the bottom of the float chamber with a brass gland nut which squeezes the small flat washer against the o -ring, both of which are fitted to the end of the tube.From your description, i believe the chances are good that either the 0-ring has perished and released the tube from its fuel tight seal, or that some one has inadvertantly pulled the supply tube out of the gland- easy to do if one is not careful when tuning/adjusting the jet nuts etc. This replacement is not a pleasant, easy job to do with the carbs in place on the engine. They need to come off- not difficult, if you have the correct size wrenches, and flexible fingers, and patience, and say the Haynes Manual, which is rather cheap, and available at many auto parts supply houses ( CDN Tire in Canada, NAPA in USA?) . You will not regret buying this do it yourself workshop guide, and as it is cheap, you will not feel bad about it sitting on the fender, or bench, getting grubby from your oily fingers. You will not need the more expensive official guide for some time, if you are just starting on the MGB. Do buy the book, as it shows how things should be, and describes how to remove the carbs.Your problem is not a big one, though you may need to buy new jets,( hopefully not) or at least new o- rings and washers. (I think the washer and o- ring are omitted from the sketch for clarity) Good luck. You will succeed.
Ken R
Ken Rich

Can I just pull the floaters off the carbs? I have the Moss catalogue and it looks as if they jut slide on... I have the HS-4 carbs. Thanks for all the help, Ill pick up the book tomorrow.
Bryan Corey

No. They bolt on. A long bolt passes through some washers, the carb body, then through a grommet and washer into a threaded boss on the float chamber. Again, i think it is almost impossible to access this bolt head with the carbs on. Check drawing fig. 3.10, P. 73 of Haynes. Bolt is # 26. I have always tightened these bolts after removal/splitting of the pair.
Ken R
Ken Rich

Does anyone know what size the bolt is on the little fuel line that connects the floater to the jet? the one that could have a bad O-Ring. Thats exactly where its leaking from, and our local MGB shop is closed. If I could tighten that it may fix it enough to start it for the time being. Also, the back carb floater fills with gas... but it doesnt shoot into the carberator... I also had a bad line though (the one that goes from the y splitter to the nubs on the carberators themselves.. could that be the problem or is that jet not firing? Its not glogged, cleaner and gas get through it fine.
Bryan Corey

Heres the deal... it is the line and the nut/washer/o-ring. The other jet doesnt seem to be firing either. Faspec motorsports carries the jets/lines through Moss for 14.95$ each carb.. so im going on tuesday to pick up new ones (could be useful anywyas, no matter what the problem is) and that SHOULD solve my problem. I went to NAPA and they can order the MGB book but dont carry them in stock. Ill check faspec when I go there to see if they carry them. Anyways... It will be wonderful to see the car running again and I am near 100% sure that this is the only prob. Does anyone have any info/websites that deal with tuning the HS-4 carbs? I need it to run OK until I can get it in for a tuneup. Thanks for all your help, you guys have been sooo helpful to this point!!
Bryan Corey

Bryan. Try Paul Hunt's web site at;
http://www.mgb-stuff.org.uk/wn_techlinksframe.htm

You can also find SU tuning information at these web sites.
http://www.mgbexperience.com/service/su-tune.html
http://www.mame.mu.oz.au/salman/S.U.html

Hope these give you enough information to get started.
Clifton
Clifton Gordon

Bryan: Those links that Clifton cited are great. From reading your posts I would like to give a short explaination of how the SU works and is different from most carbs that you may have come across.
You mentioned that you did not see any fuel shoot or squirting into the carb. You wont, at least not when the engine is not running. Unlike downdraft carbs (Holley, Rochcester etc.) the SU does not have an accellerator pump. On those carbs, if you move the throttle plates and look down inside you will see fuel being sprayed inside by this pump, which operates by a linkage to the throttle. It is there to provide a momentary blast of fuel because when the throttles are opened in the downdraft carb there is slight period of "lean" running until the main jet starts to flow.
The SU is a side draft carb, like most motorcycle carbs, uses a piston with a needle attached to vary vary the size of the jet opening corresponding to different throttle openings/engine speeds.
All carburettors use the "venturi effect" to suck or draw gas from the float bowl and into the main throat of the carb. This effect occurs as a mass of air in a larger opening (the carb throat)rushes over a small opening (the jet). The speed of this air and its volume change as the engine speed increases. Due to the friction of the air moving down the larger opening a negative pressure (aka suction) occurs along the walls of the throat. More mass and higher velocity will create more suction. This draws the fuel up out of the float bowl. Most carbs of the downdraft variety have several differnt jets with differnt size openings to handle a range of throttle openings and thus provide the right amount of fuel.
The cool thing about the SU is that it uses this same effect to also lift a piston with a tapered needle attached to continuously change the opening size of a single jet to match any engine speed. It is a much simpler device, and to me a more elegant engineering solution.
So, as long as the float level is correct, and the jets are not blocked, and the piston can freely move up and down your carbs should function well enough for you to begin tuning. Have fun!
Cheers, Andy

until the volume of air moving down the throat is enought to draw
Andrew Blackley

Andy... that helped immenseley. I figured it was such an idea of the gas being "Sucked" out but didnt know for sure. I dont understand this though. Without the engine on and combusting, there is no sucking of air through the carb intakes... therefor no flow over the jet, therefor no pull. How do they ignite in the first place? Is that what the choke does? Get fuel to the engine? I though the choke was hooked to the carbs though, I dont get how fuel gets into the engine to ingite it upon ignition.
Bryan Corey

Also, how much should hte pistons move, mine only travel about 1/4-1/2" if that... very little. Is this normal or should I dissasemble the piston assembly and clean that all out more thoroughly?
Bryan Corey

Bryan: The choke is not a separate mechanism other than some levers connected to the choke cable, but the "choking" occurs by the jets being pulled down to a lower position so that the jet opeing is larger. This enrichens the mixture on startup. Have a friend pull the choke out and you should be able to see the jets being pulled downward.
I think you have open serious misconception: the jets do not fire or ignite. They only provide fuel that is sucked into the engine, where the mixture is ignited by the spark plugs. The movement of the pistons up and down, and the valves opening in the right sequence, both draw air in (intake cycle) and push out the combusted gas (exhaust). You may want to refresh your memory on the basics of engine operation.
Finally, the pistons in the carbs should be able to raise so that the bottom of the piston is flush with the top of 1 1/2" carb opening. There is some oil in the top (the dashpot) that prevents the piston from rising too fast, but you should be able to raise the piston easily with some resistence. You should take them off (one at a time, do not mix the parts of the two carbs) and clean them.
Andrew Blackley

no, I know completely how an engine works. I just worded it wrong. What I meant, is without vacuum, how doest the fuel get to the block in the first place. Because there would be no pulling of the fuel out of the jets and into the block when the car isnt running. I didnt mean the jets ignite or anything... I simply didnt understand how the fuel and air got into the block without the vaccuum that is created when the engine is operating, and the airflow that is blowing in while the car is traveling.
Bryan Corey

Ok guys, I feel really dumb, and am extrememly thankful for the links. I was under the impression that the only lifting part was the needle, nto that the WHOEL CENTER was a piston. I noticed my mistake after reading the sites and went out, gave a little elbow grease, and they broke free, very gritty... I washed em out with carb cleaner and they slide like new now... hopefully that, my new fuel lines/jets, and the vacuum lines that come from the block to the carbs that I replaced due to crackign will be all I need to get the engine to start! Thanks so much again, this site is a very invaluable source of info for someone who is used to only working on VWs.
Bryan Corey

Hi Bryan,
Since no one has answered your other question -- initially fuel gets drawn in by the vacuum created when the starter motor spins over the engine. Once that initial charge of fuel ignites, the engine begins to run and continues creating vacuum...
Rob Edwards

Bryan: Happy to help a budding British car nut, and maybe convert one away from the nefarious "Peoples Car" ,and maybe future Porsche 356 ownership. :>)
Cheers, Andy
Andrew Blackley

Funny you mention porsche, considering VW and porsche are all part of VAG auto group, and we use Porsche parts all the time. Im actually in the process of fitting a certain set of porsche wheels on my Corrado right now. Someday I want a 914... those are cool sleek cars.
Bryan Corey

This thread was discussed between 25/05/2002 and 27/05/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.