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MG MGB Technical - MGBGT Running rich?

Hi there,
Hopefully this will be the first of many posts as i join your discussions (in a way i hope not as it means my b is having difficulty?!)...
I own a 1973 BGT which has recently started failing on me (for the first time ever!). Basically, she likes stalling upon starting, even with the 'sweet spot' on the choke, after 10 seconds the rpm drops below 10k and finally lower until the engine stalls. I have tried every choke position under the sun and even held it to make sure the lock doesnt come loose, but unless my foot stays on the gas to help her out, nothing prevents the stall.
My girlfriends father has owned a few b's in his time and thinks the problem is the carbs. He believes they need balancing and suspects only one is doing all the work. Also he says the car is 'running rich' because of this. Im inclined to agree as there is quite a smell of petrol around the exterior of the car upon starting.
Does anybody have any thoughts on my first ever MG problems?!
Lloyd.
L Williams

Lloyd, on the assumption that the car was OK, suddenly developed the fault and hasn't been touched there are a couple of points spring to mind.. Check the air cleaner elements but then most probably you may find the the float needle valves to be leaking. This enriches the mixture by raising the fuel level in the bowls. Is there any fuel dripping out of the spill pipes as this is a good indication. Whilst the choke control is returning on the dash are the jets rising against the jet adjusting nut or sticking some way down? Worth a first check
Iain MacKIntosh

I agree with Iain. SUs are very good at retaining their tune once set up correctly and not fiddled about with. Rebalancing may help, but I suspect it would only be compensating for some other defect which unless you discovered it duiring the rebalancing would remain to cause other problems. You can check the float valves by turning on the ignition but not starting the engine - if ou have an SU fuel pump that is. Depending on circumstances you should get a couple of clicks from the fuel pump when first switching on. If it has been off for several days, or was very hot when last switched off an hour or two ago, you may get a lot of clicks. If it has only been off a few moments you probably won't get any clicks initially. If the float valves and pump are good the pump should not click more than once in 30 secs. If it clicks more than this then either a float valve is leaking or a pump non-return valve is leaking back. For the former, fuel will eventually drip out of a vent pipe which should go down the left hand side of the engine to about level with the sump to drop on the ground. The later does not cause visible leaks, but if leaking back more than a certain amount it will begin to cause fuel starvation on heavy acceleration. If it is the float valves it can be a couple of causes - dirty or worn valves or a leaky float. For the later disconnect the fuel pump and run the engine until the carbs empty, then reconnect the pump. The resultant rush of fuel through the wide-open valves can often wash away the odd speck of dirt. If that cures it all well and good. If not, or it comes back after a few days, either the valve could be faulty or you could have dirty fuel. With a 73 it is easy to remove the float chamber lids to see if there is any dirt in the bottoms, to check the floats for fuel, or to replace the valves. If dirty fuel you may have tank corrosion, fitting an in-line filter may help until the tank needs replacing anyway.
Paul Hunt 2

Thanks for your thoughts,
I am very new to MG's so i can confess Iain that i dont have much of a clue in regards to float needle valves or what/where the jet is.. research needed methinks! I can however check the air cleaner elements.
Paul - The fuel pump is functioning exactly as you stated, clicks on ignition and no more than once in approx 30 seconds. I am yet to check for leaks in the engine bay, but there is a new inline fuel filter fitted so that rules out a dirty tank? its becoming quite confusing actually as she had a 'full engine service' just a month ago. maybe its just the cold weather?

L Williams

Lloyd - the new inline filter should rule out any tank dirt reaching the carbs as you say. If the fuel pump clicks are only just 30 secs apart then it may still be worth investigating the float valve and needle. My V8 (HIF carbs) was always like you describe, unlike the roadster (HS carbs) I couldn't leave it running on partial choke for very long before the revs would start to die. The pump clicks were more than 30 secs apart, but not that much longer. I had a couple of spates of float chamber leakage which first new float valves and then a new filter (standard on the V8) seemed to fix but it happened again. This was over a period of several years. The last time I changed the float valve but it made no difference, it was only then I discovered the float was full of fuel. I tried warming and squeezing it in an attempt to find the leak and hopefully repair it, but couldn't so it must have been minute and only drawing in fuel over a long time. Replaced that and set the carbs up from scratch again, and since then I find it will run just fine on the choke, gradually increasing in revs as it warms (although in practice I keep inching the choke in bit by bit so as to run on the least enrichment as possible. You could also plug both carb feed pipes and time the pump with definitely no chance of leakage, then reconenct first one carb, then the other, and see how the times compare. They should all be the same as you have already measured. If the gap is longer on one carb than the other then the other carb is leaking. It can take some time to overflow the float chamber with just a small leak.

However the full engine service a month ago should ring alarm bells - problems can often develop after work has been done. If the carbs were fiddled with the balance could well have been upset. I find it normal to have to slightly adjust the HS carbs between winter and summer as the HS doesn't have the temperature compensation the HIFs have. But this is no more than half a flat richer in winter than summer. If the carbs are touched after a full setup you should only ever adjust both carbs by the same amount in the same direction. If you really think one needs more adjustment than the other then they must be setup from first principles again.
Paul Hunt 2

Paul,
when you refer to 'clicks' do you simply mean the number of times you can hear the fuel pump click from the drivers' seat while the engine is running? If that is the case then my pump clicks twice quickly at least every 8-10 seconds while the car is out of gear idle on the driveway.
Also, i checked the float needle valves (attached to the black screw off tops of the carbs, sit in liquid?) and they look practically brand new as far i can tell, same goes for the air filters. What should i be looking for?
Thanks!
L Williams

I thought the 73 model had HIF carbs - float bowls underneath not beside. Perhaps when the carbs were refitted the choke lever was hooked into the wrong hole. Also the HIF carbs have an O ring on the choke barrel & they can leak causing an over-rich mixture.
Was the manifold gasket replaced? If it is leaking a lean mix can be mistaken for rich one - ie: won't idle & choke out floods the other 2 cylinders. I got called out to a local garage to help find that recently.
Garth.
Garth Bagnall

Lloyd,
You just found the piston dampers! The float bowls are the cylinders attached to the side of the carbs themselves. You will find the float needle valves under the lid of these, removed by the three cheese head screws around the edge. The float valve works in much the same way as the one in your toilet, except this one flushes your money away. The conical float rises on the fuel and pushes on the little needle valve in the roof, cutting off the fuel from the tank.
Where in Hants are you? Anywhere near Farnham?
Dave Smith

Hi guys,
I wish i had more technical knowledge..! im going to have to make the haynes manual go into overdrive.
so many senarios that im going to have to check out.
Dave, im in basingstoke - the whole new shopping experience..
L Williams

Lloyd,
Get a copy of the Haynes SU manual, it shows you how the carb works and how to tune it (and how to rebuild it too if you're really keen). It covers all the SU types and the sectioned drawings will help you get a mental image of what the above advice is saying and what the likely cause of the problem is.
Dave Smith

Lloyd - the clicks should be timed with the ignition on but the engine stopped, i.e. no fuel should be being used and in a perfect world the pump would not click again after getting the system up to pressure in the first second or so. As a slight leak on one float valve could take some time to actually overflow and start dropping on the floor you may need to leave it like this for some minutes, in which case it is advisable to remove the black/white wires from the coil to prevent it overheating.

UK 73s still had HSs. As Dave says you have found the dampers, which ride in oil. But even when you get to the float valves the physical appearance is no guide to whether they are leaking or not.
Paul Hunt 2

Hi Paul,
Last year I wrecked 2 VERY rusted out BGT's One had been under a tree for 14 years and only had 65,000 on the clock. Both were registered in 72 in NZ. 18V582 engines & HIF carbs and recessed grilles. Perhaps export models were updated earlier.
Garth
Garth Bagnall

This thread was discussed between 29/11/2005 and 09/12/2005

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