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 starvation/evaperation problem

Hi, Can someone please help me out with a serious re-occurring problem?
My 1800 MGBGT has been restored with the following:
New plugs, electronic ignition, coil, leads, fuel pump, in-line fuel filter, twin K&N air filters, carb floats and valves, carb shafts and butter-flys, carb gaskets and fuel pipes.
Yesterday started first time after being garaged for 4 months. Went out today for 25 minutes normal drive and it spluttered and died. I have a set of those plastic connectors that fit between lead and spark plug and showed good spark. Took plugs out and bone dry. Plenty of fuel to carb so pump and petrol flow working. Luckily Pub over the road so had meal and returned to car and it started OK. Drove for 20 minutes and same again - leave to cool down and started OK. On way home noticed that if I tried to accelerate hard it would splutter but if I gently pressed the pedal it would climb up to 4,500 rpm with no signs of spluttering.
Last year I had the same problem and I spent some time getting the floats the correct height and took it to Norfolk covering 500 miles with no problems, so why now? The carbs are HS4, I think, the ones with the built-in float chamber not the separate float chamber on the side. I also have a tubular manifold which is wrapped so little heat generated and I have new heat shield fitted. It does seem to indicate fuel evaporation when hot but why?
Any ideas would be very much appreciated
Pete
Pete Dyble

Sorry, I have twin SU H1F4 not as I said previously.
I have fitted one new float as well and used a 1.0mm feeler to set the height of it from carb casing.
Thanks again.
Pete Dyble

Hi Pete, do you have the K&N filters in the standard Cooper Can's? If not you may need to look at what needles you are using. There have been quite a few threads on fitting K&N's and needing to fit different
needles.

And of course all the obvious stuff needs to be checked - mixture, timing ect.

Andy
Andy Robinson

Pete,
What year car?
What electronic ignition?
How have you decided that fuel flow to carb is sufficient?
Have you also fitted a new petrol cap?
What happens to the Tachometer when it dies. Does it suddenly drop or die down with the engine?
If the plugs are dry after repeated attempts to start it, fuel starvation is the most likely problem.
Vaporisation sometimes happens in traffic on a hot day, at idle, but when driving at reasonable speeds the supply of cool fuel from the tank will prevent this.
The next time it stalls get out quickly and remove the fuel cap. If you can hear air rushing in the cause is the filler cap. Either wrong, unvented, one or the vent is blocked.
Allan Reeling

Highly unlikely to be evaporation on its own this early in the season, but it is highly unlikely in the UK at any time. These cars run in desert states without problems and have done so for 40 plus years, but it keeps being mentioned as a possible cause.

I've never seen how it can occur anyway - if the pump is pressurising the fuel at the tank end of the pipe then either there will be fuel in the float chambers or there won't. If there isn't the floats should have dropped and the pump will push fuel through to refill them.

It is said that HSs *can* suffer from it because of the exposed jet tube, but even then it can only be because of a faulty heat shield. But I've never experienced anything like it in 25 years in some very hot weather here and in France. Ditto in the V8 which gets hotter than the roadster, and it doesn't apply to HIFs anyway.

I did had a couple of hot-start problems in the V8 last year, but repeated cranking gave a strong smell of fuel so it wasn't fuel starvation. I suggest that is a better test, as hot plugs aren't going to show any fuel on them anyway by the time you have removed them.

It could be many things, the first step is to answer Allan's question about what the tach does when the stumbling or cutting-out occurs.

Paul Hunt

New pump. Installed the correct way up? Not having 'Out' at the top can cause cavitation that on a long run could compound on itself. I've also had pumps that will flow like Niagra falls into atmosphere but put them under load and they fail to pump once they have achieved pressure, like the points just lock up, but even with electronic ones somehow.

New coil. The right coil? A good coil? Can breakdown under loads when warm and cause a visable spark but not enough to combust the fuel.

Cheapy electronic ignition can do the above too.

Fuel filler cap (as mentioned above)

Stuttering on acceleration. Put oil in your dashpots? Have you got the right needles for the K&N? Have you got a lean mixture in general (carb set up, not fuel flow related). Ignition timing? (All mentioned above I know)

Tach status on cut out (also mentioned above...). Will the tach keep reading engine speed even though you know it's dead but it's still turning, or does it go straight to zero on engine cut? Zero on cut points to ignition issues, at least on the low tension side. No telling what the plugs are doing but they will at least be trying to spark with a tach reading.
Roadwarrior

Thanks Guts for interesting answers. I will need a bit of time to check all, but will come back to you soon.
Some reply's:
Pump has been in place for some 1000 miles so assume is fitted ok, but will check.
Needles - I don't know but would not the trouble be all the time?
Electronic ignition is Autospark, I think.
Coil was new, but if not working would not the plugs be wet with fuel as I even tried the choke to get some reaction?
Don't know about Tacho as too busy watching road to stop safely.
Filler cap - interesting.
Appreciate the other comments about timing etc, but why would it run normal for 30 mins and totally stop and then 10 - 15n mins later start and run normally?
Will report as soon as possible.
Pete
Pete Dyble

Running for 20 mins, then stalling and restartable 10 min later sure sounds like a blocked fuel tank vent.

Herb
Herb Adler

Or coil failure as it heats up, if this was the LT then you would see it on the tach, not if it were HT. Some electronic ignition systems run the coil hotter than points.

Taking a step back that's a lot of new parts, they have to be regarded with suspicion, as well as the work done to fit them.

"Filler cap - interesting." Was that replaced or not? It's an easy check, as soon as it happens again remove the cap. You should get a small 'gasp' immediately after running, but not a large one. If it is the cap incorrect in having no vent or the vent is blocked than it should start and run straight away.

Fuel: If the pump stopped working, or the float valves stuck closed (both? Unlikely). Then the next time you turn on the ignition and it starts the pump would chatter away like billy-oh for a few seconds.

"Plenty of fuel to carb" by taking the delivery pipe off the front carb? It's difficult to see a carb fault occurring on both carbs, but the floats and jets are handed. However on HIFs as the fuel to the rear carb is fed through the front carb it's possible debris is blocking the first carb *and* the route through to the second.

Whilst low damper oil level will affect acceleration it won't stop the engine running altogether.

There again you could have more than one problem.
Paul Hunt

My money is on the rotor arm. There are some bad ones out there. The plastic body loses its resistance when hot & the spark goes to earth via the dizzy shaft instead of jumping to the cap. Carry a spare & swap them over next time it stops.
G Britnell

How's the actual fuel tank? I had fun with an MGA once that would drive perfectly for 15 minutes and then conk out with refusal to accelerate etc. Filter didn't look great (not terrible though) so I replaced it anyway, and put some fresh fuel into the tank. Was fine for longer but then same result.
Tried another fuel pump because fuel flow was poor just after the problem (but cleared up again later). Same result. Blew the fuel lines out with air all the way to the tank, then had a look in the tank (had a quick look before swapping the filter the first time around too, as well as a quick peek into the float bowls). Loads of junk floating about in the tank now (wasn't there before). The pickup was sucking itself closed with debris after a few minutes of driving, then once the pressure subsided it was all fine for a while.

Something else to ponder.
Roadwarrior

A post elsewhere about a replacement tank causing problems where the pick-up pipe in the tank had an air leak.

Delivery problems are easy to check, with a fuel pipe removed from a carb and directed into a container, turning on the ignition should deliver at the very least one Imperial pint per minute and in practice about double that, in a continuous series of pulses with minimal bubbling.
Paul Hunt

Stripped carbs down and all looked well and checked manifold for cracks and tightened all bolts correctly to ensure no air leaks. Strapped a coil from my Triumph 1300 fwd to the top of the MGB one so I could quickly move the wires from one to the other. Went to Shelsey and all OK and just managed to get home when starting to miss fire and tach died with engine revs. Turned engine off and tried to start with no joy, checked petrol cap which seems normal and no start again. Quickly changed wires over to other coil and it started first time and ran no problem. So new coil on order and have chose an Accuspark one, theory being that paying a bit more would give a more reliable product...........
Many thanks to all for your time and interesting responses.
Pete
Pete Dyble

"tach died with engine revs."

Needs explanation. If the revs dropped to zero the instant the misfire started, even though the momentum of the car was still spinning the engine, that is definitely an ignition LT problem with the coil the most likely culprit.

Another diagnostic is to also look at the ignition warning light, again after the tach has dropped to zero but before the engine has stopped spinning. If that isn't glowing to start with, and only does so when the engine has finally stopped but the ignition is still on, then again the coil or its connections are the most likely cause. But if the warning light come on brightly as soon as the tach has dropped, that indicates loss of power from the ignition switch.

If the tach only slowly dropped as the engine revs came down, that is NOT an ignition LT problem, but an HT (could be coil) or fuel problem.
Paul Hunt

Thanks Paul,
New coil arrived today so will be fitted the weekend.
Will know in due course whether that cures the problem. Your comments will give me something to work on if the problem occurs again. Cheers Pete
Pete Dyble

This thread was discussed between 07/04/2015 and 15/04/2015

MG MGB Technical index

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