MGB: MGB fails when hot

Hi,
My MGB MkI (1964) is in really good condition but I virtually cannot drive it because of a fatal problem that occurs when it gets hot.....under any acceleration it sputters and has no power. Once at speed it'll cruise along, as long as you don't try to accelerate. It doesn't matter if I am going top speed or taking off from a light, it just feels as if I am running on one carby or there isn't enough spark....but only when the engine is hot. If you do stall it at the lights (due to no power!) then you can't start it until it cools down. The motor turns over well, but just no firing at all. Where I live is fairly hot and humid, and it is worse in summer, so I installed an electric fan...but, despite doing tests like running the engine at 80C instead of 90C it still starts to happen after about 15 mins driving. I've replaced quite a few things....New battery, leads, plugs, fuel pump, coil, ignition switch, points, but no luck!
The car isn't driven much (once a month ) so the carby needles might get a bit sticky?
I've got Twin SU's (HS4) and they get quite hot...when you put your hand on then float bowl its almost too hot to handle, so I'm thinking it was a vapour lock? Also, when the leads were replaced the problem went away for a few months, or that's what appeared to happen. The almost new leads had started to break down?? (maybe they got too hot as well?)

Any ideas? My next step is to replace the carbies with new ones and maybe the distributor as well!!! $$$

Thanks!
Tony

When an engine quits due to heat then starts again when cold, it can be the condensor, but this doesn't sound right in your case.
Do you have a heatshield between the carbs and exhaust manifold? Your description sounds like you ARE in fact running on one carb; try using a UNISYN to synch your carbs. Also, check your float level. It could be also a sticking float needle due to the heat.

I think Chris is right, start with a new heat shield, and set up your carbs. Where do you live??

It also sounds like a venting problem similar to what I had with my 69 with HS4s. Make sure the float bowl vent overflow pipes aren't restricted or clogged. Also (don't know if the 3 main engine has this)check the crankcase breather-the front side cover with a pipe coming out of it. If it is clogged and the breather hose feeds to both carbs, the carbs will run like crap. First check the float vents and make sure they are clear. If they are closed off you'll not hold an idle and hot restarts are almost impossible.

Thanks for your responses!

Gary.....
Heat shield was replaced when I did the rebuild (1000miles ago), and it is in good condition now (no cracks etc). My carbs were tuned when on a dyno, and the mechanics were very familiar with MG's so I think they got it right (at the time). The carbs are a bit borderline though I think. I'd say that the linkages etc are a bit suspect, but I refurbished as much as possible during the rebuild. I live in Brisbane Australia, which is hot and humid (tropical almost)....so overheating is an issue. I did crank down the engine temp (and the car has an oil cooler) but to no avail...but I realise that the heat travels around, and the carbs/lines get very hot.

Chris......I did find that replacing the condenser helped with the hot starting problem a while back, but I think this is a separate problem that has occurred since then. I will be looking into getting the carbs retuned, but a second attempt at the dyno tuning didn't show up as pointing at the carbs being the problem. I realise that dyno is showing overall power rather than whether the carbs are synch'ed, so I will have to check that out as well. The float needle comment of yours was interesting.....are you saying that the float needle is MORE likely to stick when the carbs are getting hot? Because the car is not getting much use at the moment I think that the fuel might have evaporated a couple of times and left behind residue (colouring agents and other stuff) so this is a big possibility. What is the best way to clean this out...do I have to dismantle or will I be able to do some of the cleaning by spraying carb cleaner into the air intakes? Also, how is it possible to check the float level safely when the car is really hot? I have done it once but I reckon that I must have been close to having a fire!

Cheers!
Tony

Peter,
Thanks for your reply. I will check the float bowl vent as you suggested....what is the sure fire way of knowing if it is clear?
My car has a five main motor, as well as the crankcase breather line....is there any way of eliminating this as a possible cause?....ie can I get the car hot, have the problem, block the hose off and then check for a difference?.....or, will the line have a steady flow of gas coming from it and if so, then that isn't the problem? I'm not really sure about what that breather does either?
Also...The car will hold an idle perfectly! It's just when you try to accelerate that it splutters etc. When the car is at idle, is the fuel being delivered any differently than when you rev up (and the needles open)? I know that sounds like a dumb question, but I heard someone talking about that the carbs don't have acceleration something-a-other's and pumping the accelerator when the engine is spluttering may or may do anything?
But I suppose...if the needle is sitting in one position (idle or doing a constant speed) and you try to accelerate, then the needle might stick, and the only other explanation (assuming it isn't electrical) is that the needles are opening but there isn't any fuel available. Is there any way to release a vapour lock? (maybe a release valve in the line?).
Thanks!
Tony

First it is my understanding (which isn't too technical) that the fuel is delivered the same at idle as it is at speed. There is no accelerator pump. My engine would run fine for about 45 minutes then wouldn't hold an idle and was really slow to restart when hot.
When checking it out in my driveway I found (just as the idle was dying) that by pulling the hose off the crankcase breather pipe coming up from the side plate restored my idle, somewhat. Following that for some reason I pulled the rubber hoses off of the float overflow pipes and the car came back to life. It was my fault to start with. A few month ago I decided that it was dangerous not to have somesort of piping or hoses attached to the overflow pipes on the float bowl covers; so I rigged up rubber fuel hoses to them, ran them to a "T" fitting and fed the remaining single line to the other side of the car away from the exhaust system. That part is OK, but I decided to stick a fuel filter on the end of the hose just to keep crap from getting into the hose. Well it wasn't long before that filter restricted the air flow and played havoc with the carbs. That's when I discovered that the overflow pipes also serve as air vents to the float bowl. Yeah, I know...stupid me. Air has to get in there somehow.
Anyway, just pull whatever piping or hoses you have attached to the float overflow pipes and see if that makes a difference. If the crankcase breather is clogged check the posts back about two weeks on how to clean the wire mesh element inside the side plate and/or how to replace the element with "Chore Girl" copper mesh scouring pads. That's what I did and it took about an hour. You don't have to remove the exhaust manifold to do this, just the carbs, heatsheild and radiator hose to the water pump. One bolt holds the side plate on, once loose, slide the plate forward to remove. That's why the rad. hose is pulled free because otherwise it blocks your path. Not a bad idea to at least clean that mesh element anyway. JDW and I think that soaking the cover plate in "Sea Foam" would get most of the crap out of the element inside the side cover plate.
Man this was long....sorry.

Thanks Peter.
Don't apologise for writing me a long message!...it was very informative. I'll try your suggestions. I don't have anything on those carby float bowl vents so its possible that they have blocked up with dirt or something. I'll check that out. When I get the problem I might pull the crankcase vent hose off and see if it make a difference.
Cheers,
Tony

Tony,
If you only drive the car once a month or so, it's possible you have water in your fuel, possibly from condensation in your tank. Another possibility would be that the problem is not so much temperature related as it is time related; you may have rust particles moving around in your tank that plug up the outlet after a bit of driving, allowing only a small amount of fuel through. I've had both of these things cause me headaches in the past.

No I'm not saying one will get hotter than the other, just that heat might make them stick. Check the float level when it is cold so you don't worry about fire. I'd check the posts below before replacing the needle and seat, things I didn't think about.

Good point Ken. The fuel tank was a bit of a concern when I doing up the car but I didn't replace it ($$!). I'm sure there is rust in it, even if its a bit.
I can check fairly easily to see if the pump runs at full speed when the problems occur by putting in a t-piece + tap setup...if it doesn't pump properly then the pump inlet could be blocked.

Just a simple thing, check your fuel filter, mine did the same thing for a long time.. drove me nuts..
12$
no more problems..

Boy was that a relief.. but SUCH a pain..

Actually, I don't think any of the MkI cars were equipped with fuel filters. I did install an inline on my '67 under the hood, right where the steel line connects to the flexible one. I used a clear plastic filter, it allows me to see how much debris I'm getting.

I have one of those plastic inline fuel filters....I've noticed that when the engine has been running for a while that there is virtually nothing in it at all? Is this normal?...I have a new petrol pump and filter etc and it seems to pump fast. I'm starting to think this is a vapour lock problem?? All the lines and the filter are as far away from the hot places as possible??

Tony