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MG MGB GT V8 Factory Originals Technical - Good for about 4 miles

Have recently finished a conversion SD1 V8 to my roadster,The car will run fine for about 4 miles and then start to splutter and the odd backfire, the water system is getting very hot very quickly and i am putting it down to boiling the carb, it is fitted with a Weber 500.

I have recently remove the thermostat as it wasn't opening so i am assuming a good flow of water, the twin fans are coming on, and i am not loosing any water that i know of. The inlet manifold is also getting very hot.

I have got a straight through heater valve adjustable from the engine type, i have treid it with the heater on full and this makes no difference.

I have got ceramic coated headers to reduce some of the heat.

Have i missed something obvious? Is the flow path correct? I am pretty sure everything is where it should be but really need to reduce the temp of the water, The temp gauge is not working at the moment it needs a new sender, so i am not driving it to far.

After 7 months of conversion (blood sweat and tears) everybody was right about the perminant grin on your face.

Thanks, Graham
GLG Lavis

This is a wild guess as I know little (nothing) about webers. However at one time I was playing around with a variable resistor on the temperature sensor on my EFI car. Changing the resistance to make the computer run the mix richer. I over did it. Car would start (a little rough) and run until warmed up, then as you describe.
Yours might also be running too rich.
Choke OK? Can you can make it do this much sooner if you leave the choke open?
A dodgy coil can also do this, I am told.

Good luck with it, hopefuly someone who is running a weber will post a reply.

Your coolant may be circulating too rapidly to effectively be cooled in the radiator. The thermostat also acts as a restriction to allow the coolant to spend enough time in the radiator to transfer the heat to the air.


welcome on board!

Extreme temperatures are most often due to air locks in a V8 installation.
First you should fit the right termostate, MG used one opening at 78 deg. C. for the V8.
When topping up the cooling system through the expansion tank, the engine should run and the heater should be opened too.

When I do repairs to my system, I refill into the upper hose of the heater with the plug on the radiator opened first.
When coolant exits there, I close the plug and fill the expansion tank 3/4 of its capacity, then refit the Hose of the heater.
With the 15 lbs cap of the tank still removed, I run the engine until the upper hose to the radiator becomes hot.
If you stop the engine then, you can 'pump' air locks with the upper radiator hose into the tank.

When no further bubbles come up there, everything should be O.K.

Good luck


I'm afraid I'm not smart enough to suggest a solution but I doubt the heat problem is from too much flow.

Basic engineering HVAC principles would suggest you there would be more/better heat transfer. If you want more heat either pump more water or increase the water temp difference. Also (and supporting the above) whenever I've had a thermostat go in a car (they have each failed fully open) the engine runs too cold.

Could it be that there is not enough flow - incorrect hose configuration or as Ralph has suggested.

Fix the temp gauge first, so you really know what is going on. If you have trapped air, you will see the needle suddenly rise & fall. This scared the hell out of me the first time it happened.

As Ralph said, fit a thermostat. I use a 180'F most of the time. Be sure you get a thermostat wit asmall hole in the rim and install with he hole uppermost. The hole bleeds air from the thermostat housing.

If you have a V8 style radiator, open the heater valve, remove the cap from the expansion tank, loosen both clamps on the top radiator hose, remove the end at the thermostat housing and twist the hose so you can fill the radiator by pouring into the open end. when the expansion tank is 2/3 full, install the cap. Continue to fill untill water is running out the thermostat housing, then quickly twist & attach the open end of the top hose. This almost always eiminates any air in the system. If you just can't get the air out, tap the thermostat housing for a small pipe nipple and run a small hose to T into he overflow line going to the expansion tank.

If you think the carb is too lean, causing it to run hot, back off the 2 mixture screws 1/2 turn. I am assuming the Webber 500 is 4bbl U.S. style performance carb, same as the Edelbrock 1404 or Carter 500, not the side draft 2 bbl. E-mail me if you need setup info for the carb, and do not have an instuction manual.
Jim Stuart

Thanks Guys,
a few things there for me to do. One thing i didn't mention was that the radiator hose from the top of the radiator to the termostat housing is slighly higher than the top of the radiator, could this cause an air lock?

Once again thanks
GLG Lavis

@ Jim
thanks for this explanation, never tried it this way but seems a good idea to do so next time.

@ Graham,
on my car, the radiator is also a little 'downhill' from the engine. I allways had to do the pumping as discibed above but an additional vent hose seems a good way to go and I will try this addition next time too.
BTW, with the little hole in the thermostate..., make sure it is positioned at the higes point possible and be sure you are using coolant for light alloy engines.



I like the high-flow Robertshaw thermostats.
Carl Floyd

Ditto on the Robert Shaw stat. Robt. Shaw makes the thermostats for both Mr. Gasket & Milodon. these cost around $10- $12 depending on the seller. they are High flow stats and are top of the line. i had a slight temp problem with my 5.0 Ford, but the Mr. Gasket has solved it. Their design is also different than the cheapos. excess heat is only useful in the winter!
kelly stevenson


As asked previously, is your Weber the Edelbrock 1404 series? If so, did you install the carb right out of the box? The reason I'm asking as I have the same carb on my 3.5 and it ran very rich initially.

It took a while to get the right combination of new rods and jets but now have the engine tuned as well as I can get it and I have to deal with emissions testing here in Vancouver.

Another problem I had was the floats and rods sticking causing fuel to bubble out the top of the carb and very poor running. I attributed this to dirt getting past the filter and clogging up the innards.

If you can, pick up a laser thermometer at a supplier. These tools are great for diagnosing issues. With one of these, you can tell how hot the rad inlet pipe really is.

I'm running a 180 thermostat in my car with the upper rad hose higher than the rad and thermo housing. So far no cooling issues and no air locks (that I can tell anyway).
Simon Austin

Simon, I have the same block/carb set-up as you.

Which jet set up finally worked for you?

Regards, Liam


I'm using .080" primaries, .086" secondaries and .062 x .052" rods. These are quite a bit smaller than the stock set-up supplied with the carb.

I contacted RPI Engineering on your side of the pond to see what they use. The primaries and rods are their recommendations but they leave the secondaries alone. As part of your MOT inspections, is emissions testing done?

Simon Austin


There is a date cut off - I think as my car is 1972 it is exempt from emissions testing, however the engine is 1985 so it might not be - I'm not sure which they go on.

No doubt I will find out when I take it for its first MOT, but as the engine is back out again now, that might be many moons away.

Thanks for the carb info, Liam

This thread was discussed between 02/09/2006 and 08/09/2006

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