Warm Starting Problems on 66 MGB GT

Hello all,
I have a problem when starting from warm, cold is fine, starts first time.
I suspect that the carbs are flooding or something.
Normally the pump pumps up to pressure from cold, when the car is warm, rightly the pump does not pump any extra fuel if the fuel if it up to pressure.
It tends to not pump any more when starting from warm but i'm sure it does not need any more, the engine smells of petrol, so i imagine it is still up to pressure. If anything, i suspect it has too much.
Do i need a fuel regulator, or has ithis model already got one as standard that is not working properly?
Or is it common for the floats to stick open and flood when warm?
Any light you can shed, experience with this problem would be appreciated.
thanks, Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jim; The MG fuel pump makes relatively low pressure ( about 5 or 6 lbs. if I remember right) compared with most modern units, so it doesn't ordinarily require a pressure regulator.
In hot weather they can misbehave due to the design of the cyl. head, which places both intake and exhaust ports on the same side and so the float bowls directly above the hot exaust manifold. Spridgets and Minis suffer the same disease. Heat rising from the exhaust manifold and head pipes has to go past the float bowls, and on a hot day it can get warm enough under there to start to vaporize the fuel in the bowls.
Make sure the insulating material is still on the back side of the heat shield which fits between the carbs and the intake, and make sure the float needles and seats are in good shape. Those are cheap enough and easy to replace. Check all hose connections at the float bowls etc., and make sure the gaskets under the float bowl lids are good.
I've encountered problems with some new replacement floats giving funny levels, so it's worth checking those as well.
Wrapping the exhaust head pipes with heat insulation tape, as used by a lot of racers, helps as well.
After 40 years of messing about with SUs I finally got tired of the ongoing battle last season when my pair needed yet another rebuild. I replaced them on my ' 70 B-GT with a pair of Mikuni HSR 42 flat slide carbs meant for motorcycles, and those solved a great number of fuel-related problems for me. They work very well indeed, are easy to tune, cost me less than a new pair of SUs, parts are available much more readily and economically, and they don't leak or otherwise mess me about.
While SUs can be made to produce very good power, they can also be a major pain. The choke set-up in particular is crude and prone to wear and trouble. I don't know about over there, but here in Canada it is very difficult to even find replacement jets in more than one size. Lots of the aftermarket rebuild kits and other bits for SUs are less than first class quality.
At 42 mm the Mikunis I chose are about mid way between the stock 1 1/2" SUs and the 1 3/4s used by the works cars back in the day. They are far less sensitive to the exhaust heat, give me more power and still give me an average of 30 mpg on premium fuel. They also fire up instantly whether the engine is hot or cold.
I did have to put a pressure regulator in the line for those, since they are designed to be gravity fed on bikes rather than for even the modest pressure from the Lucas pump. I also had to lower the float levels slightly to accomodate the angled mounting on the MG manifold.
My GT has a fresh engine, .040" over, with a lightened flywheel and a half-race cam, so it is a fairly brisk, pleasant tourer. The Mikunis also have accelerator pumps built in, so throttle response is improved in comparison with the old SUs.
Good luck with your problem. KH

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks Kevin,
I have just bought some insulation tape for my exhaust.
I will give the carbs a once over and replace the needles if they look bent at all, I will check the seats also.
I have just bought an inline fuel filter to make sure there is no rubbish being picked up and sticking in the float chamber.
Regards, Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jim;
I'm talking about the float needles, not the main jet needles. It's virtually impossible to bend float needles. They just get a little ridge worn in them over time, where they seal off in the seats. Of course then they don't seal and the bowls flood. They aren't in the carb bodies, they're in the float bowl caps under the float arms.
Some guys like Grose replacements, which use spherical balls to seal the inlet. I've used them and liked them, but some guys have had trouble with them, apparently.
The fuel filter is a good idea, but stick it someplace as far as you can get it from the exhaust manifold or you'll get heat troubles there as well.
Good luck.
KH Canada.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.