[T3] Loss of power at speed- could it be overheating- update

[Mark Seaton]

I think I finally found the cause of the problem that has been haunting 
me for so long! And it looks like it wasn't an obvious one, which I'm 
kind of glad about as I feel less foolish, but I think it may have been 
causing me issues since I've had the car- getting on for 10 years! 
(perhaps it's also the cause of my poor mpg figures...)
This loss of power at speed as everyone pointed out was most likely due 
to a fuel supply issue, but none of the usual culprits were found. The 
problem seemed to come and go and for a few months the car has been 
driving OK, but this weekend the problem came back with a vengeance. 
Limping along today, apparently on 2 cylinders,  I switched off the 
ignition while running and pulled over to take yet another look. This 
time the first thing I checked was the accelerator pump and sure enough 
nothing came out of the left carb tube. I took the top off the carb and 
the float bowl was empty! I removed the float needle valve and it 
checked OK- I could blow through the it with the plunger hanging down, 
but not inverted. The float was free to move but didn't seem to move 
down very far before bottoming on the main jet housing in the bottom of 
the bowl. I removed the float and noticed something I had seen but never 
really thought about before, but this time it all added up- there was a 
flaw in the casting of the carb body resulting in a small raised worm 
like "flashing" on the top of the main jet housing. It probably sits 0.5 
- 1mm above the surface, directly where the float would hit. I will 
remove the carb body and dremel it off but to get me back on the road I 
just removed the fibre washer from under the needle valve to raise it a 
little and put the carb back together. I drove off and just got back 
home with no further issues. In fact it drives smoother and more 
assuredly than ever.
I think this casting flaw and perhaps a slightly too thick needle valve 
washer, and given that on the late carbs the float doesn't seem to be 
designed to move much at all, meant that the valve was just on the cusp 
of not opening at all. Idling and short trips were OK but on longer 
trips, perhaps as the carbs warmed up and dimensions changed slightly, 
fuel starvation was the result.
All this time and I've been driving a car that wasn't working as well as 
it could because of this tiny flaw in the carb casting :-(

Anyway, thanks for all the help and suggestions from this great mailing 
list- I'm over the moon that I finally found the problem (touch wood) 
and only hope that it hasn't caused long term damage to the engine.

Mark
'73 twin carb Fasty, soon to be working as it was designed to!
UK

PS, here's a pic of the casting flaw: 
http://www.flickr.com/photos/37269790@N00/5133105029/sizes/l/in/photostream/




On 11/08/2010 16:38, Jim Adney wrote:
> On 10 Aug 2010 at 8:09, Daniel K. Du Vall wrote:
>
>> Interesting, I live in Colorado and one thing I have learned here is
>> that mechanical pump vapor lock here is quite common.
>> I lived in Arizona for 32 years and never heard as much about it as I do
>> here. In fact here you see many of the Type1 drivers with the deck lids
>> propped open at the bottom on the early models as it seems to help. I
>> have wondered if it is the altitude here and increases the chances of
>> vapor lock on the mechanical pumps ad temps tend to run at highs 90-100
>> in the summer and in Arizona its norm to be in the 100+.
> Vapor lock is a function of the temperature and absolute pressure of the gas at
> the inlet to the fuel pump. Higher altitude decreases the outside pressure AND
> all the pressures that are relative to the outside pressure, so the absolute
> pressure at the pump inlet is also reduced.
>
> Boiling of any liquid occurs when the vapor pressure of that liquid reaches the
> pressure in that liquid, so you can cause boiling by increasing the temperature
> (which increases the vapor pressure) or decreasing the pressure, or both.
>
> I'm surprised that the high altitude of Colorado produces more vapor lock than
> the high temps of Phoenix, but I don't know what the vapor pressure curves for
> gasoline look like. It's usually quite a steep function of temperature, but you
> can also get pretty low atmospheric pressures in Colorado.
>
> I did a bit of a search for this and came up with a little bit of data that
> seemed to indicate that gasoline vapor pressure might nearly triple going from
> 50 to 100 F. To get a reduction of air pressure by a factor of 2, you need to
> be above 15,000 ft, and 25,000 ft will get you to nearly a factor of 3.
>
> So this doesn't sound like Colorado should be worse for vapor lock than
> Phoenix, but I really don't have all the data I'd need to be sure of that
> conclusion, especially since they probably sell different gasoline blends in
> those 2 markets.
>