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

Mark Seaton markse at mo-sys.com
Sun Oct 31 17:32:23 PDT 2010

Cheers Mike, I guess the fact that it was running "OK" for so long meant 
I put up with it. But that's the part of my theory I don't understand- 
why should the problem become so evident now (well the past months) if 
it was there all the time?? I don't remember changing anything. Perhaps 
it's down to fuel pressure- maybe the pump lost a bit of pressure with 
age and the needle valve was so on the cusp of not opening that a slight 
loss of fuel pressure was enough to make the difference between fuel in 
carb and starvation? Hopefully removing the casting flaw will allow the 
to open properly and fix the problem for good.

31/10/2010 23:22, Mike Fisher wrote:
> WTG Mark!  At least you didn't blame it on "lousy" VW's 5 years ago
> and sell it at a loss! ;-)
> On Sun, Oct 31, 2010 at 4:15 PM, Mark Seaton<markse at mo-sys.com>  wrote:
>> 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.
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