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

Mike Fisher fisherfarms at gmail.com
Sun Oct 31 16:22:54 PDT 2010

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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