<VV> Carburetor Mystery

Thu Mar 19 12:01:06 EDT 2020

I don't know what the answer is.  Maybe you have already been down the path 
I am about to suggest.

First off, you need to look at what common ground there is between the two 
engines that could cause both to act up.  Second you need to look at what 
differences there are between the stock carburetors and the modified one 
that make the stock carburetors immune to the problem.  Keep in mind that a 
strong shot from an accelerator pump only proves there was gas in the bowl, 
not necessarily that there is enough fuel flow to keep the engine running.

The obvious common ground is the gasoline itself.  Changes could be in 
composition or contamination.  Did you start using gas with ethanol?  That 
could have changed by your choice or by a change in the local availability. 
Did you start/stop adding some magic fuel additive?  Contamination could 
have come in with a batch of gas.  It also could have been in the tank where 
it wasn't causing any problem until some alcohol (whether ethanol fuel or 
added drygas) or other "fuel system cleaner" picked it up and sent it to the 
engine.

What can be different between the modified and stock carburetors?  When the 
modified carbs were assembled (many years ago?), were now-antique parts used 
that might not be compatible with ethanol?  A sticky rubber tip on a float 
valve could cause problems.  Meanwhile, the stock carbs may have compatible 
parts.  How about the filters in the carb inlets?  Old ones tend to slowly 
loose flow capacity.  Add even a brief shot of dirty gasoline and you have 
trouble.  Your water test, although valid for what it tests, does not test 
flow though the filter or the needle valves.

These are just suggested places to look.  But whatever the problem is, the 
source nearly has to be something common to the feeding of the modified 
carburetors that the original carburetors are immune to.  Are the fuel line 
connections identical with both sets of carbs or do they have to change when 
you change carbs?

You may have already been down these paths.  But you didn't mention any of 
them.

Jim Becker

-----Original Message----- 
From: Brian via VirtualVairs
Sent: Thursday, March 19, 2020 10:14 AM
To: VirtualVairs at corvair.org ; fastvair at yahoogroups.com
Subject:  Carburetor Mystery

Over the years while working on Corvairs I've encountered a lot of 
mysterious things.  But in every case the mystery was ultimately solved. 
Solved either by extensive research into shop manuals, input from other 
Corvair guys, or just plain figuring it out by myself over time.  However, 
now I've got a mystery  that may be unsolvable.  I'm sure hoping it's not 
but I am baffled.
Here's the setup.  I have two engines I use in my track car.  The first is 
a176 cubic inch one using 3 9/16 pistons.  The reciprocating assembly is 
balanced and it has hot cam.  The second one is  3.1 liter using Volkswagen 
Porsche pistons and cylinders.  Like the first engine, everything is 
balanced and it has a cam.  Each engine has its own ignition system.  The 
only common item to both engines are the modified Rochester carburetors. 
The Venturi has been poured out to an inch and a half and the Venturi 
cluster arms extended to gain the correct distance from the Venturi walls. 
These carburetors have worked for many years on both engines.
Last summer something mysterious happened.  Neither engine would easily 
start and when they were finally running they would only run for a couple of 
seconds and then die as if they were out of gas.  This was regardless of the 
throttle position.  After the engines would die, operating the throttle 
shaft would result in for very strong accelerator pump shots so it was 
obvious there was gas in the carburetors.  Nothing I did would correct the 
problem.  Hours and hours went by with no success.
Recently I tried again.  This time using the run stand my club built.  This 
made it much easier.  I didn't have to swap engines in and out of the track 
car.  Using the1 76 in.³ engine, I first installed four stock carburetors. 
The engine started and ran perfectly responding correctly the throttle 
position.  I then installed the modified carburetors and got the same 
results as last year.  Hard starting and only running for a few seconds 
before dying as if out ofa gas.  I took the carburetors off and made sure 
every passage was clear and the fuel bowls were full.  I even tried 
different main Jets sizes although I had very little hope that that would 
affect anything.
After checking each one of the carburetors I put them back on the engine and 
experienced the same old results.  This is a mystery that I cannot fathom. 
Why suddenly after years of working perfectly did these carburetors now 
refuse to work?  I should mention that while I had each carburetor apart I 
filled the bowl with water and using my air supply blew air down through the 
Venturi with the throttle plate open.  In all four cases I did get a fine 
mist of water on my hand which I was holding on to the throttle plate.
What a mystery !  Why did these carburetors suddenly not work?  Why when all 
the passages a clear and I can get water to spray on my hand as if the 
carburetors were on the engine why do they not work when installed  on the 
engine?  Absolutely nothing was changed on the engine between the runs made 
with stock carburetors and the attempts to use the modified carburetors.
To say I am baffled is an understatement.  Does anyone have any ideas?

_______________________________________________
This message was sent by the VirtualVairs mailing list, all copyrights are 
the property
of the writer, please attribute properly. For help, 
mailto:vv-help at corvair.org
This list sponsored by the Corvair Society of America, 
http://www.corvair.org/
Post messages to: VirtualVairs at corvair.org
Change your options: http://www.vv.corvair.org/mailman/options/virtualvairs
Archives: http://www.vv.corvair.org/archive.htm
_______________________________________________ 