<VV> Turbo Motor Cycling

Hugo Miller hugo at aruncoaches.co.uk
Sat Nov 2 12:00:07 EDT 2019

Newer carbs have Viton float needle valves which are ethanol-proof. 
Older ones may get gummed up by the ethanol. Long shot, but it might 
make a difference?

On 2019-11-02 14:32, tony.. via VirtualVairs wrote:
>> On 10/30/2019 11:11 AM, Brian via VirtualVairs wrote:
>>> While on the engine stand it ran flawlessly.  Now that is installed 
>>> in the car and the Turbo with his carburetor is installed there is a 
>>> cycling problem.  The engine fires instantly and with the choke 
>>> disconnected idles but the idle fluctuates from about 690 RPM to 
>>> about 920 RPM.  The cycling is slow in other words it will run for a 
>>> couple of seconds may be as much as 30 seconds and then the idle will 
>>> jump either up or down.
>>> As mentioned all the parts are stock to include the camshaft. 
>>> Timing is set at 24° as per the shop manual.  Not being a Turbo guru 
>>> I am stumped.  Is it possible that this cycling is caused by an 
>>> improper setting of the idle mixture or is there some other issue?
>> I have seen turbo cars run rough with erratic idle caused by too 
>> much fuel pressure.  Several times.  You might wanna try a fuel 
>> pressure regulator and set it for 2.5-3 lbs and see what happens.   
>> It's odd that some Carter HVs are fine with the "stock" fuel pressures 
>> and some simply are not.   One Spyder I worked on for a friend simply 
>> would NOT idle correctly with more than 3 lbs of fuel pressure, and 
>> that was with a freshly overhauled carb.  A pressure regulator fixed 
>> it.  Another Spyder an old girlfriend owned had a similar problem... I 
>> cheated on that one and cobbled the fuel pump spring to drop the 
>> pressure via "hit or miss", got lucky and hit it the first time.  The 
>> Spyder coupe used to own (with '66 Turbo RL engine) ran fine with the 
>> factory mechanical pump unmodified and without a pressure regulator.  
>> Variances in fuel inlet valve designs?
>> tony..

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