<VV> Turbo Motor Cycling

tony.. tony.underwood at cox.net
Sat Nov 2 09:32:11 EDT 2019

> 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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