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Discussion Starter #1
Hi

I am running a 1600 engine in my 101 spider. the engine is from a 1966 giulia super. Inlet manifold is a 101 dual throat with a new DGV 32/36 5A carb with a bosch JF4 distributor.
The engine suffers from bad detonation (run-on) when I switch off the ignition.
This is with less advance but no detonation when I drive the car.
Idle is slightly lumpy but the car accelerates and runs great except for the run on which I can eliminate by turning down the hand throttle.
However if I advance the ignition significantly the idle improves but when I drive the car it detonates.( too much advance)
I wondered if I need a Bosch JFU4 distributor with vacuum advance that might advances the ignition enough when turning off the ignition switch, resulting in no run on??
Also I do not seem to find a vacuum connection on the inlet manifold?
Any suggestions welcome.
cheers robin
 

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Richard Jemison
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7,382 Posts
Dieseling

When the engine continues to run after shutting off the ignition, that typically is refered to as dieseling.

It indicates both fuel/air source in adequate amount to fire from compression/heat source.

These carbs have to make a good vacuum at lower/mid range RPMs to fuel correctly unlike side draft carbs. So you seem to have the idle screw opening the butterfly too much.

As well you should not be using extended tip plugs which can be a source of the "hot spot)

Set the initial unadvanvced timing where no more than a quarter to half turn of the idle screw on the throttle leaver results in a decent idle. Then it should die when the ignition is turned off.
If that occurs you need to modify or find a distributor that has the slower advance curve to keep the engine from "detonating" or "pinging" when driving.
Your problem might be simply too low octane fuel... or too rich fuel mixture.
 

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Discussion Starter #3
thanks Richard

The air/fuel mixture is ok, checked it with my innovator LM 1 device.

My next steps were to fill up with high octane fuel ( 95-98 instead of 91) and considering putting stronger springs in the mechanical advance.

I will follow your suggestion to fit NGK B7ES plugs instead of the NGK BP7ES.
At the moment I am running with a hotter plug NGK BP6ES but no improvement.

Personally I do not think the octane rating would make a difference, sure I have tried that before and as mentioned the car runs really great. But it is worth another try.

I will set the carb idle screw at 1/2 turn and see what happens.
If no results though could a vacuum operated unit cure my issues???

cheers robin
 
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