Carburettor Cold Start Mechanism - how to set it up?

See pages 67 & 68 in your 155/78 OM.

 

I think what people are disabling is the enrichment system on the weber carbs. Its activated by the choke lever on the console. It doesn't function very well and can cause carb problems. The device you are working on just holds the throttle open a little when the engine is cold and works great.

 

No, it's really both that get disabled -- if you (voluntarily) don't use the choke, then the fast idle cam can't really be used (as there's not enough enrichment) so it needs to be disabled. Andy has a euro (non-cat) 308 so using the choke is much less risky than using the choke on a cat-equipped US carb 308.

 

Steve
I guess each car is different. On my 78 Us I've seldom if ever use the choke but do have the fast idle cam engaged. I've used it since 1980 with no difficulties. The way its adjusted I can start it cold with the fast idle cam engaged and it idles steady at about 900 rpm which gradually increases to about 1100 as the engine warms up while driving. If I just let it idle without moving the throttle the idle speed will increase to about 2000 due to the cam not rotating. When fully hot its off the cam and idles at 975 with no AC and 950 with the AC. The fast idle cam is clocked stock, I just used the "idle speed" adjustment on the cam. Of course I live in southern calif where it rarely gets below 40 degrees. Without the idle cam engaged it will not idle when cold unless I play with the gas. I do use the choke if I'm in really cold weather as it helps due to enriching the mixture. I also have a bit of a different tune.

Main jets 130, stock 125
Idle jets 60, stock 55
Emulsion tubs, Air Correction and Start stock.
Pertronix dual module ignition in rear dizzy.
Pertronix coils
Accel plug wires with RF suppression.
Low resistance plug extenders.
Iridium plugs
Camshaft separation angle 111 degrees, stock is 108.
Derek White's adjustable pulleys and round tooth cam belts.
stock 195 degree thermostat.
180 degree radiator fan switch, I think 195 is stock.
No air pump or catalytic converters.
Timing at 34 degrees at 5K which mandates 7 degrees BTC at idle due to no idle retard points. Stock is 6 degrees ATF with R2 points.
Insulation removed from air box.

 

Understood, but you've made changes to add enrichment, live in a warm climate, and probably have your idle mixture screws set to the rich side (a stock '78 US with cats probably couldn't withstand that so people do "play with the gas" to keep it running when cold).

 

Actually I think the thing that really helps is the ignition. The spark is much stronger than stock. I forgot to mention I run about 5 thou wider plug gap also. I don't believe the idle is rich as I spent a lot of time getting it as lean as I can so it will pass smog (when I put the cats and pump) back on. 1/2 turn in on any of the 8 idle screws will start dropping the cylinder. The plugs look great after extended idle. Anyway I think each car is different, what works for mine may not work for someone elses.

 

One experiment that you might try is actually using the choke during a cold start to see if the cold RPM idle increases some. Your report that it cold idles at 900 RPM, but then increases to about 2000 RPM when warm (if the fast idle cam stays in the same cold position) is far different from the cold RPM vs hot RPM that Ferrari reports for the same fast idle cam position:

US 150/78 OM = adjust to 3400 RPM warm (at the cold fast idle cam position) so that it operates at 3100 RPM when cold
and
euro 155/78 OM = adjust to 2400 RPM warm (at the cold fast idle cam position) so that it operates at 2000 RPM when cold

Not suggesting that you use the crazy US specification, nor even the euro one, but, if you are getting 2000 RPM when warm at the cold fast idle cam position now, maybe adding a little choke will bring it up to a more reasonable cold 1600 RPM.

 

Steve: Yes using the choke (carb enrichment) when cold will speed up the idle as it is obviosity lean but for my style it's perfect the way it is. I don't have to use the choke at all here in So Calif. Even when the temp in the garage is 38 degrees (cold by our standards) it settles into the 900 rpm idle immediately and can be driven normally. I, personally, think the factory recommendations of 3100 rpm are nuts. I have no idea why Ferrari specifies such a high RPM when cold. Overall I prefer a slower idle. but without some device to compensate for the AC and no idle dash pot (mine never had one) 950 RPM with AC on is about as slow as it will reliably idle. To me it sounds better with a little cam lope.

 

I recall that the high cold idle speed was to reduce emissions when the engine is cold by pretending that 3000 was the normal idle speed. I heard a rumor at the time that some scoundrels lowered the idle speed by adjusting the little bi-metallic cam. I have had several older Ferraris and have never once used the choke.

 

Yeah the high cold "idle" may have been to pass US smog requirements in order to import the car. But its not a problem in a current smog test (Calif) due to the engine being fully warmed up when tested.