Hello All, I’m hoping somebody will be able to offer a little assistance.
I have a 1980 1275 GT with single HS4 carburetor. I’m attempting to adjust the air/ fuel mixture as I believe that the car is running rich, due to black plugs and very sooty exhaust pipe and bogging down when accelerating.
I have not changed any of the settings on the carb yet.
However I have had the timing checked and confirmed to be okay.
I read in the Haynes manual to use the piston lifting pin on the carb to check the mixture.
- So I started the car and got it up to temperature
- Adjusted the idle to 1000rpm
- Lift the piston lifting pin until I feel the piston and then lift a further 1mm (hard to judge)
At this point the engine drops in revs and wants to stall until I release the pin and the revs pick back up.
From the Haynes manual it states
If the engine RPM decreases when the pin is lifted your mixture is too Lean
If the engine RPM Increases when the pin is lifted your mixture is too Rich
If the engine RPM initially raises as the pin is lifted, and then settles back to the original RPM, the mixture is right.
So from this I would take that the engine is actually running too Lean.
However I then accidentally noticed that if I blocked the breather hose connection on the carb (which does not currently have the hose from the canister to the carb connected) the revs drop and the engine stalls
I understood from this that that by blocking this connection and reducing the air intake into the carb I am effectively richening the mixture too much causing the car to stall.
Which to me would say that I am running rich. Can someone please clarify that I am understanding this correctly?
I am now confused and have no idea if I’m running too rich or too lean. Are there any other checks that can be done?