Hello,
My Spider has been running really lean at low RPMs (I have an AEM Gauge), but the Air/Fuel screw on the 28/36 DHSA carb doesn't allow the mixture to become rich enough; after I back the screw out past 1/3 it doesn't get any richer. I checked to see it the air jets were clogged a few weeks ago, but they were clear.
Did you check the idle jet as well? It doesn't take much to block it.
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Peter Brownhill
1978 Fiat 124 Sport Spider - original owner
1977 Porsche 911S - track car
2022 Ram 4 x 4 - hauler
PCA National Instructor and Motorsport Safety Foundation Level 2 Instructor
I just checked the primary and secondary idle jets, and they were both clear. I am a complete noob when it comes to carburetors, but does the air/fuel mixture screw limit air or gasoline? When I remove the screw completely the engine dies, but both the screw and the hole are dry. Is that how it is supposed to be?
Weber carbs deliver an air/fuel mixture to the primary idle jet. Turning the idle mix screw out increases the flow of the mix and just the opposite happens when turning that screw in. You must be idling on the primary idle circuit and not the main circuit before you can adjust the idle mixture. Weber has a lean best idle adjustment procedure which I outlined below.
On a fully warmed up engine with the choke disengaged, you start by backing out the idle speed screw until it no longer is in contact with the throttle lever, then turn the idle speed screw back in 1 to 1/12 turns but no more. This process ensures you are going to be idling on the primary circuit. Then screw the idle mixture screw in until it lightly seats, then back it out 2 turns. Start the engine. It will idle but barely. Start screwing the idle mix screw in 1/8 to 1/4 turns at a time until the engine runs rougher or dies then begin backing out the idle mix screw until your engine speed smooths out and the idle speed increases. Back out the idle mix screw a 1/2 turn from there or until it does nothing to engine idle speed and then screw it back in to obtain the smoothest idle. Now you can set your idle speed with the idle speed screw which should not require more than an additional 1/4 to 1/2 turns in. According to weber, if your idle mix screw is less than 1 1/2 turns out you need a smaller primary idle jet and if it is screwed out it more than 2 1/2 turns it needs a smaller primary idle jet.
If your engine is stock and the 28/36 hasn't been re-jetted and you know there are no blocked carb passages for the primary idle circuit, then you should be able to set the idle mix according to this procedure.
According to weber, if your idle mix screw is less than 1 1/2 turns out you need a smaller primary idle jet and if it is screwed out it more than 2 1/2 turns it needs a smaller primary idle jet.
Sorry, the statement above should read....According to weber, if your idle mix screw is less than 1 1/2 turns out you need a smaller primary idle jet and if it is screwed out more than 2 1/2 turns it needs a LARGER primary idle jet.
Thanks for the reply,
I have been messing around with it, and found that when I loosened the primary idle jet, the mixture gets richer. I know that the jet isn't clogged, and a few months ago when I tuned it I was able to get the mixture way richer with the air/fuel mixture screw.
I suspect something else is clogged, but I don't have the expertise to diagnose it
A couple of thoughts. You could check the idle mix orifice for any blockages by unscrewing the idle mix screw all the way and take it out. Get a can of carb cleaner with the plastic straw and stick it into the hole where the idle mix screw goes. Open the throttle blades all the way so you can look down into the primary barrel of the carb while shooting some carb cleaner through that orifice. You should see a steady stream of carb cleaner flowing into the barrel.
Also, I'm not real familiar with the 28/36 DHSA but most idle mixture screws on weber carbs have a rubber o-ring around the shoulder of the mix screw. Make sure yours isn't missing or hasn't crumbled from age and take a look at the end of the idle mix screw itself to ensure that the tip of it looks OK and hasn't been damaged.
