[RAFO9A]

Hey guys. Been trying to chase a lean condition on my carb. Looking at the front of the engine, beneath the intake manifold I saw this capped port with a ripped cap. Can you tell me what this is for?

[b-man]

Someone drilled a hole in the valley pan and installed some sort of hose fitting for an unknown reason.

Cap it to prevent the ingress of dirt and to keep it from becoming an oil leak.

It’s not the cause of your lean condition as it’s not affecting the intake tract.

I would take off that ripped cap so you can get a better look and find out exactly what that fitting is and how it’s connected to the valley pan.

[25stevem]

When are you experiencing this lean condition, part throttle on the primary, mid throttle levels or full throttle?

[RAFO9A]

It's the idle circuit. In a cold start I can fire it up, but as the carb comes off the electric choke it goes lean. I can't drive it to see if the primary or secondary circuits are affected.

Before this problem my AFR gauge showed 13.0 - 13.5 at idle with the idle mixture screws 2.5 turns out. Now it shoots up to 16.0 - 17.0 as the carb comes off the choke. I need to go 4 turns out just to get the AFR down to 14.0 - 15.0.

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[Formulas]

your using an edelbrock intake that will accept a wide variety of carbs,
what carb are you running and are you 100% positive the base gasket is sealing some carbs require a thin adapter plate to work

[25stevem]

If with the motor fully warmed up you manually close the Choke off slowly and your idle rpm goes up then it's 95% certain you have a vacuum leak going on.

[25stevem]

I would confirm that steel line does not have vacuum.

Who knows where someone could have plumbed the other end of that tube to if you can’t see that it indeed goes harmlessly into the valley pan.

[RAFO9A]

I'm going to connect a vacuum gauge tomorrow to see if that's true. The car had been running fine until last weekend. I had the timing and idle screw mixtures dialed in too. I remembered I had a pic of the engine before it was rebuilt and it shows a hose going to the air cleaner. What's strange is that the PCV valve is ALSO going to the air cleaner. So I'm stumped on what this port is supposed to do?

[25stevem]

In that photo I don’t see the PCV hose going to the air cleaner, but going to the front of the carb like it should, on the other hand that plugged off line you have been questioning does look like it could have been going to the air cleaner and that may have been due to the valve covers used.

Without the big stock breather tube from the valve cover to the stock air cleaner base the owner it would seem was trying to still get more crankcase venting to take place with that small hose it would seem.

With the stock set up once the motor is at full throttle there is more vacuum to be had above the carb then in the intake manifold, hence the tube coming off the valve cover to the air cleaner base.

[RAFO9A]

Hey Steve, - I hooked up a vacuum gauge to that port but got no reading. I'm going to guess it's what you said in your previous post. Going to cap it and move onto troubleshooting th carb.

[25stevem]

These Demon carbs are up dated versions of the Cater Thermoquad, and as such they have a power piston and metering rod on each side of the primary’s that can be accessed from a hatch on top of the air horn.

The first thing I would do on troubleshooting your issue is to lossen that hatch screw, pivot that plate around and confirm that each piston is not gummed up and hanging up in its bore that is in the main body of the carb.

If your power piston s are not hung up then when you remove or pivot those plates out of the way the whole piston and metering rod assembly will pop up above the air horn .
If the piston is at the top of its bore but will not go down with light finger pressure then the whole assembly has moved up too high and the metering rod had popped up and out of its jet.

If I recall right there are some good videos on YouTube on how to tune these.

[RAFO9A]

I discovered a damaged air horn gasket upon disassembling the Street Demon carb. After ordering a new gasket set from Holley and cleaning out the carb, I put it back together and also capped that port from post #1. While I was at it I installed a valve cover cap that let me route a vent hose to the air cleaner. It's back to normal now. I still don't know for certain what that port was for but I'm not going to chase that mystery at this point. It seems like the lean condition was from a torn gasket and a dirty carb.

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