New Exhaust = Idle Change? - Help

[BAMO]

Pretty much what the title says. I swapped out my old rusted out 1.75" frankenstined exhaust yesterday for a 2" dual exhaust. After about 10 minutes of driving, I noticed the engine start to sputter at idle. (started to notice something amiss about 3 blocks after I started driving).

Once I got home, I adjusted the idle screw on the Rochester GV and issue is resolved (for now anyway). So the question is, would the new exhaust effect the fuel mixture in this way? I've read that it could, since the engine is now breathing better potentially the mixture could/would be too lean as a result.

Thoughts or other likely culprits? Since it is the only thing that changed from day to day figured there had to be a correlation, but figured I'd throw it out there to make sure (not that I don't have other carb tinkering to do, but just want to narrow down the gremlins, or at least their causes.

[61_FL_Apache]

I would think it is now breating better. More back pressue may keep the idle up? What were your old mufflers and what is on there now?

[BAMO]

Definitely breathing better and sounds much better too. The old muffler was just stock. Now it has two turbo style flow through jobs - much less restrictive.

[HEI451]

More air flow requires more fuel to keep the air/fuel ratio correct.

[markeb01]

As noted the problem is most likely a lean condition, but one other thing to check is the heat riser valve. I once had a new exhaust system installed and the shop installed an oversize flange that blocked the heat riser from opening completely. I didn't discover what was wrong until the engine overheated and blew the head gasket on that side.

[BAMO]

Originally Posted by HEI451

More air flow requires more fuel to keep the air/fuel ratio correct.

Originally Posted by markeb01

As noted the problem is most likely a lean condition, but one other thing to check is the heat riser valve. I once had a new exhaust system installed and the shop installed an oversize flange that blocked the heat riser from opening completely. I didn't discover what was wrong until the engine overheated and blew the head gasket on that side.

Good, then the problem is likely that and I'll make some adjustments on the mixture rather than just dialing the idle up.

On a related note (and yes dumb question) but where do I look to make sure they didn't block the heat riser (the stock ram horns are in place, nothing was touched there).

[markeb01]

If you have one it will be directly below the outlet of the passenger side exhaust manifold, above the headpipe. It will have a visible thermostatic spring and a bob weight on the pivot pin. With the engine cold so you don't burn yourself, you should be able to reach down and cycle the weight through about 90 degrees to fully open and close the valve. It looks like this:

[BAMO]

Originally Posted by markeb01

If you have one it will be directly below the outlet of the passenger side exhaust manifold, above the headpipe. It will have a visible thermostatic spring and a bob weight on the pivot pin. With the engine cold so you don't burn yourself, you should be able to reach down and cycle the weight through about 90 degrees to fully open and close the valve.

Ok, found it but mine looks to be rusted shut or "altered". So two more questions. What is the purpose of it in terms of the valve (can I just replace with one without the valve) and what is the easiest way to change, i.e. remove the manifold or remove headpipe/exhaust and come at it from below.

[BAMO]

Originally Posted by BAMO

Ok, found it but mine looks to be rusted shut or "altered". So two more questions. What is the purpose of it in terms of the valve (can I just replace with one without the valve) and what is the easiest way to change, i.e. remove the manifold or remove headpipe/exhaust and come at it from below.

Never mind. Talked to exhaust guy and whatever alteration there was left it in the "open" position so effectively there is no heat riser.

[markeb01]

Although they are handy for faster cold weather starts, not having one won't burn up your engine like it can if it gets stuck in the closed position.

[BAMO]

Thanks for the help.