Horrible Mileage - FIXED! 100%

[Sluggo]

Hey Sluggo, I looked at the " travel" pics in your gallery. Nice Photoshop job with the Golden Gate Bridge. Except for getting the shadows backwards, it looks pretty convincing. So... when do you think you'll get the bus on the road for real?

That's right. It's all a big scam. Just trying to make an alibi for where I was when....well, you'll find out.

[chitwnvw]

You don't. At least not with one air fuel ratio monitor. If you wanted a reading for each barrel you'd need 4 monitors with four senders welded into each exhaust manifold. About $500 for all that not including having the bungs welded in for the sender.

So how are you using it as a diagnostic tool? It might be showing everything is a-ok, when 2 barrels are lean and two rich...

[Amskeptic]

I think I'm just covering up an intermittent vac leak.

Your engine goes through a natural lean-out after the chokes have shut off and before the manifolds have warmed up. During this spell, the manifolds will go through a condensation period where the incoming air chills the manifold walls more than the engine heat can overcome. This condensation period occurs in all climates and is the reason behind all that factory air-preheater engineering.
Colin

[DurocShark]

This condensation period occurs in all climates and is the reason behind all that factory air-preheater engineering.
Colin

And is the biggest reason that current manifolds for centermount carbs on type 4's aren't worth the metal they're made of.

If someone would friggin just market a manifold with heat, all those Pinto carbs out there might actually work decently on our busses!

[Sluggo]

I think I'm just covering up an intermittent vac leak.

Your engine goes through a natural lean-out after the chokes have shut off and before the manifolds have warmed up. During this spell, the manifolds will go through a condensation period where the incoming air chills the manifold walls more than the engine heat can overcome. This condensation period occurs in all climates and is the reason behind all that factory air-preheater engineering.
Colin

No chokes on the Webers. Plus, it will happen after I've been driving for a few hours too.

You don't. At least not with one air fuel ratio monitor. If you wanted a reading for each barrel you'd need 4 monitors with four senders welded into each exhaust manifold. About $500 for all that not including having the bungs welded in for the sender.

So how are you using it as a diagnostic tool? It might be showing everything is a-ok, when 2 barrels are lean and two rich...

It will blip down on each cycle if one barrel is lean or blip up if one is rich. Which it does sometimes. This is why I think I have an intermittent vac leak.

[Sluggo]

This condensation period occurs in all climates and is the reason behind all that factory air-preheater engineering.
Colin

And is the biggest reason that current manifolds for centermount carbs on type 4's aren't worth the metal they're made of.

If someone would friggin just market a manifold with heat, all those Pinto carbs out there might actually work decently on our busses!

I thought that was just a problem with centermount Progressives? I have duals.

[DurocShark]

I thought that was just a problem with centermount Progressives? I have duals.

It can happen to any carb setup. It's just almost guaranteed on centermounts because of the long runner length.

With dual carbs, even if it happens, it's less noticeable because the intakes are (usually) so short. Plus they heat up faster. So as Colin said, that moment in time when the chokes just opened but the engine isn't fully warmed up is when it's most likely to happen. You don't have chokes, so it'll happen until fully warm.

If you have an IR thermometer you can watch it happening. Start the bus cold and take a reading of the intake just below the carb. Take readings until the engine is fully warmed up. You'll see lots of peaks and valleys. Possibly even some below freezing temps!

There are times when it'll happen no matter how warm your engine is. There was a good write up on a 280z site about this, but I lost my bookmarks when I got laid off so I have no idea where it was. It showed temp changes just past the carbs on a set of weber side drafts. The guy had graphed it with one minute resolution. I was really surprised.

[Amskeptic]

No chokes on the Webers. Plus, it will happen after I've been driving for a few hours too.

Same difference. Feel your manifolds the next time you slip into lean mode. Are they cool? It will happen during driving for a few hours, or after it has shut off for a few minutes after driving for a few hours?

If you ever see condensation on the outside of the manifolds, it will be happening on the inside too.

OK, so you are enthralled with this intermittant nature of a vacuum leak. . . what sort of vacuum leak can pull off intermittant symptom?
Colin

[Sluggo]

No chokes on the Webers. Plus, it will happen after I've been driving for a few hours too.

Same difference. Feel your manifolds the next time you slip into lean mode. Are they cool? It will happen during driving for a few hours, or after it has shut off for a few minutes after driving for a few hours?

If you ever see condensation on the outside of the manifolds, it will be happening on the inside too.

OK, so you are enthralled with this intermittant nature of a vacuum leak. . . what sort of vacuum leak can pull off intermittant symptom?
Colin

My barely educated theory was that as things expand and contract leaks can happen in the intake manifolds and vacuum crossover pipe. But I didn't realize icing could happen with dual carbs. I thought it was purely a symptom of long intake runners. I do always have a slight up and down blip whether lean or normal. But I guess that could be attributed to the same thing.