Carb suggestions, please!

[Thomas DeBusk]

TFI upgrade 3 months ago raised mpg all the way to 9/10. But the last 3 tanks have really tanked: 6.2/6.5/6.6 mpg. It's time to do something with the carb.

A mechanic told me not long ago he thought the choke wasn't working right and it was running rich all the time. Just recently it runs terribly until it's thoroughly warm. Could barely make 35 mph this morning. When it finally did warm up, after doing 65 mph for a little bit I let off the gas & a big backfire put out a huge cloud of black (holy) smoke!

This stock carb has already been rebuilt, about 5-10k ago.

I'm not adverse to a new carb, but need suggestions. Don't really want to do a manifold, too, but I'm open to good advice. I like the idea of a 4 bbl., but don't really want to do a manifold, too. If I do a carb, I want to retain the stock air cleaner if I can.

The motor is pushing 200k and runs strong, other than this. It does blow a lot of oil into the air cleaner. Don't know if that's screwing up the carb, but it's been doing that for the last 20k miles.

Oh, I'm almost sure I have vacuum problems, too. The heater controls don't switch between heat and defrost (for over a year, now) and the front axle went screwy for a few weeks a little while ago before it mysteriously healed. I plan to pull it into the garage and give it a thorough going-over.

Thomas

[Joe H.]

That really sounds like power valve issues, bad! The power valve gets its marching orders via engine vacuum, so the test for a blown pv is to pull the vac line off of it (on the manifold side) and see if any gas leaks out. A dollar to a donut, you'll see some.

[joe]

The power valve and the choke operation is something to check for sure. If the PV is blown go afead and replace it. Then, since you already suspect vacuum leaks, fix those first before messing with the carb any further. If you've got major vac leaks even a brand new carb is going to run bad. Fix your vac leaks first, check your ign wires, cap, rotor, plugs etc and then tune the carb last.

[Thomas DeBusk]

Thanks for the replys, so far. I'll check all the vacuum lines, for sure.

Can I get a power valve by itself? And can I change it without removing the carb.

Which side of the carb is the power valve on?

I did the ignition wires, cap, and rotor when I did the TFI stuff. Didn't change the plugs, but I don't think they're very old.

Thomas

[derf]

The power valve is under the fuel bowl on the front of the carb. I suppose you might be able to remove it without pulling the carb but I've never done that. As for getting a replacement, I bet you could if you looked around. I got one in my rebuild kit.

Oh yea, and you might want to check and/or replace your PCV valve if you're getting a lot of oil from the filler cap.

[ December 14, 2002, 10:19 PM: Message edited by: derf ]

[joe]

Thomas, I emailed you a blow-up of the carb and yeah you can buy the power valve alone.

[Bombadier]

i don't think it's possible to replace a power-valve unless you have the carb off. Maybe you could, but it would be a very tight operation, as the powervalve cover sits on the front bottom of the carb.
The biggest problem would be getting the back two cover screws off. Even then if you managed it, you'd be working by touch trying to seat the valve back in there.

As for a 4bbl carb, it defeats the purpose to use a 2bbl intake manifold with a 4bbl carb. Sure you can buy adapter plates to match the two up, but if you really want the gains, match up the intake to the carb.
I wondered about that when having my fiance's carb rebuilt. for the price of a rebuild, we could've bought a quadra-jet. but our trusty carb-guy pointed out the reasons not to do that until we had a matching intake.

[Thomas DeBusk]

Have to report back here:

Several vacuum lines at the back of the carb were loose. That was the problem. I must've left'em loose last time I had the air cleaner off. Aaaaugh! Either that, or someone else took the air cleaner off and didn't re-connect all the lines.

As usual, I had several teaspoons of oil in the air cleaner when I took it off. So I took the opportunity to take the metal tube at the rear of the manifold out and check it for gunk. Several people on the list have said it can plug up and cause oil to appear in the air cleaner. My tube wasn't just stopped up a little—it was plugged solid for most of the length of the tube.

My Wag runs runs much more happily now. I'm anxious to see if the cleaned out tube helps the gas mileage and oil in the air cleaner problems.

Thomas