Truck has started running rich & terrible MPG

[ih8vols]

Had my '79 sidestep for just over a year now. Drove it about 3 hours home when I bought it and it did great. No gas mileage issues at all.

Here's my symptoms:

• Over the past couple months, I've started noticing it's running rich. I can smell it in the tailpipe. Something else I've noticed is that it usually starts decently. However, if i drive it long enough to warm up, and then stop and get gas, when I got to start it, it STRUGGLES to start. Almost sounds like it might be flooded.
• It is GUZZLING gas right now
• There's a cheap tachometer installed in it. Idle is right at 1170. In gear, it's at about 1650. Yesterday, I took it on the interstate and it hit 5000 rpms when I hit 65mph
• The carb is VERY CLEAN. Previous owner had it rebuilt right before she sold it.

I did not grow up working on cars. I actually bought this truck to begin learning how to fix them.

The engine is the original 350. The carb is a Rochester Quadjet.

I bought a vacuum gauge today, but haven't hooked it up yet. Plan to do that tomorrow.

From what I've read, it appears the carb could need tuning, the ignition timing could be off, or maybe a vacuum leak?

Based on the symptoms I'm experiencing, what would you recommend I check first? The symptoms popping up so quickly after running great for so long is what I'm struggling with right now.

Appreciate your help!

[rich weyand]

At 65, 5000 rpms? Sounds like you were in second gear.

Which would be the case if the vacuum line from the back of the carb down to the transmission broke. The tranny thinks you are at WOT.

And the carb is struggling to add enough gas to keep up with the vacuum leak.

I'd start with the vacuum line to the tranny. Inspect it where it connects to the vacuum modulator on the transmission, and all the way back. Replace the rubber portion on general principles.

[nonstop]

Is the choke opening up? Agreed with above, but also sounds like it may be stuck on fast idle.

[ih8vols]

Hey guys, thanks for the comments. Here's a few more details I was able to piece together today. One thing that jumped out to me, I'll put at the very bottom of this post. Might be nothing, not sure.

• I drove it around the block and watched the RPM's and noticed that the truck did move into 1st, 2nd, and 3rd gears (it only has 3).
• I hooked the vacuum gauge up to the carb. I only saw 2 inlets. One inlet gave no reading at all. See my photo below for more details.
• Here's a video of the gauge with me starting the truck: https://drive.google.com/open?id=0B0...khXVTJjM2lTc1U
  
• The pressure stayed at 18,which I believe is suppose to be good.
• I crawled under the truck and checked the vacuum line going into the transmission and it looked fine. The rubber ends appear to be in good shape. The metal tube has no corrosion and isn't broken
• I was able to find the ID # on the carb - 17058218 - Built in 1978. Quadrajet (4 bbl) Federal standards for manual trans.

Here's a couple additional pics of the carb:

In the 1st photo, I hooked the vac gauge to the blue inlet on the left (video above). I tried hooking it up to the blue inlet on the front, but nothing happened.
https://drive.google.com/open?id=0B0...GZSZnpkZVdybms

Passenger Side: https://drive.google.com/open?id=0B0...1QzM1loQmpVYmc


Front view. The blue cap is the inlet that I hooked the vac gauge to and got no reading. It appears that are 2 tuning screws at the bottom of the carb. The one on the right is much further out that the one on the left. Could this be my problem? https://drive.google.com/open?id=0B0...WFlSUxZRWNVNnM


Dumb question, but nonstop asked if my choke is opening up. How do I check this?

Thanks guys!

[rich weyand]

The choke plate is the metal plate standing on its edge in the front barrels. It is shown as open in these pics, so it is opening up. Whether it is closing when cold is another matter.

Your vacuum advance is connected to the ported vacuum on the front upper left (looking back) of the carb. Better is to hook it to manifold vacuum on the passenger side, the one sticking out the side with the blue cap on it. Move the cap to the one in the front to close it off.

The two screws in front are the idle mixture screws. Before you mess with them, screw them each all the way in (not tight!) while counting turns. This will let you knew where you are so you can get back to where you are now. Then back them back out.

They probably should be out about two turns. You can start from two turns and then try this. Let the car get to operating temp. Back the screws out until you get the highest vacuum reading. Back each screw the same amount, do 1/2 turn at a time and let the engine adjust. When you no longer see the vacuum or RPM increase, then turn each screw in 1/4 turn. Adjust your curb idle speed back to the spec with the idle adjustment screw on the throttle linkage.

[ih8vols]

Rich, I really appreciate your help!

Went out and followed your directions. Turns out the mixture screws were set at about the same that I put them at.

Then I adjusted the curb idle. I backed it out 3/4 of a turn. Here's what happened to my RPM's

In Park - went from 1700 to 1300
In drive - went from 1300 to 1000
At 60mph - 5000+ due to the road I went out on, I was only able to get up to 50mph tonight, but RPM's topped out around 3600.

It seems to shift gears and drive much smoother now. Think I might need to adjust the timing a little bit, but I'm really happy with how it's running right now. Hopefully MPG gets better.

Again, REALLY appreciate the help. This is probably the "biggest" work I've done on the motor of a car. Learned a ton - which is why I wanted an old truck.

[LuvNLife]

I only mention this because it happened to me once.

On the back of that cheap tach you have, should be some sort of switch. This selects 4, 6 and 8 cylinder engines. You may have to Google the brand to find out more info, but I had one set for a 6 cylinder by mistake, and got crazy high readings like you.

I might be all wet, but it's worth a look...

Jerry

[rich weyand]

Yup. Aftermarket tach should have 4. 6. 8 selector. If that's wrong....

As for timing, I assume you have a timing light. A cheap one will work. Like $20-30 at any auto parts store.

Disconnect the vacuum to the distributor. You can plug the hole, but you don't need to just to set the timing.

Measure your timing at idle. You are probably around 8* BTDC. You should be able to go up to 12-16* BTDC. More is better as long as it doesn't knock when you get on it at low speed going uphill. Make sure you clamp the distributor back down tight.

I assume in recommending this that you moved the vacuum advance line to the side port on the Qjet.

[Shugalou]

Something else to look for is that you are coming off of the high idle cam. I have been recently trouble shooting my own choke issues.

Look to see that the little tang is dropping.