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Old 11-02-2009, 07:20 AM
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scotty1998 scotty1998 is offline
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Still Getting Abysmal Gas Mileage

A few weeks ago, I was complaining about poor fuel economy. It's been steadily dropping off from its usual 11 - 13 mpg to now, 8 - 11 mpg. Here are the things that I know I've done to the engine recently and want to pick all of your brains to think of anything else it could be.

-I've been playing with the timing. I've found a sweet spot that the rig seems to like at 16 btdc. A little too advanced perhaps but I'm not ready to chalk this up to the reason for bad economy. I can see if the timing is too retarded but this is certainly not the case.

-I've been playing with the vac advance. It's currently set so that it begins to activate at around 6hg.

-I replaced the power valve. Mostly because I fried the old one when I pulled the dizzy to clean it. Rotated it 180* out upon reinstall. Oops.

- Now IMHO, this is the most significant change I made. I dropped the accelerator pump arm to the lowest hole to get rid of annoying tip in lag that I had. It cleared the problem up but I have a sneaky suspicion that this might be the culprit since I started noticing a decrease in economy right afterward. Only... how? Doesn't the accel pump just ensure that you've got an instant burst of fuel to shoot into the carb when you need it? Unless the check valve is leaking inside, I'm not sure how this might lead to poor economy.

-I've made adjustments to base idle and curb idle. 800 rpm and 600 rpm respectively.

-Yesterday, I adjusted my choke since I knew that wasn't working that well.

- I have noticed that there's a little bit of fuel seepage on either side of the carb where the accelerator shaft bushings are.

-Adjusted my idle mix. 2 1/4 turns out on each. Oh and if I turn them in first, the engine wants to die which tells me that the idle circuit is working the
way it should.

- I do need a front end alignment.

- Tires are inflated to spec.

I've now been through 3 tanks of fuel since I first complained so I'm now satisfied that I have a problem.

Any ideas???
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Last edited by scotty1998 : 11-02-2009 at 07:29 AM.
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Old 11-02-2009, 08:47 PM
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grand_wag_85 grand_wag_85 is offline
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When you timed her, did you leave the vacuum advance disconnected and plugged off?

I'm guessing you are running the MC2150 2BBL, on both the ones I have in front of me the accelerator pump arm uses the middle hole. Are you running the factory ignition still? Also, is your emission and CTO stuff still in place and hooked up? Have you replaced the PCV valve lately? Is the vacuum steady at idle?
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Old 11-02-2009, 08:55 PM
RED-85-Z51 RED-85-Z51 is offline
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How do the plugs look?
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  #4  
Old 11-03-2009, 06:47 AM
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scotty1998 scotty1998 is offline
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Vac advance removed and plugged off when timing.

I upgraded to TFI ignition about a month after I bought her.

All vacuum is hooked up and working properly (as far as I know) as indicated by the rock steady vac reading at 19 hg.

Plugs are nice and brown looking.

I agree that the accelerator pump rod is normally in the middle hole. However, I was getting some really bad accelerator lag and noticed that the cam where the rod connects to the hole is a little worn. To compensate , I shortened the pump stroke by putting into the the lowest hole. That cleared the lag up really well but now I'm suspecting that this may be contributing to my poor mileage. Is there a known way to adjust these things? Like clearance specs or something?
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Old 11-03-2009, 07:51 AM
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Pretty sure that linkage on your accelerator pump's your gas mileage thief. Try moving it back for a few tanks to eliminate it or make it the culprit.

If you're getting a lag in acceleration it might not be that shooting more gas in is the solution but maybe it's indicating a carb rebuild or some other problem.

Steve
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Old 11-03-2009, 07:52 AM
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How long ago since your carb was rebuilt?
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  #7  
Old 11-03-2009, 09:44 AM
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scotty1998 scotty1998 is offline
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Carb rebuild was about a year ago now. I've replaced the accel pump 3 times to make sure that wasn't the problem. I still may pull the carb and check all orifices to make sure there isn't a blockage or restriction.
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Old 11-08-2009, 07:14 AM
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scotty1998 scotty1998 is offline
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I'm pulling this thread out of this dead over something a friend told me last night and something I read on another site...

First, I pulled the carb and rebuilt it again. I also put the accelerator linkage back into the middle hole on the overtravel lever. No dice, I'm definitely still getting terrible mileage.

What about a leaky fuel pump? It could be leaking into the engine correct? But shouldn't the oil on my dipstick be pretty thinned out? Also, shouldn't I get a real strong odor of gas from the filler tube? There is a slight odor of gas in there but I'm having a tough time saying, yes, that's it.

The more I think about it though, the more I like the idea. This could explain several problems I've been having including hesitation at weird times, loss of power on hill (it's been timed, carb rebuilt, fuel filter's new, etc. etc. etc.)

So, I filled the gas tank the other day and have gone 40 miles. I only have about 2/3 of a tank left. The last MPG calc showed I'm getting about 7.5 mpg. This is driving me crazy.
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Last edited by scotty1998 : 11-08-2009 at 07:16 AM.
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Old 11-08-2009, 09:20 AM
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CommanderChaos CommanderChaos is offline
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Here's an absolutely silly thought. Have you checked to see if your emergency brakes are engaged, partially engaged or sticking?
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  #10  
Old 11-08-2009, 10:34 AM
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scotty1998 scotty1998 is offline
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Not a silly thought at all but... no. They're unbound and the drums aren't the slightest bit hot after a long trip.
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Old 11-08-2009, 01:18 PM
Asphalt Cowboy Asphalt Cowboy is offline
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Quote:
Originally Posted by scotty1998
Not a silly thought at all but... no. They're unbound and the drums aren't the slightest bit hot after a long trip.


How about your fronts? Crud built up in the lines/pistons maybe?
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Old 11-08-2009, 05:00 PM
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scotty1998 scotty1998 is offline
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New cylinders all around. It stops on a dime and I notice no pull left or right when I brake. I think I'm just going to suck it up and replace the pump. It can't hurt right? I've done everything else I can think of at this point. I even went through the rubber from the fuel sender to the hard lines and replaced that today. They definitely needed it. Soft as can be and cracked. No seepage evident though. No strong odors anywhere.
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Old 11-08-2009, 05:49 PM
Asphalt Cowboy Asphalt Cowboy is offline
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I remembered something just a moment ago,,

How's your front bearings?
Years ago my GF took my highboy to the shop while I was TDY to Cali. Three months later I returned only to find my mileage had dropped from the ussual 9 down to 3-4. Found that the mechanic had the bearings in the front hubs a tad to tight.
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Old 11-08-2009, 06:07 PM
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Do you have poor top end?

You could have a mostly clogged catalytic converter -- that will put a lot of extra load on the engine, resulting in using a lot of extra gas.

Just a thought -- hope it helps
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Old 11-08-2009, 07:16 PM
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Any teen agers in the neighborhood?

Maybe time for a locking gas cap.

Just kidding. I'm running 9.5 mpgs myself. New 401/TH400/Quadratrac/Howell EFI. Besides being heavy on the foot I'm running too rich.

It'll be interesting when you figure it out.
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Old 11-08-2009, 08:46 PM
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scotty1998 scotty1998 is offline
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Well, the rig is garaged so I don't think the siphon gremlins are out. Bearings are certainly a possibility. I did repack them several months ago. However, I had it on the lift several times and the fronts spin freely with no restrictions. Also, the mileage has steadily dropped off over the past month or so. I worked the bearings before the summer. I do need an alignment but I can't imagine that it would dump out 4 mpgs because of it.

How do you know if the cat's clogged? I'm guessing you have to drop it to see and if you do that, you might as well replace it. Here's a thought. What about that dopey flapper valve that sits in the exhaust just before the cat. I can move mine pretty freely but does it just depend on exhaust pressure to keep it open or is there more to it?

I'm going to pull the pump tomorrow. I don't know if this is a symptom of a faulty pump but the rig has always hesitated at light pedal tip in. Kind of like it struggles to stay lit. If you hammer the pedal it runs ok. Lately I have noticed a definite lag at the high end... especially going up hill. I could barely keep a 60 mph up hill climb today and had to punch it until it downshifted. That is very unusual for this truck as it's always run pretty well. It does idle well enough though, and it isn't burning rich which would suggest power valve. Finally, after I shut the engine off, I can always fire it back up right away. However, let it sit for 5 min and I have to feather the pedal to get it to turn over again...
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Old 11-08-2009, 08:51 PM
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This thread makes me think I'm gonna get in way over my head... Oh well, I still want to get one

I dont know diddly about V8 engines or AMC stuff... I guess I better be ready to go broke learning.
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Old 11-08-2009, 09:27 PM
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Quote:
Originally Posted by scotty1998
Finally, after I shut the engine off, I can always fire it back up right away. However, let it sit for 5 min and I have to feather the pedal to get it to turn over again...

Read what you wrote as quoted above. The part about having to feather it to start after even 5 minutes is a big clue.

The only reason that happens is that your fuel is BOILING in the carb (or leaking out of it)-- try a thicker phenolic spacer (I've seen them up to 1" thick)

As far as your top end goes, based on what you said, I'd guess the following:

--Fuel boiling in the carb -- causes similar symptoms to vapor lock -- if the boiling is bad enough, you might even see black smoke due to the mix being extremely rich (boiled fuel +liquid fuel = too much fuel)

--Fuel pump not providing enough fuel -- also causes similar symptoms to vapor lock -- except that this will cause it to run lean.

--Clogged catalytic converter -- if your exhaust is clamped, you could remove it and put in a piece of straight exhaust pipe in its place(or just run it open -- but it will be LOUD) and go drive up the same hill again -- better performance = cat was your problem. Same performance = cat not the problem.

Hope this helps!!
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Old 11-09-2009, 06:18 AM
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scotty1998 scotty1998 is offline
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Just so I'm covering all of the bases, could valves be a potential problem for poor fuel economy, hesitation, lag?
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  #20  
Old 11-09-2009, 08:16 AM
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Yes. It would be detected with a vacuum gauge.
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