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post #1 of (permalink) Old 11-08-1999, 02:27 PM Thread Starter
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Sudden MPG drop?

My MPG suddenly just dropped from a poor 10-11 to a miserable 7.5-8.5 w/no explanation. This in on a AMC304 in '77 CJ-7. It has cooled off a little here in New Orleans but definitely not cold (45-55 degrees in the morning). I have made no recent changes that would account for this drop. It has the stock Motorcraft 2100 carb and pretty much stock everything else. Anyone have any ideas on what might cause this? I checked the choke and it seems to function normally, although it does take sometime to disengage fully (nothing new). Thanks for any input.
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post #2 of (permalink) Old 11-08-1999, 03:00 PM
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Re: Sudden MPG drop?

Make sure your thermostat is working correctly. I stuck upen thermostat will cause your engine to run way too cool and thus kill your gas mileage.

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post #3 of (permalink) Old 11-08-1999, 03:10 PM
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Re: Sudden MPG drop?

Plug wires can go bad kinda' suddenly and that could cause the drop in MPG.

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post #4 of (permalink) Old 11-08-1999, 03:31 PM
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Re: Sudden MPG drop?

check where you buy your fuel. here in the NW its oxyginated gas season and my mileage drops like a rock. some stations use this gas year round. has your idle changed? if so check your float. if your float is nitrile(foam), replace it with brass. motorcraft carbs are also notorious for accelerator pumps that crap out. they are cheap so carry a spare. check your plugs and plug wires as well.

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post #5 of (permalink) Old 11-08-1999, 03:51 PM
 
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Re: Sudden MPG drop?

If the choke is electric, check to see if the plug is connected. The choke will open with the connector off but it won't open fully. Since you mentioned that the choke plate is taking a long time to open, it reminded me of a problem I had with my carbureted Cherokee. In my case, the connector had come off while I was under there replacing the EGR valve vacuum hose, I guess. I must have unknowingly knocked the connector off the choke. Good luck!! [img]/wwwthreads_images/icons/smile.gif[/img]

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post #6 of (permalink) Old 11-08-1999, 05:27 PM
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Re: Sudden MPG drop?

I'd be checking out that choke. If it's not opening soon enough you are running rich and using more gas. As the weather cools off it is probably staying closed even longer and sucking down your gas. The thermostat suggestion sounds good also. You want your engine to reach operating temp as soon as possible.

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post #7 of (permalink) Old 11-08-1999, 07:40 PM
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Re: Sudden MPG drop?

If it dropped that much and is still running fine, I might start with a locking gas cap.

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post #8 of (permalink) Old 11-08-1999, 08:26 PM
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Re: Sudden MPG drop?

Up here in Maryland they oxygenate the gas from November through February and my gas mileage drops about 2 MPG. This might be your reason.

post #9 of (permalink) Old 11-09-1999, 10:37 AM Thread Starter
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Re: Sudden MPG drop?

Thanks for all the tips. I checked under the hood again and found that the vacuum advance line had been knocked off by the rad fan. This is probably part of my problem. I also checked the choke again and it still takes too long to fully open. The choke is thermostatically controlled(?) (ie. heat activated spring) and I'm not sure if it gets hot enough. My temp gauge indicates about 190 degrees at its highest. I put in a 195 degress thermostat before the summer and it has never gotten much about 190, even on the hottest Lousiana days. Do you think it is running too cold? Thanks again,
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post #10 of (permalink) Old 11-09-1999, 11:09 AM
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Re: Sudden MPG drop?

[img]/wwwthreads_images/icons/tongue.gif[/img] Vacuum line....HUGE contributor to MPG....that's part of it. Yes, the choke has a wound bimetallic spring which opens and closes the choke butterfly plate. The choke spring is heated by warm air from the exhaust crossover. If your spring-operated heat riser on the RH exhaust pipe is not free and working good, there won't be much heat early on. PLUS...the tube which pulls the warm air into the choke spring cavity is only able to deliver warm air IF the vacuum is there to do the job. A very small vacuum passage is supposed to put negative presssure on the choke spring housing. If that tiny passage gets crudded up, the choke stays "choked" for a long time, puking away fuel. On startup, check the tube to see if it is getting warm....if not, you need to have a carb cleanup.[img]/wwwthreads_images/icons/crazy.gif[/img]

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