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Old 03-30-2012, 01:38 AM   #1
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Unhappy so how does a carb work, anyhow?

Need help figuring out my problem, and thought just knowing how the carb works may help me get to the bottom of things. i see lots of problems like this, too.

I have a 1988 allegro with 31k miles. ford 460 with motorcraft carb e6jl ab a8a5. electric fuel pump replaced few hundred miles ago because it was burned and inop. orig owner had run separate power wire to the pump. the fuel pressure reg just replaced, hoping to fix problem. it didn't. fuel lines have the y-connection near the engine to keep fuel circulating, i guess. basically, has been running fine, except not 100% smooth, and smells like unburnt fuel at the exhaust. been like that for a year. i run at most 20mph for 6 miles each way, 1 or 2x/month, but usually run at 5 or 10 mph speeds because of the bumpy dirt roads. usually keep just under 3/4 full tank, but added 12 gals more today to bring up to nearly full.... don't trust the gas gauge.

Lately, i drive for a bit, sometimes 5 minutes, sometimes 1/2 hour, then 'lose power'.... that is, engine rpms down to idle speed, and if i keep trying to gas it, it'll stall dead. restarts immediately. drives at idle speed ok for the most part, but won't run any faster. while still rolling, put it in n and try to rev it. sometimes it revs, but runs up and down the rpm scale without me doing it, sometimes just won't rev up, and dies if i keep pushing it.

today i went around the block at about 25mph to test it, about 2 miles. 1st time ok, 2nd, 3rd, 4th times it acted up and couldn't get back up to speed. it didn't recover until i backed it 80 ft into the driveway to the house. then it revved to maybe 1500-2000rpms for a couple mins with no problems.

so, if i understood what was going on inside the carb, i might better understand how this problem might be something inside the carb rather than pump, filter, fuel lines or press reg. being intermittant makes it hard for me to analyze it.

looks like this is a common problem for these engines. any ideas out there?
thanks! am
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Old 03-30-2012, 05:39 AM   #2
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Before we get into great detail, has it ever backfired ?
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Old 03-30-2012, 06:06 AM   #3
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There are plenty of sites on the internet that will show you how a carb works. Very basically, there is a narrow area called a venturi. As the air flows through the venturi, it creates a low pressure area that draws fuel from the float bowl.

It sounds like you may have low fuel pressure or a vapor lock where the gas in the steel fuel line boils. That creates air in the float bowl and reduces power. If it does it when the motor is cold, it is more likely low fuel pressure. If it only does it when hot, try wrapping the steel line near the engine with aluminum foil and see if that helps.
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Old 03-30-2012, 07:29 AM   #4
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carb info

never has backfired, so havent suspected ignition yet. metal line from pressure valve to carb has a wrapper. eng runs 170 deg, hasnt changed since bought yr ago. power loss is something recent. thanks so far. am
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Old 03-30-2012, 07:32 AM   #5
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You may have good fuel pressure, but the flow may be reduced by a partially clogged filter.
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Old 03-30-2012, 07:37 AM   #6
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Quote:
Originally Posted by allegroman View Post
Need help figuring out my problem, and thought just knowing how the carb works may help me get to the bottom of things.
HERE is a good site that explains the workings of a carburetor - use the Next Page button at the bottom of the screen to page through the presentation.

Rusty
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Old 03-30-2012, 07:39 AM   #7
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Quote:
Originally Posted by allegroman View Post
Need help figuring out my problem, and thought just knowing how the carb works may help me get to the bottom of things. i see lots of problems like this, too.

I have a 1988 allegro with 31k miles. ford 460 with motorcraft carb e6jl ab a8a5. electric fuel pump replaced few hundred miles ago because it was burned and inop. orig owner had run separate power wire to the pump. the fuel pressure reg just replaced, hoping to fix problem. it didn't. fuel lines have the y-connection near the engine to keep fuel circulating, i guess. basically, has been running fine, except not 100% smooth, and smells like unburnt fuel at the exhaust. been like that for a year. i run at most 20mph for 6 miles each way, 1 or 2x/month, but usually run at 5 or 10 mph speeds because of the bumpy dirt roads. usually keep just under 3/4 full tank, but added 12 gals more today to bring up to nearly full.... don't trust the gas gauge.

Lately, i drive for a bit, sometimes 5 minutes, sometimes 1/2 hour, then 'lose power'.... that is, engine rpms down to idle speed, and if i keep trying to gas it, it'll stall dead. restarts immediately. drives at idle speed ok for the most part, but won't run any faster. while still rolling, put it in n and try to rev it. sometimes it revs, but runs up and down the rpm scale without me doing it, sometimes just won't rev up, and dies if i keep pushing it.

today i went around the block at about 25mph to test it, about 2 miles. 1st time ok, 2nd, 3rd, 4th times it acted up and couldn't get back up to speed. it didn't recover until i backed it 80 ft into the driveway to the house. then it revved to maybe 1500-2000rpms for a couple mins with no problems.

so, if i understood what was going on inside the carb, i might better understand how this problem might be something inside the carb rather than pump, filter, fuel lines or press reg. being intermittant makes it hard for me to analyze it.

looks like this is a common problem for these engines. any ideas out there?
thanks! am
Sounds to me like a dirty carb, a stuck float or an incorrect float setting. Does that motorcraft carb have an external provision for adjusting the float?

Depending On how much gas you have in gallons you can get some cans of seafoam AND POUR IT IN THE GAS TANK, read the directions and go a little heavy, it's normally one can per 20 gallons, that stuff works like a miracle and cleans dirty carbs from the inside. it's been around since christ was a corporal, A side benefit of seafoam is it klills water in the gas so it can be burned, works like the olf fashioned dry gas.
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Old 03-30-2012, 09:02 AM   #8
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Change the fuel filters. One on carb and should be an inline one also. Had similar issue on our 89 454 Chevy.

Good luck
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Old 03-30-2012, 09:28 AM   #9
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The issue you describe points to a bad fuel filter.

Our 1989 Ford 460 has 3 filters between the fuel pump and the carb. One is at the outlet to the pump, a seond along the frame rail, and the 3rd at the inlet to the carb. This is the original factory setup, not sure why so many but they are there.

I'd replace all of them to fix your problem.

I'd assume you have a John Deere chassis, the manual is here; 89 MANUAL
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Old 03-30-2012, 11:16 AM   #10
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I agree with the recommendations of dirty filters/carb. 31 k miles in 24 years means a lot of sitting. Try the seafoam cleaner. Also, after replacing filters, take it out and exercise it! Constant driving at 20 mph for 6 miles will not even warm it up.
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Old 03-30-2012, 11:28 AM   #11
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Carburetors suck. That's really about it. They suck.
Actually the engine sucks, pulling air thru the carb. The air movement causes a decrease in pressure or vacuum (it sucks) that pulls fuel through a jet into the air stream, into the engine's cylinders. The volume of suck determines proportionate volume of fuel sucked. The suck process also attempts to mix fuel & air.
Its a sucky situation, by design. As you can imagine, vs high pressure computer controlled fuel injection, suck technology is not very efficient at either precise metering of fuel or of air, which means both fuel economy and emissions are hard to manage well when the best you have as a control sucks.
Hope this explanation doesn't suck.
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Old 03-30-2012, 12:07 PM   #12
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Go with the filters first. then if it falls on its face when accelerating you may need a new
accelerator pump on the carb. I think that Motocraft carbs do not have a float adjustment on the outside of the carb. Holleys do.
Hope this helps .

Joe
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Old 03-30-2012, 01:09 PM   #13
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There is no such thing as suck. Higher pressure air pushes into lower pressure area, thus there is no suck, only push.
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Old 03-30-2012, 02:25 PM   #14
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You can verify if that carburetor has the external adjustment by looking at the top of each fuel bowl. There will be a large screw with a nut underneath it. Also, there will be a site screw on the side of each fuel bowl. If you have them you adjust each bowl to where fuel just reaches the edge of the site screw hole. Unlock the adjustment by turning the screw CCW and while holding the screw turn the adjustment nut CW to lower the fuel level and CCW to raise it. Also, part of your problem could be the power valve has been ruptured. As was suggested, use some Sea Foam first, replace filters, adjust float level if possible, and if these don't do the trick you might want to think about rebuilding the carb.

John
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