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Old 05-12-2009, 11:08 AM   #15
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Popping out the exhaust is caused by unburned fuel igniting in the hot exhaust headers. This is not caused by fuel starvation. Fuel startvation will cause a lean pop back through the carburator. Also an ignition misfire can cause unburned gas to get into the exhaust and ignite. The suspect in this whole thing is the Holley carb running rich either because a needle valve is dirty or sticking and that is why the question on how it idles or a power valve in the carb having a broken diaphram and bypassing fuel into the motor and running rich when going down the highway. One backfire through the carb will wipe out the power valve. You have changed everything else and I would concentrate on the carb.
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Old 05-12-2009, 11:23 AM   #16
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Originally Posted by Mike Canter View Post
Looks like you have changed all the normal failure items. At highway speed if you floor it does the MH accelarate without hesitatation or does it fall flat? How does it idle? When idling if you listen to the exhaust at the exhaust pipe outlet is it smooth, does it smell rich or normal?
It smells normal.
I can floor it and the 4bl opens and it down shifts. This is around town. It will even climb hills fine.
Once up to speed on the highway and it starts acting up it will not make a difference on throttle postion from 1/2 to full. Just stumbles.

Bob
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Old 05-12-2009, 11:36 AM   #17
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I would try changing the power valve. In steady driving these are normally closed and when at wide open throttle they open up to enrich the mixture. So if this was bad and leaking it would work normal during acceleration but when at steady rate would dump too much fuel in the motor so it would dump unburned gas into the exhaust and pop.
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Old 05-12-2009, 11:54 AM   #18
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Try driving it with the vacuum advance disconnected?

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Old 05-12-2009, 01:22 PM   #19
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Double check for a second fuel filter. I had the same problem and did not reralize I had a second fuel filter between the tank and engine.
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Old 05-12-2009, 07:47 PM   #20
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I changed the second filter the one in the carb. no change.
I changed the ign. module. No change.
I checked the egr valve it is working when I applied a vac. to it.
No light show under the hood.

I did run it down the highway toady with the air cleaner open so I could watch the carb spray fuel. It did look ok (like 4 shower heads running full blast).

---Note--- When this was brought to my house it had the same problem. I then changed everything and it is still the same.
I am soooooooooooo discouraged. I have never had a problem with a vehicle I couldn't fix.........I don't know what to do...

Bob
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Old 05-12-2009, 08:58 PM   #21
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Long shot- but check it out anyway, are your exhaust manifold gaskets failing? My dad had a '83 Ford 460 that made a heck of a racket once it warmed up and was going over 45mph, changed the gaskets and problem went away.
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Old 05-12-2009, 10:30 PM   #22
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Mine, a 1988, had an in line filter back along the frame.... change it on several occaisions.
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Old 05-13-2009, 02:33 PM   #23
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I had to replace the fuel pump on the Coachmen MH on the John Deere Chassis.
My fuel pump was in front of the fuel tank as well. It suffered the same problems.
You may find that thats your problem as well.
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Old 05-13-2009, 06:56 PM   #24
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I tried it with the vac. adv. unplugged and plugged the linewith a vac. gauge. The vac. acted normal. Same thing.
I put it in gear and loaded the motor. No light show.
I am going to check the wire routing and verify that it is good.
I am going to try and out a gage in the fuel line.
The old plugs I took out where NOT tan but NOT gummed up. They where more of a charcoal color, but dry. I have seen ALOT worse. I will pull a plug once it cools and see what color it is.

Bob
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Old 05-13-2009, 08:16 PM   #25
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If the center electrode porcelein is charchol color indicates that the motor is running rich. If you are starving for fuel the pocelein would be bone white.

One thing I have to ask here before I go on is have you tried new plugs?

If you have tried new plugs and since you have looked into the carb while driving indicates that the choke is not stuck closed. So this leaves the possibility of a bad power valve in the carb or a bad needle valve causing the floats to be too high. Again you have changed everything else. The power valves can be bought separately or you can get a whole carb rebuild kit. They are not that expensive. Holley carbs can have one or two power valves depending on the carb. Most likely you only have one in the primary side. When a motor is floored these power valves open and increase the fuel flow by ten jet sizes so if the rubber diaphram in the power valve is bad it will make the motor run real rich. If a motor runs rich it will cause unburned gas to be pumped into the hot exhaust and cause popping in the exhaust.
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Old 05-13-2009, 08:34 PM   #26
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You mentioned wire routing. You can get some cross firing if the wires are routed incorrectly. Especially at higher RPMs it might cross to a cylinder and fire the plug at the wrong time.

But if your plugs are looking a bit rich, I would be looking toward the accelerator pumps or power valve. On a Holly or Quadra-jet, the power valves can cause a world of problems ...rich or lean.

Ken
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Old 05-13-2009, 08:47 PM   #27
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Does sound like fuel delivery, but maybe check the following also
1. Distributor shaft bushing/gear worn causing bad signaling to pickup coil?
2. Are you positive there is not a fuel pump in the tank that could be bad and the one outside not strong enough? My 84 Pace Arrow had both electric back at tank and mechanical up front. Was a recall back then to put on the electric pump.
3. Sock in tank "sucking" flat, but I believe that would be more noticeable under heavy acceleration, not median highway speed lack of performance.
4. With all you have done, and if all above advice checks out, either replace the power valves or even another carb on it.

For sure, "t" in the fuel pressure gauge and watch as you accelerate and when problem occurs. If pressure stays up, obviously not fuel pump, but could still be the power valves.
Hope you find it.
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Old 05-13-2009, 08:49 PM   #28
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Had the same chassis as you and found under the air cleaner filter can setting on top of engine a vacuum plastic valve had a hole burnt through it, drove MH out of driveway and engine stalled 200 feet down the road. Could drive it back slowly but not up to speed. Got a replacement at Ford dealer. Stuff would always happen when leaving on trip with that chassis.
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