Slant Six Forum :: daily driver - random miss at idle / poor mpg

I would be thinking:
- heat in the fuel line or carb
- fuel pump getting iffy

My first thought was that it is an early winter time in KY and all over the east and the engine is running differently with the colder air and the economy has gone down due to that. It is not warming up as fast and the choke is on longer is one small contributor, and the engine may be running with the thermostat just barely open and easily 10 degrees cooler internally and perhaps that is effecting efficiency. And the colder, denser air may be effecting how the carb reacts and how much fuel it feeds in.

But I see this in our diesel trucks and recent model FI gas vehicles and may be wrong in this carbed engine.

Float level

Something stuck in fuel needle valve

(fuel overflows at idle causing rough idle)

Winter gas in MD knocks MPG down about 10%. I kind of suspect the fuel float or float valves are bad. Everything you have done to the ignition side is good. Do I understand correctly that this NEVER happens above an idle?

Sam

Where is the "fuel needle valve"? I wonder if you can get to it easily--isn't this a emmissions-era carb and everything is sealed off to the consumer?

Author:	Chief915 [ Wed Dec 10, 2014 11:55 am ]
Post subject:	daily driver - random miss at idle / poor mpg
Gents, I have a 86 D100 that's my daily driver. It's a short bed, SL6, auto, with 3.21 gears. I've had it across the scales and it weighs 3360 with me in it. I bought the truck back in July and have been chipping away at fixing things on it and I've ran into something that I'm having trouble figuring out. Randomly, probably 3 or 4 times a week, I will get a dead misfire. This always happens after I've been cruising along at 50-60 for a while and slow down to a stop. While I'm stopped, say at a traffic light, it will miss, shake, chug, and sound like its going to stall. Once I step on the gas and pull away from the light, the miss goes away and it seems to run fine again. I don't notice anything out of the ordinary, sound wise / power wise / vibrations / etc, while I'm speeding up or cruising. In the past few months my gas mileage has went down a lot too, I check it every tank. Over my first 14 fill ups, covering 3,400 miles, I averaged 16.72 mpg. Over the last 7 fill ups, covering 2,400 miles, its averaging 14.89. The routes, loads, driving style, idling time, etc has always been consistent. Since I bought the truck in July I've rebuilt the carb, replaced the fuel filter, had a valve job done which included a new intake gasket of course, replaced the timing chain, set the timing to spec, adjusted the carb with a vacuum gauge, and put in a new air filter and PCV. Once it started getting worse mpg and doing the random miss thing, I've done the following without anything helping either issue. Converted to HEI, new plugs, wires, cap, and rotor, checked and adjusted the pick up coil air gap, checked all vacuum lines for leaks, checked for fuel leaks, checked for proper choke operation, re-checked ignition timing, switched back to the factory ignition system, checked all 4 wheels for dragging brakes, and checked tire psi. What am I missing? Thanks in advance

Author:	nm9stheham [ Wed Dec 10, 2014 12:43 pm ]
Post subject:
I would be thinking: - heat in the fuel line or carb - fuel pump getting iffy

Author:	Chief915 [ Wed Dec 10, 2014 12:53 pm ]
Post subject:
Quote: I would be thinking: - heat in the fuel line or carb - fuel pump getting iffy The fuel pump has crossed my mind too..... Do you think it would cause poor MPG?

Author:	nm9stheham [ Wed Dec 10, 2014 1:10 pm ]
Post subject:
My first thought was that it is an early winter time in KY and all over the east and the engine is running differently with the colder air and the economy has gone down due to that. It is not warming up as fast and the choke is on longer is one small contributor, and the engine may be running with the thermostat just barely open and easily 10 degrees cooler internally and perhaps that is effecting efficiency. And the colder, denser air may be effecting how the carb reacts and how much fuel it feeds in. But I see this in our diesel trucks and recent model FI gas vehicles and may be wrong in this carbed engine.

Author:	Chief915 [ Wed Dec 10, 2014 1:28 pm ]
Post subject:
Quote: My first thought was that it is an early winter time in KY and all over the east and the engine is running differently with the colder air and the economy has gone down due to that. It is not warming up as fast and the choke is on longer is one small contributor, and the engine may be running with the thermostat just barely open and easily 10 degrees cooler internally and perhaps that is effecting efficiency. And the colder, denser air may be effecting how the carb reacts and how much fuel it feeds in. But I see this in our diesel trucks and recent model FI gas vehicles and may be wrong in this carbed engine. That's possible, due to the colder temps, but 2 mpg seems like a big drop to me.... I could easily be wrong however.

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Author:	emsvitil [ Wed Dec 10, 2014 6:05 pm ]
Post subject:
Float level Something stuck in fuel needle valve (fuel overflows at idle causing rough idle)

Author:	Chief915 [ Wed Dec 10, 2014 8:10 pm ]
Post subject:
Quote: Float level Something stuck in fuel needle valve (fuel overflows at idle causing rough idle) That definitely makes sense..... don't know why I hadn't thought of that! I'll take a look tomorrow. Thanks

Author:	wjajr [ Thu Dec 11, 2014 4:37 am ]
Post subject:
I don't know about KY, but in New England mid fall we get a winter blend of gasoline that knocks one or two mpg off summer numbers. Some of that loss in fuel economy most likely can be chalked up to cold stiff rotating drive train and tires, but not all of it.

Author:	Sam Powell [ Thu Dec 11, 2014 4:37 pm ]
Post subject:
Winter gas in MD knocks MPG down about 10%. I kind of suspect the fuel float or float valves are bad. Everything you have done to the ignition side is good. Do I understand correctly that this NEVER happens above an idle? Sam

Author:	Chief915 [ Mon Dec 15, 2014 8:17 pm ]
Post subject:
Quote: Winter gas in MD knocks MPG down about 10%. I kind of suspect the fuel float or float valves are bad. Everything you have done to the ignition side is good. Do I understand correctly that this NEVER happens above an idle? Sam I never NOTICE anything out of the ordinary, other than at idle. It will cruise down the road great at 55-60 mph, but when you slow down and come to a stop...it's running very rough and sounds like a dead miss. Once you begin to accelerate, it will miss until about 900 rpm or so and then clears up and runs normal again. It doesn't do this miss thing consistently.....it can go for a couple days and never do it once, then all the sudden it will do it at every stop light for two days straight. I'm beginning to be convinced that it's in the carb somewhere. I have a super six setup on the way, should arrive tomorrow, and it will be installed within the next week or so. We'll see if the problem disappears once that's on.

Author:	MoreDoorVal [ Mon Dec 15, 2014 9:02 pm ]
Post subject:
The Holley 1920 in my stock valiant had a slight off idle (1100-1200rpm) bog that would sound like it was misfiring, stumble, then pick back up. And it only seemed to do it in very inconvenient times (like trying to pull out quickly infront of a buss and such ) it took me a while to figure out but it was a mix between a small vaccume leak caused by a hole in the pcv valve gasket and the idle mixture being a bit lean, I got a new o ring for the PCV and richened the mixture up about 3/4 turn from where it was and the combination of the two solved it...

Author:	biggyfries [ Fri Dec 19, 2014 1:26 am ]
Post subject:
Where is the "fuel needle valve"? I wonder if you can get to it easily--isn't this a emmissions-era carb and everything is sealed off to the consumer?

Author:	Mike Aucott [ Fri Dec 19, 2014 1:29 pm ]
Post subject:	daily driver - random miss at idle / poor mpg
Winter gas will definitely lower mpg. It's more volatile, in other words less dense, and so doesn't contain as much energy per gallon. Also, colder air is more dense. When cruising at steady speed, much of the work the engine is doing is pushing air out of the way of the vehicle; it takes more energy to do this when the air is more dense. Mike

Author:	Chief915 [ Mon Dec 22, 2014 8:07 pm ]
Post subject:
Quote: Where is the "fuel needle valve"? I wonder if you can get to it easily--isn't this a emmissions-era carb and everything is sealed off to the consumer? The needle valve is pretty easy to get to, and not "sealed off". Follow the fuel line from the pump to the carb, there's a brass fitting where the line connects to the carb, the needle valve is under that brass fitting.

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Author:	Chief915 [ Mon Dec 22, 2014 8:10 pm ]
Post subject:
I swapped in the super six setup with a freshly rebuilt Carter BBD. The truck seems to run better all the way around. So far, after about 150 miles, the random miss has not happened again.