Q-Jet getting 5 MPG smoking like a Diesel! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-27-2011, 12:12 PM
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Q-Jet getting 5 MPG smoking like a Diesel!

I think the title says enough.

85 stocker 5.0 with a Q-jet in a G20 van.

It's a dog pretty much all over the powerband with one 'sweet' spot around 3k rpm.

It starts ok, even in the cold. Only requiring a keep my foot on the gas for about 30 seconds.

It's consuming massive amounts of fuel however. Just about 8mpg, I just did a 120 miles on 25 gallons!

At just about any speed any RPM except at idle it billows black smoke.

Where to look first? Obvioulsy rebuild it is the easy and quick answer but I'd like to gain an understanding of why or what specifically is causing this.

It's quite possible this is the origional engine/carb. This van hasn't been on the road in about 5 years also but the gas is fresh.

cheers,

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Old 01-27-2011, 12:27 PM
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Q-jet Smoke

sounds like choke is going closed on you, you may want to use a temporary zip tie to secure the choke flap in full open position and try it out. I had similar issue with a 85 gmc recently.
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Old 01-27-2011, 12:42 PM
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thanks I'll give that a shot 2morrow and get back to the board.

would be a quick fix wouldn't it?

It makes sense too. We'll see.
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Old 01-27-2011, 12:51 PM
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Another thing to check, since it sat so long, is that the fuel in it turned into trash and has gummed up the carb. You may pull the aircleaner and find fuel everywhere in the breather to find out you`ve been driving a waiting explosion. If it gummed the needle open then it`s flooding. If the choke`s not closed then more than likely the carb will have to be removed and cleaned before it`ll perform correctly. One thing I learned some time ago, a friend of mine picked up a car that had sat 6 years and the fuel in the tank, which was about half full, had completely turned into trash. After we took the tank off, cleaned it with caustic soda, the crap still wouldn`t come all the way out and the car wouldn`t run right. Since I had a identical car, we removed the tank off mine, it had been sitting 2 years but all I ran in it was 93 octane.
The fuel smelled stale, but there was no trash in the tank, it was still really clean. My friend was completely stunned at the difference. We left that same few gallons of stale fuel in the tank, bolted it in, poured in a fresh 5 gallons and he`s been running the car since, and I haven`t had to change the fuel filter or clean the carb again.
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Old 01-27-2011, 01:05 PM
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Also while I`m thinking about it, there`s also the thought of the fuel filter in the front of the carb. If it`s collapsed, trashed up severly, it goes into bypass mode. The fuel pressure overcomes the spring, and the fuel goes into the carb without being filtered. Something else to check while your in there.
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Old 01-27-2011, 01:15 PM
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Choke X2 as the first thing to check. It's not missing on several cylinders, I take it?

Black smoke is an indication of too rich combustion. This can also be caused by a float that has soaked up fuel and become heavy. This increases the float level gradually until it goes terminally over-rich. This would occur over a period of time, gradually getting worse and worse, so if one day it was fine then suddenly it was like this, it probably isn't the float.

BUT- regardless of what the cause is, you MUST rectify this ASAP!!! The raw fuel washes the cylinders clean of the oil needed to keep the rings and piston-to-cylinder lubricated. Running like this can ruin an otherwise good engine- and it doesn't take long.
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Old 01-28-2011, 12:44 AM
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I'm gonna say choke as well. I'v seen cars take one hard (almost, if not fully to the floor) blip of the throttle to get the choke off before... and it will stay on until you do it. Just something to consider..
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Old 01-28-2011, 01:34 AM
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Thanks gentlemen!

and the winner is:

Everyone!

It's the choke. The thing about a van is you can take the doghouse of easy enough obviously and presto there's your engine making some things very accessible and others not so.

So the choke is staying on, this is an '85 so the choke is electric and the bump that the throttle lever sits against is also controlled by a little electric doodat. And there's a vacuum canister on the passenger side (for the secondaries? which don't seem to open btw)

So the choke being electric I'm going to check for voltage first if that's not the issue I'll just rotate it so it's always dis-engaged.

While driving I pushed the choke open on the pas. side of the carb and kept my foot at the same spot on the throttle, it immediately responded by running smoother, not bogging and accelerated better.

Wish I'd had the idea to check 120 miles ago! That's a lot of money down the, well in this case carb!

If there's is a good source of 12v to both the choke and high idle screw thingy, how to go about disabling them? It looks pretty self explanatory, any do's/don'ts or beware of's?

Thanks guys!
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Old 01-28-2011, 05:43 AM
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If it is the choke the Spring inside the canister on the side of the carb the cover and spring can be replaced with a new one, Its kind of a little kit, Spring, cover and 3 new screws to replace the rivets if yours still has the rivets in it . Also Make sure the choke pull off is pulling in also should be on the front of the carb and should pull in soon as the engine is started .
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Old 01-28-2011, 06:18 AM
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the secondaries won't open if the choke isn't fully opened
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Old 01-28-2011, 08:21 AM
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Quote:
Originally Posted by va4cqd
the secondaries won't open if the choke isn't fully opened
As I typed I thought this...
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Old 02-02-2011, 11:57 AM
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so I ziptied the choke open and took the vac. line off the choke pull-off as it seemed to not be working properly, although it does respond to vacuum just fine but it seemed to be blocking the secondaries from opening.

They do now but it bogs before opening up properly.

It sure a big difference already though! No more black smoke and insanely high fuel usage.
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Old 02-02-2011, 12:59 PM
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The bog indicates that the secondary air doors are not set properly. I could write up the proceedure, but do a forum search, and you will find it several times over, with pictures.
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