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Discussion Starter · #1 ·
Hello Mopar Masters, thanks for the comments!

With every hose connected the engine almost stalls from a cold start till the motor gets a bit warmer. Right now it is around 12c or 54f. So around 1 minute of playing with the gas peddle to get it going till high idle kicks in and i can let the motor do its thing.

If I disconnect the hose that goes from pcv to intake there is a considerable vacuum. As a result of disconnecting the hose the van idles pretty good and eventually the rpm climbs up to much so I reconnect the hose and the idle lowers. So that leads me to believe that I'm creating a lean situation with regards to the air/fuel ratio.

On the valve cover where the pcv valve is there is no pressure coming from the valve cover. Even if I remove the pcv valve there is no pressure coming out of the valve cover. If I rev the motor, no pressure.

Im wondering if underneath the valve cover some other hole to the crankcase is plugged and maybe i should remove the valve cover and stick a wire through the theoretical hole to free up breathing.


I've cleaned the carb and other minor things that have made the motor pep up a bit but I havn't been able to get the idle while cold issue sorted.
 

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KOG
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Choke pull off diaphram leaking?
 
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The valve cover often has baffles built into them for the venting holes, so splashing oil doesn't get carried out of the vent holes and the engine looses oil, its very possible gunk has built up in the baffle.

The pistons moving up and down should create surges in air pressure in the crankcase, blowing out and sucking in very quickly. Is the air surging in and out of the Breather?

A proper working PCV will keep fresh air circulated through the crankcase and evacuate moisture and corrosive fumes out of the crankcase, that will form crud/gunk/deposits and contaminate the oil. So if the PCV is clogged you set up a situation of the inside of the motor progressively building up more and more crud inside.

When you pull the hose for the PCV, if you didn't cap off the open hose, that would create a very large vacuum leak, the reaction you described sounds exactly what you should see if you opened the hose for the PCV and allowed it to be a huge vacuum leak.

If the baffles in valve covers are clogged, I'd pull the valve covers off and thoroughly clean them up, inspect inside the motor. Its NOT that hard to do, new gaskets for the valve covers are NOT that expensive, it will benefit from a clean up in that area, and its a more proper way of addressing this issue, than just trying to poke a hole in the gunk.

A fresh PCV wouldn't hurt either, they are a flow control valve, if they gunk up and fail to work properly, it will affect engine performance.
 

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Since it runs well on a cold start when you disconnect the PCV hose, or runs well when warm when it's connected, it seems clear that there is a too-rich situation right after cold start. Two possibilities come to mind.

1) As KOG suggested, the vacuum choke diaphragm has failed and is not holding the choke open enough (or is misadjusted). Check the adjustment and also apply vacuum to the diaphragm and see if the linkage moves.

2) The carb is flooding out, and creating a vacuum leak brings the air/fuel ratio into an acceptable range. When the choke is open while warm, more air also leans out the too-rich condition.

What are you getting for gas mileage, city and highway? What vehicle is this?
 

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This is a quadra-jet and not a thermo quad, yes? There is a ballast resister connected to the bi-metal choke pull off.

Is the pcv valve stuck shut. Shake, does it rattle? Like KOG said. Choke diaphram leaking.
 

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This is a quadra-jet and not a thermo quad, yes? There is a ballast resister connected to the bi-metal choke pull off.

Is the pcv valve stuck shut. Shake, does it rattle? Like KOG said. Choke diaphram leaking.
I don't think it's a ballast resistor. There is an electric heating element attached to many bimetal springs on chokes on carbs from about mid-1970s on up, to get the choke open faster and lower emissions. But if it fails, the car will not run any richer at cold start.
 

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Bob, it could run richer because the choke isn't pulling off properly. Of course the engine runs faster and smoother with the PCV valve off, it is below the choke plate and allows more air in. Check the setting on the choke itself, it may be closed too much to start with.
 

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Discussion Starter · #8 ·
Thanks for all the reply's. My plans for the van is to travel to fix it and then travel US from Canada and check out the eclipse next summer and tour around a bit.
Total Solar Eclipse 2017 - Path Through the United States
I took the van for another ride tonight and again rough idle till warm and ok. I was driving a little more rough with it. It did stall twice... Once when I let my foot off the peddle while in drive and not moving, so trying to idle under load. Another time while slowing for a stop. Idle seems to be too rich?


Choke pull off diaphragm leaking?

Choke thingy on carburetor? I don't know how to test if it is leaking.

The pistons moving up and down should create surges in air pressure in the crankcase, blowing out and sucking in very quickly. Is the air surging in and out of the Breather?
air is not doing anything out the breather (breather meaning hole where the pcv is).

If the baffles in valve covers are clogged, I'd pull the valve covers off and thoroughly clean them up, inspect inside the motor. Its NOT that hard to do, new gaskets for the valve covers are NOT that expensive, it will benefit from a clean up in that area, and its a more proper way of addressing this issue, than just trying to poke a hole in the gunk.
I'm going to source some gaskets and do a cleanup under there.

A fresh PCV wouldn't hurt either, they are a flow control valve, if they gunk up and fail to work properly, it will affect engine performance.
I have changed the old pcv to a new pcv. Unfortunately no difference. Old pcv rattled, so im assuming it was working.

1) As KOG suggested, the vacuum choke diaphragm has failed and is not holding the choke open enough (or is misadjusted). Check the adjustment and also apply vacuum to the diaphragm and see if the linkage moves.

I'm ready to try this though I don't have a tool to make a vacuum other than my mouth and a hose. I might just replace this if it doesn't cost much.

2) The carb is flooding out, and creating a vacuum leak brings the air/fuel ratio into an acceptable range. When the choke is open while warm, more air also leans out the too-rich condition.
Possibly some of the seals on the carburetor are in poor shape.

What are you getting for gas mileage, city and highway? What vehicle is this?

I'd say the gas mileage is poor right now. Maybe (500km) 300mi on a tank. I have winter tires on it, so maybe it'd help if i had summer/all season radials on. 1985 B250 5.9L 360cuin Sportsman Prospector 3.
Van also burns some oil during startup and if I press the gas peddle all the way to 100% it kicks out a minor oil cloud (I can see it for a moment).


this is a quadra-jet and not a thermo quad, yes? There is a ballast resister connected to the bi-metal choke pull off.
I found this info about the carb. R4-4MV, GM 431

Is the pcv valve stuck shut. Shake, does it rattle? Like KOG said. Choke diaphram leaking.
pcv is new and works. I'll find a new choke diaphragm/ unit and see if that helps.


ill post after installing new choke diaphragm. see if i can get one tomorrow otherwise i'll order one.
 

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KOG
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You must have a vacuum pump (tester) in order to test or install the choke pull off. There are fairly cheap all plastic ones which will work for what you need. You'll also need some drill bits or a dial caliper to set choke opening correctly.
 

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Bob, it could run richer because the choke isn't pulling off properly. Of course the engine runs faster and smoother with the PCV valve off, it is below the choke plate and allows more air in. Check the setting on the choke itself, it may be closed too much to start with.
That's what I said in my first post, check the adjustment.
 

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You must have a vacuum pump (tester) in order to test or install the choke pull off. There are fairly cheap all plastic ones which will work for what you need. You'll also need some drill bits or a dial caliper to set choke opening correctly.
Much easier way to test without tools:
Remove vacuum hose from vacuum choke pulloff. Press the linkage in all the way, and place thumb tightly over the vacuum tube. Linkage should move out slowly (at least 5 seconds or more) or not at all. If it moves immediately and thumb is tight on the port, diaphragm is bad.
 

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On the vacuum choke pull off. It's called a rear vacuum break control. Page 39 in the link below. Item # 36. Take the vacuum line off, manually push in the rod. Place your finger over the metal port where the rubber line went over it. Let upon the rod. If the rod stays in, the diaphram in the can is good.-> Bob, you type faster than I do. ;)

Item # 25 is your fast idle cam. Spray some WD40 all around it. Make shure it's free.

On a cold engine. take the air filter off so you can see down the top of the carb. The front choke plate should be vertical. ( That back plate is for the vacuum operated secondarys. normal to be horizontal. ) Now, engine off. Operate the throttle by hand to floored position. That sets the choke. The choke plate will drop down fully closed. The fast idle cam will keep the throttle open a bit mor for a fast idle.

Now start it up, the choke plate will be pulled open around 1/8th inch or so by that vacume break control.

As it warms up, the bi-metal can # 34 will let the idle speed gradually go back to normal idle when warmed up.

http://www.gmcmi.com/wp-content/uploads/2014/05/Quadrajet_78-Service-Manual.pdf
 
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