1979 AMC 304 Rough idle.

[jafo]

I recently got this jeep and have just made my way towards the engine compartment. It's always been cold natured, meaning it runs kind of rough and dies if you don't stay on the gas pedal until it warms up.

I also noticed that sometimes, when taking off from a stop, it coughs and stumbles a little unless you give it more rpm's. It will occasionally cough back through the carb.

So I decided to take a look with the air cleaner out of the way and check some vacuum hoses and check the spark plugs etc. What I found were a few hoses cracked from age, those got replaced. I also found there is two tubes coming off the manifold passenger side to the back of the carb. One of those hoses was not hooked up. It looks like it helps function the choke. I also found back by the EGR valve a small part that is in line right before the EGR. It's some kind of vacuum pot. Hoses were good to that but the main hose going to that was also off the vacuum pot. I don't know what this pot is for, but I am thinking the EGR gets it's vacuum from or through this vacuum pot.

Could some of this be causing the cold engine running issues? I can't start it right now because of waiting on other parts. But hopefully soon, I'll be able to start it and find out. I know that a faulty EGR will cause engines to not run properly or die at idle.

I do plan on replacing the plugs and new set of plug wires also. Cap looked good inside and out as did the rotor bug.

 

[007]

The two lines on the passenger side send heat up to the choke so that it will open when it needs to. If your not sure about vacuum then replace what is cracked (I use fuel line where I can - handles the vapors, doesn't collapse and lasts longer). Plug anything else that is open and start from there. I'll update this post with a vacuum diagram from the manual.

 

[Lumpy Grits]

Check your idle full mix and be sure that BOTH screws are turned our EXACTLY the same amount.
Does the choke linkage move freely?
When was the last time the carb was rebuilt?
Have you confirmed the ign timing?
LG

 

[Hedgehog]

The short answer: Well yes it sounds like a good deal of the choke system was disconnected and the poor thing was running lean with all the vacuum leaks.

 

[jafo]

The two lines on the passenger side send heat up to the choke so that it will open when it needs to. If your not sure about vacuum then replace what is cracked (I use fuel line where I can - handles the vapors, doesn't collapse and lasts longer). Plug anything else that is open and start from there. I'll update this post with a vacuum diagram from the manual.

OK, I'll start here and go through each response.

I have a Jeep factory manual. I looked at the vacuum diagram for the AMC 304 . I started tracing lines. I have more lines than the factory diagram. The pot in question is the exhaust back pressure sensor. It has two ports. One is basically wide open. It takes vacuum from a T on the passenger side front of the intake. Thing is, this "T" is only suppose to have two lines coming off it, mine has three. Mine, runs from a "T" on the drivers side front of the intake. It is going to the correct port on the sensor. The other port on the sensor has a restricted opening. It according to the manual goes to the EGR valve right next to it. I double checked it and it is. It's just backwards from the manual. I am guessing that it really doesn't matter what port it's coming out of the manifold as long as it's a vacuum port getting vacuum.

I'm going to try an plum the engine compartment according to the manual though. It's really odd that it's this different.

 

[jafo]

Check your idle full mix and be sure that BOTH screws are turned our EXACTLY the same amount.
Does the choke linkage move freely?
When was the last time the carb was rebuilt?
Have you confirmed the ign timing?
LG

OK. Yes on the choke movement.

The carb is extremely clean. I believe it to be a replacement carb. So I have no idea when the carb was last rebuilt or by whom. The previous owner told me the engine had 18,000 on a rebuilt engine. So we'll say a carb rebuild 18,000 miles ago.

Negative on the timing confirmation. I just replaced the plugs and wires tonight. I need to confirm the engine timing, hence the popping back through the carb at times.

Another thing I noticed is this is a 2 barrel Motorcraft carb. It looks a lot like a two barrel holley. Looked at some rebuild kits and it appears to utilize a power valve or at least it looks like a power valve. That's a weak spot in a carb when the engine coughs back through the carb. It will blow them out. So, I may see if a kit will do the trick also.

I looked at the plugs. They all are dry, no build up of any kind. They are that nice medium tan shade. I think the mix is spot on really.

My intuition is telling me vacuum leak though. Whether it be intake, or base plate on the carb, it has that rough idle and wants to die when until it's warm. Warm is to me the key here. It will idle fine after warmed up.

I'll just keep stabbing away at it. I'll figure it out. I'm more in my genre now with the engine.

Short list.... check timing. Check intake for leaks. Keep replacing vacuum line. Try and find the PCV valve. Just try to seal everything up top.

 

[jafo]

The short answer: Well yes it sounds like a good deal of the choke system was disconnected and the poor thing was running lean with all the vacuum leaks.

Yeh. It helped a little I believe. Along with the new plugs and wires. I know it couldn't help finding a vacuum line unplugged. It did seem a little smoother with the foot on the gas pedal. Still wanting to die and rough idle. Smooths out with a little gas though. It idels out better after warm though.

Going down to get some smaller diameter vacuum line tomorrow and check some other things also.

Thanks for the replies. I like to throw some ideas around with others.

 

[Hedgehog]

More than likely your Carb. is ready for a good rebuild. Unfortunately running lean for a long time can be bad for any engine. It wouldn't hurt to do a quick compression check.

.... Also not to over state the obvious. It is getting cold out. Engine performance suffers in the cold. Did your idling problems pop up when the weather turned?

..... If your vacuum lines are to loose tighter lines could help. Remember too small or tight lines will fail/crack quicker. Line size alone will not change the amount of vacuum in the line.

 

[jafo]

More than likely your Carb. is ready for a good rebuild. Unfortunately running lean for a long time can be bad for any engine. It wouldn't hurt to do a quick compression check.

.... Also not to over state the obvious. It is getting cold out. Engine performance suffers in the cold. Did your idling problems pop up when the weather turned?

..... If your vacuum lines are to loose tighter lines could help. Remember too small or tight lines will fail/crack quicker. Line size alone will not change the amount of vacuum in the line.

Yes it has the same problem whether it's warm outside or cold.

I think I'll just get me a rebuild kit today and have some fun. After I check the timing. It could have some issues in the idle circuit.

The vacuum lines. What a mess. I looked closer behind the carb in front of the EGR valve and I don't know what kind of mess this guy has going on here. There's "t" connectors and large lines coming off the manifold with hose clamps. It looks like heater line hose. Short stubby pieces. It just all need to come off. It's a mess. I'll try that first while I wait for my carb kit.

I'm not sure on the compression test. It's not a bad idea. It would be one more thing to eliminate.

 

[mtnwhlr]

Is the fast idle working when the choke is on?

 

[Hedgehog]

Even stock there are some odd hoses back behind the Carb. For instance the PCV valve has two little short different size hoses. I thought it was a mess, but as it turned out it was a stock arrangement. The AMC engineers make my head hurt some times, like the heater connection and the water pump requiring different sized hoses. When I was doing Civil Engineering work as a draftsmen and on their survey crew the head Surveyor, Harry, would say, "Don't just throw details together in a plan, make it look like it was actually engineered."

 

[Lumpy Grits]

If it's spit'n back as you say. Get a spray bottle of VERY soapy water. With the engine warmed up and running at idle. Spray every hose connection and around the carbs base. Watch for the bubbles being sucked inside and the engine's idle and sound will change when there's a leak found.
First, check ALL the intake and exhaust manifolds and carb mounting bolts/nuts to be sure they are TIGHT.
Do you know how to set the idle mix? Off idle stumbles is classic sign of incorrectly set idle mix. You should have a vacuum gauge for this.
Get a timing light(inductive is the best choice)and check the timing. If neither of the above is set correctly. You're just going to go around in circles, throw'n $$$ at it.
LG

 

[jafo]

Even stock there are some odd hoses back behind the Carb. For instance the PCV valve has two little short different size hoses. I thought it was a mess, but as it turned out it was a stock arrangement. The AMC engineers make my head hurt some times, like the heater connection and the water pump requiring different sized hoses. When I was doing Civil Engineering work as a draftsmen and on their survey crew the head Surveyor, Harry, would say, "Don't just throw details together in a plan, make it look like it was actually engineered."

LOL? That's funny but believable.

I think I found my problem. It was kind of hidden. I have no idea if this part can be bought. I haven't looked yet. But the fast idle adjustment bracket on the carb is broke. The arm it controls is just laying down underneath it. So, even if the choke is working there is no throttle or butterfly control during the choke or warm up faze of the start up.

 

[Lumpy Grits]

Post some pictures-If you can.
LG

 

[jafo]

Post some pictures-If you can.
LG

I'll get some pics up tonight or tomorrow. I got going on the vacuum lines and got all but two replaced. The dang thernostat controlled vacuum switch on the upper radiator neck must have been cracked. It pulled right off with the hose. Need to get a new one of those now and another 2ft of vacuum line. I bought all they had.

Found:

1. Another vacuum line ripped open from old age right at the connection.

2. The charcoal cannister, the small cap on top was missing. Until I was replacing another vacuum line tracing it towards the cannister and poof, found the cap on the other end of the line. So, with one vacuum line going to that part of the cannist and now finding the cap off of the cannister with another vacuum line, that makes 4 lines that were basically disconnected.

3. While fixing the cannister, I noticed that there was another rubber hose maybe 3/8 diamter rolled and zip tied behind the brake booster. It was hooked to a metal line going back under the tub. It was right next to the fuel line. I assume it was the vapor line comming from the gas tank to the cannister. But there is a hard flex line someone ran from the cannister back to the tank. ??????? Yeh right. Another something to look into. I have to see what we've creatively conceived with that now. So I blew through the old line and it seems clear. Blew in and it pressurized the tank and it blew gas fumes back.

4. The pcv valve, finally.

I cleaned up the back end of the carb area. It cost me a little extra line, but it looks much bettter, less prone to kinking and popping loose.

Got a carb kit comming. Couldn't get to checking the timing and being I'll rebuild the carb, I'll hit the idle mix screws then.

 

[Hedgehog]

Dude its a wonder the thing ran at all! Just replace the PCV valve, they are cheap and need to be occasionally replaced anyways.

If the line going back to the tank is a hard green plastic material, it's stock.

In the rear of your CJ you will find a couple anti roll valves. they are supposed to prevent liquid gas from getting into the Char Coal canister and prevent fuel from pouring out during a roll over. They are inefficient and impossible to find. Someone just came out with a work around valve so I literally ripped mine out and used the new set up. Very nice, very efficient, not cheap at $65, but worth the price.

 

[jafo]

Dude its a wonder the thing ran at all! Just replace the PCV valve, they are cheap and need to be occasionally replaced anyways.

If the line going back to the tank is a hard green plastic material, it's stock.

In the rear of your CJ you will find a couple anti roll valves. they are supposed to prevent liquid gas from getting into the Char Coal canister and prevent fuel from pouring out during a roll over. They are inefficient and impossible to find. Someone just came out with a work around valve so I literally ripped mine out and used the new set up. Very nice, very efficient, not cheap at $65, but worth the price.

Ok you lost me on the roll over valves. I'll try to find them in the manual. This factory repair manual is awesome.

You got me to thinking. You're right. This thing probably shouldn't have hardly ran. I may pull some of the vacuum hook ups out of the manifold and make sure they are not plugged. Ialso wonder with the drop in vacuum, how much that's effecting the vacuum advance.

 

[Torxhead]

Jafo, hopefully the roll over and check valves are still under there in working condition and the main vent line from the check valve to the cannister is still functioning. The word on the CJ net is to use these:DeLorean Motor Company - ROLL OVER VALVE
I have installed one of those on each of the two vent lines from the fuel tank and they seem to work quite well for sometime now. If you need to replace the canister this will work on the later CJs:https://www.amazon.com/gp/product/B001DC9ZA0/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1

 

[jafo]

Jafo, hopefully the roll over and check valves are still under there in working condition and the main vent line from the check valve to the cannister is still functioning. The word on the CJ net is to use these:DeLorean Motor Company - ROLL OVER VALVE
I have installed one of those on each of the two vent lines from the fuel tank and they seem to work quite well for sometime now. If you need to replace the canister this will work on the later CJs:https://www.amazon.com/gp/product/B001DC9ZA0/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1

Thanks for the info.

Been up to my neck in cj this morning. Got my bellcrank kit in and clutch is great! 1 inch of free play in pedal and the clutch "feels" right now. On the drive though, I barely made it home. It was misfiring through the exhaust right at the point of letting the clutch all the way out. I barely made back to the house. I have to go up a small long grade or hill to the house. It was noticeably worse on the hill to the point I thought it wouldn't make it. I had to really work the clutch to keep it running. So itcs not doing well with the engine under a load especially on a hill.

The vacuum lines did help with the cold idle issue also.

Got it home....

Unplugged vacuum advance and hooked up timing gun after engine was at running temp. Rpms were exactly at 750. Emissions tag on jeep calls for 5 degrees BTDC. Found It was set at 10 degrees. So I bumped it back and the idle dropped a little but planed out fine.

Was going to adjust my idle mix screws and noticed gas dripping from either accelerator pump or power valve. Might as well just pull the carb off and go head with the rebuild. So that's where I'm at right now.

Now. With that said, there is one other issue I found while doing vacuum lines. The ignition box? Located on the drivers side fender well appears to have gotten hot. It looks like some kind of epoxy dripped out of the box and ran down the fender well. Not a little, but a lot. To me, if this epoxy was meant to protect the ignition box, and it's not all in there, then I wonder if this could lend to some of the problem with the misfiring?

 

[Hedgehog]

10 degrees advance is not unusual, the book says 5* but 360's really like to run at 10* or more, mine likes 13*. The advance is not likely to be your problem. A decrease in idle speed it normal when you drop your advance back to 5*.

It sounds like your ignition box (Ford) is shot. Upgrade to an MDS box and you will be happy.

I haven't got time to post it now, but I replaced my roll over valve with a different unit than the one Torxhead posted. The one I used is specially made for CJ's. It is easy, safe and flat out seems to work.