I may have found the problem. It's definitely A problem anyway.

I set the idle screws back down to approximately where they should be on a well tuned engine. The engine went back to its strange slow surging ways and became more susceptible to rpm changes when sprayed with carb cleaner.

It was revving when I sprayed the back of the manifold so initially I thought the manifold is leaking. I decided I better make sure it wasn't the carb or egr gaskets first. I sprayed the at the back of the EGR plate at the back and I got a little rev.

I took the carb and plate off and inspected the gaskets as I went. When I got to the manifold I realized I had forgotten that the machined surface of the manifold has a bit of pitting. That coincided with an apparent gap in the gasket. Bingo. I guess the manifold is going to come off now anyway.

Looking at the carburetor gasket, it kind of looks like the back of the base of the carb might be warped. I'm going to set a straight edge against it tonight to try to see if it is.

 

Not one to say I told you so but Nay nay na nay nay.

 

Just kidding. You did. But I was reluctant to go there since I had just replaced it.

 

Well, I think you are really zeroing in on the problem.

 

I've seen that happen a lot. There actually were two different EGR plates I have seen. One steel and the other aluminum. Obviously the aluminum doesn't last as long. You wind up with an internal vacuum leak that is very hard to diagnose.

I'd be inclined to eliminate this system as it never really worked well in the first place. Late 70's were a time of experimentation for emission equipment.

Good luck

 

The pitting in this case is on the cast iron manifold. I think I'm going to get an aluminum intake.

I checked the bottom of the carburetor and it doesn't look warped.

I'd like now to get an aftermarket 4bbl intake in anticipation of eventually getting a 4 bbl carburetor. An adapter plate should act as a spacer that would effectively replace the current egr plate height.

 

So... I shouldn't assume whoever worked on my pickup in the past knew what they were doing. Whoever did the EGR delete seems to be much to blame for my troubles.

See what some other people on the all knowing internet and I have been calling the "EGR plate?" I knew it wasn't the correct plate for the engine but figured it still was an EGR plate. Nope! It's just a spacer plate with a vacuum port for a Motorcraft 2100! NOT 2150!

I didn't notice this until I filed down the intake surface to get past the pitting and set the spacer plate back on to see how it fit. The stupid plate doesn't even cover the exhaust port hole in the manifold. DUH! No wonder the darn thing is leaking!

I have a Felpro 60529 Gasket on the way to try to fix this problem. Hopefully that will fit. Good grief.

But hey! I am optimistic that she's gonna run!

 

That should help. Big time.

 

Well, I re-installed the intake and carb a few weeks ago. I got that Felpro spacer mentioned above. It is shorter than the previous aluminum vacuum plate. Because of that, I removed the hot air lines to the electric choke, so now it's electric only. I turned the choke a bit farther to the rich side in an attempt to compensate.

Here is a poor picture of the filed manifold.


Here is a view of the Felpro gasket/spacer.



I got everything together and the problem remained albeit to a lesser degree.

It will surge when the idle screws are set at a "normal" place ~2.5 turns. It seemed happiest at 3 - 3.5 turns. It would backfire a slightly on deceleration and miss some on acceleration in cooler 50 degree weather.

I borrowed a timing light. I verified that the 0 degree timing mark was actually TDC. I checked the timing and it showed about 12-14 degrees BTDC. It's a good the timing is right because some previous idiot rounded out the distributor lock bolt. The mechanical advance appeared to be working and would advance the timing when RPM was increased. I re-attached the vacuum advance and that seems to be working as well.

I re-checked and adjusted all the carburetor settings. I adjusted the vacuum pull-down again.

I used a vacuum gauge to set the idle screws. I was getting highest vacuum at around 23 lbs with the screws at 3 turns.

I set out for a drive. It was about 65 degrees. It worked well. The only problem I noticed on that drive was a little bit of quiet backfiring/popping from the exhaust on deceleration. The throttle response was weak at 1/2 or more throttle position. Video of the drive can be seen from the link below. The backfire occurs around 1:20.

https://www.youtube.com/watch?v=u_NZ...ature=youtu.be

I parked the pickup and waited a couple of hours for sundown and cooler temperatures (denser air). I fully expected the thing to act up when it was colder.

I was right. It was about 50 degrees this time. It started fairly easily and idled pretty well, just as before. It started missing around 2000 rpm. Backfiring was worse on decel than before. Video link is below. The most obvious missfires start at about the 1:30 mark.

https://www.youtube.com/watch?v=fnql...ature=youtu.be

I'm open to any new ideas, but I've come to the end of my rope so to speak. Yesterday when it was warm out I gently drove it ~15 miles home to town. After a year of this, I'm ready to give up and fork out cash for someone else to hopefully fix this. I want my pal back.

Thanks for all your help.

 

First, this is what I get when I try to view either video:




Anyway, this may be a complex problem as there seem to be several, inconsistent, symptoms. So, one at a time...

According to this site "In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where is fails to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust pipes. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe, causing the backfire." But, you've adjusted the idle mix with a vacuum gauge so should have it rich enough.

However, where do you have the vacuum advance connected? If it is connected to manifold, as opposed to ported, vacuum then you may have a problem with too much timing as well as a issue the position of the throttle blade at idle.

As for how far you have to open the idle screws, 3 turns is quite a bit and sorta implies there is still a vacuum leak. In other words, that extra air is getting in from somewhere and you are needing to open the screw more to make the air/fuel ratio what the engine wants. However, 3 turns is not excessive so it may be that the carb is just a miser. But, it also implies that you don't have a power valve that is blown or you'd have more fuel that you wanted - so that's a good thing.

 

Sorry about the video links. I think I have that corrected.

I agree that it still seems to be running lean. I just don't know where else to turn. All I have running off vacuum are the breaks, the distributor, and the HVAC. I still have the vacuum reservoir deleted. Unless I screwed up the intake manifold installation and/or filing the machined surface below the carburetor didn't work, that should be better too.

It just baffles me how things are basically the same no matter what I change.

The vacuum advance is connected to the carburetor. This picture is now outdated as far as the gaskets and spacer plate, but the vacuum advance hose position is the same. It's the little hose in the foreground.



I'm pretty sure the power valve is good!

I wonder if this whole things stems from the missing EGR valve and problems are catching up. If carb is calibrated for an EGR and doesn't have one, perhaps that is part of the problem.

I still wonder if it's going to take a new manifold and carburetor that are meant to operate without and EGR to fix this.

I guess that's why I may let a mechanic try to find the problem.

 

What did you do with the port on the carb where the hot air from the exhaust manifold connected? That is open to vacuum in the carb so it can suck in the hot air, and needs to be plugged.

As for the vacuum advance, pull the hose when the engine is idling and see if there is vacuum at the connection. The term "manifold vacuum" doesn't mean the hose has to connect to the manifold. It means the vacuum comes from the manifold - but most carbs have a fitting on them that connects to manifold vacuum. You want a connection that has no vacuum at idle. It makes a big difference in how your throttle plate closes and, therefore, can have a huge impact in the way the engine runs at part throttle.

 

I forgot to say that I plugged the vacuum port for the hot air/choke.

When I had the timing light hooked up, there was no difference in timing in idle with the vacuum advance attached or not. With it attached, a bump in throttle would quickly advance the timing. This seems to indicate the vacuum advance is working properly.

I observed the mechanical advance without the vacuum advance hooked up.

 

Glad you plugged that port. And, it sounds like you are using ported vacuum for the advance - no vacuum at idle but you get vacuum as soon as you open the throttle a bit.

And, the videos now work. Thanks. From them I think there are two different "backfires". In the first video I think that is the rumble that sometimes comes from unburned gas in the exhaust as you decelerate. I'm not sure how big of a problem that is, nor how to fix it. But, it may be timing related - like too much timing. Not sure, though.

However, I think the issue in the second video is that the engine is way lean because you don't have enough choke on it. Notice that it happens after you've driven a bit, and when the choke has probably come completely off but the engine isn't warm. Rusty will do that at the corner about 1/2 mile away in the winter if I don't give it more choke, like I usually do in the winter. Try dialing in more choke and see what happens tonight when the air and the engine are cold.

 

I'll do that. My boy will be dying for a ride tonight with it sitting at the house all day in plain site anyway.