I have a 79 F250, 4 speed, 400 that is stock except for a edelbrock rpm intake and a holley 4160 750cfm VS carb. I don't know how many times the odometer has turned over, but it has at least once since I have owned it. The truck is my daily driver so getting it fixed (or at least finding out what could be the problem) is a big priority. It does burn a bit of oil, but hardly noticeable.

The truck has always ran great and has instant throttle response, but it has always been pathetically rich. It is a manual choke, but no matter the temperature, pump it once and it will instantly fire with no choke. It will blow black smoke at or near full throttle. I get a variable 7-9mpg. My driving conditions are the same, I have no idea why it fluctuates that much (percentage wise that is a big change). I realize that my carb is too big and has the wrong jetting (I bought the truck with this setup), and I do want to fix this. 7mpg hurts.

Right after filling up with this last tank, idle and open secondaries are fine, but with just the primaries are open I get a popping sound (not sure if it is through the exhaust or carb, but I believe the carb) and the truck starts bucking but will hold a steady speed. Get into the throttle and open the secondaries and all is well. It is the worst right at 45mph just before the secondaries begin to open. My thoughts were that I got a tank of bad gas or I got some dirt in the primaries causing the truck to run lean when I was just in the primaries.

I pulled a couple of plugs today to look at them (cyl 4 and 5), but I can read plugs about as good as I can tune carbs... Looking at all of the guides online, it seems that the carb is actually running really close to where it should be, but my timing is slightly off. This is judging from the "clean" plug out of #4. The plug out of #5 is pretty sooty and tells about the opposite. Both have too much gunk on the threads to see the heat range. I was hoping that someone on here could give me some insight. The truck was shut down after being drove about 5 miles without engaging the secondaries with about 15 seconds of idle time at the end.

I will be filling up with new fuel tomorrow as this tank is empty, and if that doesn't help, I can take the carb apart this weekend. I just want to make sure that I will not hurt the engine between now and then.

Index of /od/plugs
366 and 369 are the best pics.

 

You won't hurt anything by driving it. If your fuel doesn't work out then I'd put a fuel pump psi gage in the line and see where your at.

 

I was worried that it might be running dangerously lean and pop a piston...and I was close to correct. It continuously got worse this week and also started doing it when in the secondaries, so I stopped driving it. This is what I did in case anyone else ends up in the same situation.

I was popping through the exhaust (I have straight pipes dumping right past the cab, so it was hard to tell). I hooked a wideband up to this truck and found that I was pretty much off the chart lean at 15" of manifold vacuum on up (I idle at 21" and 15" is typical top gear cruise), but idle was still a tad rich.

I checked float level (which I probably should had done to begin with) and found that I was running 1.5" below the sight hole in the primaries (leaving just under .5" of fuel in the bowl) and .5" in the secondaries. I had adjusted the floats just a couple of months ago. Assuming that my float needle valves were stuck, I rebuilt the carb today. The needle in the primary was stuck, although the needle in the secondary was still moving. There was a ton of crap in the secondary fuel bowl, but not in the primary.

I went on ahead and dropped it down 2 jet sizes (72 to 70) as I know before I was running pretty rich at cruise and installed a quick change spring kit for the vacuum secondaries. I left the power valve the factory 6.5 for right now, but I have a feeling that I will be going to a two stage power valve when it is all said and done for this application. I also installed a new fuel pump just for good measure.

I have rebuilt more holley carbs that I can count, but I have never really tuned a carb before, just get them close so they run good and let it go. I guess I am too young to really grasp reading plugs, but it will be considerably faster using a wideband. This should be really interesting.

 

This is the BESTEST PLUG READING INFO EVER!!!!!!!!!!!!!!!!!!!

sparkplugreading

 

That is a pretty good explanation, but as it states up top, it is for WOT tuning. My concern is low-medium throttle and cruise, which I have found is a lot harder to read plugs on.

Since this thread was bumped again, I will update my experience for anyone's benefit.

This has been real interesting. I have had more carbureted vehicles in my life than fuel injection and raced strictly carbs until last year....but I have never had anything throw me for a loop quite like this one did.

After reinstalling the carb I drove it all weekend and it ran excellent (it is my daily driver). I was getting just a hint of surging under accelerating, but hardly noticeable. I drove it into work that night, and upon leaving my driveway I noticed a lack of power. I had to slip the clutch quite a bit to get it moving, but we drove away fine after getting up to speed. About 2 miles down the road, a single sputter and she was DOA, and it was pitch black. I coasted into a church parking lot. Next morning I went to pick it up and she fired right up. About a mile down the road, a few sputters and she was dead. Towed the rest of the way.

I spent that whole week diagnosing the problem. No spark. I followed the ford diagnostic procedure step by step and it came up with the duraspark box...no real surprise. I replaced it with no success. I took a look at the pickup and it was bad despite passing the ohm test. Upon trying to replace the pickup, I found the vacuum advance was cracked off of the distributor. Of course, being a 400 the distributor was froze in the block (I knew this already as I tried to adjust timing before). After trying everything to get the dizzy out with no luck, I dropped the pan and beat it out of the bottom. I am a diesel mechanic and work on semi engines, 11L+ all day long and lets just say, I won't do that again. It lacked a whole lot of fun.

With a new distributor and a new coil (after starting the truck, I found that it was shorting out through the case the hard way), she ran great. Drove it to work that night. Huge hesitation off of the line. Acceleration was real slow, even killed it a couple of times, running stupid lean. Brought it home and adjusted the accelerator pump linkage. 5x better, but still a slight hesitation, so the next day I adjusted it a little more. 6x better, but not quite there. I gave the idle mixture screws an extra 1/2 turn and it was pretty decent, although idle was now a tad rich.

Then I started focusing on cruise. I was getting a slight surging. I pulled the plugs and found them to be incredibly white. I am running really lean. I put the jet sizes back to 72 (this is the only thing in the carb I changed with it apart). Surging at cruise is gone, but hesitation/surging is back under acceleration and pretty bad again.

My conclusion currently is that I didn't have a carb problem originally, the pickup in the dizzy was starting to go out and I had a travelling miss. This caused my wideband to read lean. Other than being a bit rich, there wasn't a single thing wrong with the carb.

I have no idea what my problem is now. I have the carb set just like it was originally, same jets, same PV, same turns out on the idle screw...but it still runs like it is really lean. I just don't get it. I have adjusted this carb "by the book" with the single exception of pulling the carb off and checking how much of my transition slot is showing. I didn't do that because it is a PITA and I have been fighting what I think to be bigger problems.

Is it possible that too much of the transition slot is showing causing my acceleration issues? The truck won't idle below 750rpm (stock is 650), so the idle is adjusted higher than it should be...but this is also an aftermarket carb in the first place.

 

Yes it could be possible that the slots are showing too much, my 750 DP was that way. Could not get it too idle for crapand had a bad hesitation, thought it was a cam issue (big cam). I backed the fast idle out then fiddled with the mix screws, then set the fast idle and idles good and no hesitation. Could be your running on the "run" circuit of the carb, not letting you set it up properly.

You said you adjusted the accel pump linkage, double check the pv (make sure its working), also look real good for a vac leak. (I am sure you prolly did that). But that would really mess with ya! also cause lean conditions too if the leak is big enough.

 

One quick test I read is to check to see if running the mixture screws all the way in will kill the engine...and it does. I may be pulling fuel from the transition, but it isn't enough to keep it running. This truck has no choke and I know that is absolutely killing the drivability when cold. When I got the truck the mixture screws were 3.5 turns out...amazingly rich. I bet the exhaust out the pipe was explosive. The mixture screws set properly at 1 1/4 turns out, but drivability actually went down with the lack of choke.

I swapped jets yesterday and checked the PV while I was there. I replaced all of the vacuum hoses while I had the dizzy out. Nothing more than the booster and vacuum advance since this engine is pretty bare. The only other vacuum port is manifold vacuum which I have running to a gauge in the cab hoping for some insight.

I am pretty sure I have the wrong PV, well, I know I do. It is the stock holley 6.5, but I pull 20-21" at idle. I noticed that right around 10-12" of vacuum, depending on throttle position, I start to surge. The numbers say to use a PV half your idle vacuum, and it seems to be right. However, at this point it runs tremendously better with more throttle than less. I had to take the wideband back off for the race truck and it is a serious pain switching them over so I am running blind again on my a/f. Surge says lean. I was considering getting a 2 stage PV, but I can't find a whole lot of information on them. Application is fuel economy and big vehicles, so I think it might be a good choice.