This might get a little long winded, but I'm gonna guess the more details I give, the easier it might make it for someone to narrow my problem down.
1977 F250 with 351M
-New parts-
Edelbrock performer 400 intake-non egr
Edelbrock 1406 carb
Edelbrock air/fuel ratio gauge and o2 sensor
Heddman headers
ignition module
complete distributor including ignition pickup
distributor cap and rotor
spark plugs (factory heat range autolites)
spark plug wires-(forget name, but GOOD set)
ignition coil
air filter
fuel filter
factory mechanical replacement fuel pump
pcv valve
valve lifters

Ok... problem is this... runs and idles great when cold, but then when it warms up it idles *severely* rich (your eyes water from the exhaust) and the a/f guage indicates this also but not when cold (after choke comes off). The fuel mileage is absolutely horrible... even for this motor 5 mpg is off the wall and I don't drive it with my toenails in the 4 bbl at all. The carb is brand new but nevertheless has been checked for anything abnormal such as float adjustment, etc. and I have been up one side of it and down the other with jets, needles, springs, etc. to no avail. If you lean the carb out enough to get the a/f gauge to show proper (or at least close to proper) ratio, it either won't idle at all or idles like it has a rowdy cam in it... which it doesn't... verified stock. I adjusted the carb edelbrock's way... same thing... adjusted it for best vacuum... same thing. Check for *any* vacuum leak revealed absolutely none. Compression test came out excellent as did cylinder leakdown test. Timing is dead on @ ten degrees, vacuum advance giving 10 additional and centrifugal giving 10 on top of it all. Fuel pressure is a steady 6 psi. Vacuum gauge reading at idle is 18 inches with needle fluctuating slowly and rythmically down to 16.5 to 17 then back up to 18. To me, this seems like a sticky valve, but you could just about eat out of the inside of this motor (however I will not rule it out). When the gauge flutters, the idle changes accordingly, of course. What can be seen of the cam lobes through the lifter bores shows *very* little wear and absolutely no signs of a lobe failure. Valve springs show no breakage or collapse and rocker arms and pushrods are in excellent condition and when engine is run with valve covers off, everything is oiling fine. Lifters were replaced due to slight clatter from several and they needed to be eliminated as a problem. A mechanical oil pressure gauge (snap on diagnostic gauge) shows cold idle oil pressure around 55 psi and warm idle oil pressure around 28 psi (idle being 750 rpm) but at warm idle, the oil pressure gauge needle will fluctuate several pounds almost exactly like the vacuum gauge. I simply cannot find the oil pressure specs for this motor and am wondering if this has something to do with anything because it seems like as the engine warms up and the oil pressure drops, the problem(s) get majorly worse. Also, engine temp runs normal at 195 degrees(checked with thermometer), but header tubes checked with infrared thermometer show different temps even when checked at several different distances from block which sounds like an unbalanced carb but both idle mixture screws are within a 16th of a turn of each other for idle-best mixture and seem to be functioning normally when turned in/out. More than likely there is some other diagnostics that I have performed and forgot to mention here, but I'm about out of ideas as to where to go next except for the oil pressure issue (if there in fact is one). Oh... almost forgot... if I bump the base timing up more than a couple degrees, it pings on acceleration/high rpm cruise... don't know if this matters or not but didn't want to forget it. Also, I bought the truck with the carb, intake and headers on it and simply replaced them because carb and intake were improper ones for this engine and headers were rusted out. Edelbrock gave me the suggestions for carb and intake based on my information. It has been verified that the engine *is* a 351 modified and the heads are also correct for the engine. I can't help but feel that this is somehow a valvetrain issue but I'd like some other opinions because maybe there's something simple that I'm overlooking. Any hints, tips or suggestions (short of buying another truck or engine) would be greatly appreciated!

 

Did you try a different known good carb ?

 

Yep... had a professionally rebuilt edelbrock 1406 lent to me and once installed, all the symptoms were the same. Another friend lent me a 750 cfm edelbrock and as before, all symptoms were the same when installed.

While doing some more "digging" here in the forums, several topics made mention of timing chain "slop"... hmmmmm... wondering if the valve timing might be a little off due to a sloppy chain? Could this possibly be causing some of the problems?

 

i think you might be on to something with the vacuum...something is causing it to vary too much...should be very tight at idle...valves are not adjusted too tight i assume...feel for you and hopefully folks here will be able to help.

 

I definitely agree that the vacuum should be tight at idle... the bouncing needle is sending up a red flag for me. When the new lifters were installed "dry", I torqued the rocker stud nuts to 20 ft. lbs. according to ford specs (those I found) on the low spot of the cam lobes so as near as I can tell the valve adjustment should be correct.

 

AFB carbs don't tolerate alot of fuel pressure. Go over 6 psi and it will overflow the bowl. Maybe look into that angle. Don't know how that would be related to operating temperature though... Maybe try 4 pounds and go from there. Just an idea.

 

Did you try running it without the pcv valve hooked up ? What are the plugs looking like ?

 

You mentioned 'tuning' the carb... Have you calibrated it yet? If you are at a high altitude you will need to lean the mixture down by a jet and metering rod change. I changed three stages lean to get mine to run right at 3000 feet altitude. It was very rich too, which is why I calibrated it. Fine now.

 

It shouldn't run rich no matter what else is going on. I always use an off-road needle valve on an Edelbrock. I also used a Carter Electic pump to gaurantee 5psi fuel pressure.

 

Thanks to all for the suggestions! I'll try to answer the questions in one reply here. I think the next thing I'm going to try is an electric fuel pump with regulator to make absolutely certain the carb is not being "flooded" with too much pressure. I haven't tried running it without the pcv valve unhooked, although I did remove it from the valve cover on the off chance any oil vapor would be causing preignition or detonation and nothing changed. The spark plugs are a little bit of a mystery... tan on one side and nothing on the other side. This almost leads me to believe that the fuel is being burnt unevenly in the cylinder, but I'm not sure. I have attempted to calibrate the carb, but it would seem that any calibration makes one symptom better and another worse or creates a whole new problem. The truck does ride like a stagecoach, so I will definitely be acquiring an off-road needle and seat for the carb to ensure this is not contributing to the problem since on a rough road, when your foot is bouncing slightly on the throttle you can watch the air/fuel ratio meter act accordingly (richens slightly because of the accelerator pump I'm assuming). Does anyone think that if the cam sprockets and chain have enough "slop" in them that the valve timing may be off enough to be causing some issue? Also, since I haven't been able to find them yet, does anyone know what the warmed up idle oil pressure specs are for this engine? From everything I've read, 28 psi at warmed up idle is acceptable, but I'd rather have it confirmed. Again... thanks to all for the great suggestions... I'm going to go through everything that's been suggested and hopefully get this problem solved! I'll keep you updated!

 

non egr intake. How about the distributor....non egr spring?


nevermind....you said "idles" rich when warm.

 

This engine sounds just like mine only mine is worse. Terrible idle and part throttle operation. Worse when warm. RPM rises and falls with vacuum readings. Runs great off the mains. Fuel bowl is not flooding because I've operated it with the top off the stock carb just to verify what was going on. Great compression.

Things I've done to try to fix problem.

New cam gears. New intake manifold gasket. New egr valve. New GM style HEI distributor and wires. Adjusted the idle circuit every way imaginable. New fuel pump. Checked and double checked every possible vacuum leak source including PCV.

You and I are in the same boat. If you ever figure out what to do please post.

P.S. My 50 yr old professional mechanic brother and his partner are both stumped too. I've been running this truck in this condition for about 2000 miles over two years. I recently pulled my plugs because It was barely running. Two rear plugs looked perfect. Front six were fouled with carbon deposits and wet. I really wanted to find 4 good and 4 bad so I could blame a bad idle circuit but the two good plugs were from two different sides of the carb. I've seen the rear two plugs on some other engines get oil fouled from oil leaking down from the heads but never seen them be the best of the lot.

 

Bad Power Valve?

 

You might just have to start from scratch. I would get the dial gauge out, pull the valve covers, and measure max lift of each pushrod to rule out camshaft issues. I would also put a degree wheel on the crank and check cam timing. As far as oil pressure, you won't likely find any specs, as it will never be the same on any two engines. 28 psi at idle is fine though. Vehicles equipped with just a low oil pressure light will sometimes flicker when idling to show pressure is low then go out when you are moving. You could spend a lot of time on this trying to find the problem, but you will feel rewarded when you do. You could essentially tear the engine down and rebuild it just to measure main bearing tolerances.

 

I think both of you guys are going to find intake gasket problems.