I'm thinking valves. Did it take a lot of revolutions for that one cyl to come up to 110? Its a stumper. I wish I lived close so I could take a look at it. I love puzzles.
I had a similar problem with my '77 Maverick. It turns out my idle screws were almost completely vibrated out of my throttle arm. It seems you covered some of the major things maybe it could be something as simple as that.
If you have access to a leak down gauge. Wait for the engine to run bad then turn it off. Use the gauge to determine valve/ring condition. could be a bad spot (carbon/crack) that only shows up when the valve rotates to a certain position. Have you checked cam thrust? that could affect ignition timing. How about timing chain play? Flat lobe on cam? leaking/concave lifter? bent/deflecting/weak pushrod? Worn/deflecting/cracked rocker? Someone did mention the ground cable. Very good thing to check. Jumper cable from neg battery to engine block can eliminate that possibility(or cure your problem). Motor getting hot and vapor locking? Had a Jeep that did that and did the real old school trick on it. Put WOODEN clothes pins all the way up the fuel line. One of those things you'd never think would work but, does.(wrapping the line in solid copper unshielded wire works too) If it is POSITIVELY not a fuel problem(have you checked the fuel pressure and volume?), vacuum leak(btw have you checked the brake booster if you have one), or a spark problem. It must be internal engine problems.
Thanks for the ideas. I'll check the ground cable. It starts right up but maybe when it is running.....Had a battery that went bad on a Falcon six. When it got hot the plates would open up. Never noticed it until I made a short trip and car wouldn't hot start. Got a jump then when waiting for a train the idle dropped and the generator cutout relay dropped out and motor quit. It was running on the generator only when it got hot. No slack in the timing chain. Replaced the fuel pump yesterday, and rubber hoses. Now it runs bad from the get go, missed even before it is warmed up. I put some Seafoam in the intake, crankcase and tank. Maybe that will loosen whatever is sticking and make it better or worse. I haven't run it for long after that. Will try to get it hot and circulate the stuff thru the system. I'm about ready to change the head just for drill, or at least take it apart and change the lifters. It doesn't make any mechanical noise at all so I'm reluctant to tear it apart. Would eliminate any valve problems. Wonder if Pick-a-part has any cars this old. Next step is to drain the tank and get some different gas to eliminate that. This will be a good "What went wrong" article for Skinned knuckles when we fiigure it out. John, still puzzled in Kalifornia
Two examples: 1) 05 Mustang 4.0 v6 ran terrible, but no noise. broke a valve stem, dropped a valve onto the piston. The piston drove the valve through the cylinder head like a nail. ran on 5 holes but, not one mechanical noise. 2) 04 Dodge Ram with a 5.7 hemi. Common problem, broke about 2 truns off of a valve spring, letting the valve float. ran ok with a little miss at idle and got worse as you drove but, again no noise. If you changed the fuel pump and the nature of the problem changed, I would believe it was a fuel system problem. Have you checked the fuel line all the way back to the tank? including the metal lines. these cars are old and those metal lines can rot from the inside out. And, even a pin hole in a fuel line that doesn't leak when you are not running can pull a little air in when you are. Put an inline clear filter in and watch for bubbles and/or debris.
I can't beleave you haven't found the fix yet. Have you honistly considered maybe just another engine all together? I'm sure you could find anouther 6 or v8 with just as much trouble as your putting into finding out what's wrong with this one
i just reread the whole post. i still think its a carb problem. i suspect both carbs used have the same problem. it does seem to be geting excesive fuel. now when you said that the idle screw had to be turned out 5 turns, were you talking about the mixture screw or idle speed screw? if you unplug the pcv hose and leave it open to create a big vacuum leak how does it run if it does?
Today I checked the grounds from the battery to the engine and to the frame. Wiring is OK. Haven't ran it for very far since the Seafoam treatment. Maybe tomorrow I'll take a tour of the neighborhood and see if I can get some answers, or at least get daa on when it runs bad and if it is temp related. There is a rubber hose between the gas line and the fuel pump. I think I can Mickey Mouse a can under the hood and run off that. fill it with NEW gaas from a different brand of gas station. This will eliminate the fuel system. I agree that is SEEMS like a carb problem. However that wouldn't explain why it ran good for awhile after it was adjusted the other day. With new filters and gaskets there was no way that the carb could suddenly mess up. On the Carter RBS the float bowl is to the side. Four screws and you can drop the bowl. I did that and the level was good, gas was clear no dirt ot water. It would be easy to give up and put in a new engine but I would feel real stupid if it was something simple like bad gas and the new nmotor ran the same. If I can find a cheap head at pick-a-part I might get it redone then swap that in. Too many parts needed for a V8 swap, which is what Mikey wants. A dead rusted out east coast donor car would be the way to go for the V8 conversion. The 250 C4 trans will fit the small block, I believe. More tomorrow. Don't you love a mystery? John and Mikey
Latest update: We spent the day going over everything, thinking a fresh look might reveal some problem we missed. Runs worse than ever. Number six cylinder failed the leakdown test, something like 20 percent. I think that the bad rings are causing a real problem with the manifold vacuum. We changed the ignition system AGAIN, swapped carbs, etc. Nothing helps. I'm tearing it down. John and Mikey
