I don’t think this is a carb issue. I think the timing is off or there is a value or lifter issue. I am not much on vintage cars but did these come with hydraulic or solid lifters? You might have a sticking lifter or valve.
I once had this issue with a mower engine. It was a Kohler Command commercial line. I bought it for $10 because I could tell it had compression and turned but was being sold as scrap metal. It would start and then die with pops and bangs out the intake. Raw gas was just being spit out the intake and into the air cleaner. There was actually one big pop that shredded the air cleaner once.
This ended up being a sticker hydraulic lifter because the previous owners had neglected the engine. I ran fresh oil and found I could keep it running if I kept the RPMs down. I let it idle for a while and it really smoothed out. I had to be careful mowing under load with it for a while, otherwise the lifter would overpump and the engine would die with pops out the intake. Eventually the engine cleaned itself out with fresh oil and the issue was no more. I changed the oil once more at this point and everything was fine from then on.
Sometimes this classic cars sit for a while between use. I am wondering if something got sticky from not being used often.
Being an old fart, have you tried timing it by ear? If I recall correctly retard it until it slows, then advance 1/4 turn. Maybe the timing mark is off for some reason. 6 degrees sounds low to me, but I do not know this engine.
How many miles? Has the timing chain ever been replaced? Was it with a cam/crank gear with multiple keyways? Many of these have a TDC, and ± a few degrees. Sure the correct key way was used?
Are all your cylinders getting hot? If you have a temperature gun, you can read the temp of each exhaust port. This can help isolate if it a specific cylinder that is not firing correctly.
If you recently replaced your plug wires, it is possible that the cam is a 302 grind vs 351. They are interchangeable but have different different firing order. My stroked 302 to 347 has a 351 cam which has a different firing order.
With the valve cover off and engine turning over, are all you valves fully opening? I once had a new cam during breakin on a 347, mushroom the lobe on the camshaft where the valve was not opening. Those older engines don’t like the non zinc oil that is made today. I think the last oil rating that had zinc and phosphorous in was SL rating and we are now 3 or 4 generations past that now. It got phased out about 12 years ago about the time I had a cam breakin failure.
Have you replaced your cap and rotor? I have seen carbon cracks that have cause the spark to be off.
Its a long shot as well but I assume you replaced your plugs. It is always possible that you bough a brand new but defective plug. It has happened to me more than once.
Another thing to consider is a leaky intake gasket. The SBF had a crossover port where the exhaust gases would cross through the intake manifold. A potential bad gasket could be causing exhaust gas to enter the intake port. I had a pesky smoking 302 engine. I burnt a lot of oil but never through the engine and never fouled a plug. It was leaking oil from the valley directly into the exhaust port in the crossover. That took me years to find.
Could be your firing order. Are you aware that Ford numbers their cylinders differently than other manufacturers. On a Ford, the cylinder on the drivers side front is #1, then 2, 3 and 4. The first cylinder on the passenger side is #5, then 6, 7 and 8.
I got a 69 Mercury pretty cheap back in 76 with low miles because the owners son put on new plug wires and followed the Ford firing order but the Chevy number order. It did exactly what yours is doing. I spotted this as soon as I opened the hood, corrected the wires and showed him that all that was wrong and gave him an opportunity to back out. He said a deal is a deal so I got it. He had been told by three independent garages that it needed a valve job.
hmmm … that’s a tough one. I have a Ford 302 from the same era. The only time I had blowback like that into the intake manifold was
when one of the spark plug wires wasn’t making a good connection to the plug.
one spark plug wire was inducing a spark into another plug wire (cross firing)
the idle timing was way too far advanced or way too far retarded
Since your compression and leak down tests passed on all cylinders, I think what I’d do in that case is first replace all the spark plugs, then if the problem remained, carefully ohm out the connection from the distributor to the spark plug on all 8 cylinders. You may have a bad connection somewhere. Beyond that, my guess would be a valve or valve timing problem, quite possibly a broken intake valve spring. Common sense says to double-check the spark plugs are connected to the dizzy in the right order of course.
If the heads were off there’s always the possibility that one of the valves has too little clearance. There was no adjustment on most of the Winsor engines and push rods of varying lengths were used to get the proper clearance. When heads are removed the push rods, rockers, etc must be labelled to be replaced in original positions. With ~zero clearance a cylinder’s compression will be OK to start but oil pressure will prevent full seating when the engine is running.
If the vacuum gauge is reading below 18 inches, at warm 1200 RPM, and is steady, you have a very tight engine; a major vacuum leak, late ignition timing, or late valve timing. I researched the cam numbers on a Hoeerlin stock cam. Using a check point of 0.050 inches, the intake opening point is 5 degrees before top dead center and the closing point is 39 degrees after bottom dead center for a duration of 229 degrees. So get a degree wheel, determine TDC of the cylinder being tested, set the pointer to 0 degree on the wheel, turn the crank over until the cam reaches the beginning of the lift point, and read the numbers. If cam timing is not the problem, you are going have to find out why the engine has so low a vacuum. The transition and idle circuits of the carburetor will not work at low speed and power levels at this low a vacuum.
Another idea, less likely, but still possible, the ignition timing is not actually as it appears (via a timing light ) b/c of a damaged harmonic balancer.
Before we go blaming parts, I’d make ABSOLUTELY sure that the distributor and the camshaft are correctly timed. As I asked before, can the distributor be installed one tooth off?
Not sure how it works on OP’s engine, but on my 70’s era 302, there’s a certain amount of leeway on the dizzy install. As long as the the vacuum gadget doesn’t get in the way of something else, and the rotor is pointing to the number one position at tdc 1 compression stroke, after that just set the base timing (6 deg btdc), should be good to go. The TDC compression stroke should be verified with a wooden dowel in the cylinder reaching a peak extension. At that crankshaft orientation the harmonic balancer should reads 0 degrees. A dial indicator on the dowel works well to get the exact top of the piston lift. Make sure it is compression and not exhaust stroke of course.
If everything else is within spec the timing can be run back and forth searching for the best setting and working from there. There’s no ECM trying to control anything so just sweep the distritutor back and forth and if something interferes with sweeping while things are improving just move the wires back in sequence to give an additional 20* of available change.
But this sounds a great deal like a valve clearance or valve timing problem to me.
Wow the knowledge pouring out of these posts is incredible. I have tried pulling dizzy and moving g it every tooth possible, and always are TDC on 1. The balancer is always pointing to TDC afterwards. I have not used a degree wheel. I know this has to be something simple because of engine performance at higher rpm’s?
Dazed and confused!
There is not really any leeway on installing a distributor as far as the teeth are concerned. To be sure, use a marker on the side of the distributor where the tower on the cap for the #1 cylinder is. Remove the cap and rotate the crank pulley until the engine is at TDC for the compression stroke for #1. The rotor should point directly at the mark you made, or very close. Also the points should be open.
If it is not close, you could be cross firing to two cylinders at once. If it is correct, then rotate the engine backward to the specified timing mark for idle with the vacuum disconnected. You can rotate the distributor back and forth to where the points are just opening. A quick check is to turn the ignition key to the run position engine not running (not started) and you will see a small spark across the points just as they open.
Tighten the distributor down and your timing should be correct.
Ironically the Jeep 4.0 engine had a cross fire problem for a year or so @keith. A technical service bulletin advised widening the slot in the mounting base of the distributor. That wasn’t a common problem with older cars but it did occasionally happen due to the design and/or poor manufaturing tolerance of the cap and rotor. A uniformly high firing spike on an oscilloscope could quickly bring attention to an excessibe air gap in the cap.
I’ve never done the experiment, but it seems like you can always rotate the distributor to compensate for getting it installed a few teeth off from where it was before; i.e. as long as the rotor is pointing to the number one plug at tdc-compression. Is there a reason that wouldn’t work, other than if you are too far off it will be physically impossible to rotate the dizzy enough to get the rotor pointed to number 1 plug? Conceptually it seems like if you could rotate the dizzy a full 360 degrees you could install it whatever orientation you like, then point the rotor to one of the plug positions and call that number one, and wire the spark plug wires from the top of the dizzy to the SP’s accordingly.
Over the years I have dropped distributors in many engines paying no attention to being in time. Once fully dropped into place and the hold down bolt snugged down the crank is brought to TDC and the wires installed with #1 wire aligning with wherever the rotor is pointing then the engine cranked and if it attempts to start a little twisting of the distributor will get it close enough to run. If the engine will not even attempt to start cross the wires and it should start.
Long ago dealing with engines on test stands I was required to repeatedly remove distributors and crank the engine over, remove the plug wires and then while being timed install the distributor and get the engine running. There were V-8s from the Big 3 there and all were scrambled, unscrambled, started and timed to spec and the operation tested on a Sun Machine with the spark displayed on an oscilloscope and there was never a problem with being 1 tooth off on one of those engines but those engines are real antiques today just like the OP’s '70 Mustang.