Have recently acquired a 71 Ranchero Squire with 31,000 original miles. Untouched throughout, belonged to a Ford dealer/collector. The 351C under the hood has never been apart. When it was delivered to me, it appeared to be starving for fuel. Turns out that the fuel tank was full of crud. Entire fuel system removed and cleaned, along with complete rebuild of the 4300 4 bbl by a well-know carb shop. Full tune-up done, including electrical, filters, fuel pump, etc. No fuel problems now. BUT, the thing idles so bad you'd swear it was running on 6 cylinders instead of 8, or the firing order was screwed up (it's not). Nothing has helped. Compression test normal, as expected. No vacuum leaks found. Motor revs fine out of gear, but not much under load. I'd appreciate any and all suggestions as to where we go from here. This is likely the nicest unrestored 1971 Ranchero in existence, and it currently can't be driven.

 

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Maybe try replacing the fuel filter again. Possibly a piece of debris could have worked it's way into the new filter, and it's a cheap place to start. Also double check the distributor timing to make sure it is correct.

 

A couple of questions. You said it revs fine out of gear but is it still rough? If so I'd take a very hard look at the distributor cap. A fine, hairline crack can be very hard to detect and cause a major problem...BTDTGTTS (Been there, done that, got the t-shirt) OK...so that was only 1 question.

Second thing to check is the placement of the spark plug wires. In the firing order of the 351 #6 cylinder fires right before #5. If the wires are run together all the way from the distributor to the cylinders you can get inductive firing between the 2 which can cause a rough idle. To solve this you have to reposition the wires so that they don't run together for too far.

This is of course assuming you understand the cylinder numbering on Ford vs. GM V-8s and that Ford has 2 different firing orders for older V-8s. Fortunately all of Fords 351 cid motors have the same firing order.

 

I thought the 351M and 351W have different orders. My chilton's lists the "351" as 1-5-4-2-6-3-7-8, but when I wired my 351 Windsor that way, it wouldn't start...because that firing order was for a 351M. The firing order I needed was actually 1-3-7-2-6-5-4-8. I'm not sure which firing order is that of the 351C, but with my experience with my chilton's ambiguity, I'm not making any assumptions.

 

Chiltons is wrong. The 351C, M/400 and W all have the same firing order 1-3-7-2-6-5-4-8. The 289/302 and 429/460 firing order is 1-5-4-2-6-3-7-8.

 

It appears you are right about the misprint. My 400 is wired the same way as my 351 Windsor. Chilton's has it backwards...they have the 351 firing order listed under 390, 427, 429 and 460. That's a pretty bad place to have a misprint!

 

Rough idle can indicate valves not seating properly. this may not show up in a compression test. Try a leakdown test.
Bad valve seating will smooth out at high RPM.

Exhaust system blocked? A blocked exhaust system will show up under load, but not at idle or unloaded. The same is true of a blocked fuel filter. Neither of these should affect the idle.

 

Since it revs fine it seems that the idle circuit of the carburetor is suspect. Check the idle mixture screws. It doesn't matter if the people who rebuilt it are experts or not, debris could have still clogged something in the carburetor or there is a bad part like the power valve or maybe a gasket not sealing good.

Something else rings a bell here. If the secondary throttle plates are not closed enough at idle this will cause a poor idle condition because this is essentially a vacuum leak. I'm not trying to make your carburetor rebuild team look bad, I'm just naming everything that I know cause poor idle without getting into the engine's internals.

Also when the engine is at idle the vacuum is higher, so if there is a vacuum leak it will have more of an effect on fuel mixture at that time. There are plenty of places for a vacuum leak: vacuum hoses, intake and carburetor gaskets, any vacuum diaphragm (especially the power brake booster), PCV valve. One place often overlooked is inside the dash where there are even more vacuum lines and diaphragms than under the hood. I have had the vacuum hose loose from the transmission modulator valve under a car before, so check this line as well.

I see where you say no vacuum leaks found, but trust me you are just kidding yourself if you haven't installed new vacuum hoses yet.

 

Thanks to all for the suggestions.

A closer look at vacuum lines revealed that the two lines for the dual-diaphram distributor had been reversed (port to retard, manifold to advance). Putting them right stopped the severe idle shake, and some toying with initial timing has made huge improvements.

That distributor, though, surely seems to be a good place to create driveability issues. We will likely be testing different settings, as the 15 lbs of idle manifold vacuum certainly raises havoc with the initial factory setting of 8 BTDC.
Any thoughts on this issue? Because of the nature of the vehicle, I do not want to "upgrade" the distributor.

Thanks again to all who responded

 

The vacuum advance is supposed to affect the timing. It's going to be even more prevalent on a dual diaphragm system. That's why Ford has you disconnect and plug it before setting the initial timing.

One of the best mods you can make to a points vehicle without modifying the distributor is to put in a Pertronix ignition module. It simply replaces the points & condensor and it all fits under the stock cap and makes a huge difference in driveability.