I own a '79 Bronco with a 351 Windsor in it. It is recently rebuilt, polished valves and heads. New 4 barrel Holley, all new everything but the block! Anyways, there's a shade less than 10k on the rebuild and I did a tune-up. New spark plugs, new oil/filter, adjusted idle, new cap/rotor. Checked all wiring, rewired the remote cable on the radio, and added a 12v charger socket. My point is, a week goes by and I'm driving down the freeway at 65-70... when she starts misfiring in Drive. I slow, put it in 2nd, and she stops. It seemed to start at around 2000 RPM, in Drive, sounded like only one cylinder. I checked all spark plugs, cap, rotor, wires, etc. I found what I think would be 1 Left cylinder, or front/driver's side spark plug... was loose. I tightened it and everything seemed to work just great. I fired it up this morning, and it just got worse. I had to drive 20 miles in 2nd, at 20mph or less. Please help? I want to do it right. I've never blogged before, will I get a direct reply... experts would be appreciated. Thanks, S

 

interesting motor. i say this because the 351w was never put into a 79bronco from the factory. It did however come with a 351M which is a completly different motor so are you sure on what it is?

what type of spark plugs are you using? are the plug wires routed in such a way that they are not touching or crossing each other which may cause crossfire. What is your timing set at and i am also wondering if perhaps your fuel filter might be clogged or the fuel pump is going out.

we will get this figured out no worries there.

 

Well, it's a rebuild... not sure if it's the original block. It does have 351-W on the bell housing, all components of it are a 351, some 302 parts (compatible). I'm 99.9% sure it's a 351 Windsor. I put Autolite 45's, p/n BSF42C, spark plugs in it. A MasterPro Ignition cap and rotor. It's all related to this last tune-up I did. I don't believe it's the fuel filter, and no wires are crossing. I will check on the timing, I have a hunch it may be that. The fuel pump is rebuilt/new.

 

actually most 302 parts wont work on a 351w. the heads, carb, pistons are about it.

 

I understand that, I didn't specify what parts... it doesn't matter. I am prying into her tonight and I will fix the problem if it's the last thing I do!!! Thanks for the help...

 

hows the carb working? has it been rebuilt lately? if its a holley they tend to act this way if the powervalve pops.

when you set the timing be sure to plug the vacum line that runs to the vac advance.

 

I've been suggested to re-build the carb. It doesn't seem like too much work. But, I just replaced the riser gaskets between the block and the carb. I also replaced the valve cover gaskets, and guess what! Roller Rockers!!! Pretty cool, anyways. I think I'm going to have to take the carb apart, I'm going to have a local shop diagnose it tonight. So I can let you know tomorrow what they say.

 

If you found out what caused this misfire then please let me know, I have a similar problem with my 69 351 windsor. Mine is intermittantly firing and then misses for a few seconds then fires again on 5 and 6. Interestly this is in the firing order 6,5. I've been told it could be wear in the distributor shaft bush, but when I force it move across by hand it doesn't move enough to close the points. Spark plugs, leads, distributor cap, coil all new.

Also, its running really rough, cam shaft bearing? I'm no mechanic, but any help is appreciated.

cheers, Gerard

 

Have you tried all the usual suspects yet? Spark plugs, wires, firing order, rotor, cap, points? A tune up in other words? Carb been rebuilt lately? You'd be amazed at what a difference a good carb rebuild will do to the smoothness of your engine.

The first thought off the top of my head for the orginal poster was going to be a ground came loose, but he's been gone so long it's irrelevant.

 

Spark plugs, leads, rotor, distributor cap, coil, all new. With engine running I also tried a wire connecting positive coil terminal directly to battery to check that side of the system, but without any improvement. Carb has been rebuilt, also new fuel pump.

Not sure how many miles on her when I got it, I didn't think it was too many so I just changed the rings and big end bearings while in was in place, and reconditioned heads with unleaded valve seats. ( had some assistance with this from a friend whose a mechanic)

It drove 600 miles without any problems, but after sitting for a couple of months this rough idle and running developed.

thanks for giving it some thought.

 

I'd say one of your new plug wires has a problem, measure them with a DMM they should be about 5k ohms per foot and should pass the wiggle test.

 

Points, what about the points? Are they pitted? New? Rebuilt?

It might be that the gas has gone stale, can you hook up a fresh can of gas to your fuel pump and run that thru the carb?

 

leads, points and fuel are fine, but sorry my earlier diagnosis has changed. I was using a timing light to watch the firing on 5 and 6, and was repeatedly getting that missing, I also confirmed this by simply removing the lead whilst it was running and noticed no change in the idle, therefore I concluded that the problem lay there, however I've just tested it again with another cheaper style timing light that doesn't have the battery connections and can you believe it, its firing fine. But its still really running rough. Therefore, I now really think its blocked lifters, cam shaft bearings, maybe the timing chain has jumped a tooth. Geez its driving me crazy, looks like I'm going to have to get real dirty ......again.

 

Check the routing of the spark plug wires, on the 351W a couple wires cross to prevent misfire. I'm not sure which ones but I'm thinking its 5 or 6.

 

its plug 7 & 8 that usually have the cross fire issues. you might start looking for a wrecking yard 351w with low miles that you can simply remove your current motor and drop a lower mileage motor in so your up and running quickly.