Alright motorheads, here's my problem. I have a mud truck, 72 Bronco with a 351w, C6, and 205. When it was sitting there (pre-test drive), it ran like a champ, sounded great, and hit all 8 cylinders. Drove it from the house to my buddy's shop (approx. 1 mile). Goosed it a tad, and it started hitting on 4 cylinders instead of 8 at idle. Bought a new ig. module, no help. Got plenty of fuel. Electric fuel pump, so fuel getting there isn't a problem. I spoke to a guru friend, and he said POSSIBLY cap and rotor. Plugs are all good, and I'm pretty darn sure spark is getting to all 8. Funny thing, is it hits perfectly on every other cylinder. Plug wires are all good. Above about 2k rpm, it starts hitting all 8 again. That might work out, tho, save fuel at idle, or else just increase idle speed to 2k, right? Another thing I just thought of. I'm running a dual plane performer intake with a Corvette carb. The carb worked fine previously on both this engine and a 460. Is it possible that, at idle, one of the two planes on the intake isn't receiving enough/any fuel? If so, that might indicate a vacuum leak on the plane, or a carb adjustment issue on that venturi of the carb. This clears up @ 2k rpm, so it may be a vacuum issue. I just can't figure out why this started after I goosed it a little. Since this is an off-road truck that will only see idle speeds 2 seconds before hitting 5k rpm, this isn't a HUGE issue, just something that has been bewildering me for a couple weeks. Any words of wisdom are greatly appreciated.

 

first of all why do you have a cheby part on a ford!!!!! just kidding... It sounds to me like vacuum, test it with a vacuum guage.... and make sure u have vacuum befor you do anything else... and then make sure you are getting spark, it might be your vacuum advance if you have one... Just do the easy stuff first... make sure you are geting the 3 most important things - Air (vacuum) Fuel (carb) and Spark (ignition) , so check for vacuum, make sure your carb is working (if you have another truck you can stick it on put it on there and see) and Ignition, replaceing parts will get you nowhere unless you know that the part is not working.... It could also be your distributor..... Just do the main 3 things befor you do anything else, check for air fuel and spark.

 

Have you tried adjusting the idle mixture? If you don't have a vacuum leak it sounds to me like you got a piece of crud stuck in one of the idle mixture passages. Blocks the fuel at idle but when you get the RPM's up the main circuit takes over and it hits on all 8.
You could try to take the mixture screws out and see if you can blow the piece out but you might have to go deeper (fully disassemble, clean and rebuild).
Just a thought.
Greg

 

Well, I've got three 4-bbl carbs off of mid-80's F-250's, but all are in dire need of a rebuild, and I got this one for free. A Predator is in the works, though. I'll put that on my list of things to check on. Sounds like I should be on my way to getting my V-8 back!

 

And in conclusion- I took it out for another small test drive, and now it's hitting on all 8 again (for now). I guess something might have just gotten in the way somewhere in the carb. Just another one of those unsolved mysteries...

 

well... sounds like u got some gunk in ur carb... If I were u I would clean it out and replace your fuel filter... maybe flush ur gas tank... so then u wont suck up and crud.. You prob just had a pluged jet... but that usaly floods the fuel bowl ...? do you have 2 side hung floats or just one?

 

i believe i just have one. I haven't taken it apart yet because I'm not entirely sure just what kind it is, and to get it rebuilt would cost more than an out-of-the-box edelbrock or holley, so I'm just going to run it as long as I can, then ditch it for a Predator or one of those new Off-road Holley carbs I read about in 4 Wheel and Off-road. I am probably due for a new fuel filter, though. And there's no telling how long this carb was sitting before I got my hands on it.