So I just had the spare motorcraft carb rebuilt after I broke the fuel filter off in the previous one. Ran great at first. Got it nice and hot then took it for a test drive down the road. Came home and let it sit an hour or two. Went to take it for another ride and ran like ***. Popping, sputtering, and a backfire or two. Thought it may have been due to low gas so waited til the next day to try it again. Picked up some nice fresh gas and poured it in to make it to the store. Put some more gas in so I could do a little driving in it. Had to make a trip 2 miles down the road to pick up an engine. On the way it started running bad again. Pulled over to check some things out and nothing was wrong so I changed nothing. I jumped back in and it ran fine to where I was going. It did have a rather hard start when leaving. Had to peg the throttle to the floor for it to start up. Then the trouble ride really began. For the whole 2 mile ride home with the engine I had picked up in the bed, the truck would not go above 20 due to backfiring and sputtering. I have adjusted the air screws all the way in and out with no change. I have checked the float and adjusted it. I have tried a new coil. Plugs wires cap and rotor were just done. New fuel pump and tank. I'm at a loss with what to do. Accelerator pump seems to be working. With the engine off I can hit the throttle at the carb and it definitely sprays fuel. Could this be a POWER VALVE problem? If so how would I check and go about replacing? BTW this is my 1968 F-100 with a 351M in it. I can give it gas while idling and with more than 3-5 seconds of constant throttle of any amount it starts popping and shooting flames out of the exhaust. Any ideas?

 

If it was a Holley older then 5 years I would say that you for sure blew out a power valve. Holley carbs would blow that valve any time the engine backfired, but since 2005 Holley has gotten the pressure check valve in the carbs fixed to solve that problem. In a motercraft I cant say. At any rate your symptoms tend to make me think that the first place I would look is that valve. It is Cheep and easy to change.

I also think that your timing chain is wore out. Easy check. Start the engine and let it warm up. Use a Timing light and look to see what the timing is set at. At Idle it will hold steady between 3 and 10 degrees past TDC, depending on engine and condition of your chain. While watching the timing marks, have a friend gently rev the engine and hold it at around 2500 RPMs for a moment and then gently idle back down. You should see that your timing will advance smoothly and once you get to the target RPMs and hold, the timing will hold steady. When you idle back down the timing will return back to the setting as before. If you notice that thie timing marks are not steady while at idle (like bouncing between two numbers giving you a range, like 5 to 8 degrees past TDC for example) or does the same during acceleration and deceleration, then you have a timing issue. Many things can cause timing issues, but only a loose chain will cause a bouncing timing mark. Solution: Replace Timing set. That means Chain AND Gears.

PS....... If you have trouble getting the engine to start and stay running at idle then it is possible that the engine "jumped time". This means that the chain has gotten so loose that the crankshaft "advanced" in the timing chain. this causes the valve train to be "retarded", and you will have to adjust the Distributor to compensate. (in short the chain slipped) If so, replace the timing set. If the crank/cam timing gets to far out of whack, then pistons tend to hit valves. Best case your replacing valves, but I have seen a few valve heads break off. At that point you looking at scrapping the entire engine including the block.

 

I'm not thinking it has jumped time. 25k on a crate engine? It runs good when its getting the right air and fuel. I swapped the points out and it ran good then I remembered I forgot to readjust my air screws. They had been backed out like 5 turns earlier trying to lean out the mixture. I turned them in then back out one turn and its back to doing the same stuff again. How many turns back on the air screws is too many?

 

OK well you didn't say how many miles was on the engine in your first post so thanks for clearing that up. It would also be nice to know what size engine, and the model carb.

Air Fuel mix, Screw in to lean and out to make rich. So screw them all the way in and then back them out 1.5 turns. You may also need to adjust the float as well. My Holley has a screw on the front of the bowel that I remove, then the adjustment screw is on top. I would adjust the screw so that the fuel level was just to the bottom of the front screw hole. This may or may not apply to your Motercraft carb. It has been to long since I have even seen one I can't remember. Once the float was adjusted right, then set the engine idle screw to 800 RPMs. Followed by leaning the fuel/air mix one screw at a time. You need to do this slowly, as even 1/8th of a turn makes a big difference, but there is a bit of a delay between adjustment and the impact on the idle. You will notice the engine RPM increasing. When you pass the "sweet spot" it will slow down again and then get rough. If it only has one fuel/air mix screw it will kill the engine all together. If it has two, when you got the screw adjusted to the point you have the max RPM do the same to the second screw.

I would ask that you post a picture of this carb in this thread so I could see just what it is and how we can adjust it for you.........

 

So I snapped a few pictures of the carb but they wont upload. I scrounged around a previous thread of mine and dug up a pic of the one I broke the filter off in. They are really the same carb so this picture will do all the justice needed. It is just a motorcraft carb. Nothing special. I did say it was modified 351 that is in the truck. I need to run grab some more gas before I can really do any more adjusting but in the meantime I am still trying to figure out whats going on.
When I bolted the carb on after rebuild it ran like a dream. Idled perfect and accelerated like a nice new truck. Came home and let it cool down. Dad came home and asked to go up the road in it so we did and it did fine for the first mile. Turned around to come back and started running bad. No backfiring prior to this and no changes made. Just turned around. Started running like it wasnt gettng gas and I figured it has to do with being low on gas. Next morning I put some gas in and it ran fine again. I started to take it down the road and it started acting up a little over a mile and a half down the road. It was running really rich, backfiring, sputtering, flames from exhaust. Babied it back home since I couldnt give it really ANY gas or it would act up. Went to get a new coil thinking maybe it was spark related. No change. Adjusted air screws, no change. Adjusted float. No change. Put in a new points set, and it ran nice. The normal 2 mile test drive seemed to show all was good. Remembered my air screw were turned out some ridiculous amount of turns so I turned them back in all the way then out 1 turn. Ended up adjusting them out to about 2 or 3 full turns and still running like poop.
Seemed like at about 4-5 turns it ran good but that seems a little too much.
If it is in fact related to the POWER VALVE, where can I get the parts, where is it located for replacement, and how does one go about replacing it? This would be the first time taking anything off the carb minus removing the float haha. Any advice is of course appreciated and I do thank you guys for the responses.

 

Modified 351....... That carb....... I am sorry but I think I am going to have to defer this to the other members. That carb looks like nothing I have ever seen before and I think it might be to small for that engine. I am not saying that it could not be adjusted to work, I just am not sure how.

After reading your last post however, I suddenly don't think it is the carb at all. Does this truck have an automatic or manual transmission?

 

Automatic trans. Not sure what the difference would be based on the trans as it does it in park and in neutral and in gear....but I can't say whatever you're thinking is nowhere near right so shoot buddy

 

Thats a very simple carb, to replace the PV remove the carb, turn it over and remove the cover plate underneath (4 screws) right under the float bowl, the cover has a gasket, the power valve screws into the carb body and has a gasket underneath it, very simple. If the cover is full of gasoline the valve is leaking....Cheers You may have to buy a whole OH kit just to get the valve, should be a tag on one of the top screws with the carb# on it, I think some may have been stamped on the base......


The tag is right on the top of the carb in your pic

sounds more like you have some crap in the needle and seat or the float has sunk......Try this, plug the fuel line somehow, pinch it in front of the pump or plug it at the filter, run the eng until it runs completely out of fuel, reconnect and start the eng, sometimes you can wash any crap out of the needle and seat, it's a longshot but it cost's nothing but a couple minutes

 

I found the valve last night but didn't pop the cover off. There's some thongs its doing that makes me think it may be related to the power valve. But seeing as though sometimes it runs ok and others not...I question that being the problem

 

Make sure the points haven't closed up or the condenser has come loose....

 

Just replaced the point set. Condenser was tight but maybe its bad?

 

Can you recheck your Gap on your points?

 

I rechecked the gap on the point set. Ran it, ran like crap. Readjusted again. Ran it, ran like crap. Adjusted a few more times, and it started running nice. Took it down the road to the normal test drive turn around. I pulled into the turn around, backed up some, pout it in drive to pull out and it started acting up again.
Things that have been REPLACED
Plugs
Wires
Cap
Rotor
Point Set
Ignition Condenser
Coil
Gas Tank
Fuel Filter
Fuel Pump

Idk what more I can play with. Power Valve, Jets, Needle and Seat...?
I did adjust my air screws in the out about 1.5 turns and it did have some effect on the idle. So I just to a degree this could mean its not power valve related. It is however still backfiring, sputtering, and popping. Couldnt go over 5 mph back home.

 

I have been thinking that you may be burning out your points. I have been asking the odd questions in an effort to figure out how. The only thing I can think of is over reving your engine. Points dont like to run more then 4500 RPMs. Maybe putting something like this in would help you. http://www.summitracing.com/parts/cr...1700/overview/

 

I have thought about that. I played with the carb some more last night and checked again for vac leaks and such. Sprayed some carb cleaner around the mounting base and found that where the choke rod hooks to the carb, there is a small leak. I'm really considering just buying a new one and being done with it being the possibility of the carb