Hello i have a 400m, new holly 4150 and my problem is thier a small kind of miss...me and my friend went through the valves and ignition to try to figure it out (and thier was a small valve train knock at the time). After putting it back together it ran good for about 30 minutes then 2 spark plugs wires gave out. After i put new wires in it it ran decent but thier was a small miss in it. I dont believe it is an ignition miss because it has all new distributor wires plugs cap rotor and it doesnt act like a miss, it runs strong and gets up to speed good, I believe its the carb ,ive heard power valves go out in holley like none other . Any help is appreciated. THANKS!

 

Power valves only go out on Holley and stock Motorcraft carbs when you have an intake backfire. You said your carb is new, which means it will have a check valve to protect against that. It is not your problem because believe me, you'd know if the power valve was out.

Check the gap on your plugs. Check the resistance on all your plug wires (7Kohms per foot max). How did you check that the 2 plug wires gave out? Did you ohm them out? Quantitatively check the ignition. You didn't state a year - does it have points? You can run the engine and pull plug wires one-at-a-time to isolate the problematic cylinder. If you pull a plug wire and the engine runs the same, that cylinder isn't firing for whatever reason. You can also clamp an inductive timing light onto each wire to make sure it's firing.

Carb problems don't cause a miss. The same carburetor feeds all 8 cylinders. A miss is one specific cylinder not firing which throws off the balance of the engine and you can feel it. Running rough, rough idle, stalling on acceleration and backfiring do not constitue a miss; they are problems that can be connected with a miss but they are not misses themselves. What specifically makes you think it has a miss? Answering that question will help pinpoint the problem. If your ignition checks out good (having installed new parts without verifying anything does not satisfy the requirements of determining if your ignition system is good) and you find that one cylinder still is not firing, you must run a compression test to verify and then perform a leakdown test to isolate where your compression is going.

Also, I didn't completely follow what you said. Your post is that your engine has a miss but your next to last sentence says "it doesn't act like a miss." Please clarify.

 

Sorry i wrote it in a hurry im at work...it runs great it just seems like it cuts out a little but very small...but it just now broke down on me at work and it acts like its getting no fuel and dies and doesnt run very long.. the carb is a
"re manufactured" off ebay not brand new

 

hmmm , didnt you say you had trouble with some loose spark plugs? cause it could be related....

 

Well i messed with it today...and it started idling at 100 and actin like it had a vacuum leak out of no where, i reved it up, it fixed the problem for an hour..then it suddenly dropped again and started dieing and acting like it had a vacuum leaki...if i rev it up i can keep it alive, but if i dont it idles super low and dies..some times it wont idle so low and its driveable.. what else could it be within the carb?

 

With the engine idling, take a spray can of carb cleaner and spray suspect areas. If the idle jumps up, a leak is there. Common places to look are the power brake booster (if you have it), vacuum lines, manifold vacuum tree, and most of all - the carburetor base.

Also sounds like your float level is too low.

 

Ok. I will look for the leaks but if none come up, what could it b on the carb? Is it still possible that the power valve went out? It has back fired before when i first got it on thier.

 

A power valve is basically a diaphragm that has fuel on one side and vacuum on the other. When the diaphragm ruptures, vacuum will draw fuel directly and you will have a severe rich condition. You will be running so rich that the motor barely runs, like a stuck float, and you'll have black smoke out of the tail pipe. This condition will not only affect idle, but acceleration as well. Holley carbs unfortunately have this reputation now that everyone wants to assume it's the power valve. Replace it if it makes you feel better, but I don't think that's your problem and there are some other non-carb things to check first before you do it.

As for the carb itself - did you check the float height (my previous post)? Have you tried adjusting the idle speed and mixture? Where is the choke at? If you check all these adjustments and it still has problems, go ahead and replace the power valve since you said it backfired. Since this was a rebuilt carb off ebay, it's not for sure that it will have the check valve to protect the power valve. However it is unlikely for an intake backfire to destroy a brand new power valve.

 

It should be about rright on the idle screw...and as the float level i believe it is good to. It was running decent idling at about 600, running pretty strong, then it just cutout and started idleing at 100 and dieing etc..i have a 750 edelbrock carb off my friends truck im going to put that on and see if it solves anything.
And also i lent my 4150 to a supposed "friend" he ended up using it for a few months on his 454...and it wasnt exactly in the best condition and he was constantly messing with it..i believe when he had my carb he treated it very badly as we looked over it and thier where a few things that had gone south..when i got it back the deal was since he borrowed it he would tune it for my 400..but he didnt uphold his end of the deal and i got stuck with a carb tuned for a 454 on my 400.. on the first start up of my truck with the carb it had some intense back firing..when i got my truck running it still had back firing issues and carb issues...i finnaly got it tuned with a help of my friends dad and then this happen. This is also why i was thinking it was a power valve.. also i dont have much piping off of my manifolds so i couldnt see if thier was black smoke..but lately i have been smelling a very odd strong smell of something and i couldnt put my finger on it...also it runs exactly as if it was running to rich. Thank you for all the help
so far!