I have a 400 small block. The car has been running great until just a couple of days ago. It just started missing. I started looking around for something obvious and got shocked by one of the spark plug wires. So i changed those, thinking that was the problem. But it was'nt so i changed the spark plugs themselves and put a new distributor cap and rotor in. and still no change. I started tinkering around with carb (650 doublepumper holley) by messing with the fuel/air mixture but still no change. and last i touched the timing some. and yes you guessed it no change.
Am I missing something here? Any suggestions would be greatly appreciated.

 

doesn't sound good man check your oil for coolant last time i seen something like that it was a head gasket leaking into the compression chambers

 

At what rpm is the miss? If it idles ok, but starts to miss above that, you could have a piece of trash in the carb transfer slots. When all the tuning stuff is checked, the next thing to do is a compression test. This will let you know if a cylinder (or cylinders) is down. Possible causes for that include the above mentioned head gasket, burnt valves, broken or stuck rings, etc. Another thing to look out for is a broken valve spring.

 

It runs rough when im easy on it and seems to clear up some if I romp down on it.
Someone told me to take the valve covers off and see if there is anything different in there. but i'm really not sure what to look for.

where do i look to check for trash in the carburator. I have an inline filter but its clean.

 

If it runs better when you get on it, it's probably not a spring or rocker or anything like that. It sounds like trash in the idle feeds or transfer slot. Or it could be a blown power valve.

If it's blowing black smoke, it's probably the power valve.

Did you check the float levels? If one of the inlet needles has trash in the seat, float levels will be high, causing roughness at lower rpm's.

If not, try this. With engine off, turn the idle mixture screws in all the way (lightly), then back them off 1 1/2 turns. Start up engine. Turn in each screw 1/8 turn at a time. Should idle best around one turn out. If you can turn in one screw all the way without affecting idle, that side has the trash in it's circuit.

In that case, the carb needs disassembly to clear out crap. Or you can remove that mix screw and shoot some carb cleaner in the hole to try and clear out the trash.

I don't like the clear screw together type inline fuel filters. The ones I've used weren't very effective. I prefer the homelier Fram plastic housing type. And they have a nice big pleated paper filter in them. Cheap too.

 

Thanks alot for the reply. I'm gonna try that tommorow. I'll let you know what the deal is with it.

 

my guess is a valve spring problem(floating) could be weak, Your best bet is to do a Vacuum test, This test alone should help u discover your problem. If nothing appears then do a dry and wet compression test

Just to note I did have the same problem as you are mentioning, i was going crazy looking for this problem and it was a a header gasket leaking(didnt make any noise, but it would missfire) check that as well.

 

DUDE...YOUR MOTOR IS MISSING????? How do you MISPLACE a 400ci small block??????

 

Yeah, I always find my stuff in the sofa cushions. :-)

Seriously, Try to narrow down which cylinder is missing. Put some spit on a finger an touch a header pipe to see if it sputters, like testing the iron you wax a snowboard with. Which tube sputters the least is the cylinder that is missing. Then look at things for that cylinder like plugs and plug wires. If its not a certain cylinder look at something common to all cylinders like the carb.

 

Well I got some carb cleaner and sprayed some behind those fuel/air mixture screws. but i guess that was'nt enough cause it did'nt fix it. I also popped off those valve covers and I did'nt see anything obvious.
I guess my next guess is to tear that carb apart and clean it up a bit.

Also I have a MSD 6-AL hooked up to it. do you think i would have to get a high performance coil to go with it.

Thanks for all the help!

 

Hey Thomash, where the hell did you learn your shadetree knowledge from? I've never heard of spitting on a header to find out which one is missing. And NO i dont think its anything like checking the IRON that you wax your snowboard with. You might need to watch some more horsepower tv or something. (SUNDAY'S AROUND 1000).

Oh if you were serious about that, then just ignore what i wrote.
Thanks for the help!