carb or ignition

[gotmopar]

i've been fighting this prob for some time now and go'in
ape over it. I have a 400ci with RPM int.,750holly,MSD,
erson cam, hedman headers, 9.0:1, and i live at 6000ft above.My prob is that RPM does not matter it just feels like i lost a cylinder or 2 while driving at any speed, and it does this off and on with'in seconds. it runs perfect and then starts to miss and then it runs good again, back and forth constantly. all parts are new plugs,wires,coil,carb....etc.! it did not do this when i first put motor in after about 2000mi i've been fighting it ever since 3000mi later. any ideas?

[George G. Leverette]

What is the Temperature when this happens?, could be choke pull-off, vacuum lines, do you have smog gas in your area? maybe the carb or ignition should be calibrated. A lot of what if's. Why not check the exact conditions when the problem occurs and post back.

[Mr. Trans Am]

Not sure if this applies w/ an MSD box but if you are still running the mopar ECU box that may be the culprit. THose things cause all sorts of maladies. If you are running one, change it out, theyare cheap. Also be sure that it is well grounded, in fact a bad ground will cause even a good box to act strange. I always run a ground wire straight to the battery and I have had zero ECU troubles on all of my Mopes ever since.

[72satellite]

Is the MSD unit a model 6A or is it just a MSD coil. I had a MSD 6A box on my satellite and after several thousand miles the engine would begin to miss with rpm's over 2,500. I bypassed the MSD unit and the engine ran fine and when I reconnected the MSD unit it started all over again.

For some reason the MSD unit was not working properly. I returned the unit to MSD and they sent me a replacement unit, so I would try to bypass the MSD box first.

[rumblefish360]

I have had a couple of problems with the MSD box. Same unit as yours. MSD was reallllly stand up about it. They sent me a new box and repaired one. It was very fustrating. <sp>? However, it's all straightened out and works great.
I also had problems with the coil. I remounted the coil striaght up and down away from the engine and havn't had a problem since.
With the MoPar box's, I had a problem once. Not since then. It was a defective box out of the wrapper. Would only start once.

[gotmopar]

it seems to be fuel related after playing with it more today. while driving and hold steady it won't do it. i had a coil proplem earlier that would mess up the plugs after cuople miles and it would run rough. so one prob solved, changed plugs AGAIN and new coil which led to this one. now if i hold steady throttle in any position it wont do it, but any kind of acceleration even just barely it surges like fuel starvation, so i started poping in bigger jets #70 to begin with (i have quick jet change bowls) now i'm on #76 it got better but still randomly does this surging, i can accelerate smoothly now,and faster,so had to order more jets #78,80,82,85 so if the primaries want bigger i imagine the secondaries want the same. by the way my MSD is an older direct 5A(12yrs old,yes 5A) replacement plugin for the stock ECU, and i have no choke(it's a milled down holley with K&N air horn) or emissions crap in the way. timing chain is a new double roller,and the distributor i rebuilt with new bushing, it's not loose(i did check),and i am running a holley blue pump with an inline pressure gauge that reads 6.5 psi at all times so that should not be an issue either.
So thanks for the replies it helped me find the bad coil and chase the next problem down. Any more suggestions would be helpful and considered.

[stick4406]

where are you at with your power valve-this is going to matter more at throttle tip-in than the main jets-the jets may just be masking the problem-run at steady cruise w/vac gauge hooked up- note vacuum reading i.e.14inches-split number in half for proper power valve size..i.e. 7.0.

[BigBlockBabe]

I had a really bad problem with missing that started off and on at high RPMs and was soon happing all the time. The wires were a mess, so I replaced them with a set of 8.5 MSDs, which seemed to fix the problem until a month later the engine started running rough again. So after much searching, it ended up that one of the new MSD wires was arcing near the distributor (defective boot or something), and after replacing it everythings been fine *crossing fingers*.

[Jims451]

If you are using a mechanical fuel pump, check the fuel lines going from the pump to the gas tank.
My pet peeve is the rubber fuel line at the gas tank. I have had 3 vehicles with split fuel lines in this location which allows the fuel pump to suck air in along with fuel, so it acts like it is vapor locking.

I live in the Denver area and I might be able to check it out for you, but you might have to meet me at a friends house since we will be pulling an engine out of his truck this weekend.

[Union Man]

If you have Vac. advance try disconnecting it and plug carb. I had a similar problem on a 340.I do not know why it worked for me. But it did.