my 287 pontiac, has developed what i would call a dead miss, right around 2500 rpms (no tach...just guessing on the rpms). maybe dead miss isn't the right term, because it seems to be firing on all 8. it just runs rough at that point......and the point at which it does it, is the rpm that the engine is usually in while driving it normally. it does irt while driving, and while sitting still with the trans in neutral. before that point, and after that point it runs great.... it has new points, a new rotor, new cap, new wires, new plugs, and a new coil. the problem was there before i replaced everything. could it be that the vacuum advance is not working or stuck ? the weights for the centrifugal advance are not stuck, the cam for the points does not look worn out...and i have the point gap set at 17-19 thousandths. what else should i be looking for ????
does it seem like it stumbles for a couple hundred rpms when it hit 2500, then starts to make power agian? i dont think its the vacuum advance because if it was stuck it wouldnt run any better above 2500 as it would be running out of timing. check the weights closely and look for any irregulaity in them. my guess would be its int he timing theres a slight possibility that it could be a fuel problem but if it was you would be poppin ans spittin as soon as you started making power agian.
no, it doesn't stumble it just runs rough (for lack of better words). i have ac plugs in it, accell wires, stock distributor, cap, rotor and points, a stock ac delco coil and all that stuff is new (other than the distibutor). the gas tank was boiled out last year, all new steel 7/16" fuel line ran from the tank, to the pump, to a mr gasket fuel block, and then rubber 7/16 lines out to the carbs. the carbs are rochester 2 g's (2 of them) and they were rebuilt last year. the stock mechanical fuel pump is also new, and i put it on this past summer. it has 7 psi of pressure. fuel filters are also new. i do notice that at like 55 mph...the engine surges back & forth, and it's a constant surge. it never goes away, or never does it every once in awhile...it does it all the time, and it's a steady rhythmic surge. it never dies, never stumbles, never backfires or coughs. it's just an annoying rough running condition, and the engine surge at that certain rpm. not sure if this has anything to do with it but the exhaust is new. due to the way the frame is, and the fact that the drivers side exh. manifold exits to the front....it has a Y pipe off the manifolds, a single 2.5" pipe ran back to about the middle of the car. from that point, it's split with another small Y, and dual pipes ran all the way to the back bumper. no mufflers...just the straight pipes. i don't know if the lack of backpressure could be causing this or not since there are no mufflers on it. i just had the exhaust done recently, and i noticed this problem shortly after that. it didn't do it right away, i drove it a few times after the exhaust was put on...and then one day i noticed it. that may not even have anything to do with it...i don't know.
I would look at the wire that connects the points to the coil. As the vacum advance pulls in the wire could be breaking. dirt t
well if it had anything to do with the vacuum advance when you got over 2500 it wouldnt get any better or smooth out any. if the timing cant advance in the upper rpm range when you got over 2500 it would get worse until it died. if it was a backpressure issue you would have a whole lot of trouble off idle and have a hard start condition. however there may be an issue with exhaust gas turbulence causing restriction in the system. if the exhaust gasses are having problems flowing properly is will actualy cause something similar to a blockage in the system. its likely but im still more inclined to thing that you are having a problem with you timing. have someone check your timing you may have a flat spot in your timing curve.
i'm running dual rochester 2 g carbs, and nothing has been done to the jetting. they are both "factory" stock rebuilts. i've had the dual carbs on it for over a year, and this problem just became noticable in the last month or so. if anything, it's running too rich because i am not running progressive linkage....both carbs function together at the same time. i checked for vacuum leaks with starting fluid, and found nothing, i even changed the few vacuum lines that are on the engine. i am wondering if turbulence in the exhaust or intake is causing this....but it makes no sense because all that stuff was done prior to the problem showing up. i'll have to go back, and pull the cap off...check the wires in the dist., and re-check the point gap. i drove it today, and noticed that at a higher rpm (probably 4500 or so)...the ignition is breaking up...like the points are bad. i may have the point gap set too wide. i didn't have time to check it today...all i had time for was a quick drive. would a bad ballast resister do the same thing ??? not that mine is bad, but you never know. i don't know how to check it other than visually looking at it. i am also getting a timing light to put on it this weekend. i'm in no hurry to fix this...winter is almost here and i don't drive it in the winter. thanks for all suggestions, but if anyone has anymore, i'll be glad to try them. oh...yes it does have a fuel pressure regulator, and i adjusted the fuel pressure down to 5 psi...and it made no difference.
I'd check the wire from the points to the coil inside the distributor. It's a little scrawny wire and they can get rotten and crumbly inside due to corrosion and vibration. When the wire flexes to a certain point, if it's real crumbly inside the wire, it might cause those same exact problems. You might want to use a dwell meter to check your point adjustment in case it's way off.
There's more potential effect from the vac than the actual advance...one of the hardest problems for me to figgerout was here once. The actual pivot point where the plate moves can wear to the point that your breaker gap can change greatly at certain advance positions, then come close enough to right to work at a greater/lesser throttle position that changes vacuum. Easy to eliminate--jut disconnect and plug vac line. The actual effects of the advance usually won't cause roughness, just a bit more or less power.
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