unicorn_squad

so I rebuilt my friends motor a month or two ago. motor started up first try, runs pretty good. pulls hard at high rpm.

problem is, at low rpm.. (like, below 4k rpm or so), the engine jumps/misfires. at partial or full throttle... the car feels like its on a bump road because the engine is misfiring.

motor is 100% stock 87' N/A. all factory emissions equipment are still in place.
spark plugs are brand new, timing is dead on.
I tried switching out the primary injectors, but it didn't help... thinking the injectors are clogged/leaky...

I also switched out the lower intake manifold/port actuators, as the old ones weren't functioning properly. it has more torque now, but it still jumps at low rpm.

also, when cranking the engine over to start it, it usually misfires a couple times before it starts up. then it starts up and runs good, for a little bit, and dies. have to use the throttle to keep it idleing for the first 30 seconds to a minute, then it idles no problem by itself. I tried adjusting the cold idle stop thingy as well, but it didn't make any difference.

anyway, pretty confused as to what the problem is.

also, it failed emissions, just on hydrocarbons. total allowed was 3.0, it had 4.5 or so.

hopefully someone will have some sort of idea....

rotorpower27

how many miles are on it since the rebuild?

unicorn_squad

hmmm... about 300 or so...

rotorpower27

well theres your problem, or part of, break in period is 500 miles and being easy on the motor.
all of the new seals take a while to seat in, and doing hard pulls during that time is not good.

i have about 1500 on my rebuild, and i still feel like every mile helps the motor run smother and put down more power

unicorn_squad

the problem is at low rpm. not high rpm.

if the problem is just simply that the motor is not yet broken in, then that would be great... although I really don't think thats it as I've never heard of this problem before, and its not something thats easy to go unnoticed....

if you're trying to say that the motor is messed up because it has had hard pulls on it, and thats why it has this problem, then please don't bother posting in this thread again. I've built quite a few motors, and the closest thing to a 'break in' I have done on any of them is a 3rd gear pull on the dyno, and they all still run absolutely great. maybe they won't last as long, in the long run... but at this point it is not an issue.
thank you for trying to help though.

rotorpower27

your welcome