i usually drive around the neighborhood and my max speed is usually 40mph. one day one on my friends pulled up on the side of me at the stop light and started revving. i never pushed my z hard before so i was curious how it would do. so we just raced to the next light. my max speed was like 55mph. after that i noticed the car started to pull weak and the exhaust sounded like a low pitched rattle. i was goin like 25 all the way home because i couldn't get it to go any higher. when i got home i checked everything and it looked ok. but i noticed the plugs were a black, some of them were a little oily and only the #1 plug was normal. any advice?

How is your fuel pump? do you have a FUEL pressure gauge?

There's no way it's fuel-starved. Check your ignition, for starters. It might be missing. Does it run quite roughly?

It still runs smooth and revs fine at idle but when trying to drive it down the street, it starts to struggle and sounds really rough.

One thing I've learned (and others have verified) is that the ignition has a harder time sparking through higher fuel/air densities/pressures -- e.g. with an engine with a higher compression ratio or when a stock engine is under load. Your ignition might have gotten weak, e.g. with some shorting inside your coil. I've recently replaced a lot of my ignition and find the engine fires much better under load, particularly at low RPMs. (It used to miss quite a bit.)

What does the spark look like, if you spark from a plug wire to ground?

I assume you have a points system. You might swap out the points and condenser to see if that helps. Your coil might have gone bad. Check the resistance of your drop resistor and high tension wires.

I'm not saying it's definitely ignition. It could be other things too, but ignition is an easy place to look.

Edited November 10, 200915 yr by FastWoman

i really think it could have something to do with the ignition although i just changed my points and condenser and spark plugs like 2 months ago. maybe something happened to the timing? also, what do you mean by sparking a plug wire to ground?

It might be a good idea to have a look at the points. Inspect their condition, check for carbon tracking, soot around the points, and pitting. The contact surface should really have no flaws

It sounds like your plugs are fouled... when was the last time you replaced them? A set of NGK copper plugs are not much money if they are old. You may also have some carbon buildup if you're really only cruising at 40mph. You've never jumped on it on the freeway onramps?

I think what Sarah means by sparking the plug wire to ground is you pull a wire one at a time while the engine is running and hold it close to a good ground (the cylinder head). You will likely be able to see the spark jump to ground.

the plugs were fouled so i replaced them. there is a slight improvement but still low on power. its starts to stumble and lose power at anything over 3k rpm.

Distributor's Vacuum Advance Plate.

Remove your Dist Cap and disconnect the vacuum hose going to the carbs at the carbs.

Suck on the hose, you should see the distributor's point plate advance and when you release the vacuum you've created with your mouth (or a vacuum pump if you're squeamish), it should slide back to it's prior position.

If it doesn't advance, or it's spotty jerky on the return leg, then you may have lost the little bearings inside the roller plate.

Remove the old, replace with a new. If you're careful you won't even need to adjust the timing.

FWIW

E

post deleted

Edited November 14, 200915 yr by clindayag1
dumb question

Distributor's Vacuum Advance Plate.

Remove your Dist Cap and disconnect the vacuum hose going to the carbs at the carbs.

Suck on the hose, you should see the distributor's point plate advance and when you release the vacuum you've created with your mouth (or a vacuum pump if you're squeamish), it should slide back to it's prior position.

If it doesn't advance, or it's spotty jerky on the return leg, then you may have lost the little bearings inside the roller plate.

Remove the old, replace with a new. If you're careful you won't even need to adjust the timing.

FWIW

E

i took the plate off to check out the small bearings and i found that the weights on the shaft were rusted and wouldn't move. so i went to the junkyard and got a distributor that was in a better condition than mine. swapped out the rusted one and put in the newer one. i also put in new points and condenser. when i started the car, it idled really rough and when i tried to give it some gas it would stall. :disappoin

Make sure that the timing is on and that everything, EVERYTHING, in that used dizzy is in good shape. Don't just assume that because it looks better that it IS better. Looks can fool even the best mechanically inclined person.

Jan