i have a 1972 240z and it's got a couple problems.

it was running great a while ago, but i had the carbs adjusted and it wouldn't hold idle. i also couldn't rev it up or it would die.

i adjusted the carbs again thinking maybe it was flooding the engine, so i made it leaner. it runs and drives now, but i can only rev it to 3000 before it sputters, backfires, and and stays at 3000.

any suggestions as to why this might be and how i could go about fixing that?

-Matt

I strongly recommend buying the SU video from www.ztherapy.com it tells you just about everything you would ever want to know about adjusting your carbs.

Also, search the forums.

Are you sure the problem is in the carbs? Check the point gap, or dwell if you have a meter, and make sure the timing isn't doing something strange around 3K.

There could be many things that would cause this sort of thing, including the fuel pump, so you have a lot of ground to cover.

You violated rule number 1 - never adjust the carbs on a good running engine! Why does everyone figure they need to fiddle with them all the time. Once they are set properly. leave 'em alone!

I second the ZTherapy video. Go through the entire routine, start to finish. Don't skip around.

Try and rule out ignition first. Check your plugs for fouling. Maybe you dialed it too rich. Fouled plugs won't readily clean themselves. Clean or replace and set your fuel mixture right. Get a timing light and see if the spark is becoming erratic at 3K+. If it is then check the distributor shaft for play and check the points gap and dist cap. (assuming plugs are good) Check the timing to see if the distributor has shifted.

If all is well with the ign system then you can move on to fuel issues. Change the filter and check your fuel level in the float bowls. Hopefully you don't have tank rust.

If it's not a fuel issue it might be a mechanical issue. Be certain that the oil pressure is OK. If you overheated the engine the pistons can seize. In some cases the engine will run for a short time but then dies once the pistons have expanded from overheating.

Grasping at straws here

Jim

i had dave (zsondabrain) adjust the carbs for me, because i could rev, but under load it died. i'm not especially good with mechanics, so how would i check the oil pressure? because it runs for about 20 minutes and then the engine dies..so that might be the problem.

Sounds like ignition issues to me - check the coil, plugs etc we had a zed here in NZ (my old 240z) that got the same issue and it turned out to be the newish coil I installed before I sold the car died. A new coil corrected the issue in this case.

i i'm not especially good with mechanics, so how would i check the oil pressure? because it runs for about 20 minutes and then the engine dies..so that might be the problem.

If it runs twenty minutes then it's not anything mechanical. (This leaves ignition and fuel as supsects) Your oil pressure guage in the dash would tell you if you had spun a bearing. You need roughly 10psi per 1000rpm. Check the oil level. The car can run on as little as 2 quarts but this is not advisable.

It sounds like ignition or fuel. Ignition is the first suspect. See last JimmyZ post.

You will become a good mechanic in time if you own an old Z like this. Not because they break down but because they are so easy to understand. Just grab a Haynes manual ($15 or so) and read the descriptions of each system.

It could be that your coil is bad or the ballast resistor is wired wrong. I'm sure Dave can get it figured out. When it dies feel the coil to see if it's hot. Check the coil voltage when it's running. It should be at 6 volts when running and 12 volts during start. A $10 voltmeter is all you need for this.

Good luck,

Jim

Sure, throw Dave under the bus!!

Ha ha. I had a similar issue where my car would idle fine and free rev forever, but upon acceleration under load, it would bog and sputter and die. I had the fuel tank boiled and lined, and hadn't had a problem since (well, besides running pig rich because I adjusted the carbs rich unknowingly counteracting the restriction from the tank)!

Please mark me down as +1 for the Z Therapy video! While you are watching that video, be sure to save up the $600 or so for a completely reconditioned set of SUs from them!