HELP!! SU carbs suddenly..

[astrohog]

I am finally getting my 1971 240Z into reliable running order. I have had her for about three weeks and have replaced the following: tires, exhaust, plugs, wires, distributor cap, points, condenser, alternator belt, battery and terminals.

Today, I thought I would take her for a spin as it is beautiful out, so I called my buddy (1973 240Z) and we met up, decided on a route to take and hit it. He had asked me to get up to 80 mph on the freeway so he could check his speedo, so we did...then 85...then 90, when I went to back off the throttle, the rpm's would not drop!! I had to use the brakes to get the r's down and got off the freeway. When we stopped and popped the hood, the mechanical linkage looked fine, no binding, broken springs, etc. The engine would start fine, but idle at 2000-2500 rpm's. when I would give some throttle, it would just stick at the highest rpm that I would feed in with the pedal. I limped her home this way and I am stumped.

I pulled the carb covers ( I have the Weiand covers) and started her up. As I fed in throttle to say, 3000 rpm and let off I could feel the piston oscillate a little like it wanted to drop back to idle, but would not, it would stay and the rpm would hold at 3000 and want to continue increasing.

I am no carburator expert, so I am not sure where to go from here. Anybody have any advice??

Thanks for any help!!!

regards,

Jeff

[beandip]

What kind of carbs do you have ? You couldent just turn the ignition off ? Or is this another issue ?

[jmanz6]

The fuel nozzle/nozzles could be bent as well as the needles for the nozzle. Make sure when you move the piston by hand that it will drop freely. Disconnect the linkage to take that out of the loop. If it works by hand with no linkage attached, then reconnect the linkage and see if it still works ok. I think there is some mechanical binding. Take the covers off and check the needles for straight. If they seem ok, check the nozzles for straight.

[mlc240z]

check the above first.

also, if you had the domes off, check that you didn't accidentally swap domes. they're supposed to be matched to the piston and i would think they might bind if swapped.

[zack_280]

Remove the air cleaner and hold the piston up with your finger then let it go. It should fall freely. It also may be possible that your butterflies (or one of them) are sticking. Are the linkage return springs in good shape?

[ezzzzzzz]

Quick note...when testing the piston drop remove the damper from the piston dome. Otherwise, the damper will restrict piston movement and you'll not get the classic "thunk" or readily see the needle hanging in the nozzle (if that's the problem).

[astrohog]

I have not had the domes off, so I know they are not "switched". The piston drops as it should. At least I think it drops as it should! I don't really have any experience with these, but it seems to drop "smartly" as one of my manuals says it should.

As this car is all original and appears to never have been messed with really at all, I think it is probably time for the carbs to come off and get overhauled. I might as well learn em now, cause I ain't getting rid of this car any time soon!!

If there are any other ideas, I would certainly welcome them, and thanks for all the responses so far!!!

Regards,

Jeff

[zack_280]

Ztherapy has a good video for rebuilding/refreshing your carbs. I recommend it. I was in the same boat as you this past winter and it helped me a lot.

[1 Bravo 6]

Hmmm,

Since nobody else asked this question, I'll play the fool and ask the obvious.

Is there oil in the dome ???????????????

Rick.

:devious: :devious:

[mlc240z]

i would think that no oil would cause the dome to "plummet" and cut off fuel too quickly. if anything the oil would have to be really thick to prevent the piston from dropping.

[zeiss150]

check to see if the choke is stuck open. also check if there is a vacuum leak through the throttle shafts. does it get rich when this happens? if so then the problem may be in the nozzles needles or floats. If you "tap" the throttle does the problem go away?

[kenz240z]

Hi Jeff, post some photos of your Z when you get a chance. You've gotten some good advice already about the carbs, but there is something else I wanted to point out. You had a good list of items you've replaced on the car, but I saw no mention of the brakes or brake system. If your Z had been sitting for awhile before you got it, I'd highly recommend that you drain all the old brake fluid out of the lines, calipers, wheel cylinders and master cylinder. Make sure the master cylinder, calipers and wheel cylinders are clean and in good working order. Replace/rebuild any that are suspect. Also, replace the flexible brake lines at each wheel. Once the brake system is in good working order, fill with new fluid, bleed the brakes and enjoy!

While you are at it, you might consider replacing the radiator hoses and thermostat. Unless the previous owner provided service records for the car, there is no way to know what maintenance was done in the past. Better safe than sorry. Treat your Z right and it will serve you well as a fun, little sports car for many more years!