Hi fellow Z lovers,

I recently bought a ‘71 and am using it as a daily driver. The car is generally in pretty good condition and runs well for the most part but sputters/jerks/misfires(?) while going uphill.  This usually happens while within 10% of 65 MPH/3500 RPM. So far it hasn’t completely died yet but it doesn’t feel, sound, or look good when it happens. I’m worried about it.

The car has a rebuilt, non-original L24 with about 45k post re-build.

What should I be looking out for?

I got mine at Lowes. Seems like it's 3/16" maybe?, 1/8th"? Take something with you to compare it to like a golf tee or a nail. It's on a roll in plumbing and you buy it by the foot. 2ft would be plenty I think.

I found my old thread from I did this. You may be better off turning the mix knobs down and looking at the fuel level under the dome. I have an extra set mounted to a wall and adjust the floats there then transfer the float and lid assembly to the pair on my car. So if yours are in your car try this first.

Post #7

 

I’m happy to report that after an afternoon and a half, my carbs are tuned! Thanks to everyone for the help! I had a blast figuring it out; definitely a lot easier than expected.

8 hours ago, shoehorn_hands said:

I’m happy to report that after an afternoon and a half, my carbs are tuned! Thanks to everyone for the help! I had a blast figuring it out; definitely a lot easier than expected.

What did you find?

Final cause of the problem?

Yeah really! Way to let us all hanging here!! 

Seems to have been a fuel starvation issue. I started going through the “quick and dirty” guide that siteunseen linked. Once I got to setting the fuel mixture, I started to notice significant improvement.

I thought I had gotten it all the way dialed in yesterday afternoon (I couldn’t replicate the issue). However, this morning on my commute, I noticed it sputtering again. It doesn’t happen nearly as easily or as violently as it used to, but I can get it to start to misfire pretty much exactly at 65 MPH/3400 RPM going uphill. I have incrementally (1/8 to 1/4 turns per adjustment) increased the fuel mixture since this morning and the misfiring has gotten even harder to replicate.

If it misfires only under those specific circumstances, and has been generally improving as I increase the fuel in the mix, would that indicate that I need to continue incrementally increasing the fuel in the mixture?

I guess an alternate question is: will the car definitely start to backfire from the exhaust when I’ve gotten the mixture too rich?

 

*edited for clarity

Edited September 14, 20213 yr by shoehorn_hands

I didn't even think to mention this earlier. Are you still using points in your distributor? If the gap and dwell aren't right, that could weaken the spark enough to make the car bog down going uphill. I was working on another guy's 240Z not that long ago. I got the carbs set up pretty good, put it bogged down going uphill. Another friend set the dwell, and it improved the car for a while. I finally convinced the owner to get a Pertronix. Once I installed that, the car would run uphill like a scalded dog.

I don't recall any talk of adjusting the float levels (the primary mixture adjustment). The easy way to check them is remove the domes and pistons, raise the mixture screws to the top, then lower them 9 1/2 - 10 turns down from the uppermost position. The fuel level should be close to the jet height at 9 1/2 - 10 turns down. Then raise the jets back up to 2 1/2 - 3 turns down from the top. How many turns down are your mixture screws now?

13 hours ago, Mark Maras said:

I don't recall any talk of adjusting the float levels (the primary mixture adjustment). The easy way to check them is remove the domes and pistons, raise the mixture screws to the top, then lower them 9 1/2 - 10 turns down from the uppermost position. The fuel level should be close to the jet height at 9 1/2 - 10 turns down. Then raise the jets back up to 2 1/2 - 3 turns down from the top.

This doesn’t seem too difficult.

How many turns down are your mixture screws now?

Well… I didn’t think to keep track of that until I was about 5 adjustments in. I’d say I’m roughly 4-6 turns down. ¯\_(ツ)_/¯ 

For what it’s worth, I’d say the problem is 95% resolved. I pretty much have to make a conscious effort to linger exactly in that 65/3400 “danger zone” until it starts sputtering. Once it starts sputtering, it generally seems easier to make it do it again, but still not necessarily “easy” to do.

17 hours ago, SteveJ said:

I didn't even think to mention this earlier. Are you still using points in your distributor? If the gap and dwell aren't right, that could weaken the spark enough to make the car bog down going uphill. I was working on another guy's 240Z not that long ago. I got the carbs set up pretty good, put it bogged down going uphill. Another friend set the dwell, and it improved the car for a while. I finally convinced the owner to get a Pertronix. Once I installed that, the car would run uphill like a scalded dog.

I am not using points. When I bought the car, it had a Pertronix flamethrower already installed. I did replace the stock ignition coil with a Pertronix one and like it so far.

10 hours ago, shoehorn_hands said:

This doesn’t seem too difficult.

How many turns down are your mixture screws now?

Well… I didn’t think to keep track of that until I was about 5 adjustments in. I’d say I’m roughly 4-6 turns down. ¯\_(ツ)_/¯ 

For what it’s worth, I’d say the problem is 95% resolved. I pretty much have to make a conscious effort to linger exactly in that 65/3400 “danger zone” until it starts sputtering. Once it starts sputtering, it generally seems easier to make it do it again, but still not necessarily “easy” to do.

Check the float levels. you may find that elusive 5%.

11 hours ago, shoehorn_hands said:

I am not using points. When I bought the car, it had a Pertronix flamethrower already installed. I did replace the stock ignition coil with a Pertronix one and like it so far.

Good to know. That eliminates one of the extra variables. Don't forget to check the valve lash and timing, too. Those are important to have correct before futzing too much with the carburetors.