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‘71 sputters going uphill


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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?

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Might be the carbs - have you tuned them? They might not be balanced, or the mixture could be off. I had similar issues, and a good tune helped a lot. Though I never had sputters from hills and hills alone - it was more a rpm/throttle input thing in general. My mixture was off, I reckon. Sputters are much rarer now that I've gotten things pretty dialed in.

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I had the same problem going up steep inclines. The front carb would run dry and then it only ran on the back 3 cylinders. I could roll the car backwards and cut the wheel making it perpendicular in the road, let the front carb fill back up and it ran fine for another mile or two. My floats were way off but once I got those adjusted properly I went straight back to Cheaha Mountain and "tested" it. Pulled up that thing like a mule, no sputter whatsoever.

The quick and easy way to get an idea where your's are is the clear hose method.

float level.png

Edited by siteunseen
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1 hour ago, siteunseen said:

I had the same problem going up steep inclines. The front carb would run dry and then it only ran on the back 3 cylinders. I could roll the car backwards and cut the wheel making it perpendicular in the road, let the front carb fill back up and it ran fine for another mile or two. My floats were way off but once I got those adjusted properly I went straight back to Cheaha Mountain and "tested" it. Pulled up that thing like a mule, no sputter whatsoever.

The quick and easy way to get an idea where your's are is the clear hose method.

float level.png

So you would do half a K turn and block traffic while your carb refilled - lol

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Sputtering at 3500 rpms while climbing a hill sounds like fuel starvation to me. Especially if it will rev to high rpms in neutral (no load). Fuel filters, float levels, carb balance, fuel pressure, and fuel volume, are where I'd start looking.

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11 minutes ago, shoehorn_hands said:

Thanks for the help! What is a good source for information on how to tune the carbs?

https://www.youtube.com/watch?v=F4JVwqu8dgo&t=343s&ab_channel=LivingWithAClassic and https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/ The video and guide are how I learned. I was running in circles for a while on my first tune, then everything kinda clicked. 

 

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On 9/4/2021 at 4:25 AM, siteunseen said:

I had the same problem going up steep inclines. The front carb would run dry and then it only ran on the back 3 cylinders. I could roll the car backwards and cut the wheel making it perpendicular in the road, let the front carb fill back up and it ran fine for another mile or two. My floats were way off but once I got those adjusted properly I went straight back to Cheaha Mountain and "tested" it. Pulled up that thing like a mule, no sputter whatsoever.

The quick and easy way to get an idea where your's are is the clear hose method.

float level.png

Where does one find a clear fuel hose like this?

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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.

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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

 

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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?

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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 by shoehorn_hands
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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.

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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?

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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.

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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.

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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%.

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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.

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