High RPM punchless

[trout_hound]

Alright, so my 72 stock 240 is painted, mostly restored, and ready to roll. The engine was recently tuned up and the valves adjusted, and the SU carbs apparently synched by the mechanic I used (who I trust, as I've known him for years). The coil was replaced, as that was causing the car to hiccup heavily once it got warm. It doesnt do that anymore, but for whatever reason, it revs great up to about 4,000 rpm then it just seems to bog out. No high rpm torque at all. My nephew who has owned some z's and is a pretty good mechanic drove it and suggested that the carb floats are the problem, and that the carbs arent getting enough gas. Its true that when you hit that spot when it starts to flatten out, if you wait a few seconds for what may be the bowls filling, then it has a bit more punch. Do you guys think I'm on the right track? I would hate to spend all that time tearing the carbs apart if thats not the issue.

Thanks.

DT

[Zforce]

My car does the exact same thing, If you are driving in town there is no problem, but take it on the highway deep in the throttle and BAM, between 4k and 5k it falls flat. And like you said lift up a bit and it gets better.

I got a stock air cleaner off ebay with some holes drilled in the top of the housing, it was only 20.00 bux so I tried it and picked up about another 500 rpm but the bog is still there. So today I orderd a pair of SM jet needles from Ztherapy and will give them a try.

[trout_hound]

Did you install the new needles? Did that help? I may go that route.

[ajmcforester]

OK if you think it is fuel start with the basics.

When did you replace the fuel filter and how is it, maybe try a new one.

Does the gas smell right, and when was the last tank put in

Check to see if you have proper fuel pressure, pumps go bad sitting.

Are the inside of the fuel lines clean

To save you from taking the bowls apart their is a bolt on the bottom you can hook a to. This can be used to measure the float level to see if they are correct. Also what kind of cars does your mechanic specialized in? I will let you know every Z car a bought the carbs were tuned by a trusted mechanic of the last owner and they were not in tune at all. The last one the last owner was shocked how fast the car was when I roughed tuned it before driving off.

[Zforce]

I put the needles in but have not had the time to evaluate them yet. I still need to do some tuning first. One thing I noticed it idles alot better and there is way less fumes. After installing the SM needles I had to crank down the idle speed and it seems to idle best 3 1/2 turns out on the mix nuts.

[Travel'n Man]

I had the same thing happen and this winter I corrected my problem. If the floats are not set right then there will not be enough gas for the heavier volume for higher rpm demand. You don't need to break your carbs down - just remove the float bowl lid and the float will come off with it. Be sure to do your research - set you bowls correctly (it may take a couple of attempts to get it dead on, and you're set - you may want to check your air flow before you finalize). I would recommend when you are in there to replace your float valve (if they have not been replaced) and have a couple extra float bowl gaskets on hand (once you take yours off they will look like a pringle potato chip).

You will be travel'n over 5K RPM's before you know it - one more thing - you may get better air flow from the original air breather that Datsun's came with stock (with air horns included) vs the stand alone air filter system. I corrected mine this past winter with faster acceleration and more power.

Edited May 7, 2010 by Travel'n Man

[trout_hound]

Thanks for the help, guys. I'm fairly positive its fuel related. The gas is good, but I'll have to look at the lines. I haven't messed with carbs since I rebuilt the Webers in my old '69 911T long ago, but I think I can handle it, especially if I pull the trigger on a Z-Therapy rebuild kit (which includes the video). I'd at least like to really enjoy driving this baby for while as I may have to sell it this summer if my job situation doesn't improve.

[tlorber]

Mine has a similar issue. It does not bog, but as I get slightly above 4k it just does not seem to pull much. If it were starved for fuel, wouldn't it sputter a bit?

I also have a tiny exhaust pipe-which I'm man enough to admit-(probably 1.5", and less around the 90* bends) and I'm running an L28. I plan on replacing the exhaust, but could this symptom be from exhaust not getting out fast enough at high RPM's?

[Gary in NJ]

Mine has a similar issue. It does not bog, but as I get slightly above 4k it just does not seem to pull much. If it were starved for fuel, wouldn't it sputter a bit?

I also have a tiny exhaust pipe-which I'm man enough to admit-(probably 1.5", and less around the 90* bends) and I'm running an L28. I plan on replacing the exhaust, but could this symptom be from exhaust not getting out fast enough at high RPM's?

A restrictive exhaust will reduce high-rpm output, as will fuel starvation and poor ignition. It's best to evaluate on system at a time. If you can confirm that your ignition is operating well at high=rpm, and that your carbs are delivering adequate fuel flow, then the exhaust (knowing that it's 1-1/2 inches) is a good place to look.

[deadflo]

I've never had the problems you mention, but I am curious about the SM needles. Hopefully Bruce will chime in here. I thought they may only be useful if you had done some other mods like head work or a cam where you needed more enrichment at higher RPM's. Would they give you more performance with a mostly stock setup?

[Z-Luke]

I'm chasing a similar problem. After cleaning the carbs and putting seafoam in the tank, I can get to 5000 rpm pretty good, but then there is SHOCK AND AWE coming out of the muffler. Explosive, cuncussive afterfire. Oddly enough before the Seafoam it would just wheeze out at 5000.

I'm suspecting the points being worn. As I've found in other parts of this forum, i'm going to replace the ignition components (and I mean all of them) before I go back to fuel. The floats were even and pretty high, the needles were not sticking, and the pump delivers a lot of gas by eye.

I'm getting my Pertronix with new coil next week along with new plugs, cap, rotor, and points/condensor just because. New plug wires are already on the car. I'll let you know how it goes.

[tlorber]

Gary, thanks for the reply. I do have an electronic dizzy that I plan on swapping to replace the points one that is currently on there, but the existing one looks to be in decent shape. I'm a big fan of one variable at a time so perhaps I'll wait til I have access to a timing gun and swap the dizzy first since I already have it.

I plan on a new exhaust regardless. I was just wondering if I should brace for disappointment after dropping $430.00 on it.