Thread: Bogs down when letting clutch out

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Hey guys. New to this forum, so hello! I recently bought an old Datsun Z with a 350 and a T-56 (ridiculous, I know). The motor and tranny seem like they're in good shape, however since I've bought the thing, I've noticed that it struggles really, really hard get going from first gear. I can tell i'm on the border of chirping the tires every time I need to get going from a start, just because if I used a normal amount of gas, it would immediately die. I also end up riding the clutch for a very long time, and I'm sure the clutch wont last long at this rate. I'm very new to the SBC and carburetor's in general, so any advice would be appreciated. It idles fine (once it gets started) and doesn't stumble if I just rev it in neutral, so I think its getting enough fuel.

I've driven stick for nearly a decade, and have driven many different manuals, so I don't think its just me. Does this sound like a clutch issue?

Describe the carburetor, make and model.
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What is the gear ratio in the rear end? What is the timing set at?

First thing I'd do is change the spark plugs, and don't get Champions!

the 5.0 in my truck had the same problem. never quite figured it out but all roads pointed to cam timing. i would bump the timing a few degrees and see if it helped.

Make sure your in first gear and not 3rd------

check brake system for stuck parts------especially residual preasure from faulty vaves

Kinda what I wanted to say. Jack it up and see if you can spin the driveshaft by hand. easily. maybe the brakes dragging?
drum brakes? over adjusted and dragging?

ask the person you got it from how it works ?

Also----possibility of brake fluid types being mixed over the years causing complete caous will the rubber/silicone components

But who cares?? He ain't been back to check for answers????????

Sorry it took me so long to come back. I posted this question on a different forum site and didn't get any responses after a week, so I kinda thought it might be slow on here too. I'm very thankful for all the input, however! I was under it a while ago and I know the driveshaft took some force if I wanted to spin it by hand...Does that indicate the brakes are locked up? I'll have to go out and get all the details for the carb. I know nothing about the gears in the rear end. I'll also go check all the spark plugs here in a minute

Good to see you back. As long as you could spin the shaft, by hand, your good. If you lean on each tire ( with the car off the ground) and push to turn it, it should move rather easily. If it does, your probably good there. While the car is jacked up, use a piece of chalk and mark the tire, I like to make an arrow that points straight down to the ground.
With that arrow pointed down, make a mark on the driveshaft and right in line with that mark, mark the pumpkin (housing / other side of the u-joint)
Now turn the driveshaft slowly by hand or strap wrench / whatever , but count the turns the shaft makes and don't make but 1 revolution of the tire!
Obviously - if the driveshaft turns 4 times then you've got a 4:1 ratio, if the shaft turns 3 1/2 times but the tire turned once, then you've got 3.50 :1 ratio, etc etc.

This will get you close enough for now. ( as far as knowing the ratio )

If you have a limited slip differential, you can tell by putting the trans in neutral, emergency brake released, and raise one tire off the ground. If the wheel spins easy, rotating the driveshaft, then it's an open differential. (Or if it is a limited slip, the clutch packs are completely gone).

When I originally read the post my first thought was also the rear end ratio (possibly combined with tire height). I didn't post that response because after looking up the ratios for the Z cars, to me it looks like the gear set should be deep enough to easily get the car moving especially with the 1st gear ratios of the T56, provided it's the original rear end.

Here's a link to the site I used......by the way it would help to know what year the car is.

Transmission and Rear End Gear Ratios


If you don't find an issue with the brakes dragging (either front or rear), I would start looking at the engine.

If you brought the car into my shop with the complaint you have this would be what I would check.

I would first start by verifying it is a 350........a lot of 305s have been sold as 350s to the unsuspecting over the years. You can check this by looking up the block casting number located at the upper drivers' side rear of the block near the bellhousing area.

I would do a quick visual check of the intake, carburetor and exhaust looking to see if they are a good match for the street and not the drag strip and that the exhaust is not overly restrictive.

Next check would be to listen to the engine, basically to get an idea of how radical the cam is...... the more radical the cam the less bottom end torque you have to get the car moving. I would then check the carburetor for any obvious issues (vacuum leaks, sufficient accelerator pump shot, relatively well tuned, or that a computer controlled carb has not been installed on a non computer controlled engine). If the engine has an EGR installed I would also check to verify that it isn't stuck in the open position (and that it's NOT plumbed directly into a full time vacuum source).

I would then move on to the ignition and verify the condition of distributor cap, rotor, wires and plugs. When I have the distributor cap off I would verify the vacuum advance and mechanical advance are both functional. A stuck mechanical advance could easily be the problem. I would then check the timing and in addition to the static timing look for around 32-36 total degrees of advance at 2200 to 2500 RPM.

If nothing obvious has shown up to this point I would likely do a compression test looking for low cylinder pressure. Chances are you will have found the problem by now. If not the final check I would recommend would be to pull the timing cover and verify the cam has not been installed in a retarded position.

Well that's my 2 cents anyway.



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Lots of great advice! After you've checked for brake drag and gone through Mike P's list I'd change the fuel filter, and then look at the accelerator pump on the carb and see if you're getting a strong and consistent volume shot of raw fuel as you move the throttle from idle to WOT with the engine off, looking down the venturi's. I'd assume that the car has been running ethanol blend gas, and when you say you recently bought the car chances are it sat idle for a while? Today's blended gas destroys older fuel system parts, especially if left to separate and become corrosive.

Just another $0.02 worth to consider.

Wow! All great advice! Wednesday and Thursday are my two days off this week, so i'm going to start from the top. I had no idea you could tell the gears in the rear end without opening the pumpkin, so i'm excited to find out about that.

I'll take a video of her running and see if you guys can hear anything that sounds funny.