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heyitsrama

4 Screw Dashpot SU Configuration

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Howdy Yall,

I've been driving my Z for a couple weeks, (its too much fun, dont want to drive other cars now). Ran into an issue that I cant seem to get myself out of.

I'm getting what appears to be a lean condition on the carbs, the motor will produce a backfire/bog down when running between 2.7-3.5k RPM, and if i get to 40 mph it really drives rough.

I parked the car for 2 weeks and have been trying to get a baseline tune on the carbs, based on information that I'm finding on the web, but am a little confused from the information that I'm reading online regarding the float settings:

Idle ~6-750RPM @ 20mmhg on the flowmeter

apparently for this carb (1970 4-screw), the instruction set that came with the carb rebuild kit states that I should be at 10.38mm on both front and rear carb, online readings indicate that the rear should be configured at ~2mm lower than the front, but the 1970 4-screws have equal length float arms, and the new floats that I got have longer floats needles in the front, while the float needles that came with the carbs are equal length front and rear.

When I end up placing a new (aftermarket) float needle into the rear carb, and set it to ~10.38mm FROM THE FLAT PLATE THAT THE FLOAT NEEDLE SITS ON, i end up with a sunk float needle, which leads me to believe that the configuration is incorrect.

I think adjusting the float level correctly would be the first thing to do in order to get the mixture configuration correct, does anyone have some input, or maybe some settings that work for their 4-screw? Should I switch the manufacturer for the float needles? Really want to drive it :)

 

Edited by heyitsrama

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Ah someone has yet to respond to this, but I thought I might as well give an update to what has happened so far.

I set the front carb to 13/32" clearance from the top plane of the float chamber, and according to some online images, i added 2.4mm to the rear clearance, for a total of

16/32", I set the idle around 700-800 ~25mmHg on the syncrometer, the motor pulls a lot better, and less sputtering, although on right turns the engine complains a little. I think leaning out the jets might help address the dieseling issue that I'm having when turning off the motor, will further test tonight. 

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 Bruce Palmer (Ztherapy) shared this tip a while back for checking float (fuel) level in the float bowls. Run the engine at idle, turn it off. re-set the mixture screws 2 1/2 turns down(factory setting), remove the 4 screw domes and the pistons with the needles, I temporarily set the pistons, needles down, in the cowl slots), look straight down at the fuel jet (orifice) that housed the needle. When the floats are adjusted properly, the fuel levels will be 1/16" below the tops of the jets. I find this method much easier than pulling the float bowl tops, measuring, re-setting and still wondering if I got it right. Let us know what you find.

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5 minutes ago, Mark Maras said:

 Bruce Palmer (Ztherapy) shared this tip a while back for checking float (fuel) level in the float bowls. Run the engine at idle, turn it off. re-set the mixture screws 2 1/2 turns down(factory setting), remove the 4 screw domes and the pistons with the needles, I temporarily set the pistons, needles down, in the cowl slots), look straight down at the fuel jet (orifice) that housed the needle. When the floats are adjusted properly, the fuel levels will be 1/16" below the tops of the jets. I find this method much easier than pulling the float bowl tops, measuring, re-setting and still wondering if I got it right. Let us know what you find.

Mark ill give that a try, is there an accurate way to check if the fuel level is at 1/16" below the top? I assume eyeballing it to a reference would be "okay" How many times do these 4-screw jets turn out? I think mine does almost 3.5 turns before it bottoms out, however on some threads I hear that people are able to get more than that?

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 Eyeballing the fuel level is close enough. I don't recall how many turns of adjustment the mixture screws have. My original 4 screws had adjustable stops below the mixture screws. The stops you're referring to were probably adjusted down farther or removed to allow more mixture screw adjustment. 2 1/2 turns down and the fuel level (floats) set 1/16" below the jets is what you're shooting for.

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Valves were adjusted almost 1000miles ago, checked while motor was "hot", timing was set around same time, but i should probably double triple check the adjustment.

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currently its set to 0 mechanical, and the dizzy is retarded all the way. With the timing gun, can see the mark on the crank pully pass by after the 0 mark on the timing bracket. 

 

I inspected the carbs fuel level, it appears that the fuel makes it to the top of the front jet, (i can blow over the jet and see it move down then up), however on the rear carb it does not appear to make its way to the top. I cleaned out the jet, and readjusted the mixture to attempt to tune out the little stutter that it has, on the driveway it seems okay so far.

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My Z's run best at 17 degrees advance at 1000 rpm. Start the car up with the timing light and advance the timing until the engine runs the smoothest. Your problem may not be carbs at all......

Edited by Diseazd

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10 hours ago, heyitsrama said:

currently its set to 0 mechanical, and the dizzy is retarded all the way. With the timing gun, can see the mark on the crank pully pass by after the 0 mark on the timing bracket. 

 

I inspected the carbs fuel level, it appears that the fuel makes it to the top of the front jet, (i can blow over the jet and see it move down then up), however on the rear carb it does not appear to make its way to the top. I cleaned out the jet, and readjusted the mixture to attempt to tune out the little stutter that it has, on the driveway it seems okay so far.

 Sounds like the float level is too low in the rear carb. Fuel level should be 1/16" below the jet at 2 1/2 turns down.

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