Datsun Roadster Forum-311s.org

I can't seem to get my SU's dialed in. I just replaced my needles and nozzles. Carbs back on the car, seems to tune up just great. All balanced, air velocity is right at idle and 2000-2500 RPM. Car seems to drive okay, until I get up around 4000 RPM with my foot on the peddle, it just stops pulling and sputters, even mini-backfires. Plugs look okay, but my timing light appears to be broken, so I can't check that side of the spectrum. Suggestions?

My car stock 2L, with the smog removed and the recurve kit done, if that might have any thing to do with it.

Any help would be great, I just don't know which way to go first.

Thanks,

That sounds like you are going lean at the high rpms. It could be from the wrong needles, or too small fuel line, or the float bowls being set wrong. Try taking it out and driving it hard till it falls on its face, than turn it off and coast to a stop. Pull a plug and look at it.
I hope that will help some.

Will

I believe I got the right needles, will check the float levels tomorrow. My first thought was not enough fuel. Keith Williams suggested I put springs inside the fuel lines to keep then from kinking, which I did, but I think I'll try taking those out, the fuel lines didn't look like they were gonna kink.

If I were to drive it hard until it falls on it's face, which I'm not sure exactly what that means, then pulled the spark plug, what would it look like if it were to lean? I know there is a whole science to spark plug reading, but I think I was learning how to blow stuff up in science class

Thanks,

lean is white

The electrode will be white. Black means that it is really rich. Drive it hard till it starts doing what ever bad thing it does and just turn it off and coast to a stop and pull one plug from each of the carbs (1&3 or 2&4) and look at them.

Will

I am guessing but it could be one of two things: you may have the carbs too lean or the timing is off because it is not set properly (16 degrees before TDC at idle or the centrifical advance is not working properly). Apply a little choke at 4000RPM to see if that helps. If it does, then you are too lean.

Okay, so I've been working on my problem everyday, for about 3 hours a day. I have gotten almost no place.

Today I tried taking the car out and driving it hard until it start sputtering, then turned it off, coasted to a stop, and pulled the plugs. They looked fine, not burned and white looking. The tips were white, at the points where the spark arcs. But the rest seemed a bit carbonized, say "oily but dry"?

I have been trying to dial in the float levels. I don't have the tool #???? that I see refered in a few places. I have the Just SU Carbs video from ZTherapy, and he talks about having 9/16" space from the top of the float to the inside edge of the float bowl cap. He is talking about it when you have his ball type gross jets installed, which apparently he doesn't stock anymore. But I have gotten it as close as I can, and no help. I now have the floats adjusted as high as I can get them. Keith Williams showed me a rough way to tell if your floats are adjusted properly, which was to pull off the domes and look at the top of the nozzle. You should see fuel at the top of the nozzle. I don't. If I pull the choke down, thereby pulling the nozzle down all the way to the lowest point that it will go, I see fuel there. This is about 3/8" to 1/2" below the point where the nozzle would sit flush if it were all the way up. Any suggestions on adjusting my float bowl levels?

I have been thinking about the possibilty of my vacuum advance somehow being screwed up. At times it seems quite likely. For instance, it only happens under load. And it always happens at the same RPM, about 3800. I can't get this car to do it unless I'm driving it.

I also haven't been able to turn the nozzles down more than 3 ful turns, they run into the stops, which I can't adjust without taking the carbs off the car again. I've been trying to avoid that, only because my hands aren't meant to articulate like that.

At this point, I'm kinda stuck. I will borrow a vacuum gauge tomorrow, but don't really know what to do with it. I will also try swapping my old needles back in, maybe somehow that will halp. The only reason I started this whole process was because I was having some problems at idle. But the car ran great. Pulled great, all the way up 6K and above, made it to Shasta and back getting about 29.6 MPG.

If I sound desperate, it's because I am. I have run fun run this sunday, and would really like to drive my own roadster. If I could buy a rebuilt set of carbs tomorrow, I probably would, along with a new electronic ignition, just to get that doubt taken car of as well. But I can't, no time. I've cancelled work on Saturday, so I can work on my car. I have a friend coming by sometime on saturday who is a Volvo SU guy, I'm hoping he can help. If anybody else would like to take a stab at it, come on by. The fridge will be full of beer.

Well, I don't know what else to add, but thanks for the advice so far and hopefully for the insights and help to come.

With the Mityvac see if your distibutor still holds vacuum. If not the that needs to be fixed.

Everything below is what I have read and have been to taught to me by Keith at Shasta the other week.

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Make sure your chokes and throttles are bottoming out when the throttle is released. If not, get some carb cleaner and fix that.

Exam if your throttle shafts are leaking air by spraying some carb cleaner and/or WD40 on it at idle and see if the causes anything to the idle. If so, your thottle is letting in more air at idle and causes the mixture to be rich when the throttle is down. Fix the paper gaskets or swap out for the ball bearing upgrade..

Keith mentions that you need to get your carbs balanced first and foremost before tuning them. Get them balanced at idle (about 700 RPM) and at 2500 RPM.

After that you can then dink with the mixture. Be sure to test the air rich and air lean with your fingers by pushing up and down the carb piston. If idle picks up when you push the piston up you're rich. If idle picks up when you push the piston down you're lean.

When you done with that, take your hand and completely block off one of the carbs. Your engine should idle rohgly with one carb functioning.

When mixture is finally set test drive it.

At this point you gonna either be satisfied or become the questing perfectionist. If your satisfied - STOP. If not, go on young man...

It is best to have a air/fuel ratio monitor ($?) or exhasut analyzer ($100).
http://www.team.net/www/morgan/tech/tuning.html

So when you step on the gas to accelerate at moderate to heavy load in the entire RPM range. This will accurately tell you if you are lean or rich at the instant. If you so, then you can start hunting for that elusive perfect needle. That is another chapter.
http://www.jetlink.net/~okayfine/sutech.html
http://www.jetlink.net/~okayfine/su/suneedles.html
http://www.jetlink.net/~okayfine/su/sun ... chor222774

Download an Excel file containing all the needles (Brit and Datsun), suitable for sorting or framing.
http://www.jetlink.net/~okayfine/su/excellneedles.zip

When it is all done, you never want to swap out the SU for nothing. You can now drive in any elevation and your air fuel ratio will be on target...

You will be probably be looking to swap for a more aggressive CAM next.

Joe,
Call Mike Young let him know the story, he may be in the City today. I think it something simple rather than a complex problem:
415-337-8144 San Francisco, CA
707-928-4029 Lake County, CA

415-987-8441 cell phone

jibha wrote: Keith Williams showed me a rough way to tell if your floats are adjusted properly, which was to pull off the domes and look at the top of the nozzle. You should see fuel at the top of the nozzle. I don't. If I pull the choke down, thereby pulling the nozzle down all the way to the lowest point that it will go, I see fuel there. This is about 3/8" to 1/2" below the point where the nozzle would sit flush if it were all the way up. Any suggestions on adjusting my float bowl levels?

That indicate that your floats aren't set up correctly. Keep in mind that the 9/16's figure is where the float valve closes, not where the float rests. Another way to check the level is to remove the fuel lines between the float and carb (carefully, there's fuel in there) and replace them with a legnth of clear hose, say at least 6 inches long. Attach the hose only at the float bowl and secure the other end up above the float bowl. Make a mark on the float bowl 23mm below the top of the bowl (NOT the top of the float cover) Crank the car until the floats are full and then look at the clear hoses to check the actual level of fuel in the float bowls. It should be 23mm below the top of the bowl. If it's low, you'll be lean. High, you'll be rich. Given what you've indicated above, you'll be low.

Well, my car is now running great. I'm not sure how this happened, but my timing was off by 10 degrees. As I said, my car was running fine, I just changed the nozzles and needles. I had a friend come by today with a working timing light, and my problems seem to be solved, after a bit of final tuning I'm ready to go. I guess this just proves what they say about doing a standard tune up before adjusting your carbs.

Thanks to all for the advice, I'm now happily tearing up the roads again, and on my way to Sears for a new timing light.