Running Rich

[Randalla]

Hi guys,

Been on this site for a long time but just making my first post. I have owned 8 240-Z's all with the original L-24, till now. One of my current cars is a 1971 240 Z with an N-47 head that I converted to four screw SU's. The car runs terrific with great power but I believe it's running too rich because I smell fuel and fuel economy is in the 17 mpg range. I'm running an electric fuel pump (3 1/2 pounds pressure) and currently have the jets screwed out only one full turn (spec is 2 1/2). Timing is set to factory spec and the floats in each carb are at spec as well. At this point I'm leaning toward changing the needles out (SM or N-54) to see if that makes any difference. I'm not yet sure if the needles are stock or have been modified in any way. Any other ideas on what to check to obtain a leaner run condition without adversely affecting performance? Oh, also currently running the standard spec NGK plugs but considering a hotter range NGK plug as well.

[Gary in NJ]

The number of "turns" for the nozzle height has little to do with the factory setting and everything to do with the float height. The factory setting is just a starting point.

Are you running the stock airbox?

[Bruce Palmer]

After we talked I had a chance to talk to Steve Epperly the owner and resident "straight poop from group" about what you and I discussed. With everything I told him he said you may, in fact, have N-58 needles in your carbs. He called them a California smogger needle and just ran rich and crappy at all settings. Why a smogger needle would be thin causing a rich condition makes no sense to me but just to know there was one piece of crap needle in use at least opens the door to that being a partial culprit.

We also agreed that a lower float setting and a lower nozzle setting would help lean thins out a bit too.

Oh, and you're not extrordinarilly high where you are, are you? For folks above say 5'000' we can and do at times supply undersized nozzles to compensate for the lack of air.

Good luck identifying the needles you have.

[Randalla]

I am running the stock air box, good question though as when I began running ITG's on my 1972 Z it required a substantial adjustment of the mixture. Bruce, thanks for the follow-up. I'll check to see what needles I've got later this afternoon and see if that might indeed be the culprit. I'm only at about 2000 feet here in AZ so no problem there. Re: the lower float and nozzle settings, since they are directly related to one another, as Gary points out, not sure how that might lean things out. Correct me if I'm wrong but lowering the float would result in it shutting off fuel sooner thereby creating a leaner condition. On the other hand, lowering the jet richens mixture. Seems like doing both leaves me where I was, in theory, because one move would counteract the other. Am I wrong?

[Bruce Palmer]

I think the lower float and lower nozzle thing warrants a try. Some oif this stuff can be over thought. High float and high nozzle puts the fuel level up closer to the air stream so it's easier to pull excess fuel into the air stream. Lower both down the tube and the same amount of air will pull a lesser amount of fuel regardless of where on the needle it is taking place.

[Randalla]

Well I checked the needles and there are no identifying marks to tell me what the profile is. I decided to lower the needles in their seats to line up the needle shoulders flush with the bottom of the piston. They were flush with the slot in the bottom of the piston. This should have the effect of leaning out the carbs slightly (dropped the needles a little less then 1/16th of an inch.). Also will try dropping the floats and jets tomorrow to see if the changes help at all.

[Duffman]

Randy, have you tried the clear hose technique to see how high your fuel level is in your float bowl and in your nozzles? See Arne's comment #7 in the following SU post:

http://www.classiczcars.com/forums/showthread.php?37771-su-adjustment&highlight=float+bowl

[Randalla]

Nearly have the rich mixture thing under control. Dropping the needles down, readjusting the floats (a little lower) and dropping the jets to 2 1/4 turns seems to have put me in a good spot. Will know better after checking mileage and inspecting plugs after a few hundred miles.

[Bruce Palmer]

That's good news. We thought that combination of dropping the float level and the nozzles at the same time would work to get you where you needed to be.

[Randalla]

Thanks for the advice Bruce. I actually think the biggest issue was the needles being buried too high in the pistons. All three items have to be synced together, so adjusting everything in order (needles then floats then jets) got it done. Thanks again!