burntz Posted January 9, 2006 Share Posted January 9, 2006 Hello Guys, First let me break down my set up. 73 240z. F54 block, stock with the flat top pistons, N42 head with a mild port & polish. Rebuilt 70 SU carbs with the stock mechanical fuel pump. Mild cam, about a 270 duration. And headers with a 2 1/2" exhaust. I recently installed an O2 sensor on my exhaust. And it's reading rich. About 12:1. Where it should be at about 14.5:1. I turned my mixture screws from about 2 1/2 turns to 1 turn and it was still reading rich. So I'm thinking maybe my floats are out of whack. I adjust them. And I make sure my choke is properly adjusted. And the reading is still rich. No difference. I turned the mixture screws down to 2 turns and left them there. I don't want to mess things up anymore than they could be. The needles in the carbs are N-27's. What can I do to lean out the fuel mixture? Are the needles I'm using causing this rich mixture? Any ideas? Thanks Quote Link to comment Share on other sites More sharing options...
AdrianZ Posted January 9, 2006 Share Posted January 9, 2006 can't you just tune the carbs? Quote Link to comment Share on other sites More sharing options...
burntz Posted January 9, 2006 Author Share Posted January 9, 2006 can't you just tune the carbs? Do you mean adjust the mixture nut under the carbs? I did, I turned them all the way up. Then at idle it would be too lean and at about 2k it would be too rich. Quote Link to comment Share on other sites More sharing options...
EZ-E Posted January 9, 2006 Share Posted January 9, 2006 Are you still running the stock plugs. You might want to go with one step hotter plug, that might help your rich condition a little bit, though I dont see how you are not getting any real adjustment at all from your settings, you might also want to try a lighter weight oil for the carbs that will let the needle rise a little bit faster. Quote Link to comment Share on other sites More sharing options...
Silent Posted January 9, 2006 Share Posted January 9, 2006 is it a wide band or narrow band o2? mine is set at 12.8:1 afr at wot idle it sits about 23 something to one. the car is rarely at idle, so this doesn't bother me. what about runnin an egt gauge to see where it is at 6k rpms in 4th. or going through the traps to see if it is too lean {14.5:1 seems way lean to lean} Quote Link to comment Share on other sites More sharing options...
burntz Posted January 10, 2006 Author Share Posted January 10, 2006 Are you still running the stock plugs. You might want to go with one step hotter plug, that might help your rich condition a little bit, though I dont see how you are not getting any real adjustment at all from your settings, you might also want to try a lighter weight oil for the carbs that will let the needle rise a little bit faster. I'm running NGK BP6ES. Which would you recommend? And what exactly does running a hotter or colder plug do? And wouldn't the needle rising faster allow more fuel? I currently have ATF 20WT in there. Thanks Quote Link to comment Share on other sites More sharing options...
Silent Posted January 10, 2006 Share Posted January 10, 2006 I'm running NGK BP6ES. Which would you recommend? And what exactly does running a hotter or colder plug do? And wouldn't the needle rising faster allow more fuel? I currently have ATF 20WT in there. Thanks bp7es would be a hotter plug bp5es would be a colder plug Quote Link to comment Share on other sites More sharing options...
AK-Z Posted January 10, 2006 Share Posted January 10, 2006 I know this is a stupid question and I know that you already checked but, just to make sure. Did you check to see if the sensor is at fault? Quote Link to comment Share on other sites More sharing options...
EZ-E Posted January 10, 2006 Share Posted January 10, 2006 Silent you have it backwards, it is bp7es would be a colder plug, as like what I run in my turbo set up. bp5es would be a hotter plug, one step hotter then stock. Burntz please forgive me as you are correct, it has been a while since working with SU's, the lighter weight oil will allow it to rise faster, and I am incorrect. I was thinking the opposite and writing the same lol. Quote Link to comment Share on other sites More sharing options...
Silent Posted January 10, 2006 Share Posted January 10, 2006 Silent you have it backwards' date=' it is bp7es would be a colder plug, as like what I run in my turbo set up. bp5es would be a hotter plug, one step hotter then stock. Burntz please forgive me as you are correct, it has been a while since working with SU's, the lighter weight oil will allow it to rise faster, and I am incorrect. I was thinking the opposite and writing the same lol.[/quote'] DOH! i was early in the morning when i typed that out Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 10, 2006 Share Posted January 10, 2006 oil weight will change transient response, you will see transient leans on throttle application. doubling up the spring weight will show a more positive response. welcome to the world of the "metered vacuum leak"!!! BTW, there is really no "set" number on the "Turns Down"---ideally it is set based on lean or rich best idle, and the setting of the needle height sets the rest of the rpm range. Quote Link to comment Share on other sites More sharing options...
Pyro Posted January 10, 2006 Share Posted January 10, 2006 I assume you are talking about the idle mixture. A bigger cam like the one you have will need a richer mixture to idle. And the engine will need more initial timing, something like 15 to 18 degrees. Adjust the fuel mixture at idle so that the high idle rpm (highest vacuum) is obtained. Too lean a mixture with that cam will make a misfire. You could lower the needle position slightly. Quote Link to comment Share on other sites More sharing options...
george.bryant Posted January 12, 2006 Share Posted January 12, 2006 do su carbs have jets? maybe you need differet needle & seats... i did on my 2l pinto when i put 45mm dcoe sideys on it Quote Link to comment Share on other sites More sharing options...
burntz Posted January 13, 2006 Author Share Posted January 13, 2006 The sensor is new. I'm probably going to take it somewhere and have the exhaust gas analyzed to see if my sensor set up is accurate. If it continues to run rich I guess I'll try looking for a "fatter" needle. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 13, 2006 Share Posted January 13, 2006 For best power you want somewhere right aroud 13:1. Stoichiometric is 14.5:1. You don't really want stoich at WOT. Your basic tuning choices are needles and nozzles, springs in the domes, and oil. That's pretty much it. As you've already figured out the adjuster on the bottom only adjusts the mix at idle. Once the piston lifts up it's more about the needle profile. There have been some really good threads about tuning SU's in the last few months, with links to some British car sites where they talked about the springs in depth. A little search should bring them up. Quote Link to comment Share on other sites More sharing options...
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