'73 240z su carb adjustment

[chocobosteve]

i was working on my front carb today, because of a problem with the float sticking, so fuel came out of the jet in a steady stream. i got that working again, but now i think that the carbs were adjusted to work with the other one, which was way off. if there's any way to do this without money, i would be very happy, but if i need to shell out cash, i'm willing.

[AxtellZ]

I'm not really sure what you're asking:confused:

Can you be more specific as to what works or doesn't, or what- you mean the carbs were adjusted to control a sticky float?

I'll help if i can, just not really sure whats wrong!? JA

[chocobosteve]

okay, here's my dilema... both of my carbs are backfiring, and it all happens at the same time. the car won't idle, it just backfires, then kills. it does, however sound fine when the rpm's are around 2000+. i asked around yesterday, and i was told that the timing may be the solution, and i will look into that, but any other possible solutions would be appreciated.

[Royce]

Sounds like there may be quite a few issues. I think I would start with a tune up(valve clearance, points, cap, rotor, plugs) and go from there. Or just go through properly setting up the carbs, either buy the ZTherapy video or grab a manual and go from start to finish. It is certainly possible to balance the carbs at 2000+ RPM without getting the idle right. It will be a challenge to set timing or the carbs if the other is out of spec. But if you get the carbs close you should be able to get the timing set.

No quick fixes here.

[chocobosteve]

thanks for the help! i looked further into it today, and discovered that i had some bad vacuum hoses, which is quite a relief. the car sounds great now, and is ready for the road again.

[beandip]

Are you running the stock flat top SUs ?? if so why? anytime you are trying to tune multicarberated engine you MUST have access to a uni-sen or simular ballancing device . with out ballancing the carbs first you cannot tune and set the mixture. yes leaking vacume lines will grossly effect the fuel mixture, but if you have messed with the mixture with the vac. line problem . you now need to retune the mixture. I dont know about flat tops but with round top SUs at least poping back through the carbs when you fully open the throttle quickly is refurred to as "lean pop" and is caused by the air fuel mixture being set too lean. If you need some help with setting the round top SUs just e-mail me and I'll walk you through it .

[Phacade]

I realize it is an expensive option, but I highly recommend Z therapy round top SU carbs. There is just no subsitute. I paid about $900 for everything but it was well worth it. I have never had another problem with them. Scott at Z therapy walked me through just about everything with them, mounted them on a polished intake manifold and included alot of hardware to get it installed. If you have the flat tops, you really have no idea what you are missing!

[BadDog]

I can sympathize with you... I've got a '73 that I just got a few weeks ago, and was dieseling and backfiring a lot. I have '72 carbs, though...

Anyway, I've never tuned an engine before, but I watched about an hour of the Z Therapy video, and with a Unisyn and a screw driver, I got my carbs adjusted pretty well. I also decided to check my timing, as it was still backfiring after balancing the carbs. Turns out that my distributor wasn't tightened down enough, and my timing was set to just *after* Top Dead Center (Agh!!!)

With the timing the way it was, and 1 carb not flowing any air at idle (!!) I'm suprised she even ran. She runs pretty well, but I still think she's running a little lean, because I didn't adjust the carbs much past the "lean backfire" phase, she doesn't seem to pull that hard past about 4500 RPM, and I couldn't see my ColorTune in the sunlight outside (I don't have a garage yet). Heck, I even cleaned out the domes and pistons a little (I was trying to check/adjust the needles per the Z Therapy video, but they wouldn't budge )

I'm not sure if I'm brave enough to tackle checking my valves and adjusting them, but they seem to make a lot of noise. Other than that, she runs pretty well. In fact, my on-choke idle is up about 1,000 RPM to about 2,500 (yikes!) but I think I can fix that this weekend but bending or straightening the lever...

Anyway, I guess my whole point is that $100 of equipment and video instruction goes a long way with a 240... and you can feel like you've accomplished something, too!

[beandip]

You say that your idle is at 1500 and you have to bend somthing to get it down?? You better watch the video again !! There is no reason to bend anything. Adjusting the valves is important and usually done before doing the carbs, but be aware that you should check the carbs after the valve adjustment. I really recomend that you do the valve adjustment cold . unless you are experienced in doing it . As the engine cools the adjustment will vary , but if you do it all cold it will be consistant. Then check the clearence after you warm the engine if you think it needs it. Be aware a valve set to tight can burn the valve, if loose it will only click. The same with the carbs if set too lean , that can can burn valves , too rich the only the mileage suffers. Just a thought. Just one more thing to concider to warm up an engine completely for doing this stuff it takes the equilivent of driving the car about 4 or 5 miles , not just watching the temp gage go up to normal, The best of luck to you and yours.

[Si|v3r72]

since we're on the subject of carbs....

I tuned my carbs with the ZTherapy video and the shop manual last week. The thing that confused me the most was the idle nut. I'm not sure I was turning it in the right direction (pretty sure, just not 100%). The ZTherapy video kind of skimmed over the actual tuning and mixture nut adjustment pretty quickly so I was a bit confused.

My questiojn is, looking down at the top of the carb, when you turn the idle nut clockwise are you making the mixture richer or leaner? The book said to turn to the right to tighten, but it just seemed like it should have been the opposite to me.

[beandip]

when looking down at the top of the carb. and turning the adjustment clockwise you are richening the mixture. If you were to turn the mixture knob counter clockwise all the way to the stop and then clockwise 21/2 turns you would be at the starting point to do the tuning , then stick your finger in to the throat of the back carb and lift the piston , disableing it, and adjust the front carb untill the engine runs roughly . If when you lift the piston the engine dies the front carb is too lean , repete the whole operation on the rear carb and you are vary close , now if you "blip" the throttle , that is floor it and release it quickly and you get a pop out of the carb , this is a lean pop , turn out that is turn the adjustment clockwise 1/4 turn . do this untill no lean pop is experienced and you are good to go. Enjoy Gary

[BadDog]

Well I had trouble getting my colortune not to show Rich at idle without also having lean pop... I think my needles may need to be adjusted.. Problem is I couldn't get them to budge . NO biggie, if I'm running a little rich for a while, so be it. I only get to drive the once or twice a week anyway, plus I have to sort out a few other issues (like getting the steering wheel centered )