240z tuning/spitting problem

[dylan777]

I know people have probably asked a similar question but it's easier if I explain exactly what the car is doing to get an appropriate solution.

 

I've been having ignition problems for a while, it ended up the two wires inside the distributor had a bare spot and were grounding out. Took them apart and reconnected the wires and shrink wrapped them. That fixed the problem for a bit. Also since I bought the car it's been running rich, I know how to adjust it and all but once I adjusted the su carbs fuel it sounded good at idle but when I went for a drive it ran well for 10 minutes and started to bog and once it went back to normal it would spit out of the carb and backfire. Did that while I tried to get the car back home. Is the car just lean? It would spit then run fine for a minute and do it again. It was doing the same thing in the spring but it was because my timing was off, and if it was my timing again it wouldn't idle very well. When I stop the car at a light it idles fine and first gear pulls fine but once I shift it starts doing it again. Is this a timing/ignition issue or a mixture issue, does anyone know the factory set point for the fuel adjustment on the bottom of the su carbs?

 

I apologize if this is on here already but I can't find a similar post. I'm not interested in arguing with someone I just want to solve the problem

 

Thanks

Edited August 21, 2015 by dylan777

[Bob_D]

Hi Dylan, I just fixed a similar problem on my 240. I have a '73 but with '71 SU dome-top style carbs. In my case there were several steps to fix this issue. 1st was my spark plug gap was way too large (even thought the plug color looked fine). I had put in a Pertonix electronic system and eliminated my points, and at the advice of a friend had opened up the gap on the plugs to .040. Turns out this wasn't the right thing to do for this particular engine. It really likes .035 (running NGK "stock" plugs). Second, my damper oil was low in both carbs. I removed my domes, cleaned everything, reassembled and used actual SU damper oil that I purchased from Black Dragon. It seemed a tad thicker than the 10W30 I had been using. Finally while I had the carbs synced fine at idle it turned out they weren't synced properly at 2500RPM, and required some adjustment of the balance screw (page EF 20 of the '72 factory service manual - if you don't have one you can find one here http://www.xenonzcar.com/s30/fsm.html HIGHLY RECOMMENDED). After doing these things my car came alive like it has never been in the 3 years I have had it. Idles smooth, accelerates strong, and the rich smell out of my tail pipe is gone. In addition it purrs on the freeway where as before it seemed to surge a bit. Finally my annoying 4500 RPM "wall" that the car seemed to hit has vanished, and it will rev all the way up to 6K no problem.(I don't have the guts to go any higher than that!) Hopefully this helps you out.

[seattlejester]

Welcome to the forum.

 

Do you have a synchrometer and a way to determine AFR? Unless you have both, you are just going to be guessing without a trained ear, and it really only takes a little adjustment to make the carbs go from ok to bad.

 

Symptoms of backfiring out the carb point towards timing. Do you have a timing light?

 

It is pretty hard to help without some numbers. What timing is your base timing at, does it scale correctly with the revs, what is your AFR, what is your fuel pressure, what is the flow reading.

 

Carb oil is another consideration, straight 20w seems to be the popular choice, this is available as 3 in 1 from most hardware stores.

 

Need hard numbers to really find the problem, or else we are also just guessing.