SUs and a 280 degree cam - what needles?

[lombrozo]

Hi folks! I am bringing my dad's original 73 240Z back to life after 23 years in the garage. It has a Racer Brown 280 degree cam, and I installed SUs and manifold from a 72, and Pertronix electronic ignition.

What should I be using for needles in the SUs? I have it idling well, and balanced, but seems to be lean on the top end. I am gradually richening up the nozzle settings, but wonder if I am going to need different needles with this cam.

Thanks!

[beandip]

I am running SUs on my ZX engine with a cam and headers etc. I have found so far the SMs and 47s are way too rich . I am running 54s at present and so far all seems good , I am going to set the needles deeper in the vacuum piston in an effort to rich-en up a little for the higher revs. With SMs I was getting fuel in the oil because of the added fuel was washing away the oil on the cylinder walls. The 47s are a Nissan needle for a '70 240 and is also vary rich as well. Gary

[jmortensen]

I ran the 47's when I was running SU's with a .490/280 cam with my 8.3:1 compression L28 dished/E31 combo. I also ran Marvel Mystery oil in the carbs with 280ZX EI and no vacuum advance. I had 2.5" mandrel exhaust with a Super Turbo muffler. I think this is the official beandip recipe for failure, but it was really quick and I loved the way it ran. A little more compression probably would have been a good thing, but once it got up into the rpms it hauled arse. Got good gas mileage too with a ZX 5 speed and 3:70 rear gears.

[beandip]

With the vary rich needles I can understand the use of Mystery oil or ATF in the dampeners. I have never said that the car didn't or wouldn't perform. I am running 9.5 compression and a P-79 head on a flat top engine. It ran vary strong with the richer needles but I did find fuel in the oil and it was too rich . Where I live , normal driving on the streets , I don't have the RPMs in the upper part of the power band and mostly around 2K to 3500. Since I changed back to 54s my oil is staying clean and free from the fuel smell , stink is more like it . I can only relate to what I have personally found on my Z . But it has been confirmed by a person that I vary much respect that is in the business of building these engines and restoring Z for a living . He has been doing so for over 25 years that I know of . No flame intended . Gary

[nealkeith]

The old school method of determining proper jetting, once you have gotten close, was to run the car and kill it at certian RPMs then pull the plugs and read them.

[jmortensen]

Reading plugs is good if you're good at reading plugs. You have to look around before you can find someone who actually knows how to do it. It is not a common skill.

A far more accurate method is to install an O2 sensor bung in the exhaust and hook it to a gauge or better yet a voltmeter. I tuned my Mikunis in with a narrowband O2. If you have money to spend a wideband O2 is more accurate than the narrowband I used. I never did this when I was running the SU's. Kinda wish that I had now...

I was teasing you beandip. Seems no matter what the issue, ignition, carbs, cams, whatever, I've had the exact opposite experience. I don't know what to attribute that to, but I think it's kind of funny... :-)

[Go240Zags]

I've recently had an L28 F54 built up that was bored .20 over with flat top pistons and rebuilt P90 (looking at 9 - 9.5 compression) plus 280ZX E12-80 dizzy and ceramic coated 3-2 headers (unsure of brand). I have Z Therepy rebuild kits with SM needles and the "Just SU Carbs" video for my 4 bolt SUs. Since I have the stock cam that was in the slightly milled P90 will my SM needles be too rich. I was told i could get the cam reground for a little more lift? for about $100. Should I do this? Not trying for a drag racer - just a quick car down the highway and in the twisties.

[beandip]

Check your PMs , Zags

[jmortensen]

Regrinding the cam is a good idea, there are lots of companies that will do it for you, including Delta Cams which is here in WA state. I think they charge $65. I'd go large on the cam, as stated before my .490/280 worked well with the SU's. I think Delta has something right in that range. Delta will also resurface your rockers for $3 each if you want. For lifts over .460 you'll need different valve springs, I'm using Schneider springs, and you'll be REALLY close to having the retainers hit the valve stem seals. You can use a Ford 2.8L V6 valve stem seal to allow for a lot more clearance, they're made from a better material (Viton), and they're readily available for $20 at any parts store. Despite the need for different springs and seals, the extra extra power from the increased lift is well worth it IMO.

[Bruce Palmer]

... The first thing one should check is to make sure the fuel nozzles are spot on. The opening in the nozzle should be round and measure .100". A lot of used carbs have holes worn egg shaped which means all meaningful metering bets are off, no matter the needle.

Rich or lean can also be a function of float level. Ultimately when all is said and done with tweaking of your SU carbs you should be able to remove the dome and piston and look directly down into the nozzle and see the static gas level within a sixteenth of the top of the tube. If the gas level is way down the nozzle things will probably be lean and if gas is puddled up on to of the nozzle, things be fat. All the float level does is to set the fuel level in relation to the top of the nozzle.

In the How to Modify Datsun Engines and Chassis from '73 by Bill Fisher and Bob Waar, the first 240Z tuneup upgrade included distributor spacers to control the advance curve and solid core plug wires, as well as richer SM needles so this profile has been used succesfully for a long time.