'72 240 mixture question (still poppin)

[Guest bastaad525]

Okay guys... been goin thru this for months now....

 

Every so often, usually when it's hot out, and usually if I've been idling for a long time at a light or whatever, if I suddenly jab the throttle, I get a loud pop thru the carbs. Many people on here have told me that this is caused by only one thing, and that is the carbs are set too lean. So... every time I would get one of these pops, I'd richen the carbs up by a 1/4 turn, in hopes of getting rid of this problem. I had ended up richening the carbs by one full turn more rich than what I had set them at for peak power the last time I visited the dyno. It really did seem to have an impact on the car, it felt sluggish, and was starting to foul plugs. And still, I'd get the occasional pop. So, to finally solve any argument at all about the mixture of the carbs, I went down to a smog place two days ago and had them do the sniffer test on it under load, and as I suspected, the car was running WAY rich. So I tuned the carbs back to as close to perfect as I could get them, under load, and now there is no question that I am DEFINATELY not running too lean. Well today I was sitting at a light and just nudged the throttle and there's that pop thru the carbs again... GAAAAAAAAh this is gonna drive me nuts. Can someone here please tell me any other possible or probable causes of popping thru the carbs? It happens rarely, maybe once every few days, it's not like a regular every day thing. Here is my setup, in case anyone wants any of this info:

 

L28 block bored out to 2.9 with flat top pistons

 

N42 head, unshaved, only minor port and polish, stock size stainless, polished valves

 

Mild cam, 480 lift, 270 duration, set on the second timing mark as per Pierre of Pierre Z's recommendation (to help with some of the low end torque loss) and yes I've checked and double checked that it is lined up with the right link in the timing chain, and have triple checked my valve clearance adjustments

 

Newly rebuilt (as of about a year ago) Ztherapy SU carbs, with the SM fuel needles, checked and double checked for any kind of vacuum leaks and have found none. I've also recently gone thru the carbs completely to make sure they were still adjusted and performing properly (checking the floats and float valves, for instance)

 

Electronic distributor from and '80 ZX, running thru an MSD 6A ignition and Blaster 2 coil, initial timing set at 9 degrees, fully advanced at 30 degrees.

 

That's about all i can think of that might have anything to do with this. So please, someone please give me some kinda idea as to why I'm still getting this 'lean pop' thru my carbs. It's gonna drive me nuts real soon

[Clifton]

What weight oil are you running in your carbs? Just a possibilty, but it could be too light allowing the piston to rise to soon, before there is adequate air flow and this is giving it a lean condition for a brief second at low rpm and quick full throttle.

[Guest bastaad525]

I had considered this. Originally I was running ATF in the carbs as most people do, and I figured maybe this was the problem so I switched to the next thicker oil I could find which was straight 30 weight. The first thing I noticed right off the bat was that the car was VERY sluggish up to 4k rpms with this oil in the car. So I took it back out and put the ATF back in and the car ran good again..... then just out of curiousity I tried Norm's method again of taking out the dampers...... effectively removing the oil from doing anything, and the throttle response improved even more. So right now I'm running them like that, and yeah I figure they aren't helping my popping problem at all.... someone suggests it may have more to do with the cam then anything else, as the higher lift cam is effecting my vacuum at idle (not getting enough suction) and... well he went into great detail that I couldn't possibly remember everything he said but it seemed to make sense. Which has me thinking that maybe there will be no good fix... though he also suggested the key to fixing it could lie in my distributor setup, primarily the vacuum advance, which as luck would have it mine is disconnected as the vacuum canister had long since given up the ghost and I didn't consider it important enough to mess with......

[Lockjaw]

Maybe you are experiencing the first signs of vapor lock. I would put my money there. What kind of fuel pump are you running? You could also have the float levels a little low.

[Guest bastaad525]

Just the mechanical pump..... how would I know if it's vapor lock?? I did wrap all the fuel line in aluminum foil (makes it look real show car, you know?*snicker*) to help with this problem. The floats are adjusted exactly to where Ztherapy recommends, checked and double checked that and also that the needle valves weren't sticking open or closed.... everything checks out.

[DAW]

Three things come to mind: 1) Your damper spring may be too stiff for the low-end response you need. You can get SU damper springs from a company called APT. A lighter spring is like shaving weight from the piston; it's easier for the initial tip-in airflow to lift it, which richens and improves throttle response. 2) The SM needle may be too lean at low speeds. I've tuned SU's including custom modifying the needles and SM needles aren't universally the place to start. In some applications the SM's are too lean at low end and too rich at top end. This can be dealt with by turning them down carefully using a drill press and extra fine sandpaper (technique described in "How to Power Tune SU Carbs") in specific increments which correspond to the physical location of the needle in the jet where the over-lean condition occurs. The SM isn't always the place to start due to the transition profile. One thing you could try as a primitive test is to raise both needles into the pistons slightly. The shoulders of the needles are set flush with the base of the piston. You could try retracting them slightly (like 1mm). This will richen across the board. This is just a test, but if 1 or 2mm improves the low-enf response, you know you need to tune the needles. Related to this is the position of the jet. Idle stoichiometry may not ensure that throttle response will be good 1,000 rpm above that. I've found that it's better to put up with an idle quality that is a bit rough in order to have a symmetrical, optimum mixture at higher speeds. Count the turns on each jet setting and record, then run them fully in and set both at 1 1/2 turns and work richward by equal increments of 1/4 turn. Once you stop noticing improvement, stop or you will over-richen at high speed. 3) Vapor lock is a possibility. Are you running the fuel return line? The vapor cannister could help you. Tee the vent lines from the carb bowls and run the line to the carb port of the canister. Fuel tank goes to the marked port on the cannister and dist. vac port goes to the port on the carb that has it for the source of ported vacuum.

 

I'm not sure that these suggestions will do it, but I've tuned out similar problems using them. Good luck! DAW

[Guest bastaad525]

Thx DAW that's a lot of stuff to consider. Well the springs.... I dunno.... I have that SU book and had given thought to the springs but it just becomes an issue of one more thing to look at and change. Anyways these carbs came from Ztherapy..... and I know they've sold carbs to guys with setups like mine, so I'm thinking they probably already put the best spring in there........ then again they also recommended the SM needles to me! I will say that I am almost 100% convinced that the SM's are not the best needles for my car. I can't wait til I can get some more dyno time and see exactly what these needles are doing throughout the rev range and take that info to a place like Rebello and get some good needles made. Another thing about the springs, you're saying that I need a lighter spring to help them raise faster off idle... see I thought the whole thing was that you wanted them to raise slowly, initially, as THIS would actually enrichen the mixture at first, acting as an accelerator pump, and that this was the role of the oil in the pistons. I know I thought that this made no sense, it seemed to me this would lean out the carbs, but apparenetly the extra vacuum of the engine is trying to suck more air and run lean, but since the pistons dont pop up fast enough to allow the air in, the engine sucks the only other thing it can get at, which is extra fuel. Wouldn't going to a lighter spring then in effect lean me out for the first second of acceleration, and then just cause me to run rich everywhere after that? I do find that I get much better throttle response and what feels like more torque if I run the carbs with no oil in them at all. And the throttle response feels great at low end and off idle. Anywho, about the SM's running too lean at idle though I find it to be the opposite..... they appear to run richer at idle. Case in point, when I tuned at the smog place, I tuned to get close to stoich mixture at different rpms and speeds, under load, and after getting it about right, it was always a bit rich back at idle. This explained why my spark plugs were always covered in soot when I putt around town, but if I run it up on the freeway and do a 'clean cut' they look just right. Well I do think that these SM's may be part of the problem either way or at least are not allowing my car to hit it's full potential or to be properly in tune....... DAW what needles do you think are a good place to start for a 2.9 liter non stroked engine with mild cam and high compression?

[Lockjaw]

What thermostat are you running? If you are running a 160, you could be to cool and need to step up to a 180. I run 180's anyway.

 

I have the SM needles in my SU's on my 2.8 with a nice cam and some compression, and I don't have a problem with them. I ran some stock needles, but it did not want to turn the rpms the SM needles do. While they may not be ideal, they are richer needles then stock, which it seems to me you need for an engine that is .5 liters larger then the carb was designed for.

 

I get no popping with the jet nozzle(whatever the term is for the thing) adjusted per the Z therapy specs, which is to get the car warm and adjust for the highest idle.

 

Do not rule out that you could have a vacuum leak causing a problem, and some carb cleaner sprayed around likely places should change your idle if you find a leak.

[72zcar]

I have the SM needles in my SU's on my 2.8 with a nice cam and some compression,

 

I have heard of using different needles in the su carbs. What are the SM needles and where can I get them? Thanks

[Lockjaw]

http://www.ztherapy.com/

[Guest bastaad525]

nope I've sprayed around for vacuum leaks like 10 times. About tuning just for the fastest idle, some time on a dyno showed me that this is not tuning for peak power. I ended up richening the mixture nuts by 3/4 turn from the fastest smoothest idle setting, to reach my peak hp and torque #'s (an overall gain of 4hp and 8 ft lbs at the wheels over the fastest idle setting). I am already running the 180 degree thermostat. About the SM's... they are a compromise more than anything, yes the pull harder than the stock 240 needles, and yes they pull LONGER, but they were never intended for a Z engine. So if the stock 240 needles are off than these things must be WAY off..... personally I am now a firm believer that a custom engine requires custom needles, and I'd like to get a set of 240 needles in my carbs, dyno it with a five gas test under load, then take that info to a place like Rebello and have them custom work my needles based on that info.

[DAW]

Re the damper springs, they are color-coded by pressure. Their pressure pushes the piston (and needle) down. If you lighten the spring, you can improve the throttle response FOR A GIVEN AIRFLOW. That's why you don't create over-lean by letting the piston raise more swiftly, the more significant factor is the piston's weight and it takes larger increases in carb/engine airflow to lift the piston so much that it creates a lean bog. Altering the rate of rise or initiation of rise raises the needle faster and covers the throttle opening with fuel supply vs inertia of rest piston lag. I believe this is why you saw improvement when you removed the oil-damper plungers...because it let the pistons initiate rise more quickly. If you've got yellow-code, e.g., springs, don't go out and buy some more but if you aren't sure what springs you have, then try the lightest springs and go from there. You'd be better off using light springs and oil-plungers than removing the plungers. I use ATF but I've recently tried synthetic multiple weight (low temp range) oil. I'm not sure if it's better than ATF but it's no worse.

 

The needles you start with when custom tuning should be slightly thicker than what you will end up with, i.e., you are going to remove material to richen...you can't add it back (so remove sparingly). Most recently I used needles from smaller SUs from an L4 SSS dual SU setup. I liked the profile on these vs stock 240Z needles which tend to be lean at cruise rpm. Remember, a smaller carb may have a smaller jet and when you transfer the needle to a larger jet it will be richer but you retain the profile. The tuning of the needles is an empirical endeavor and can't be done without the car. The companies you refer to are good and reliable but no one can tune your car without your car. You can't assume that out of the box the carbs are going to be perfect for your car. Did the carb source ask for your altitude and outside temp range you operate at? DAW

[Guest bastaad525]

No they did not, twas Ztherapy I got the carbs from, and from what I gathered they send out every set with the same springs and with the SM needles. Getting ahold of all different manner of springs for these cabs is gonna be a pain in the ass...........

[DAW]

Think British sports cars. You could pick up a British sports car racing magazine at a bookstore and look for ads selling SU carbs & parts for sources. The Hitachi SUs you have are called HS series in England. APT is one source (http://www.aptfast.com) and for HS carb dashpot springs they list: 12-oz. Green ($6.79); 8-oz Yellow ($6.79): 4.5-oz. Red ($10.78); and 2.5-oz. Blue ($11.98). If your springs have a color-code then you'll have a reference point. If not, then you'll have to trial and error. However, I can almost guarantee they didn't use 12-oz. springs, more likely 8 or 4.5-oz. Given your compression ratio and the type of problem you're having, you could just put 2.5-oz. springs in and go from there. This may also allow you to bump your initial timing up from 9 to 12 degrees which will further improve throttle response (without the ping).

The next step if the lightest springs aren't enough would be to lighten the piston. Some British racers try removing the spring altogether but you get problems like when you removed the oil-dampner plunger...too eratic. DAW

[Guest bastaad525]

About the eratic performance or fluctuation, after removing the dashpot oil, is this something I should notice? I've been paying close attention since removing it, but the car seems to run fine, and again, is more responsive. I know Norm, whom I got the idea from, runs his a little different, leaving the oil in and then also adds a heavy screm into each piston to keep them from fluctuating, so I'm assuming this fluctuation is something I should be able to notice? What exactly will it be noticeable as?

 

Also, on advice from the Zcar.com board, I finally got my vacuum advance canister replaced and re connected it, so I have vacuum advance running again, and so far it seems to have solved the problem (oddly enough, I wouldn't have thought timing would have anything to do with this). It's been well over a week of hot weather and I haven't heard one pop, and this is after leaning the carbs back out to where I had set them when I dyno tuned. However, I am still going to pursue trying some different springs, now that you've pointed out a place for me. I'll call Ztherapy and see if they can tell me what springs they used, and go one step lighter, and today I'll put the oil back in the dashpots. *sigh* I wish that the dyno place I had gone to was still open .... they were very low priced, and they let me do my own tuning between runs. Then I could see exactly the effect all this stuff will have. I dont have $150/hr to let some OTHER guy tinker with my Z! DAW, since you're pretty up on the technical aspect of all this, I was wondering, do you know any reason why the vacuum advance being disconnected would cause the 'lean pop'? I'd appreciate if you do if you can put it in laymans terms for me so I can understand what the heck is going on

[DAW]

Throttle response is directly related to ignition timing. You want to run as much initial advance, generally speaking, as you can (without causing ping somewhere else in the rpm range) to get the best throttle response. The "popping" you describe is due to lean-out on throttle tip-in, which amounts to poor throttle response.

 

In general, when you tune the SUs, you'll synchronize airflow, record where the jet ht is (# turns on idle mixture knurled ring), run each one in and then back 1 1/2 turns to start out with. When you adjust these keep track of the turns by referencing to the large and small steel balls imbedded into the rings. Snap the throttle open off idle (you can do this with the car parked). If there is hesitation, advanced the timing a couple of degrees at a time until hesitation is minimized. You now work with the needles by checking at progressively higher rpms identifying lean spots you find by hesitation on throttle snap. Eventually you will be removing the plungers and marking a straw or lightweight balsa or plastic stick at about 1/8" intervals and placing them into the dashpot where they bottom out in the piston. Revving the engine will cause the pistons (and the little marked sticks) to raise a specific amount corresponding to the specific rpm. The mark on the stick is calibrated to a specific location on the needle which can be mic'd. Hesitation at a particular rpm/piston ht/needle at the jet location, means the mic'd thickness needs to be reduced to allow more fuel flow at that point. You may be able to record your needle thicknesses at the standardized intervals, noting which intervals are too lean (thick) and look in a British racing catalog to find a needle which suits your carbs/engine based on this tuning right off the shelf. However, turning the needles down using a drill press is not that bad and it's highly gratifying when you finish the tuning.

 

Look at that book again on SUs. I think he may have some needle profiles and part # in the appendix.

 

Once you get the higher speed lean pops and sags tuned out you will have done it through richenning measures which will reduce ping, so you might be able to advance the timing a bit more (keep the total within reason). When you go to fine tune the idle mixture, try to keep both jets even and don't get lost or move them too much because you can get way out of whack trying to chase a few rpms for best idle. Remember you are moving the entire jet which has an effect throughout the rpm range.

 

As to the plungers, I don't think that their removal will effect driveability as much as removal of the damper springs (that's what I was referring to). I think lighter springs and lightweight damper oil should be done before needle changes. It would be great if you put in some "blue" springs and run 3-in-1 or sewing machine oil or whatever in the damper plunger wells and your problems disappear! DAW

[DAW]

P.S. When I referred to removing the damper plungers I actually mean removing the oil and replacing the plungers thereby eliminating their function. You noticed improvement when removing the oil...but remember, you reduced some resistance to piston rise by doing this and noticed improved response...the more significant factor is that you lightened the piston by removing the oil. Switching to a lighter dashpot spring should have the same effect. DAW