-  Miscellaneous Carburettor problems
-  tuning advice 
-  including some info on triple Webers and 
-  on advancing distributor curves
-  O2 sensors
-  SU mixture adjustment

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On Fri, 28 Oct 1994, Jeremy Butts wrote:
>
> Also, once warmed up, my Z idles a bit high, the tach reads 1500 rpm,
> although it does not feel THAT high, i still need to hit the gas when I
> release the clutch.  I have the idle set screw adjusted all the way down
> and the high idle is not stuck on.  I also tried adjusting the carbs, but I
> could turn the main adjuster almost all the way in before the engine died
> of fuel starvation. There was no difference in idle anywhere elso.  I have
> no vacuum leaks, and throttle response is good.
>
> thanks
> Jeremy
> 
Jeremy, check to see if the throttle safts in the carb(s) are sticking 
and this is holding the idle up. If by turning the throttle shafts by 
hand, you can lower the idle, then this is probably the case. What causes 
this is somewhat difficult to explain without an visual aid, but here goes;

The throttle shafts float longitudinally (side to side) in the throttle 
body. The throttle plate is the only thing that locates the shaft 
longitudinally and it only does so when the throttles are closed. As the 
throttles are opened, the plate no longer holds the shaft centered in the 
body, and the shaft will shift to one side or the other. As the throttles 
are closed the throttle plate will re-center the shaft. It does this by 
rubbing on the bore of the body. This will, over time, wear flat spots on 
the sides of the plate, next to the shaft. It will also wear the bore of 
the carb body. As this wear becomes worse the throttle plates will start to 
stick when their flat spots come up against the flats worn in the body. 
Rebushing the carbs and putting in new throttle paltes will not always 
fix this. If the wear in the body is bad enough the sideways motion of 
the throttle shaft (when the throttles are open) will force the plates in 
contact with the wear spots on the body and the new plates will stick as 
well. What needs to be done is the shafts need to be located 
longitudinally in the body by some means other than the throttle plates. 
When I rebuild the carbs I drill  and tap the "dead" end (the end oposite 
the threaded end) of the shafts for a small screw. I the use small 
plastic shimms (washers) between the head of the screw and the boss on 
the body to set the "end float" of the shaft. I also put a similar shim 
on the other end of the shaft, between the body and the interlink/nut 
assembly. This positively locates the throttle shaft in the body 
longitudinally, regardless of throttle position. I usually leave about .010"
end float so as to avoid binding problems.

A less dificult problem that can cause your symptoms is poor lubrication 
between the balls and sockets in the linkage, or wear at the "J" hook and 
clevis in the linkage. I also once had a car where the screw which holds 
the lever to the short shaft (the other end of the short shaft from where 
the clevis is attatched) was loose and the linkage was slipping. The last 
item I've ever seen cause this is a sticking advance weight in the 
distributor (or a loose or sticking vac advance).

Hope something in here helps
Marc Sayer
Performance Engineering


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On Mon, 31 Oct 1994 lthompson@turtle.frco.com wrote:

> Yaosa!  I finally have a way to get rid of that annoying kick the throttle
> before braking problem.  Hey Marc, another astute z-car'er (Pete at
> paraska@oasys.dt.navy.mil) said his carb problem were related to 
> the fuel jet nozzel sticking when going from choke-on to choke-off position.  
> Have you run into this before?  I don't think this is my problem because
> kicking the throttle gets the idle back down, only to have it stick again
> at the next light (without touching the choke).  Regardless, after I get my 
> front end back together, out go the carbs to look at jet nozzels and throttle 
> plates.  Thanks!  This list saves me alot of time diagnosing problems like 
> this one...
 Yes I have and it can come from several sources;
1) the needle is bent and holds the jet down as it tries to rise
2) The jet is dirty and does not move freely in the jet holder
3) The hose is stiff and is not letting the jet rise freely

The first is the least likely in Hitachi SU's (more likely in British 
SU's for some reason). The second and third are the more common 
posibilities and each one accounts for about the same per cent of cases. 
If you recently replaced the hose, or if the hose is very old you might 
try disconnecting the hose and see if the jet moeves freely. If it does, 
its the hose, if not pull the jet out and clean the outside of the jet 
and ID of the jet holder. I have seen a few cases where the choke 
activation linkage (on the carb body) was binding but only on really 
dirty carbs or one that someone had just put back together (incorrectly), 
so unless one of these cases applies I would look elsewhere first. If you 
have to pull the jet examine its OD, the jet holder ID and the needle, 
for nicks, gouges or any surface imperfections that might be hanging 
things up. 

Oh yeah on some systems the fast idle cam will have a flat spot in it 
that holds the idle up (again mostly British SU's).

I agree though, it doesn't sound like that's your problem.

Marc Sayer


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On Fri, 4 Nov 1994 JWW7118@ACS.TAMU.EDU wrote:
> I'm running a L28 motor with triple Weber sidedrafts, Nissan Comp header, 
> and mild cam ('rally' grind from Motorsport).  I also am running a Mallory 
> Unilite that does NOT have the vacuum unit on it and a matching Mallory 
> coil.  My question is this;  What should the advance curve look like??  
> It's been a while since I checked it, but I think it is at about 6 deg at 
> idle (approx 1000 RPM) and goes to full advance by somewhere around 
> 2500-2700 RPM with a total advance of about 38 to 40 degrees.

Jeff for tripple Webers to work best you want lots of initial advance but 
not a lot of total advance. A good all purpose curve for your set yp 
would be 20 degrees at idle (with 0 degrees of mechanical which means a 
static setting of 20 and a reduced mechanical curve so that the 
mechanical advance doesn't start to operat until 1500 RPM or so), and 34 
to 36 degrees total advance, all in by about 3500. This means a 
mechaincal curve of 0 degrees at 1500 and 14-16 degrees at 3500. I have 
screwed around with Webers and ignition curves for years and this is the 
best all around curve I've come up with. Have the advance curve set as I 
described and then set the timing at idle for as much advance as the 
engine will stand without detonation or until max advance is 36 degrees, 
whichever comes first.

Hope this helps
Marc Sayer


--------------------------

> 
> >>>>> "Seth" == Seth Wolpert <wolpert@eece.maine.edu> writes:
> 
>     Seth> Hello Z-car enthusiasts.
> 
>     Seth> For a while, the engine in my `72 240Z has been sluggish.
>     Seth> While it goes from 60 to 80 well, it won't come off of idle
>     Seth> quickly unless I apply a bit of choke.  Even then, the
>     Seth> pickup is not typical of a 140 or so horsepower engine in a
>     Seth> 2400 pound car.  I was once even beaten out off the line by
>     Seth> a Jeep.  I'll never live down the shame of that.
> 
Seth have you checked the ignition timing, mechanical advance and vac 
advance for correct setting and operation? To do this you will need a 
"set back" (also called a dial back) timing gun that will let you plot 
ignition timing at various eng speeds. Also check the oil in the dashpots 
of the carbs. The symptoms you describe are typical of carbs with too 
little or too thin a gade of dashpot oil. The oil acts as an accellerator 
pump on those carbs (I'm assuming you still have the SU's if not what are 
you running?). If your throttle shafts are worn or there is some other 
problem causing a vac leak you will often get this sort of problem, so you
might check that too. How old is the motor and have you done a leakdown 
test on it? Are you running the 3.36:1 rear gears? If so I would 
recommend switching to either the 3.56:1 or the 3.7:1 gears. The 3.7:1's 
are great if used with a 280Z 5 speed trans. If the motor is getting old 
I wouldn't rebuild it, I would put an L28 motor in it instead. The 
appearance is the same but the performance is much better.

Let me know if this helped

Marc Sayer 
Performance Engineering
msayer@efn.org


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On 4 Nov 1994, Walters Steve wrote:
> I can't answer this question, but it brings up a question I have on the same
> subject.  I have nearly the exact same engine modifications on my 240Z engine
> (carbs, cam, header, and 2.5 in. exhaust).  However, I am currently using the
> stock distributor with vacuum advance.  I need to replace the distributor and
> am wondering what is a good way to go.  I am considering removing the vacuum
> advance but I could use some input on what distributor is a good choice and
> whether to retain the vacuum advance.  BTW, I also have added electronic
> ignition (optical pickup) which I was planning to re-use.
> 
Hey Steve get a stock electronic distributor and have it recurved. I 
posted the curve I use on the list today. The electronic has several 
advantages
1) it has a larger diameter cap than ths points dist and so can handle 
higher voltages
2) parts are easy to find, unlike the Mallory distributors whose parts 
are often special order items and not in stock with local shops.
3) upgade parts such as brass cap and rotor set are available and not all 
that expensive (I think I get less than $10 for the brass cap and rotor set)

As a guide I get $125 for a recurved stock electronic dist. with a $30 
core charge. You can get two styles of this dist for the 6 a remote mount 
box and the ZX type with the box on the side of the dist. Both seem to 
work just fine.


Marc Sayer



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Subject: Re: O2 sensor/carb adjust


You have to be carefull using an O2 sensor to set mixture etc. with
triple carb systems. If placed in the general plenum of an exhaust (as
almost all are) it reads the agregate mixture. You could have one very
lean cylinder and one very rich cylinder and the O2 could read ok. The
only way to do this right is to put an O2 sensor fitting in each
cylinders pipe and then take readings from each cylinder. You can use
one or two O2's and then read plugs to assure uniform adjustments, but
don't depend on an O2 to assure a balanced adjustment. Also make sure
the O2 is removed durring operation or run only lead free fuel. There
are other limits on what types of products work with an O2. For
instance many of the silicone sealants will ruin an O2 if used on the
engine. You have to make sure all the products you use are listed as
O2 safe if you are going to leave the O2 in all the time. There is a
company that sells a feed back loop, O2 based, on the fly, mixture
adjustment system. I don't know how it would work with multi-carbs,
but I would bet it can be made to work. The best way to adjust is with
a gas analyser (an old fashioned 2 gas will work), a plug reading, and
a cylinder balance test. But hey an O2 will work if you are csmart
about it. A conpany called Gunsons sells a DIY (do it yourself)
tailpipe sniffer that uses an O2 sensor to give a reading of
mixture. It is self contained, and runs less than $100. It has the
draw back of giving an agregate reading too.

Marc Sayer


--------------
Subject: Re: SU mixture adjustments


On Wed, 9 Nov 1994, Scott Gemmett wrote:
> The problem I had was that although the mixture screws were turned equal
> number of turns, I can raise the front carb piston all the way up without
> any real change to the idle. But when I lift the rear carb pistion
> juuuuust slightly the engine dies almost immediately.
> I did try setting the mixture nuts differently (did not spend too much time
> on this) and it did not seem to have any effect.

OK real breifly, the fact that the adjusteres are turned equal numbers of
turns is not the important thing here. That adjustment will set the jet's
position on the needle. By setting them both the same you are setting the
jets the same distance from the jet bridge, not on the same positions on
the needles. Needle positions can vary one carbs to another for several
reasons;
1) the needle holders in the piston were not set the same
2) different positions of the pistons due to differing throttle settings
3) different positions of the pistons due to differing piston to dashpot
   clearances (same vacuum signals set the pistons at different heights
   at the same throttle settings
4) minor variations in bodies leading to altered locations of jet bridge,
   piston chamber, etc.

Also needle/jet wear can require the needles to be set diferently from
one another. Throttle body/shaft/plate wear (or any other vacuum leaks)
can give false readings with the synchronisers which can lead to the two
throttles being set differently from one another and this will alter
piston position.

Balance the air flow through the carbs and ensure that the pistons are
both at the same height (there is a cheapie tool set that lets you do
this). Adjust the mixture "screws" to achieve best idle. I don't trust
the "lift the piston" test alone. After doing that I turn one of the
mixture "screws" in slowly until the ilde drops a fixed amount (say 200
RPM) and I keep track of how much I turned the mixture screw to get this
RPM drop (say 1/2 turn). I then set that mixture "screw" back to its
original position and adjust the mixture "screw" on the other carb the
same number of turns in as I did the first one. If the idle speed drops
the same amount and my lift the piston test was good too, I'm done. If
not, then I adjust that mixture "screw" until I get the same idle speed
reading as I did with the other carb. I then note the difference between
the number of turns on the two carbs. Lets say that to get the 200 RPM
drop on both carbs I had to lean out the first carb 1/2 a turn and the
second carb 1 turn. This tells me that there is a 1/2 turn difference in
settings between the two carbs. Since the combined mixture is about right
(best idle setting) each carb must be off by half the amount, but in
opposite directions. So carb number 1 needs to be set 1/4 turn richer and
carb number 2 has to be set 1/4 turn leaner. now I go back and try the
lift the piston test and the change the idle speed with the mixture
"screws" test again.  If this doens't do it I then try the "richer/leaner
test". It goes like this, carbs are balanced, piston heights are equal,
mixture is set for best idle. Turn the mixture "screw" on carb #1 IN
until idle speed drops (say 200 RPM) and count the turns. Turn it back
out the same amount (were back where we started). Turn the mixture "screw"
OUT until the same (200 RPM) drop in idle speed is acheived. Compare the
In and OUT adjustments. If they are equal cool go on to next carb. If not
(say 1/2 turn IN & 1 turn OUT) then I split the difference (1 turn + 1/2
turn = 1.5 turns half of that is 3/4 turn) and run the mixture screw back
IN (I haven't touched it since I ran it OUT) 3/4 of a turn. Repeat for
other carb. This sets the mixture screw in the middle of the range of
adjustment plus or minus 200 RPM.

> Overall the car runs fine, and the spark plugs do not look any different
> between the front and rear bank of cylinders.

The adjustments you've made are close to right, the biggest difference
will be at idle and light throttle and this is not reflected well by plug
color. Additionally, some gas additives now in use alter plug color to a
uniform tan and mask mixture. If you can't get the carbs set right look
for worn throttle shafts or other vacuum leaks. If these are present you
can not get a proper adjustment and will have to live with the best you
can get. In that case worry about the normal running pahse more than the
idle phase. A lean idle will be an anoyance but a lean full throttle will
ruin an engine.

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