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Discussion Starter · #1 ·
figure this one out. If you gently accelerate to WOT, it winds out fine. If you clutch drop, speed shift, or otherwise accelarate harshly, it totally starts sputtering, and takes a minute to pick up, unless you back out, and then tip in slowly. I HATE L-JETRONICS!!!!ARGH!
 

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Discussion Starter · #2 ·
> figure this one out. If you gently
> accelerate to WOT, it winds out fine. If you
> clutch drop, speed shift, or otherwise
> accelarate harshly, it totally starts
> sputtering, and takes a minute to pick up,
> unless you back out, and then tip in slowly.
> I HATE L-JETRONICS!!!!ARGH!

Sounds just like a problem I had. Turned out to be my alternator of all things. Not producing enough current for the fuel pump to provide enough fuel. I posted messages on here and know one could figure it out. Someone said I had some major problem. I know for sure this was my problem. Right after I changed my alternator, I took it for a test drive, and no sputtering or cutting out on hard acceleration. I haven't had any problems since. This may not be your problem, but check it wouldn't hurt to check your alternator output. It does sound like not enough fuel being supplied. Also you could check fuel pressure under hard accelaration. I don't know how you would do this.

Later, Leslie
 

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Discussion Starter · #3 ·
> figure this one out. If you gently
> accelerate to WOT, it winds out fine. If you
> clutch drop, speed shift, or otherwise
> accelarate harshly, it totally starts
> sputtering, and takes a minute to pick up,
> unless you back out, and then tip in slowly.
> I HATE L-JETRONICS!!!!ARGH!

Yep, it's crap all right, that's why I scraped it and went back to carbs. Less parts to go wrong and cheaper when they do. You might take a look at the variable resistor that's on the throttlebody and see if it's getting worn. It tells the brain the throttle resistance that say's if it's this much resistance, it's this much throttle. I'd take a OHM Meter to it and see if it opens up or just doesn't read right through the throttle range. I don't know the resistance values but it's not that important for a general check to see if it's reading a smooth change through no throttle to full. Other than this I don't have a clue what else can cause it besides maybe a bad connection to this unit also, I know when you really get on the engine the engine shifts a bit and might jiggle a loose connection and when you go slowly the engine doesn't jerk on the motormounts and it might not cause a bad connection to show it's self. Good luck anyway.
 

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> figure this one out. If you gently
> accelerate to WOT, it winds out fine. If you
> clutch drop, speed shift, or otherwise
> accelarate harshly, it totally starts
> sputtering, and takes a minute to pick up,
> unless you back out, and then tip in slowly.
> I HATE L-JETRONICS!!!!ARGH!

Im going to have to stick up for EFI even if it is a bitch to find the problem but carbs can do some weird things also

One day doing about 60 on my DirtBike I hit my driveway and went maybe 4 feet off the ground but Im guessing 10 long when I hit the ground it just wanted to die inless I gave it less then 1/2 gas so time to 1. try run it like **** and hope it blows the jets out or 2. pull it out and clean it. My point is both have there problems but Carbs are just alot easier and cheper to fix but things happen to both...
 

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Discussion Starter · #5 ·
btw the fuel filter is good, and it doesn't lean out if you gradually push the pedal to the floor, just when the car jerks/surges forward. Ie letting the clutch out quickly , peeling out, shifting quickly, tromping on it in 1st/2nd etc...
 

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Discussion Starter · #6 ·
> btw the fuel filter is good, and it doesn't
> lean out if you gradually push the pedal to
> the floor, just when the car
> jerks/surges forward. Ie letting
> the clutch out quickly , peeling out,
> shifting quickly, tromping on it in 1st/2nd
> etc...
does it just do it when you put a heavy load on it? I sense something major. I wish I knew but try to describe a little more.
JN
 

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Discussion Starter · #7 ·
FI blows SUs away if you know what you are doing

> You might take a look at the variable
> resistor that's on the throttlebody and see
> if it's getting worn. It tells the brain the
> throttle resistance that say's if it's this
> much resistance, it's this much throttle.

The little black box on the side of the TB is NOT a variable resistor, it simply tells the ECU to kill that gas while coasting (above 2200 rpm or so) and to richen the mixture about %30 at WOT.

Your problem sounds exactly like a lean mixture (carbs will do the exact same thing if you run them lean). Possibilities:

1. Vacuum leak (check the boot between the MAF meter and the TB)
2. MAF meter out of adjustment (somebody messed with it, try loosening the spring)

Incidentally, don't worry about messing up the MAF spring settings (simply mark it before you move it). It is very easy to adjust the mixture in this way. It is also better to run it a little rich than lean (just like carbs).
 

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Discussion Starter · #8 ·
Re: FI blows SUs away if you know what you are doi

> The little black box on the side of the TB
> is NOT a variable resistor, it simply tells
> the ECU to kill that gas while coasting
> (above 2200 rpm or so) and to richen the
> mixture about %30 at WOT.

> Your problem sounds exactly like a lean
> mixture (carbs will do the exact same thing
> if you run them lean). Possibilities:

> 1. Vacuum leak (check the boot between the
> MAF meter and the TB)
> 2. MAF meter out of adjustment (somebody
> messed with it, try loosening the spring)

> Incidentally, don't worry about messing up
> the MAF spring settings (simply mark it
> before you move it). It is very easy to
> adjust the mixture in this way. It is also
> better to run it a little rich than lean
> (just like carbs).

Hey, don't forget lack of fuel pressure. Happened to mine. Would lean out at high rpms only at full throttle when the most gas is needed. So if I stepped on it to accellerate hard in any gear or if I tried to reach my top speed. If I let off, I could accelerate clear to redline. Turned out to be my alternator. Vacum leak wouldn't only cause problems on full throttle. It was worse when the lights were on. I was checking everything else before I found out this was the problem. It was still providing enough to keep the battery charged, but just barely.
 

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Discussion Starter · #9 ·
Re: FI blows SUs away if you know what you are doi

> The little black box on the side of the TB
> is NOT a variable resistor, it simply tells
> the ECU to kill that gas while coasting
> (above 2200 rpm or so) and to richen the
> mixture about %30 at WOT.

> Your problem sounds exactly like a lean
> mixture (carbs will do the exact same thing
> if you run them lean). Possibilities:

> 1. Vacuum leak (check the boot between the
> MAF meter and the TB)
> 2. MAF meter out of adjustment (somebody
> messed with it, try loosening the spring)

> Incidentally, don't worry about messing up
> the MAF spring settings (simply mark it
> before you move it). It is very easy to
> adjust the mixture in this way. It is also
> better to run it a little rich than lean
> (just like carbs).

It's been so long I forgot it was on the airflow meter housing instead of the throttlebody. Sorry about that, glad you came by to correct my mess. It's been quite a while but now that I think about it that was what was wrong with my airflow meter back in 83,It's potentiometer on the airflow meter went out and seeing the cost I put carbs on soon after that. I guess it was messing around with that throttle body switch stuck in back of my head when I put a big throat TB on. It's been that long sence I've ever seen the fuel injection and it's the last time I'll see it that's for sure. I know one thing for sure, after 200,000 mi it's a bunch of expensive trouble. I got dual webers cheaper than the airflow meter cost and I like the way it runs just as much but it's the low cost of maint. that I really care about and not getting stuck on the side of the road because of that screwy fuel injection.
 

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I have EXACTLY the same problem...

PLEASE OH PLEASE!!!!
IF YOU FIND OUT WHAT IS IT, LET ME KNOW
I WANNA FEEL THE V1 COMPRESSOR BEFORE GOING TO THE MONSTER T61

IF I FIND OUT TODAY I AM SURE I WILL LET YOU KNOW ASAP

I ruled out the flow meter and the ecu (tested other units) so dont worry it wont be too expensive.
what i am checking now is the thing a friend posted about voltage for the fuel pump at high boost. I am considering getting a bigger pump at the same time.

I even installed new coil with no success

Please comment on your findings

CarloZXT

> figure this one out. If you gently
> accelerate to WOT, it winds out fine. If you
> clutch drop, speed shift, or otherwise
> accelarate harshly, it totally starts
> sputtering, and takes a minute to pick up,
> unless you back out, and then tip in slowly.
> I HATE L-JETRONICS!!!!ARGH!
 

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Re: FI blows SUs away if you know what you are doi

> The little black box on the side of the TB
> is NOT a variable resistor, it simply tells
> the ECU to kill that gas while coasting
> (above 2200 rpm or so) and to richen the
> mixture about %30 at WOT.

> Your problem sounds exactly like a lean
> mixture (carbs will do the exact same thing
> if you run them lean). Possibilities:

> 1. Vacuum leak (check the boot between the
> MAF meter and the TB)
> 2. MAF meter out of adjustment (somebody
> messed with it, try loosening the spring)
SUS RULE< YOU CAN HAVE YOUR FI. I SMOKE FI'S
> Incidentally, don't worry about messing up
> the MAF spring settings (simply mark it
> before you move it). It is very easy to
> adjust the mixture in this way. It is also
> better to run it a little rich than lean
> (just like carbs).
 

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Discussion Starter · #13 ·
Re: I have EXACTLY the same problem...

> PLEASE OH PLEASE!!!!
> IF YOU FIND OUT WHAT IS IT, LET ME KNOW
> I WANNA FEEL THE V1 COMPRESSOR BEFORE GOING
> TO THE MONSTER T61

> IF I FIND OUT TODAY I AM SURE I WILL LET YOU
> KNOW ASAP

> I ruled out the flow meter and the ecu
> (tested other units) so dont worry it wont
> be too expensive.
> what i am checking now is the thing a friend
> posted about voltage for the fuel pump at
> high boost. I am considering getting a
> bigger pump at the same time.

> I even installed new coil with no success

> Please comment on your findings

> CarloZXT
GuyZ,
Interesting answers. First of all if you can find a mass air flow sensor on a stock L-Jettronic system I will eat it for lunch. It is an air flow meter. A potentiometer that measures flap position in order to estimate air to engine. A rebuilt air flow meter is about $150.00 but if you can seriously get dual Weber set ups for less I will take 100 sets. The black box on the side of the throttle body is a throttle position sensor which richens mixture at idle and WOT. The black box as described by your respondant is the BCDD. FI is so reliable that it is almost guaranteed that problems blamed on FI are other than FI such as vacuum leaks, electrical problems including corroded connectors on the FI connectors. A toilet is a simple mechanical device, when one needs repair most people can fix it. A carb is much like a toilet mechanically. As to eating FI cars for lunch, I have programmable FI and I top out just short of 150mph with an otherwise stock L28 motor. Hope your hungry.
 

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Discussion Starter · #14 ·
Re: SUs don't atomize fuel very well

> FI, IS GREAT IF YOU CANT SET UP A
> CARBURATOR, I EAT FI Z'S FOR LUNCH.

Am I correct in assuming that you have a 71 240z? If so, then you're relying on the power/weight advantage of the lighter 70-74.5 chassis. I like the fuel system on the 240z and have worked on one extensively myself, but it does not match the efficiency or power of even the L-Jetronic FI system. Carbs cannot atomize fuel as well as an injector. If you drop an L-24 SU engine into a 280z, it will be a dog. If you drop an L-28 FI into a 240z, it will scream, especially if you get an 81-83 engine which has a 40 hp advantage over the L-24.
 

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Re: SUs don't atomize fuel very well

> I like the fuel system on
> the 240z and have worked on one extensively
> myself, but it does not match the efficiency
> or power of even the L-Jetronic FI system.

I think the fuel system on the 240Z is very
simple, lightweight, and, in many instances,
very effective. As far as them playing second
fiddle in the efficiency/ power department,
especially to the L-junktronic, I must strongly
disagree. Todd Trumbley, the long lost SU god
of the Brazos Valley Z Club, claimed to have
gotten above 30 MPG out of his SU's and my last
L28 powered 240Z would get in the high 20 MPG
range on road trips between Houston and Austin
or San Antonio. On a '75 280Z that I had, with
it's stock, 80K mile L28, 2.5 exhaust, and
K&N cone filter, it would run consistent 16.30's
in the quarter, about .7 better than it did
bone stock. A year or so after that, the injection began doing strange things like running
and dying cold and at different intervals, thus
leaving me stranded. After paying a couple mechanics to fix the problem, which resulted in
the car staying running, but now exhibiting poor
driveability, I decided one weekend to try this
old set of SU's I pulled off of a 240Z in Pick-A-Part on my 280Z. I also replaced the 280Z fuel pump with one from a carbureted RX7. The difference in torque was amazing and it was much easier to light up the tires. I know I'm comparing this to a dying L-jet system, but under similar conditions at the dragstrip, the SU
powered '75 280Z would run more than .5 sec faster
(and with 5 or 6 MPH more trap speed) than it's
former injected self. I noticed during the swap
that the car lost quite a bit of weight that was
associated with all the injection stuff.

After my experience, I read in an old Grassroots
Motorsports magazine (or AutoX, as it was called
back then), that champion Solo2 Z pilot, Bill
Breedlove was running a 1979 280ZX in CSP with
an SU fueled L28E. He claimed the system was
lighter than the car's former intake set-up
and there was a noticable power increase. You
be the judge. Remember, in the 70's, most of your factory cars were switching to injection because of economy and emissions, not horsepower.

> Carbs cannot atomize fuel as well as an
> injector.

You know, I hear everyone say this and preach it
and preach it, but a LOT of guys with GT-40
injected Mustangs pull that stuff off and stick
a good intake and carb on and run faster times
in the quarter mile. Most of your seriously quick
drag cars are carbureted, too. I will agree that
some of the mechanical fuel injection systems,
the recent OEM systems on newer cars, and these
fully programmable, custom injection systems are
great stuff, if you can afford them. Yes, in many
cases they are superior, but are not WORLDS better
than carbs, except for special applications like
turbocharging. As far as bang for the buck, simple
horsepower, it's hard to beat the carbs.
 

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Discussion Starter · #17 ·
Re: FI blows SUs away if you know what you are doi

> I modified my Su's to make 280hp, any ?'s

Sorry, you would neet something in the area of %100 volumetric efficiency to get that much HP out of a Z motor, which means building the **** out of it or turbocharging it with at least 15lbs boost. There's a guy in my area with a 600cfm holley on his 240z who thinks he's god (can't even outrun a stock 280zx), but simply dumping fuel into a stock motor just doesn't make more HP.

In the case increased volumetic efficiency, I imagine simply matching the needle to your fuel requirements would suffice for SUs, where as with FI you need bigger injectors, MAF, TB, and lots of other stuff.

Question: I've recieved a lot of messeges praising SUs, but all I normally hear is how great Webers and Holley conversions are and how much SUs are crap. Any clarification on this issue? (I like side draft much better than down draft, for simplicity, but I don't know how they compare otherwise)
 

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Discussion Starter · #18 ·
Re: SUs don't atomize fuel very well

> Todd Trumbley, the long lost SU god of the Brazos Valley Z Club, claimed to have
> gotten above 30 MPG out of his SU's

And monkeys fly out of my but.

> the SU powered '75 280Z would run more than .5 sec faster (and with 5 or 6 MPH
> more trap speed) than it's former injected self. I noticed during the swap
> that the car lost quite a bit of weight that was associated with all the injection stuff.

Don't forget that you had 2.5 exhaust in addition to SUs. I will admit that the stock FI TB and MAF probably starve the engine, but it is relatively simple to swap them out for larger units. I have heard claims that a TB and MAF swap in addition to 2.5 headers and exhaust will get an otherwise stock Z to 6.6 (0-60).

> be the judge. Remember, in the 70's, most of your factory cars were switching to
> injection because of economy and emissions, not horsepower.

FI increases overall efficienfy. Because the fuel is more completely burned by an FI system, emissions are lower AND power is increased. Where FI runs into trouble is when injectors max out and/or restrictive MAF meters and TBs starve the engine. I believe that Nissan deliberately used a restrictive intake system and small injectors for economy. (smaller intake increase air velocity and fuel atomization). There's a BMW that tackles this problem with a (3 way) intake system that uses 3 different intake sizes (and lengths) for different RPMs, maximizing efficiency at each level.

> You know, I hear everyone say this and preach it
> and preach it, but a LOT of guys with GT-40
> injected Mustangs pull that stuff off and stick
> a good intake and carb on and run faster times
> in the quarter mile. Most of your seriously quick
> drag cars are carbureted, too

I agree that for many years, US car FI systems were junk. I think the early 80s corvettes were the most underpowered since the 50s, I saw a post from one guy who claimed his 81 280zx had more balls than his 81 Vette.
 

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Discussion Starter · #19 ·
Re: FI blows SUs away if you know what you are doi

Your letter was interesting. You assumed that the motor that my Su's are on is stock. You are right that simply dumping more fuel into an engine would not cause it to respond very well. I have a flowbench, Depac dyno system and use numerous engine simulation programs on a daily basis. This takes a bit of the guesswork out of designing a package such as the one I built for my street car. The motor that the carbs are on is a 3.0L that I built at work with extensive portwok, high squish/high swirl chambers and a cam that I designed on the computer. The modified SU's that I do flow about a third more air than their stock counterpart and work very well. the motor pulls from 3800 to7500 with 270 ft/lbs of torque and 280 hp. If I put on a set of Triple sidedrafts I could easily top 325 hp (on pump gas) but I thought it would be cool to make the engine appear stock from the outside except for the header. In rergards to your question about which is better sidedraft, downdraft Holley or what, I think the best carb setup is a set of triple Mikunis. An individual runner/cylinder intake tract is easy to change the length on via the velocity stack therefor making it easier to properly tune the resonance of the intake pulses. When the piston is on it's way back up the bore on the compression stroke the intake valve is of course still open. By geting the length of the runner right you can time the pulse of high pressure to arrive at the valve so that it may hang open for a moment more, increasing cylinder filling and increasing volumetric efficiency. I hope that I have not bored you too much with information you may already know. It was nice to hear from you i'll talk to you later. Where are you from by the way, i'm in the bay area (Oakland), and what do you do for work.

take it easy, Mike
 

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Discussion Starter · #20 ·
Re: Head Work Cost?

I am a CS undergrad at Virginia Tech, and deliver pizza to pay the bills. I've got a 75 280z that in really bad condition, an 81 280zx that was subjected to my first attempts at welding and painting and is my daily driver, and an 88 300zx that recently developed trans trouble (needs a new overdrive gear set) and needs some front end body work.

> The motor that the carbs are on is a 3.0L
> that I built at work with extensive portwok,
> high squish/high swirl chambers and a cam
> that I designed on the computer. The
> modified SU's that I do flow about a third
> more air than their stock counterpart and
> work very well.

I don't have a lot to spend on hobbies these days, but how much does it generally cost to port a Z head and shave it a bit? I've got an extra P79 that I yanked the exhaust sleaves out (for better or for worse, cracked 2 exhaust valve guides) but I don't know if it worth the cash to port it, replace the exhaust valve guides, and have it milled. I would like to put together an engine with 10:1 or so compression mainly as an learning experience. I have access to a set of SUs that I may also use if I can't come up with a better intake for my FI (I've got a 60mm TB and MAF meter, but don't know what to do about the manifold. I would love to have a new one made with larger and longer runners, but that's gotta cost $$$).
 
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