Weber jets??All who live for their triples please read this

[toecutter]

Not Webers but just in case someone is looking for 40mm DHLA Dellorto info on settings my progress sheet to date.

 

Sulio

Tripple Carb Info Sheet - updated 28 May 2008.pdf

[luvemfast]

Is putting triple 40mm Mikuni's or 45mm Weber's going to give me much more power to a stock JDM L28 N42(block) N42(head)? Which would be better?

I have been told that it would be a waste of time and money to bolt these on with the motor as is.

What do you recommend that I need to do to it?

 

Can I just shave the head and put in a cam with springs and port it?

or

Do I need to rebuild a motor with flat top pistons and aim for 10:1 compression with cam, springs and porting?

Sorry for 20 questions, just want to make sure that I'm heading down the right road.

Then I can move on to jetting

Thanks,

Simon

[zredbaron]

yes, upgrading carbs can produce more power than stock intake. [assuming of course that you tune them!] if they are too far out of tune, your power can DROP. problem is, if you have too much carb and not enough engine, your driveability will suffer. sure, your peak hp might be a tad higher, but your accelleration (ie power band) might be worse. with a stock setup, 45mm is too big. also be aware that the venturis inside the carb matter more than the external number (40 or 45mm etc). the external number determines what range of venturis will fit, and what shape they are (with the weber dcoe, anyway).

 

as for your other questions, you can do whatever you want. the question is, what makes the most sense for you personally, and what is the most bang for the buck?

 

we can't advise you unless we know your intentions for the car (racing? cool street car?) and your budget. everyone wants more power, but to what extent and how much are you willing to pay for it? NA power is raw and very appealing, but it's also very expensive...

[JMortensen]

If you follow the formulas for figuring carb size that you'll find in Weber and Mikuni manuals, they'll tell you that 44's or 45's are the right size for the L28. The old Mikuni adverts showed a 15% gain with 44's compared to stock FI on an L28 if I recall. Other than that one point I agree with Mark. The engine is a system, and you'll get the best results by building it to match your goals from top to bottom. Putting one aftermarket part (especially triples) on an otherwise stock engine won't get the best results from that part or the engine as a whole.

[luvemfast]

we can't advise you unless we know your intentions for the car (racing? cool street car?) and your budget.

My car is on a classic historic registration which means it doesn't get driven much. Mainly to and from club events.

Then there is the motorsport, sprinting, hillclimbs and motorkhana's. Which is what its all about really. I would sacrifice city drivability for track speed.

 

Putting one aftermarket part (especially triples) on an otherwise stock engine won't get the best results from that part or the engine as a whole.

I agree.

Hence my question about headwork. Will a cam, porting and shaving (to increase compression) be enough?

 

If I was to run 40mm Mikuni's with the largest choke would that be ok?

[JMortensen]

I agree.

Hence my question about headwork. Will a cam, porting and shaving (to increase compression) be enough?

 

If I was to run 40mm Mikuni's with the largest choke would that be ok?

That should be fine. I'd go 44s.

[zredbaron]

Will ___ be enough?

 

no such thing...

[luvemfast]

no such thing...

 

Sorry, I meant to ask whether this would show me decent gains, or is a new motor with flat tops etc, the only way to get results.

 

Basically I have intentions in the long run to rebuild a spare L28 as a "hot" motor to keep from car to car.Budget permiting.

In the meantime, I would like to do bits and pieces to my current set-up that I can transfer over to the future one.

 

I understand that I must sound a little simple. Trying to do this at work right now and don't want to waste too much time.

[zredbaron]

absolutely. pretty much any upgrade(s) will show 'decent gains.' its all relative anyway! obviously headwork will show a larger gain than exhaust, but as long as you realize the relationships and how upgrading one area of an engine inevitable shifts your weakest link to another area of the engine, then by all means go for it!

 

your questions seem to have gone away from the scope of this thread, which obviously is intended to discuss triple carbs only. i'd recommend you do some reading and searching, there are a LOT of threads with the exact same questions asked. if you still feel a little unsure, start a thread in the L6 forum if you feel you need to.

 

that said, as per your hot motor to swap out between a few cars, i agree with jon. your proposal of head work and carbs should do just fine for what seem to be your intentions. don't forget to open up your exhaust a little to keep up with your head/cam/intake. i personally like headers with a magnaflow universal 2.5" muffler, but hey, that's me. good luck out there, i hope you get what you're hoping for.

 

[again something that doesn't seem to exist. damn pursuit is so damn addictive though... hah.]

[josh817]

I have a question that I can't seem to get over...

 

I bought some 40mm carbs for my stroker. Did my own porting so I'm not sure if what I did was mild or hot. Took off about 3-4mm on the ports. Anyways, I always get mixed feelings with if I have the right size or not. Of course if I had the money to get 45mm I would and then if its too big I could just choke it down. However, I didn't go that route and I want to be able to change venturi size if needed. I am unaware of what is in the carb right now but if it doesn't work out and I need something bigger than 36mm, I'd rather not go out and have Rebello make me 38mm venturis. With that said I was looking in my "How to Modify Your Nissan/Datsun OHC Engine" book and it has this:

Look at "SCCA GT-4 (formerly GT-3)" It says OR no venturis.

 

 

So basically I ask myself, could I run 40mm straight through or does this just not work? The venturi is to speed up airflow as it goes into the carburetor correct? Is this only possible with race engines because maybe they have a ton of vacuum to draw air and fuel in? I wouldn't think so due to their cams and the lack of vacuum from overlap or something but I honestly have no idea. This is something I MAY try out since it won't cost a dime to do and if it doesn't work I'll just put back whatever size was already in it. I would like to know some of your opinions though before I dig in.

 

 

 

 

EDIT: Alright I read through that chapter and it gave me a formula to figure out throttle-bore size and outer/main venturi size. Of course this is for a race motor and modifications will make these numbers vary but its used as a rough guide. I am only using a Stage IV cam so thats .495"/290º with a 3.1L stroker.

 

Lets see if the picture turns out well so you can see the formula.

 

Alright so there it is. Basically for a 3.1L RACE motor with max USABLE RPM @ 7000RPM I'd need a 50mm carb with 39mm venturi. If I don't run any venturi at all in my 40mm carb I can get close to that 39mm it gave me, but without the effect in which a venturi is suppose to give... Dragonfly on this board said Rebello can make 38mm venturis for people but I have a feeling it will be costly and only fix the problem of needing a larger carb partially. I would prefer not to invest largely on the wrong carbs when I could sell them and use the money meant for making the wrong ones less... wrong... and buy the proper ones.... If that makes any sense.

[Dragonfly]

Dragonfly on this board said Rebello can make 38mm venturis for people but I have a feeling it will be costly and only fix the problem of needing a larger carb partially. I would prefer not to invest largely on the wrong carbs when I could sell them and use the money meant for making the wrong ones less... wrong... and buy the proper ones.... If that makes any sense.

 

The cost was not bad but the time waiting had a lot to be desired. The reason I had them made was because my carbs were to small for my engine and since I had traded SU's for the tripples it was worth it for me to invest in making them right for me rather than just buying the correct carbs... So to me your statement makes perfect since and as long as you are in a position to do so you should buy the correct parts rather than modify the incorrect ones.

 

Dragonfly

[JMortensen]

As you noted, you'd still be missing the velocity stack or funnel taper of the carb body that the larger carbs had. On that same p 109, How to Modify has some pics of bored triples that ran without venturis. You can't really tell from the pictures, but they were opened way up, and that taper essentially takes the place of the venturis. We had a guy post here about 4-5 years ago. He had a set of 44's and the holes on the outside were bored out to something like 56mm or 58mm. Memory of that post is a little weak, sorry, but the point is they were bored way out. My question is would there be enough intake velocity at low speeds to make the carbs work effectively, or would you only really get proper function when the engine rpms were really high. I suppose it couldn't hurt to try, but I suspect the answer is no.

[josh817]

Ah ok so they only do maybe like the front half to open up, get a smooth transition from big to small and thats whats up. Before learning about carbs I always thought that engine vacuum sucks the fuel in and I wonder if thats just enough to draw in fuel and air. Whats the worst that can happen, no start? Rough running? I'll give it a shot later on once the motor is broken in.

[zredbaron]

first, that book is for all out race cars. it recommends things like tube framing, tack welding your diff, etc etc. your table says *road racing.* sounds to me like you drive your car on the street (since you are "breaking in" your engine). am i right? if so, you DEFINITELY want to undersize peak hp venturi calculations, whether using the hot rod your datsun book or the weber book. your fuel economy and driveability depend on it.

 

i think jon mortenson is right, and i'll expand on what he's talking about.

 

running a carb without a venturi is a very uncomfortable idea for me. yes, you are somewhat correct in that the piston's downward motion 'sucks' the fuel and air in. (technically, the lower pressure in the cylinder allows atmospheric air to PUSH through the carb). as this air passes through the venturi, the venturi speeds up this air (since the diameter tapers). this causes a localized low pressure to be felt, which allows fuel from the carb's float to be pushed (or sucked if you prefer that concept) through the main fuel circuit (ie jets etc) and mix with the air flowing through the venturi. this mixture is immediately passed into your manifold runners.

 

also note that the 'no venturi' line is under the 4 cylinder section and not the 6 cylinder section in your image. very odd.

 

if you remove the venturi, you are using the carb in an unintended (and un-engineered) manner. the phrase "you aren't going to out-engineer the engineer" comes to mind. with no venturi, you are using the engine vacuum to draw in fuel, vice the incoming air doing the work via the venturi. not only that, in my weber dcoes, the walls are somewhat sharp and have jagged little niches, which are completely smoothed out by the venturi (again, by design!). turbulent air won't flow as well, which inhibits how much fuel/air is in the engine, which inhibits power production. (you want your head smooth, don't you? same concept!)

 

i don't see how it could be remotely efficient in any way.

 

that said, as you mentioned you can try it without consequence. the worst that could happen is it won't start or would run like crap. i predict it would start, though it may be hard to find the sweet spot with the pedal. it's *possible* it may even make more power at wide open throttle (if your current venturis are way too small), but i also predict that mid-throttle positions would be poor, and transitions from the idle circuit to the main circuit would be very very poor. it would likely be very very rough.

 

also of significance is how you intend to race, if that is indeed what you're doing. if your racing is high rpm wide open throttle (drag racing, most road racing), then yes, follow the book and go for max hp. however, if you plan on using different pedal positions more than WOT, you should use a slightly smaller venturi in favor of power band / driveability. for instance, my 40mm dcoes are limited to 36mm venturis which is significantly inhibited according to the weber book formula, but my application is autocross. i use 2000 to 7500 rpms and i need power there at all times. sure, my peak hp is a little lower than otherwise, but my low end torque and throttle response are extremely stout. i rarely get to use full pedal.

 

i've found smaller venturis easier to tune. they're just more responsive to changes, whether from jets or from your foot.

 

and yes, i do think 36mm are too small for me, even for my application. i'd experiment with 38 and 40, but hey, 36 is what i've got for now. perhaps i'll give rebello a call for these 38s, but i don't see how its possible in the 40dcoe. the 36s are extremely thin.

[josh817]

Ask Dragonfly (I really need to learn peoples REAL names). He posted a picture of his on my stroker thread. My motor is meant for a hot street motor. Possibly a track day once and a while. I'm just trying to think of a poor mans way to run larger. Either way I'm sticking with these carbs for a very long time because I'm not dumping anymore money into the car just now. I'm just afraid I'm going to find like 34's in the carbs or something. >_

[Dragonfly]

Here are the pictures that josh817 is talking about:

 

 

 

 

As you can see the 38mm are quite thin but what is virtually impossible to see in the pictures is the taper. The taper is moved from the stock location by a small amount and the taper angles are different in order to get as much effect as possible out of such a thin venturri. They work quite well but are very delicate when outside the carbs.

 

Dragonfly

[Daeron]

Here are the pictures that josh817 is talking about:

 

 

 

Fence rail and dremel FTW

 

Seriously, though, it is good to see that your question about the venturis is finally answered; I've been curious myself since the first time I saw you bring it up.

[PalmettoZ]

Awesome... carbs are fairly new to me, but i'm learning quickly. Any tips in jetting is greatly apreciated, i've only found a couple really useful sights for tunning webers, but the jets are so expensive i'm hesitant to try new ones.

-Ed

(don't envy, the gas price kills me)

 

Hey Ed-

I had Webers (4x2 IDF's) on my 302 Ford on a Cobra replica about 10 years ago. Same carb as DCOE's but in a vertical position. I too was trying to get the jetting dialed in and came up with an idea that worked great. Since I had extra jets I was able to experiment with them. I bought a set of very fine drill bits that were a half mm difference in size in the range that I needed. Then I took the main jets and heated them with a soldering gun and filled the holes with solder.

Then I drilled the holes with a dremel at slow speed starting with a half mm smaller and trying them out. This took about 4 tries at gradually different diameters until I got the correct size that I needed. Then I ordered this size and it worked perfectly.

I also went down a size in the idle jets doing this same process. If you have some extra jets you might want to give it a try. It will get you very close to the size you need.

[Lazeum]

Hello guys

 

My car was ticking for a while now (but I don't drive it often and strong because of that) when I was accelerating from 2k rpm.

After some attempt to fix that I've tried to check any reason why it is happening (exhaust leak, air horns vibrating, etc.) I ended up this weekend playing with the idle jets. Until I noticed the carbs were not tight at all on the intake manifold. I was able to turn every bolt by 5 turns before to tighten them.

 

In the process I've changed the idle jet since I did believe my problem was coming from the transition circuit (running lean). I went from 50F9 to 55F9.

 

btw the setup of the car is as followed:

240z with L28 F54 block, N42 head, flat pistons, mild Scheider cam from MSA, 3 40DCOE, 3-2-1 turbo headers.

 

Regarding the DCOE's, I have

Mains 130

Air correction 180

Emulsion tube F11

Idle jet 50F9 before, now 55F9

Venturi 36mm

 

I haven't check yet accelerator pump jet and secondary venturi.

 

Besides the transition, the car runs strong after 3000-3500 rpm as long as you are gentle with the trottle before.

The car is shaking like crazy after 6000 rpm, I don't know why yet (so I don't go that far).

 

 

The problem as of today is that the idle is very chaotic now. it runs at 300-400 rpm; the car stinks, shakes. I need to have the choke fully open to let the car idle at 1000 rpm.

 

Could this be because of the change of the idle jets????

I assume no, I believe it is because of the air leaks that might exist with the carbs not tighten. Since the car was running approximately ok before, I assume the previous owner tuned the car accordingly.

I think the synchro between carbs are all messed up as well since they were synch with erratic leaks.

 

Please advice

 

I am currently reading a lot of stuffs about Webers, I will try next weekend to resync the carbs and to adjust the idle from scratch.

 

I'm also afraid I will have to review the rest of the setup afterwards, I might be running very rich since there is no more extra supply of air with air leaks removed.

[Zmanco]

55Fxx is awfully large for an L28. When I had triples I ran with 45F6 (albeit I'm at altitude). IIRC, the F number determines how much air is bled in and that's probably where you should be making changes. You might want to go back to a 50F6 if you were too lean with the 50F9.