jimmyjones000 Posted May 26, 2011 Share Posted May 26, 2011 Just wondering what the typical MPG should be on an l28 w/ triple 40 mikunis? Right now I'm getting around 8-9 on 91 around town, and I'm a semi hard driver. Haha More info: 1970 240z 75 l28 n42 ( stock) no smog or air unit 75 n42 head w/ upgraded cam (12 @ timing) 40mm mikunis w/ air horns and mesh covers Electronic ignition 8mm spark cables 6 to 1 ceramic header w/ 2.25 exhaust Mechanical fuel pump at 4 psi fuel pressure Spark plugs are light brown and not wet Idle at 1000-1100 Ill post a couple pics soon Quote Link to comment Share on other sites More sharing options...
Robftw Posted May 26, 2011 Share Posted May 26, 2011 With a lead foot what do you think? The cars 30 years old. Quote Link to comment Share on other sites More sharing options...
TimZ Posted May 26, 2011 Share Posted May 26, 2011 With a lead foot what do you think? The cars 30 years old. I would think that you should be able to do better than that, lead foot or not. That's about what I get with 600+ hp and E85... Quote Link to comment Share on other sites More sharing options...
310z Posted May 26, 2011 Share Posted May 26, 2011 I have not checked my in town mileage but I have checked my highway mileage. While driving to the Canby Car Show & Swap in Oregon USA last year up I-5 for 100s of miles, averaging 75 mph and many blasts beyond 100 mph with a 3.1 L and triple Weber 45s I averaged 24 mpg on more than one tank of gas 91 octane. Quote Link to comment Share on other sites More sharing options...
240zip Posted May 26, 2011 Share Posted May 26, 2011 8 to 9 MPG? Sounds low, but also within the realm of possibility I have 42DCOE's, 2.8L, E31, headers, and a moderate cam ... more like 15 to 18 MPG, but I need to check and that's very mild driving, not racing. I suspect you're running way too rich. Have you hooked up a wide band AFM and checked? Quote Link to comment Share on other sites More sharing options...
Pharaohabq Posted May 26, 2011 Share Posted May 26, 2011 Yeah I agree, Sounds like you're rich to me too. in my stock 78' running like crap I was getting 18 in town for the 3 weeks I was driving it before I tore it apart. Granted that's EFI and not built, but still I would expect 10+ at least. Quote Link to comment Share on other sites More sharing options...
240zip Posted May 26, 2011 Share Posted May 26, 2011 My triples ... FYI ... no comparison between EFI and DCOE's with respect to mileage. Two different animals in terms of fuel efficiency. I suspect that much of this is due to improper jetting and tuning. In my experience, many people set their DCOE's to be overly rich. Also, few people really dial-in their DCOE's with the proper chokes, emulsion tubes, and jets. The expense, dyno-time, and expertise all make people want to move to EFI (ITBs) or other set-ups. Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 26, 2011 Share Posted May 26, 2011 (edited) 40 PHH Mikuinis on stock L24 in 240Z AUTOMATIC with 4.11 differential got 28mph steady state cruising 60-65mph over a 357 mile known loop. Same car in-town would get 17mpg. 44PHH Blowthrough Turbo, L28, Late ZX 5Speed, 3.36 gearing , averaged over 26K miles of daily commuting got 17mpg. Highest was about 20 when cruising, could get 4 to 5 at the track when on the boost a lot. Changed to 3.7 gearset, didn't change anything appreciably. 40 DCOE 18's, stock L28 in 240Z with 3.7 gears and late ZX 5 Speed, averaged 12-15 on the freeway, and 8-10 in town driven spiritedly. Sold them to some idiot who wanted Webers and refitted my Mikuinis, what an expensive mistake and overhyped combination. Mileage restored to 24mpg on the freeway, with 17 combined. Before any comments are made on misjetting et al: horiba five gas analysis AT REDLINE with John Concialdi and Steve of AEM doing the tuning and jetting. It may have been my foot, but even in steady state I rarely got above 15, maybe 16.2mpg on occasion. They just rely on the main jet too much in a Weber compared to a Dellorto or Mikuini and the economy suffers accordingly. They have to be jetted rich unless you start chamfering throttle plates, drilling progression holes, blah blah blah... If I don't have to do that with my Mikuini to get better mileage I don't see why I should have to do it to a carb that is supposedly 'better and more refined'... Also JC and Steve commented on what I could expect my mileage to be, and they were spot-on! Power as shown on the dyno was not appreciably different from the Mikuinis... 40PHH Mikuinis unless seriously misjetted should return nearly identical if not slightly better fuel economy as SU's on the same engine. If you use the HP the mileage will be less. But with a light foot, their jetting arrangement provides for exceptional economy. They were designed as an OEM Fittment for the L24, and their OEM design heritage (Solex) shows in drivability and fuel economy. If you're getting single digits, I'd look at the plugs. You have to be in the Main Jet a LOT to get that kind of mileage! Edited May 26, 2011 by Tony D Quote Link to comment Share on other sites More sharing options...
jimmyjones000 Posted May 27, 2011 Author Share Posted May 27, 2011 (edited) With a lead foot what do you think? The cars 30 years old. Haha, well I'm keeping it under 3500 , 95% of the time , usually in the 2000-2900 range Edited May 27, 2011 by jimmyjones000 Quote Link to comment Share on other sites More sharing options...
jimmyjones000 Posted May 27, 2011 Author Share Posted May 27, 2011 I have not checked my in town mileage but I have checked my highway mileage. While driving to the Canby Car Show & Swap in Oregon USA last year up I-5 for 100s of miles, averaging 75 mph and many blasts beyond 100 mph with a 3.1 L and triple Weber 45s I averaged 24 mpg on more than one tank of gas 91 octane. 24 MPG would be nice Quote Link to comment Share on other sites More sharing options...
jimmyjones000 Posted May 27, 2011 Author Share Posted May 27, 2011 My triples ... FYI ... no comparison between EFI and DCOE's with respect to mileage. Two different animals in terms of fuel efficiency. I suspect that much of this is due to improper jetting and tuning. In my experience, many people set their DCOE's to be overly rich. Also, few people really dial-in their DCOE's with the proper chokes, emulsion tubes, and jets. The expense, dyno-time, and expertise all make people want to move to EFI (ITBs) or other set-ups. Clean looking, did the air filter help with any of the fuel smells? I'm just running mesh covers Also, do mikunis need to be super fined tuned new from factory? Mine are brand new from factory back in the 80s I believe, I got them from someone that had them boxed in there garage, were sitting for almost 10-15 years ( never been used ). Well techinally they came with my other 240z , production date 01/1970. , but they were obviously purchased later on by the guy. All I really did was adjust the idle air to about 4-5, and adjust the fuel supply till the idle was great and the plugs stayed at a light dry brown color Quote Link to comment Share on other sites More sharing options...
240zip Posted May 27, 2011 Share Posted May 27, 2011 I think TonyD understands the Mikunis, I only speak Weber. My carbs were OEM on a Maserati 3500 GT. They had the following baseline settings. 32 3.5 135 F15 155 55F2 45 200 They're fine from 2500 RPM and up but right now are too rich at idle. So after attempting to lean out the idle, we're likely going to swap the 55F2's with 55F5's. Right now the plugs are black with soot. I had Rex Chalmers rebuild mine, the cost was $95 per carb which was cheap given the quality of his work. He's been helping remotely with some of the tuning aspects. I had rebuild kits otherwise those are an additional $45 per carb. On the K&N filters. I put in the air horns and tried a few different combinations. In the end, it seemed the mid length air horns and the K&Ns gave the best response and AFM. Quote Link to comment Share on other sites More sharing options...
madkaw Posted May 28, 2011 Share Posted May 28, 2011 I thought 4psi was almost too much pressure for Mikunis? Are you running vacuum advance, if not, I would think you could tweak that timing up a bit. There is something a miss to get that bad of MPG-IMHO Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 28, 2011 Share Posted May 28, 2011 (edited) ESPECIALLY if you are only going to 3500rpms! You are juuuuust kicking into the main jet. under steady cruise conditions, Mikuinis are on their IDLE jet system, they usually run a 55-65 idle jet. They have extensive progression holes to allow low throttle angle operation on the idle jet ALONE. The BIGGEST problem I see with people running Mikunis is they undersized or didn't change the idle jets. They get a "lean pop" so they start upsizing the mains. WRONG! Upsize (or at least check the idle jets sizing) and then concentrate on accel pump volume and squirter nozzle size. With the proper accel shot, you can run idle jets to get GREAT fuel mileage, and on a quick WOT shot the accel pump will mask a very long transition (meant for street driving where you are on the verge of lugging the engine) until the main system is properly drawing and supplying fuel. If you are jetted correctly, on level ground in fifth gear (late ZX box) and a 3.36 differential you can be rattling along in top gear at 1500 rpms(so that is about 40-45mph)and go WOT.... The engine should not lean pop, but should just start pulling---slowly at first, and faster as you approach around 3000 3500 when it should really come alive (comes on the cam)... I don't have any with jet sets in them now but if you could take a look at your jets, the idles, mains and air correctors it might give me an idea where you are or what is wrong. Unless you are completely and continually stomping on it to WOT and just pumping the hell out of the accel pump circuit..... Something definately sounds wrong if you are only going to 3500. If you short-shift an L24 with a 3.7 gearset and a late five speed I have seen guys with 40's break 30mpg if they keep it around 60-65mph. If you have the wrong float level or excessive pressure on the fuel (normally 3.5psi) you will get crappy mileage. A high float level causes the main to tip in too early, and really screws up how much is delivered in the idle circuit as well. To the Weber issue of being over-jetted: I have found guys with 45 DCOE's running main jets bigger than our 320HP Bonneville Engine!!! If we could make that power with a 140 or 135 main jet (depending on altitude...) what the hell are you doing with that thing in a STOCK L28??? Answer is always the same: "It was lean popping so we kept increasing the jet size till it stopped"... They get 8-9mpg as well!!! <EDIT> and the timing comment is spot on: make sure you are running the vacuum advance with the fitting from the first carb, or crank in some advance! Without vacuum advance the car will be a dog and won't get crap for cruise fuel mileage, you will be using a lot more throttle angle than necessary and tipping into the main like being in the wrong gear going uphill! Edited May 28, 2011 by Tony D Quote Link to comment Share on other sites More sharing options...
Lazeum Posted May 28, 2011 Share Posted May 28, 2011 (edited) To the Weber issue of being over-jetted:I have found guys with 45 DCOE's running main jets bigger than our 320HP Bonneville Engine!!! If we could make that power with a 140 or 135 main jet (depending on altitude...) what the hell are you doing with that thing in a STOCK L28??? Answer is always the same: "It was lean popping so we kept increasing the jet size till it stopped"... They get 8-9mpg as well!!! <EDIT> and the timing comment is spot on: make sure you are running the vacuum advance with the fitting from the first carb, or crank in some advance! Without vacuum advance the car will be a dog and won't get crap for cruise fuel mileage, you will be using a lot more throttle angle than necessary and tipping into the main like being in the wrong gear going uphill! Tony, My first guess with lean pop would be to act on the idle mixture. Am I right? Since it occurs with throttle closed how could some people believe the main jet would solve their problem? I would also tend to play with idle jet if playing with the screw won't help much or make the idle runs very rich. Regarding your comments on main jet being way too big, I think you also need to put into consideration the head & the venturi size. I was running 140 on my stock N42 head with low timing (28 total to avoid knock; flat tops & stock N42 with mild cam was not a good combo) & 40DCOE w/ 32mm venturis. AFR were around 12.5/13.0 at WOT, engine was very healthy. When I have swapped the N42 to a P79 port/polished by Braap with Rebello street cam, I had to go down on main jet down to 125 (what I had in stock) and AFR was around 10.0/11.0. I came to the conclusion new head was flowing way more air, vacuum in carbs was way higher. I believe there are 2 ways of fixing this: smaller main (110? - far away from the rule of thumb: Main = 4 x venturi's size ) & bigger venturis. Bottom line is my engine combo is out breathing my current carb setup, venturis size has to go up (34mm?). Same goes for your story, 45 DCOE might have too big venturis for the engine you've seen. So they've compensated "lack" of vacuum with big jets. Your engine is most likely flowing way more air. Edited May 28, 2011 by Lazeum Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 28, 2011 Share Posted May 28, 2011 As I said, float level affects when the main jet will also transition. If there isn't sufficient vacuum in the main venturi (and we don't discuss going to a smaller booster venturi) a bigger jet supplies more fuel...but closing down on air bleeder will have similar effect, but on the top end may make it 'too' rich---thing is on a stock engine where will the peak RPMS go and will they ever get to that 'draw point' where the air corrector starts enrichening??? Shift, CTRL Arrow Up...tab Arrow Down shift, CTRL Arrow Down.... Screw air correctors, emulsion tubes, booster venturis, main venturis...blah blah blah... I really don't like talking about this crap, it seriously brings me down. I don't want carbs and I'll give help to a point, but if you want to discuss WHY one combination may be one way or the other---I AM NOT YOUR GUY! I made my decision to go EFI because this is a ROYAL PITA and results in compromises that I find unacceptable. If you simply are satisifed with a great steady WOT pull, carbs will do it and do it rather easily. Beyond that, get out some serious time and tuning skills and have at it. Me? I'll hook up the laptop, have someone drive to my direction, and start playing with the fuel AND ignition lead on the same screen with the tips of my fingers faster than you can remove the damn jet covers! Apologies to Dr. Seuss but: "I do not like car-bu-ray-tors. I do not like them in a box, I do not like them with ITG Socks. I do not like them here nor there, I do not like them ANYWHERE! I do not like car-bu-ray-tors, I do not like them They Be Damned! They do not have them on Big Trains, The do not have them on Commercial AeroPlanes! I do not like car-bu-ray-tors, I do not like them They Be Damned!" Quote Link to comment Share on other sites More sharing options...
240zip Posted May 30, 2011 Share Posted May 30, 2011 Try them and you may I say ... Some helpful links ... http://www.teglerizer.com/dcoe/webertune.htm'>http://www.teglerizer.com/dcoe/webertune.htm http://www.tjwakeman.net/TR/WeberDCOEinfo.htm http://7faq.com/owbase/ow.asp?HowDoI%2FTuneWeberCarburettors http://joltfreak.tripod.com/id28.html this one has a link to a DOS program for jetting http://www.teglerizer.com/dcoe/ Quote Link to comment Share on other sites More sharing options...
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