chino Posted September 17, 2005 Author Share Posted September 17, 2005 wow thats a whole lot of info. thanks guys. i will be buying some books over the net so ill know more about these carbs and i am goin to get it tuned but its gonna cost a lot. yes i live in vancouver. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 20, 2005 Share Posted September 20, 2005 I'll agree with JM, our L24 with Triple Mikuini 40PHH's routinely got 28mpg, which was FAR better than it EVER got with the SU's! The Mikuini was designed for OEM duty: DAILY Drivability. Most High Performance cars in the JDM had them straight from the factory. The myths and legends about multiple carburettors ard dispelled pretty quickly once you drive, say, a 1973 Toyota 2-TG Powered Corolla. Toyota drivability and reliability on a twin PHH carbed DOHC four. Same for the Fairlady 432.The linkage was rigid, and you spent more time attending to the cam chain tensioner than you did on the carbs! The Mikuinis are very simple compared to what you have to go through with Webers or Dellortos. I know, I own all three! If you are getting bad gas mileage, I'm with the gent above who says it's fuel pressure. Many a time I have bought carbs that "didn't work" or that had "bad sets of floats" and in the box was an EFI pump. You use that pump on these carbs without a regulator? "Yup!" Uh....O.K.... Uh, I'll give ya $150 for the set, being they got 'bad floats'! LOL Anything over 3.5psi and you can sink the floats. I have never had to screw with my float bowl level---though a bad air cleaner causing a differential between the float bowl and the inlet horn will cause really rich running also. Good Luck, they are a good carb, and will drive really reliably once you figure out what happened to make them go rich! Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted September 21, 2005 Share Posted September 21, 2005 ...a bad air cleaner causing a differential between the float bowl and the inlet horn will cause really rich running also. Now this I am learning today. Quote Link to comment Share on other sites More sharing options...
Guest hybridz-MO Posted September 21, 2005 Share Posted September 21, 2005 If your exhaust is all stock (manifold on back), what setup should I go with if I have trip 44's? I think I have mine tuned (the carbs) pretty well, but I think the exhaust is holding me back. Headers from MSA? I've got an original L24 is what the Mik's are sitting on. Quote Link to comment Share on other sites More sharing options...
Gollum Posted September 22, 2005 Share Posted September 22, 2005 I'll agree with JM, our L24 with Triple Mikuini 40PHH's routinely got 28mpg, which was FAR better than it EVER got with the SU's! I'm getting gyped... I'm getting 20MPG on a completelly stock 81' ZX... Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted September 22, 2005 Share Posted September 22, 2005 A mikuni/solex manual for sale on eBay : http://cgi.ebay.com/ebaymotors/Datsun-Roadster-240Z-260Z-280Z-Solex-Service-Manual_W0QQcmdZViewItemQQcategoryZ80739QQitemZ4577354299QQrdZ1 Quote Link to comment Share on other sites More sharing options...
JMortensen Posted September 22, 2005 Share Posted September 22, 2005 Just keep in mind that "manual" is about 20 pages with lots of pictures... more of a pamphlet. What it does do is give you a good idea of what the circuits in the carburetors do. I think the How to Hotrod book is better, gives you jet sizing formulas to use to get in the ballpark right off the bat. Quote Link to comment Share on other sites More sharing options...
briann510 Posted September 23, 2005 Share Posted September 23, 2005 get a book on the carbs.its probably from a float level thats too high.those things arent the hot set up for a street car.to make them work they need to be jetted on a dyno or install a wideband o2 system and learn about jetting carbs.those carbs have many adjustments.stock su's with some mods work better for street. They ARE the hot setup on a street car. ours run perfect and they were jetted damn close before it even went on the dyno. only thing we had to change was 2 steps down on fuel jets and a couple steps up on pumps once on the dyno and we probably could have left them and been fine they were that close. the only time we actually did a mpg calculation on them was at 21 mpg on a 100 mile trip. we have over 6K on the engine and have never had to resynch them nor rejet them since. Quote Link to comment Share on other sites More sharing options...
Guest hybridz-MO Posted September 25, 2005 Share Posted September 25, 2005 Do I need to do anything with the exhaust (see earlier post). I'm all stock, except for the Mik's. Quote Link to comment Share on other sites More sharing options...
briann510 Posted September 27, 2005 Share Posted September 27, 2005 Do I need to do anything with the exhaust (see earlier post). I'm all stock, except for the Mik's. many people say adding a header doesnt do a thing for a stock engine on the Z's. i figure it cant hurt any and it looks much better than the stock exhaust manifold also. on ours a header is needed for sure, but a totally stock engine probably not anything gained performance wise. Quote Link to comment Share on other sites More sharing options...
HakosukaJD Posted January 11, 2006 Share Posted January 11, 2006 I know...old post, but I'm going crazy trying to figure this out. L28 w/ triple sidedraft PHH44 Mikunis main air 225 main fuel 165 Pump 45 Pilots 60 The floats are set to 12.5mm Carbs all have the same pull. Unfortunately, I don't know what cam I have or what pistons are in it (I trying to obtain a borescope to at least determine flattops or not....) I am getting between 4 and 6 mpg. This is based on known driving distances (basically work and back). On a single trip to work (10km), I can WATCH the gauge go down. The car doesn't seem to care how hard I'm getting on it....always gets sh*tty mileage. So what gives? I haven't found any fuel leaks (and no smell of fuel) while its running (or not running for that matter). I'm still having an issue with carb spitting and engine stumble if I step too hard on the pedal. Under normal acceleration I would get it around 3500RPM. I went from 50 to 45's on the pumps, which shifted the stumble up to around 4500 RPM and its a little better on accel. I had 175's on the mains and went to 165's. I think I'm still running pig rich (as evidence by the black stain on the rear garage wall despite never running the car in the garage except to pull in and out). The fuel pump is an old aftermarket model, but appears to be built for the Z's....haven't been able to test psi on it..... No matter what the Cam or pistons, I would think I could generalize it to at least get 10mpg....and figure it out from there, right? I have both Rebuild and Modify your Nissan OHC engine books....as well as the Mikuni manual. I understand the basic workings of the carbs and how to adjust them. I'll fine tune later....is there a baseline jetting to start from? Mikunis only have 4 jets to change' date=' pilot, main air, main fuel, and pump nozzles. That's it. They are MUCH simpler than Webers, but you can tune a little bit finer with the Webers. About the only other things going on with the Mikunis is synching them and possibly changing the lever for the acc pump. You can change the jet blocks, but everyone says the OA blocks that most Mikunis come with are the best anyway. If you know when you're rich or lean, you can figure out which jets are going to affect that part of the rpm range and you can then adjust the jets and fix it. I struggled for a while until I got an O2 sensor installed, from there it was pretty easy to get them reasonably well dialed in. It is impossible to diagnose problems over the internet though, especially with someone who knows nothing about these carbs.[/quote'] Quote Link to comment Share on other sites More sharing options...
briann510 Posted January 11, 2006 Share Posted January 11, 2006 Something is definatly amisss there with 5-6 mpg. We get double then some at least. make a hard run and pull a plug without driving it even a couple seconds. just pull over and yank the plugs. Better yet pay $65 bucks for a pull on a dymo and see exactly where you are at. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2006 Share Posted January 11, 2006 Why don't you know what size pilots it has? They're right there under the cover next to the jet blocks... I'd suggest you get a fuel pressure regulator. I'd be interested to know what pilots are in it. Are they synched? I know you say they have the same pull, but that doesn't mean that they're synched. I can get over 20 mpg on mine. Something is WRONG with yours if you're getting 6 mpg. Brian's suggestion of a trip to the dyno is a good one. At least there you'll KNOW what's up. Quote Link to comment Share on other sites More sharing options...
Tony D Posted January 11, 2006 Share Posted January 11, 2006 One of the reasons that Mikuinis get such great mileage is that the PILOT jet is usually all you are running on below 3000rpms. Unlike webers where the transition ports aren't really that big, so they jet the mains richer and use a smaller booster to tip in the main circuit earlier. Once you go on the main circuit, you will be lucky to break into the 20's. Generally pilot jets are between 55 & 60 I can't remember where mine are now, 220 airs, and maybe 145 or 155 mains. It's been so long I can't remember. I agree, though, 5mpg isn't good. I had a blowthrough turbo setup that got that at the track with 44PHH's. But that was at the track. If you drive like you are at the track, and have rpms up appreciably above 3200 continually, the mileage will suffer... Quote Link to comment Share on other sites More sharing options...
HakosukaJD Posted January 11, 2006 Share Posted January 11, 2006 Sorry....had to go home to check...numbers weren't in my head for that one.... Pilots are 60's. I'll try to get to the dyno next week. I can get it done at SuperAutobacs for about 5k Yen....(40 bucks). I'm just trying not to use a whole tank of gas on the way . There's no smell of fuel around the carburetors in general...unless you stick your nose up to a horn..then its pretty strong. Normal? I thought syncing the carbs was alternating between adjusting pilot screw and throttle stop screw until I'm getting the same draw across the horns. Is this wrong? Is there something else? Or course, that does lead to a question, what is your starting point for your pilot screw? Seat it and back off how many turns? My manual doesn't really tell me....that I could see..... Why don't you know what size pilots it has? They're right there under the cover next to the jet blocks... I'd suggest you get a fuel pressure regulator. I'd be interested to know what pilots are in it. Are they synched? I know you say they have the same pull' date=' but that doesn't mean that they're synched. I can get over 20 mpg on mine. Something is WRONG with yours if you're getting 6 mpg. Brian's suggestion of a trip to the dyno is a good one. At least there you'll KNOW what's up.[/quote'] Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2006 Share Posted January 11, 2006 Synching is adjusting the linkage, not the pilot jets, until the carbs flow the same amount. If you have one carb barely open, and the next open 10% and the next closed entirely the car might idle at 900 rpm, but its really running mostly on the middle two cylinders. Synching might help you out quite a bit. Pilot screws should all be set exactly the same at 1.5 turns out from the stop. Then you adjust the LINKAGE until the carbs flow as close to the same amount as you can get them. Tony is exactly right that when you're on the freeway cruising along you're running on the pilot pretty much all the time. In fact if you're not on a racetrack you're probably on the pilot and only the pilot 80% of the time. Only when you really open it up are you going to hit the other circuits in the carbs. If you have the pilot screws all jacked up to try and synch the carbs, you'll be putting a whole bunch of fuel in some cylinders and barely any in others. Your jets sound reasonable. You've got a pretty large air and a larger fuel jet than I do, but that might make sense on your car depending on how the motor is built. Quote Link to comment Share on other sites More sharing options...
shire240 Posted January 11, 2006 Share Posted January 11, 2006 I'm still having an issue with carb spitting and engine stumble if I step too hard on the pedal. Under normal acceleration I would get it around 3500RPM. I went from 50 to 45's on the pumps, which shifted the stumble up to around 4500 RPM and its a little better on accel. I had 175's on the mains and went to 165's. I think I'm still running pig rich (as evidence by the black stain on the rear garage wall despite never running the car in the garage except to pull in and out). What size are your venturis/chokes? I was having a similar stumble until I went up a size on the venturis. IIRC I went from 32 to 34 venturi. Quote Link to comment Share on other sites More sharing options...
HakosukaJD Posted January 11, 2006 Share Posted January 11, 2006 The outers are 41mm. I know there are different size outers....but do they have different size inners as well? A "friend" actually took the outers out because the car seemed to be running so rich. I'll put them back in and see if I get a noticeable difference. I guess I let him play with too many things at once....makes it hard too really determine the problem. What size are your venturis/chokes? I was having a similar stumble until I went up a size on the venturis. IIRC I went from 32 to 34 venturi. Quote Link to comment Share on other sites More sharing options...
HakosukaJD Posted January 11, 2006 Share Posted January 11, 2006 Your jets sound reasonable. You've got a pretty large air and a larger fuel jet than I do' date=' but that might make sense on your car depending on how the motor is built.[/quote'] I'm starting to think my cam is a lot more aggressive than I originally thought. It looks like the carbs are set up for a great deal more flow. The original setup was thus: Air 220 Fuel 190 Pump 50 Pilot 60 Outer Venturi 41mm Unfortunately, I don't remember the original advance....but it was pretty far as I recall (especially for someone used to working on RB's) Quote Link to comment Share on other sites More sharing options...
srgunz Posted January 12, 2006 Share Posted January 12, 2006 Earlier you said your mains were 165's. Now they are said to be 190. Is that correct. Way too big. If your venturis are in fact 41, the car will be totally undrivable for the street. 41 is the largest that Mikuni makes. I recommend size 34 for the street. Then drop the size of the main and air jets. My mild L28 current runs the following. Venturi 34 Main 135 Air 180 Pilot 60 Quote Link to comment Share on other sites More sharing options...
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