I hate it when I'm right :D
Hi Martin, I believe you are right. uneven floats and poor throat to throat balance can be partially overcome by going rich. I have a very open flowmaster exhaust and after fitting it i went to 60 idle, (from stock 55), 140 main, (from 135) and 200 ac, (from 220) to get rid of the backfiring, popping back through the carbs. It runs better but still is a bit "sluggish", is stinko at idle and gets 15 mpg on the highway. So, to get max power, get better throttle response and up the mileage, I am going to: pull the carbs, overhaul them with new needle and seat, port match the intake to head and the intake to carb, Use two 4 tube manometers, (like in motorcycles) to get the carb balance spot on, And lean on a friend with an infra red tester to get the header pipe temps even with idle mixture adjustments. While the intakes are off, this might be a good time to do the valve adjustments, so all of the holes are pulling the same. Next step would be to use the LM-1 A/F ratio meter to get the jetting just right, (i've had it two years, just been a bit busy with other stuff). I recall an old post recommending an initial 10 degree advance, (close your eyes, you smoggy purists) with a bushing on the weight shaft to limit total advance to something reasonable like 32-34. Anyone recal the specifics??? IIRC, 10 degrees is what the carbed boxers ran and it yielded a more snappy response, (fatter low end torque curve). (all of the smog stuff is on the bench so i can polish it all up for the next owner ;-) I would welcome any suggestions on how to tweek it a bit more. thanks, chris
60???????? phew...that makes NO sense to me!! I'm getting 18 miles per IMPERIAL gallon with city driving now, which is about the same as yours on the highway! keep us informed of what you find?
Hello Chris, Looks like we have a lot in common. I too have the Flowmaster exhaust. My jetting is described earlier in this post. Valve lash adjust and timing are spot on. So using Aircon's input data I am going to 55, (from 60) 135 (have been in there all along) and 185 AC from 200 (220 when I first got the car). But my car is stinky at idle and I am going to try the 55 idles. I hope I dont have to go through a complete carb rework (float levels etc) to make it work. If it only pops through the carbs when the motor is cold it will be OK. When the motor is cold the mixture will be too lean if the mixture is set for a hot engine. (that is why they have choke circiuts). Fortunetly in Southern California we dont need a choke, and can start the motor with a nudge on the accelerator pump in the carb. Then it takes a little tickling on the accelator to keep lit, untill it warms up a bit. I never actually get on it untill I have 180 degrees OIL temperature. The car naturally pops in the mufflers on over-run (even reportedly "shoots blue flames" on overrun from a high RPM full accleration run) but I sort of like that. Part of "Tweety's" music. I do not understand your manomters methodology. For variuos reasons I cannot measure my MPG, but I would not be surprised with 15mpg. I have set up the distributors with the 10 degrees and the limiting bush inside. Works well. Martin
Well, to complete the story, changed out the idle jets (to 55 from 60) and air correction jets (to 185 from 200) yesterday. No other adjustments made. (Mains are/were 135, 36 emulsion tubes). This means I leaned out the idle circuit and richened top end. It works realy well. The idle is smoother, reponse from lite throttle is good, and upper end (6500 and above) is sharper. Very pleased. Thanks Aircon, for the info and inspiration. Martin.
what do you think of this? http://www.fcavic.com.au/308/exhaust.htm 31 miles per imperial gallon-10.9l/100km-22 miles per US gallon I reckon that's pretty good. Now...here's the problem *sigh* when i'm cruising along a highway, the car's running fine, but if there's a bit of an incline, or i wish to accelerate to a higher speed, when i squeeze the throttle, all's well initially, then it run out of fuel and goes very lean...i keep squeezing and then it goes on to the mains and pulls like a train. The 50 idles are giving great fuel economy, running at an average of 14:1 a/f on cruise. i DON'T want to go to the 55s, as it runs on 12.8:1 a/f on cruise, but of course, is so rich that it covers that lean running part. Does anyone have any idea what i can do to get more fuel at the point of that flat spot without messing everything else up?
Hi Aircon, I have a set of 52 or 53s I haven't even installed yet, but if you want to borrow them for a quick trial, I could ship em out monday. How about bumping the float level an "RCH", or fattening up the main, just a taste,so it reaches "down" a bit. I am betting most of your economy is in the 50 idles. hth, chris p.s. I would scuba dive for rotorooter if i could get 22 mpg, (with good driveability)
Hi Chris, I could drill out the 50s myself if i was sure that would fix it. I tried 140 mains, but that made no difference....so i'm using 135s, which is spot on for my car. Thanks for the offer of shipping them, but I'm in Australia, so it's a bit impractical! also...the mixtures are excellent at cruise, i don't really want to mess it up by putting in a larger idle jet if there's some other way of fixing the problem. i don't quite understand why it's running lean at that point....but then, i don't understand carbies! what would raisiing the float level do, and why?
Raising the float will raise the level of gas in the float bowl, makeing it easier to pull fuel into the airstream. I have not done this but have heard that raising the float will richen the mixture across the board. Having little experience with fine tuning carbs, I have heavily relied on experts, like Russ Turner (SNJ), to show the way. chris
i wouldn't like to do that. cruise and full throttle mixtures are almost perfect, it's only the flat spot at light acceleration that's the problem. i could just fuel inject it
Everything is a trade off. You have done an amazing job I think of tuning for efficient running - as evidenced by matching fuel injection mpg levels even - with the expected side effect of a minor spot in the transition. You are really at the gnat's ass tuning now. I like the idea of the 53's.
i was hoping for perfection! you're right, it IS a minor spot. i could just live with it. since i'm on a roll...maybe i'll try going a bit bigger with the idles, then. i just don't like the idea of my freshly honed bores and new rings having an overly rich mixture pouring down them just for the sake of getting rid of this little flat spot.
What emulsion tubes are you running, F24 or F36??? What size venturi??? Changing float settings screws everything up. These carbs can feed cylinders far far larger without starvation, so float level wont help, it will just screw everything up. In fact everything ive read says the float level is extremely critical, so keep it set to standard settings. It just needs to be jetted properly. As far as drilling out jets, Weber jet sizes are numbered for how the jets flow, not exactly as to thier true drill size. Like a 130 main for example, it flows like a 130 jet "should" flow. Ive thought about drilling some of mine, but without a flow bench to match them, you would just be punching holes in the dark. By the various books, verturi diameter in mm X 4 equals approximate jet size. 32 mm venturis which are common to these cars would then equate to a 128 main jet, so we commonly see 130's. Another rule thrown around is add 60 to the jet size for air correction jet. so with a 130 main, you commonly see a 190 AC jet. These carbs are extemely sensitive to changes. If you change the muffler, somehow the carb knows and everything is thrown out of whack. Ditto the aircleaner, valve clearance changes, valve timing, etc., etc... Birdman noted that even after several hours of soaking, his idle jets were still giving troubles. After soaking them overnight and blowing them out his troubles went away, which shows that cleanliness is very critical. There is an excellent book by Pat Braden through HPBooks on Weber carbs that anyone playing with these cars should have. I truly believe the emulsion tube has as much to say about how these cars run than any other single jet. I do notice that they call for totally different e-tubes for alcohol use, which makes me wonder if a 10% alcohol mix in our fuel could change things enough to require a special e-tube that doesnt exist? While there is a great deal of information regarding DCOE's with a wealth of e-tubes to chose from, the DCNF's are pretty "out there". Is there a better carb that would be easier to find parts for and play with, a 44 IDF perhaps?
I will simply add on to what Paul has said in that you've got it working great - would think hard before any large changes. The Braden book is ok and I have it, but I also like the small manual that Pierce Manifolds puts out. Now, this is not in any book, but just in my experience: The ET's are kind of the 'personality' part of the carb. They can very roughly be described as rich (F24) or leaner (F36) which is determined by their width and how much gas they let sit in the emulsion well at any one time. In my own little world, where the ETs are a set point for the entire spectrum they also seem to affect the throttle response to small changes where the Accel jet is not a factor. Now, clearly Ferrari historically went from the more performance oriented F24s to the more emissions friendly F36s as documented in all the manuals. If you are running F24s, I would not change. If you have F36s, you might consider changing to the F24s. Yes, you will have to rejet slightly, perhaps with a larger air correction (which as Mike Kuhn describes, acts as a brake on fuel delivery at high venturi vacuum signal by acting to 'short circuit' it a bit). You will notice a slight, but definite change in how the car responds, especially at smaller throttle changes. Now, if this is what causes your lean hole and you have an F36, then the F24 might be worth a try. Now for those a bit more experienced than me, they may recommend adding or enlarging one of the transition holes to 'reach up' with the idle/transition circuit. That takes a bit more experience and tutiledge than I have so far, but is the more definitive answer. Alternatively, many advocate that you can go up to a 55 idle and drill a small hole in each butterfly which leans the mixture at lower throttle settings by allowing more air per vacuum signal but allows the larger jet to reach up and cover the transition spot as the butterfly opens. Again, a bit above my current experience level, but one day it is inevitable that I will learn how to do this. It's one of the beauties of Webers.
i'm starting to think that the amount of work involved to gain VERY little isn't worth it. that small stumble at the transition to the mains is so minor that i think i'll just live with it unless one day, i find a definitive answer to its cause.
wow....i sure was misguiding everyone! my car runs almost perfectly on the idles. the stumble is NOT because it's going lean. it stumbles when the main circuit is starting. i suspect they may not all be coming in at exactly the same time or same amount...the SMALLEST change in throttle one way or the other and it's straight back to the idles and around 14.5:1 (or so) or onto the mains and 12.5:1 or so. does this help analyze the problem for anyone? i know i'm still confused!
This is great info!! I put a new exhaust on my '78 and have been getting a lot of popping while on the idles. From there to the upper ranges it seems about right and then seems to lean out again at higher RPM. I started pulling things apart and so far have found 125 mains, 200 A/C's with F36 tubes. As soon as I get the rest of the air box off I'll pull the idles and see what they are. I suspect they are 50's or even less. I'm going to try getting it close and then maybe take it to the local dyno shop to check the A/F mixture. Thanks for all the info guys!! John
Any chance that backing out the idle mixture screws a small amount would help? If you are choking down the 50's too much with the mixture screw some additional volume might help you thru the progression zone. Maybe a stupid idea, but it's free. Where is the dyno shop taking the AF mixture readings in your exhaust system?
Funny you should say that. I turned out all the mixture screws 1/2 a turn yesterday and virtually eliminated that stumble. i'm not sure what that's done to everything else yet though. from the tail pipe http://www.pless.com.au/cars/gt4/rebuild/images/BILD0156%20(Large).JPG
