ATOYOTA Posted March 20, 2013 Report Share Posted March 20, 2013 Hi all, I'm in need of a carby tune on a dyno. The carburetors are Mikuni CV ITB's (off a GSX1100R I believe) and they're strapped to Phil's twin cam 4K. They're currently running very lean due to changing the air filtration system and it looks like it's gonna need a whole new set of jets/needles to be selected. I need to know two things: Is there someone around Brisbane who is capable of getting these just right, and what am I expecting to pay? I've been told it's not gonna be cheap, but I'd really like to know how much roughly it will cost. Thanks! Quote Link to comment Share on other sites More sharing options...
altezzaclub Posted March 20, 2013 Report Share Posted March 20, 2013 Assuming you know how they work, and they are similar to SU carbs... Is the idle mixture OK?? That sets the deepest level of the needles in the carbs, when they are right down in the jets and fuel flows past the maximum diameter of the needles. This is decided by lifting a slide slightly and listening to the response of the motor. As soon as you open the throttle the slides are sucked up and the thinner parts of the needles are exposed, allowing more fuel up the jets. So if you are lean when running, but fine at idle, the needles are too thick further down. They can be easily re-shaped with some #400 'wet & dry' sandpaper and a micrometer. New SU needles can be bought in Sydney, but I don't know where you would find a selection of Mikunis. Selecting them is a trial and error process, so the tuner needs a stack of the various different tapers to fit and try against an oxy sensor. Have a play through this website- You could measure a Mikuni needle every 1/8th of an inch down its length and see which SU needle it compares to. I had AAP needles to start with, then bought ABH, then sanded my AAP (lean) needles to get what I wanted. http://www.mintylamb.co.uk/suneedle/ What I did is here- http://www.rollaclub.com/board/topic/42407-the-girls-ke70/page__st__30 There are a couple of other good sites around about SU needles, the MGs, Jags and Datsun Zs use them, but I don't know how much you will find about Mikunis. You are right about it not being cheap, I'd LOVE $75 for the hours I spent on this work. The first dyno guy cost me $120 and did nothing. I reckon I know more about them now than he does. Let me know how you go, or if you have a question. Quote Link to comment Share on other sites More sharing options...
ke70dave Posted March 20, 2013 Report Share Posted March 20, 2013 This mob sounds like they might know what they are talking about. they don't mention your specific carbs, but if they can't help you id hope they would know someone who can. http://www.auto-carbs.com.au/index.html pretty close too, near wooloongabba ish. Id just give them a call eh, ask them how they are going to tune it, dyno or guesses? also how do you know its running lean? Quote Link to comment Share on other sites More sharing options...
ATOYOTA Posted March 21, 2013 Author Report Share Posted March 21, 2013 I've called the bloke who runs that shop and he told me he uses vacuum gauges as opposed to a dyno. I have no doubt in the world that he knows his shit, seeing as carbies are his business, but the thing I'm dubious about is how well it'll go under load post-tune. I might end up trying him anyway. His work is reasonably priced and he does know everything under the sun about carbies. Plus it doesn't hurt that he's literally around the corner from my work. I know the basics about carburetors, like how the venturi, jets, float and throttle plate work together, and I am eager to learn more about them, but to attempt this with as little knowledge as mine would be like learning to swim when your ship capsizes. I mean, there's four of the damn things! What I should have done was get my hands on a set of twin side draughts for the old 4K and tinker with them but then the twin cam came along. The car runs... just, and only when I choke the filters a bit with these socks I've got. I should be able to limp it up there at 1/4 throttle without burning anything out. Idle mix is still just a tad lean, but it's fine in the meantime. Quote Link to comment Share on other sites More sharing options...
springersrolla Posted March 21, 2013 Report Share Posted March 21, 2013 ive used old mate at the gabba and hes tops. i would probably run the car down on a trailer considering theres a few dollars of engine if you do burn something out. could you not swap back to the old air filter style to see if everything returns to normal? what does old mate at the carb shop think about it all?? other suggestion would be a bike tuner but again no dyno. i know there was a bloke who was a guru round slacks creek but not sure if he was all over itb's Quote Link to comment Share on other sites More sharing options...
altezzaclub Posted March 21, 2013 Report Share Posted March 21, 2013 he told me he uses vacuum gauges as opposed to a dyno. Yep- that will synchronise the 4carbs so they are all sucking the same amount of air. What you need is an oxy sensor in the exhaust to read the mixture while you drive around, which is not hard to organise. Hopefully he will have a portable AFR meter for doing that. I'm passing through BNE next Tues & Wed, wouldn't mind a look at it if you have the car around at lunchtime. Quote Link to comment Share on other sites More sharing options...
ATOYOTA Posted March 22, 2013 Author Report Share Posted March 22, 2013 ive used old mate at the gabba and hes tops. i would probably run the car down on a trailer considering theres a few dollars of engine if you do burn something out. could you not swap back to the old air filter style to see if everything returns to normal? what does old mate at the carb shop think about it all?? other suggestion would be a bike tuner but again no dyno. i know there was a bloke who was a guru round slacks creek but not sure if he was all over itb's I've spoken to several carby guys, including one who was so switched on about bike carbs (one of dad's mates) he could tell me exactly what was going on before I even started up the engine. He says Mikuni CV's are high maintenance, and that one of the jets goes a bit oval-shaped every 5000km and needs replacing :/. I'm still sticking with them, but if they're too hard to keep in some sort of usable, reliable form I'm just gonna sell them and buy a set of flat slides. Yep- that will synchronise the 4carbs so they are all sucking the same amount of air. What you need is an oxy sensor in the exhaust to read the mixture while you drive around, which is not hard to organise. Hopefully he will have a portable AFR meter for doing that. I'm passing through BNE next Tues & Wed, wouldn't mind a look at it if you have the car around at lunchtime. A wide band O2 sensor is in order, but I just can't afford the money/time right now to get one. It's #1 on the list as soon as I get the fuel system sorted and the engine running sweet. Phil recommends an Innovate LM1 wide band sensor for this engine, but that may as well be hieroglyphs to me because I've never dealt with them before. All the same, I pick up on this sort of thing pretty quickly so I'm sure I'll learn it. I'd love to have a chat and a show & tell if you're around. Will you still be in the area after/around 6pm next week? It'd be good to meet & greet with one of the elders of Rollaclub. Quote Link to comment Share on other sites More sharing options...
altezzaclub Posted March 22, 2013 Report Share Posted March 22, 2013 I'd love to have a chat and a show & tell if you're around. Will you still be in the area after/around 6pm next week? Ah, sadly not. My bro & I are going from Coolangatta up to Kingaroy on Tuesday and back on Wednesday, so we're passing through Brisbane sort-of morning on Tuesday and afternoon on Wednesday, its 4hours either way. Thursday is back to Orange before easter, one big 1000km hit. One of these trips I'll get some time to spend up there and catch up wth a few nice Qld cars. The difference between the Mikunis and SUs is that the SU jet is adjustable, so you can richen the whole mixture range by screwing it down, or lean it all by screwing it up. They both run tapered needles, but SUs have that bit more adjustability with whatever needle you have. I assume Mikuni sell larger jet tubes as well as a range of needles, you just need to go through them to get the one that suits. The early SUs had the needle go straight down the tube of the jet, and you adjusted its position so it never touched the sides. For some reason they later put a spring mount on the neede so it rubbed on the side of the jet, which must have worn the needle slightly over time, making it richer. Mine were sprung mounted, but I converted them back to fixed needles in case they wore oval. You might fnd the Mikunis the same, if the needle drops cleanly down the jet they don't wear. Quote Link to comment Share on other sites More sharing options...
ATOYOTA Posted March 23, 2013 Author Report Share Posted March 23, 2013 One of these trips I'll get some time to spend up there and catch up wth a few nice Qld cars. Well, I guess you could still drop by and have a look at it, but it won't be running due to the battery having shat itself, and I won't be around because I'll be at work. I'd like to get some full slide carbs one day to replace these, but from what my dad tells me (he's into bikes) they're basically like a button. It's either fully open for straights, fully shut for braking, or slightly feathered through corners. Can't imagine what that paired with a button clutch would be like on a hillstart. LUURRRCH. Quote Link to comment Share on other sites More sharing options...
altezzaclub Posted March 23, 2013 Report Share Posted March 23, 2013 Ther is lot on the net about Mikunis. What sort are yours? One of these? http://www.customfighters.com/forums/showthread.php?t=36637 ..and it seems you can richen it by moving the needle up into the piston with the washers. That is equivalent to screwing the SU jet down to richen it. http://bikeadvice.in/fit-kn-filter-rejet-carburetor/ Quote Link to comment Share on other sites More sharing options...
filfrederick Posted March 23, 2013 Report Share Posted March 23, 2013 the needle circlip position only changes the midrange mixture, the main jet still controls the mixture when the needle is completely out of the seat. you could put larger mains in it, i think i gave you a few sets and one should be drilled out to 210 from memory and i think it has 180s in there at the moment, with the larger mains you could bring the circlip up to the second lowest groove with only one steel washer under it, i think it is now on the lowest(richest) and an extra washer to five it a bit more. I would be very careful mucking around with mixtures without a lambda sensor or a lot of experience reading plugs. I still have those 39mm Keihin FCR flatslides hanging around too.... -phil Quote Link to comment Share on other sites More sharing options...
ATOYOTA Posted March 24, 2013 Author Report Share Posted March 24, 2013 Ther is lot on the net about Mikunis. What sort are yours? One of these? http://www.customfig...ead.php?t=36637 ..and it seems you can richen it by moving the needle up into the piston with the washers. That is equivalent to screwing the SU jet down to richen it. http://bikeadvice.in...jet-carburetor/ the needle circlip position only changes the midrange mixture, the main jet still controls the mixture when the needle is completely out of the seat. you could put larger mains in it, i think i gave you a few sets and one should be drilled out to 210 from memory and i think it has 180s in there at the moment, with the larger mains you could bring the circlip up to the second lowest groove with only one steel washer under it, i think it is now on the lowest(richest) and an extra washer to five it a bit more. I would be very careful mucking around with mixtures without a lambda sensor or a lot of experience reading plugs. I still have those 39mm Keihin FCR flatslides hanging around too.... -phil Oh yeah, I forgot about those spares you gave me. I've laid them all out into their 3 size groups. I'll pull the main jets out and see how big they are in comparison, and probably go for the overdrilled ones. Those carbies look like slide type. Mine are butterfly valves. http://www.dudeworld.com.au/ARTICVCARB.HTML The first one here looks close to mine. That article on tuning is pretty helpful. Very easy to understand. I gather I need to take the bowl off to get at the jets, that should be pretty straight forward. What about slow jets? Do these have them too, and do they need to be upsized/drilled out? How do I get to the needle/slide so I can adjust the heights? Quote Link to comment Share on other sites More sharing options...
filfrederick Posted March 24, 2013 Report Share Posted March 24, 2013 don't pay too much attention to the numbers on the jets as some have been drilled, best to measure them to get sizes. Quote Link to comment Share on other sites More sharing options...
ATOYOTA Posted March 24, 2013 Author Report Share Posted March 24, 2013 (edited) I can tell the difference by eye. There's three different sizes in that bag. Carbies had 160's in them, and I swapped those for the overdrilled ones. I haven't started it up yet because it was absolutely pissing down outside and my battery has died in the arse. I figured out how to get at the needle to adjust it, but where is the idle mixture screw? Is it one of the screws in the actual throat of the intake, or the vertical one right at the front, on the bottom? Edited March 24, 2013 by Senseless Sensation Quote Link to comment Share on other sites More sharing options...
filfrederick Posted March 24, 2013 Report Share Posted March 24, 2013 idle mixture is the one underneath on the engine side of the float bowl cover, just make sure they are all adjusted the same. you should be right to drive it round to get it tuned just stay out of the high revs and keep an ear out for knocking. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.