I have a 32mm Mikuni VM round slide carb (two of them) on a 1964 T120 650 with 1 3/4 Inch straight exhaust pipes. The carb has the large hex jets in it... and is running 170 main jets, and 55 pilot jets... it is very rich!!!! I am fairly certain the pilot jet needs to be "leaned" out to somewhere around 35's, not sure about mains. Is anyone else running this combo, and can offer good baseline settings?
Hey Jethro - I have a similar set up on my '72...here's what I found works best. Main 220 with needle in highest (richest position) - I may try a 230 with needle in middle or second to highest as it seems to "hunt" a little bit around 1/2 to full throttle. Your pilot sounds WAAY to rich. I've found a 27.5 to work best for my bike (Boyer with K&Ns, drag pipes with 2" slip-in baffles). Anyway, depending on your intake, I'd drop the pilot to somewhere between 22 and 30. Hope this helps!
Furies and Apaches and Interceptors, oh my!
Re: Mikuni carb recommendations pls... #79652 05/30/0511:29 am05/30/0511:29 am
I would recommend you buy "the book" on Mikunis for Brit bikes as sold by Victory Library (www.victorylibrary.com)
The tuning parts to get it right include selection of: main jet needle jet needle needle clip position air jet pilot jet pilot air screw setting slide (cutaway)
Tuning a Mikuni is not nearly as easy as tuning an AMAL as there are perhaps 50 different needles, 50 different needle jets of two distinct styles. Amals have one or two of each. I spent literally years trying to get my BSA Hornet with Mikuni's tuned in. I got close then I got the book, bought the recomended jetting and tried again. I had to deviate a bit from the suggestions but the recommended testing help decide which way to go. It was the correct starting point that helped me the most. The jetting from Sudco where I bought the BSA specific kit was out in left field.
Thanks gents... LD, I did buy the SUDCO book, and it was a major bit of work just to find out WHICH VM carb I had!! LOL. How hard would it be to stamp the bloody carb with a number? Anyway, they recommend the 200 main to start with, and I DO have a 25 pilot jet sitting on the table. Thanks for the info Jeremy.... what I'm going to do is clean the carb while it is apart, use the 170 mains for now, put in the 25 pilot jet, and see if I can't use the idle screw, air screw, and jet needle clip to "adjust" from there, trying to get it close. Is the 170 main jet richer than the 200 recommended by SUDCO?
i'm running a 25 pilot and a 200 mains on my 750, the 170 is leaner than a 200. i was running a 190 and had a lack of power on top so i went back to a 220 and it wouldn't hardly run when you opened up the throttle so i dropped down to 200's and its better. i've been working with a real rich condition too but after alot of playing with jets i'm convinced its a ignition problem. i'm sure with your 55 pilot that your way out of the ballpark there, do you know what your needle and needle jet are?
When given the choice between two evils I picked the one I haven't tried before
It is not the Sudco Mikuni manual but that gives you what's available. What you want is the Victory Library's book "Mikuni Tuning for British Twins". it not only tells you how it all works but gives you a process to find the right jetting. You should buy the book just to get the process.
Here are some starting numbers:
pilot 20 to 25 needle jet 159P2 or 159P4 needle 6DH2 slide 2.5 main 220 air jet 1.0
The book helps you select the changes from a starting point. Don't waste your time until you have the book. Really, I screwed around for five or six years trying to get to the best jetting. Victory's book got me there in two half days in my garage. The first longish ride convinced me that my long term effort was a complete waste of time. I had to deviate from the starting jetting a little but I got there quickly with the books help. I didn' t realize how poorly my bike ran until I rejetted it this time!
The needle jet and needle were the most critical as that is where any of these Brit twins run most all of the time. The main is 7/8 to full throttle which we almost never get to. The idle air screw is easy as the best idle gives you that. The slide cut away is tougher. Install the air jet called out and no change will be necessary unless you have a highly modified engine and a dyno to test it on.
Again, buy the book. You won't be sorry. It's actually pretty cheap.
Re: Mikuni carb recommendations pls... #79656 05/31/0510:31 am05/31/0510:31 am
Hey 'Panic', I bought your treatise a while back, excellent work, interesting approach! However, mostly geared towards big twins. What would you suggest as a starting baseline for 28mm VM Miks. on a 867 Trident? I realize cams, ports, etc. will change the end result; but a starting point would be appreciated. Thanks, Tom
Re: Mikuni carb recommendations pls... #79658 06/01/0511:47 am06/01/0511:47 am
Thanks for the info... Panic, that link was bad, but I'll try to find the book this evening through Google.com. Also, for general info, try Carb Parts Warehouse in Cleveland, Ohio if you have a problem with Mikunis, n need parts. They are very sharp. I just ordered a new set of Pilot Jets 25 through 35, and mains between 170-200. I currently have the p-4 jet needle, which is what was both recommended in this setup for the Triumph I have, and I'm sure the needle jet is "standard" also. I'm installing the 25 pilot jet, and 200 main jet, the jet needle in the "middle" position with the clip, and run from there-- I'll post back. Thanks again.
Re: Mikuni carb recommendations pls... #79660 06/05/051:41 am06/05/051:41 am
Just a quick update, because I know ppl like to get some feedback on posts...
I went with a 25 pilot jet, and 180 main jet. The needle was a p-4, which is what was recommended. I did NOT take out the needle jet, which I probably should have. Anyone have a recommendation there? Mikuni tuning book recommends changing the needle jet BEFORE ever touching the jet needle. THe bike is still running rich. Got about 29 miles to the gallon on winding, twisty country rounds, with some cruising thrown in. AND the plugs were definitely rich. Running the N3C's for plugs. Also, is th ere a special tool for removing the needle jet?
Re: Mikuni carb recommendations pls... #79661 06/05/051:50 am06/05/051:50 am
No just spanners will do. Remove the bowl undo the big nut above the main jet and pull the whole assembley out. Now just hold the big nut in the spanner and use a smaller one on the top jet. If these are well used carbs renewing them will help lots. Well tuned mics need choke to start even warm read lean. I get 55 mpg but a buddy with the same set up gets 60 mpg doing 60 MPH. norbsa
norbsa 1960 TR6 1963 Super Rocket 1965 650 Star 1966 441 1968 Thunderbolt 1969 Twinkle 250 1972 Fastback 1974 Roadster 1970 S.S Way too many BSA's not named http://decentcycles.com
What kind of air cleaners are you running? Alot of folks run the thin pancake filters which, although they fit in a limited space, the back of the filter is too close to the mouth of the carb for performance. This tends to act as a partial choke when higher air flow is required (i.e., hold the palm of your hand 1 1/4" from a shop vac hose). You may find yourself jetting around improper air flow.
Before you get too concerned with your main jets, you need to get the jet needle/needle jet configuration resolved as this controls the bulk of your typical riding range. For the most part, if mine didn't even have main jets in them, I'd rarely miss them as I shoot for strong mid-range and WFO is only seen a few times a year.
Several tuning guides give you the silly little exercise of marking your throttle at 0, 1/4, 1/2, 3/4 and wide open. It is not silly and often times when you think your at 1/2 throttle....your not. You've got to know what range or circuits your riding in before you can correct them. Also, I recommend buying sparkplugs and lots of them for this tuning process. The NGK plugs (B8ES) are cheap and readily available. If you have access to a bead blaster, you can recycle the sooted plugs and the blasted plugs take color quicker than a new plug.
Start at the bottom and work your way up. If your sooting your plugs in the mid-range, its hard to read your plugs for WFO. I found out the hard way that no one can give you the answer as no two bikes are identical in every respect. If you do your homework, you should end up with your carbs set up in the middle of their range (i.e., needle clip in center and air screw in the middle of its range). This will give you room for finer tuning, which will come later.
The Sudco book is only useful as a jet listing. The Victory book, which is essentially Panic's, is the work guide to go by. It's not a book of answers but a book of how to arrive at an answer. If you end up with a few extra pieces of brass when the process is over, it's a small price to pay for what your going to get in return. If you are getting 29 mpg, your riding a dog or plug-eater, so-to-speak and an AMAL equipped bike will eat you alive. When your tuning allows your mileage to break into the 50+ mpg range, you'll start to see why the Miks are so desirable. I have two T120R's...one with Miks and one with new Amals. Both run good for the different configurations, however, the Mik equipped bike will literally blow the other off the road...hands down. When I'm riding with my buddies, I have to consider which bike I'm on before I start "testing" them a little.
As far as simplicity, the Amals can't be beat and are my choice for running-in a fresh re-build. As far as performance, the Miks are the best money you can spend, especially, if you buy them from someone who didn't have the patience to dial them in properly. You'll never realize how many flat spots Amals have until you ride with a well tuned pair of Miks.
Bottom line...it's not a quick process but well worth the effort. Do not settle for close as you will be leaving performance on the table.
It's a shame that someone doesn't build an AMAL replica with the tunability and wear resistance of a Mikuni.
I like to post follow ups, and this one has taken quite awhile, because it IS difficult to get the Mikunis tuned, but I just wanted to say THANK YOU for all the great advice!
The bike is running great, and starting on the first kick, finally.
It's a 72 650 on a 64 triumph rigid frame, with dual vm-32 Mikunis and drag pipes. The pipes have a small bolt just forward--4 inches--of the back of the pipes for a little bit of back pressure. I'm at 1000 feet altitude, and it's summer here. This is what I ended up running:
200 main jet 25 pilot jet 2.0 air jet 3.0 slide 159 p-4 needle jet 6F4 jet needle
I operate 80 percent at 0-1/4 throttle. Still runs a little rich. I'm putting in a jet needle that is slightly leaner at those settings.