Technical mikuni tuning question

[s3a]

Hey all,

I've got a 1975 s3 that I'm in the process of swapping the carbs on. I will add that I am running it on premix and jetted accordingly in previous setup. It has stock pipes and uni foam pods. I ran through the tests before starting this, compression is 145-150 acrossed, all 3 cyl have good spark, no bottom end leaks etc. I bought 3 vm26-8074 spigot mount carbs brand new off eBay for 100$ ish a piece usd., 35mm i.d. rubbers with clamps, and fabricated some aluminum intake flanges that are ported to match the round to square style stock ones. I mounted them straight off the engine for clearance of the bowls, they are sitting at about 22-23 degrees, float height compensated. I used the pilot and main jets from my previous setup (stock carbs with some tweaking, 22.5 pilot and 95 main). The needle jets and needles I'm using are the ones from the new carbs (5 series needle instead of 4, so I dropped the needles to compensate). I do have a pretty good understanding of tuning mikunis as I have done a few from scratch before on old snowmobiles, but there is one factor that I think may affect me and I'm not sure about. Both the stock and new carbs have a 26mm through bore, but they have different size slide bores. The stock carbs have a 27.5mm slide bore, and the new ones are 30mm. They both have 2.0 slide cutaways. Just wondering if that would be throwing anything off with my tuning? I've had the bike running to test the float levels under vibration and a quick ride up the road. I set the air screws at 1.5 turns to start because that's where I had them before. It starts and runs alright, but idles high once warm and seems lean up top. Thanks in advance

Ian

[s3a]

Update

I discovered something today. My stock carbs had large round mains #95. The new carbs have hex mains (the ones they came with were way too big) so I had ordered some #95 hex and fitted those without even comparing. Today I compared the two jets and the rounds actually have a larger hole, which got me thinking I should do some research because I always thought a #95 was a #95. I found a conversion chart for hex to round sizes , and ordered some genuine mikuni hex mains in a few different sizes. That should sort out my lean issue up top, as for the pilots I also ordered some 25's to see if I can get rid of the high idle. Any info is appreciated im sure someone has been through this before, not necessarily with the same carbs but with the same headaches ! Lol

[Walms]

Make sure you have the early S3 needle Jets 2-0 I believe.
My KH400 had the later version 6-0 and I couldn't get the mid-range jetting sorted no matter what pilots and mains used.
As far as which mains to use, that's pretty much pipe/ inlet dependant.
For Wirges pipes, I ran 97 mains (round ones are proper for the stock carbs) and 105's with more aggressive pipes.
As I remember, 22 or 25 pilots.
FWIW, don't pay so much attention to what is stock.
Kawi was able to get the stock KH400 to work with massive needle Jets and very small main jets. This was likely only meant to pass emissions and had little to do with performance

[zambia]

You can find a chart online which gives you the relative comparisons..and yes, a 230 hex jet is nowhere near twice as big as a 115 round.

[zambia]

[s3a]

That's the same chart I found! I ordered 105,110,115 and 120's in packs of 4 hex head genuine mikuni mains. See if I can get it sorted out up top. I had those cheap wire mesh cone filters on it for the last few years and just recently picked up some uni foam pods so they should flow a little better.

As for the needles/needle jets, I tried fitting the stock S3 needle jets (220 0-2) into the new carb bodies but they are about a 1/4" shorter in length than the new style. I'm going to try it with the new needles/ needle jets and see how it works. They are 182 N-8 jets and 5F21 needles. I compared the needles and although they are a little longer they have a very similar taper

[Walms]

Here is the link to stock pilot jets for the S series bikes
3cyl.com/mraxl/manuals/sseries/sman56.htm

[s3a]

With the stock carbs, stock pipes, running premix and cheap cone filters I went from 20 pilot to 22.5 pilot (I thought 20 was stock, but I guess it's 22.5 according to that manual) with the air screw set at 1.5 turns and it worked great. If only I could have kept the carbs mounted tight I would have kept that setup, I just figured the spigot mount type with rubbers would be way better, but it's turning out to be quite a pain! lol
Also having issues with setting the chokes, I bought the updated cable choke adapters (mikuni mk-412) that fit the thread of the new carb bodies but the factory s3 plungers don't slide up high enough when the choke lever is applied. I'm experimenting with some old plungers now, lightly chuck them in the lathe and turn down the upper part about .025" so it will fit inside the cable choke adapter

[s3a]

I had the bike running for 10 minutes or so, seemed to run great when it was cold but once it warmed up it got very boggy in the lower end/mid range. tried adjusting air screws out some and it helped a bit but really didn't do much. I have the needle clips in the highest slot (leanest) and I'm thinking the needle/jet is the culprit. Now I have a question for you guys, since the stock needle jet is a 220 0-2, and the new one is a 182 N-8 , would a 182 0-2 be the same inside diameter as stock? They are available in the US, just wondering how the coding of them works. From what I understand the 220 and 182 part is the "series" of jet as in it fits vm26 carburetor , i read that somewhere but maybe I am wrong. Just wondering if an 0-2 needle jet is an 0-2 needle jet regardless of the series of it. Another question, my stock 4EJ4 needles are a fair amount shorter in length than the 5F21 needles that came in the carbs (about a 1/4" or so), so I'm assuming I probably couldn't run them and expect the same results?

These carburetors say on the mikuni site great for 200-350cc. My 400 is 133cc per cyl so I'm assuming I will for sure have to jet down the needles... Just wondering how to figure out what I need thanks

[mraxl]

Jul 2, 2018 12:36:37 GMT -5 s3a said:

I had the bike running for 10 minutes or so, seemed to run great when it was cold but once it warmed up it got very boggy in the lower end/mid range. tried adjusting air screws out some and it helped a bit but really didn't do much. I have the needle clips in the highest slot (leanest) and I'm thinking the needle/jet is the culprit. Now I have a question for you guys, since the stock needle jet is a 220 0-2, and the new one is a 182 N-8 , would a 182 0-2 be the same inside diameter as stock? They are available in the US, just wondering how the coding of them works. From what I understand the 220 and 182 part is the "series" of jet as in it fits vm26 carburetor , i read that somewhere but maybe I am wrong. Just wondering if an 0-2 needle jet is an 0-2 needle jet regardless of the series of it. Another question, my stock 4EJ4 needles are a fair amount shorter in length than the 5F21 needles that came in the carbs (about a 1/4" or so), so I'm assuming I probably couldn't run them and expect the same results?

These carburetors say on the mikuni site great for 200-350cc. My 400 is 133cc per cyl so I'm assuming I will for sure have to jet down the needles... Just wondering how to figure out what I need thanks

Since the bore is larger on your new carbs you will be getting more air than stock carbs so you will need jets larger than stock to offset the air increase. None of the stock sizes will be appropriate. Needle jet diameters are all the same regardless of series. I don't know of anyone who has put newer VM26's on a 400 so you're plowing new ground on jetting for them.
Seems a waste to spend so much time & money to figure out jetting when some VM 28 or30's would give more power for the same effort. The OEM carbs could have been easily fixed to fit tight on the manifolds.
There is a good bit of info here for you to get a better understanding of what you are trying to do: kawatriple.com/carbtuning.htm

[s3a]

I had tried just about everything with the stock carbs to keep them tight, the bike has 24,000 miles and the internal gaskets in the carbs were severely worn. I cut the slot out on each of them several times to the point the carb bodies were getting weak and would break. Tried applying a good sealer to the intake pipe and putting them on, regardless of what I did they would come loose within a tank of fuel (about 60 miles). I tried researching about different carb options and found nothing really for the 400's, other than some people putting PWK's on bigger triples. So I decided to dive into it and I will admit I'm regretting it now but im in too far to turn back. Just trying to figure out where I should start to jet down the needle/needle jet. I'm not sure how they are sized

[Johnny O]

This is what I did to my 400 and it runs unbelievable..

www.tapatalk.com/groups/kawasakitriples/s3-400-triple-lift-porting-t11697.html