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Mikuni Carbs on T160 #602446
06/01/15 5:58 pm
06/01/15 5:58 pm
Joined: May 2013
Posts: 208
Fl., U.S.A.
Thunderbutt Offline OP
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Thunderbutt  Offline OP
BritBike Forum member

Joined: May 2013
Posts: 208
Fl., U.S.A.
Just scored a used set of Mikuni carbs in nice condition for my T160 for less than it would cost to re-sleeve and purchase new slides for the Amals currently on the bike. The Mikuni's came off a triple but figure I would ask other members with this set up what they are running for jets and needles before I re-decorate the piston tops with air conditioning holes. I'm at sea level, running stock head pipes, pea shooters and will try to fit the T150 air box. If I cant fit the air box I'll use the K&N Pods. The carbs are currently fitted with 120 mains, and 5F21(?) needles with circlips in middle groove. Didn't pull the low speed jets. Already tried the search function but didn't locate anything definitive. Any help much appreciated.

Jerry

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Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602450
06/01/15 6:11 pm
06/01/15 6:11 pm
Joined: Mar 2006
Posts: 3,771
Norfolk, UK
L.A.B. Online content
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L.A.B.  Online Content
BritBike Forum member

Joined: Mar 2006
Posts: 3,771
Norfolk, UK
Originally Posted By Thunderbutt
Just scored a used set of Mikuni carbs


Type?(VM?)
Size? (26?, 28?, 30?)

Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602455
06/01/15 6:40 pm
06/01/15 6:40 pm
Joined: May 2013
Posts: 208
Fl., U.S.A.
Thunderbutt Offline OP
BritBike Forum member
Thunderbutt  Offline OP
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Joined: May 2013
Posts: 208
Fl., U.S.A.
L.A.B.- pretty sure they are 28's, using tape measure looks like 1-1/8th inches across. Not sure about the type as there are no markings on the bodies. But, they use the rubber sleeve to manifold and they have the taller squared brass fitting protruding up into the airway and around the needle.

Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602457
06/01/15 7:00 pm
06/01/15 7:00 pm
Joined: Mar 2006
Posts: 3,771
Norfolk, UK
L.A.B. Online content
BritBike Forum member
L.A.B.  Online Content
BritBike Forum member

Joined: Mar 2006
Posts: 3,771
Norfolk, UK
Originally Posted By Thunderbutt
Not sure about the type as there are no markings on the bodies. But, they use the rubber sleeve to manifold and they have the taller squared brass fitting protruding up into the airway and around the needle.


VM?

http://www.mikuni.com/pdf/vmmanual.pdf

http://www.sudco.com/mikuni.html

Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602460
06/01/15 7:06 pm
06/01/15 7:06 pm
Joined: May 2013
Posts: 208
Fl., U.S.A.
Thunderbutt Offline OP
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Thunderbutt  Offline OP
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Joined: May 2013
Posts: 208
Fl., U.S.A.
Yup, just like the center one in the pcture

Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602463
06/01/15 7:15 pm
06/01/15 7:15 pm
Joined: May 2013
Posts: 208
Fl., U.S.A.
Thunderbutt Offline OP
BritBike Forum member
Thunderbutt  Offline OP
BritBike Forum member

Joined: May 2013
Posts: 208
Fl., U.S.A.
L.A.B.- scratch that, they must be 26's. Wearing my spectacles I just went over the carb body and found the #26-8074 stamped into the body up by the choke assy.

Thank you very much t the two links.



Jerry

Last edited by Thunderbutt; 06/01/15 8:35 pm.
Re: Mikuni Carbs on T160 [Re: Thunderbutt] #602658
06/02/15 6:22 pm
06/02/15 6:22 pm
Joined: Dec 2013
Posts: 4,294
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
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Joined: Dec 2013
Posts: 4,294
ohio, usa
i have concentrics on my T150, but mikunis on my T120.

i'm really interested in what you learn. on the T120, they upped the milage, throttle response, and were easy easy to jet.


"Are bikes so nice as all that?" asked the mole, shyly...

"Nice? They're the only thing," said the Water Rat Solemnly, as he leaned forward on the handlebars. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about with bikes."
Re: Mikuni Carbs on T160 [Re: kevin roberts] #620504
10/04/15 6:19 pm
10/04/15 6:19 pm
Joined: Dec 2013
Posts: 4,294
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
fefsa

Joined: Dec 2013
Posts: 4,294
ohio, usa
i bought these very nice mikuni carbs for my 1974 T150. essentially brand new, thanks again thunderbutt. they're really nice, and with a new crankcase filter from britcycle they bolt right on.

as they came they're set up like this:

pilots 30
slide 2.0
needle 5F21-3
needle jet 182/P-2
mains 120

i've finally had time to pay attention to them today. the 120 mains seem lean-- the engine wont take full throttle, and picks up a little speed if you roll the twistgrip back into the needle. most people see to run130s or so. so ill get some bigger ones and check

midrange seems fine-- clean, sharp, no hesitation or signs of richness. it will rev to 7500 more or less on command. so i'm ignoring the needle jet and needles for the moment.

the pilot circuit, however, is also lean-- and the jets are 30s, which make the plugs soot up in the VM32s i use in my T120 (i use 25s in that, a morgo 750). at constant low throttle the motor bucks and misfires a little, and good idle is with the screws way in. i put a colortune on all three plugs to check, and the flame was blue-- lean-- with the idle air screws turned way in from 1 1/2. couldn't get a rich yellow flame until i was about a half turn from bottom. still, the bike idles clean and steady at about 800, even with these.

i'm guessing that there are seriously plugged passages in the idle circuit. the carbs are very clean inside, though. 30s certainly seem like they should be rich enough. i didn't notice the pilot jets being plugged up.

today was really high air density-- RAD was 106, so the jetting might be a bit richer on more normal days, which around here seem to be about 102.

stay tuned. i still have the 26mm concentrics sitting in a box, and it only takes about 20 minutes to switch from one set to the other. i have a new AMAL choke cable coming from barnett, and when i get it i'll be interested to actually do some back to back comparisons on the two types of carbs.

Last edited by kevin; 10/05/15 6:40 pm.

"Are bikes so nice as all that?" asked the mole, shyly...

"Nice? They're the only thing," said the Water Rat Solemnly, as he leaned forward on the handlebars. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about with bikes."
Re: Mikuni Carbs on T160 [Re: kevin roberts] #621958
10/15/15 8:09 pm
10/15/15 8:09 pm
Joined: Dec 2013
Posts: 4,294
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
fefsa

Joined: Dec 2013
Posts: 4,294
ohio, usa
these old miks were spotlessly clean on the outside, but the jetting issue had me puzzled for a bit.

called up marino at map, and he looked into his mikuni kits for the T150. he said what i told him i had in the carb was how they were delivered, at least by him. generally i would expect that be on the rich side.

so i took the mains out and looked at them very closely with a lighted lens, and there was mung inside them, duh. so instead of 120s i was effectively running something much smaller. took a very fine wire and very gently rodded the brass jets, then blasted them clean with 120 psi.

did the same with the pilot jets, although there didn't seem to be any blockage to speak of inside them. obviously the brass jet itself must be the limiting constriction in the fuel for the pilot circuit, but the system was still running lean there.

anyway, took the bike out and full throttle was fixed-- no hesitation on WFO, and no change in mixture quality as i rolled off the throttle into the needles. so that's done, for the moment.

once the mains were clean, it was apparent that the needles weren't right after all, and the bike blubbered just a bit between half to three-quarter throttle. didn't need to look at the plugs to know what that was. the clip was set in the number 3 notch- right in the middle-- so i opened up the carb tops and dropped the needles one notch into the number two groove. the correction was instant. no more rich motorboating, clean acceleration when rolling the throttle from 1/4 to 3/4, and likewise a clean transition from 3/4 to full throttle on the mains. so that's done as well, for the moment.

the pilots are still a problem. i have a bunch of pilots for my T120 that fit these little 26mm carbs, so i took one and replaced the 30 pilot in the right hand carb with a 35. i had the idle air screw bottomed in its bore in order to get a rich flame (colortune), but with the 35 there was essentially no change. so it isn't the jet that's metering the idle fuel, it's whatever the restriction is wherever it is. i've already taken all three carbs apart and shot the idle circuit passages full of cleaner and compressed air, but clearly they're still munged up.

needle jet seems fine, and the single-taper needle seems okay as well. next week i'll take this machine to the airstrip and look more closely at the mains and needle, but for the moment with the changing meterology, it looks like i'm as close as i can get.

still waiting on barnett to finish the new choke cable for the old amals i have for this machine. frankly, though, i'm no great lover of the concentrics, especially with the weird gantry and complicated spring and lever linkages they need to have on the triples. the mikunis are bone simple, easy to diagnose and tune, and seem to have few idiosyncracies that i haven't already encountered. so even when i get the cables back, i'll likely leave them in the box they're in now.

Last edited by kevin; 10/15/15 10:11 pm.

"Are bikes so nice as all that?" asked the mole, shyly...

"Nice? They're the only thing," said the Water Rat Solemnly, as he leaned forward on the handlebars. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about with bikes."
Re: Mikuni Carbs on T160 [Re: Thunderbutt] #622649
10/21/15 10:32 pm
10/21/15 10:32 pm
Joined: Dec 2013
Posts: 4,294
ohio, usa
kevin roberts Offline
fefsa
kevin roberts  Offline
fefsa

Joined: Dec 2013
Posts: 4,294
ohio, usa
interesting.

these things are puzzling.

i took the pilot jet out completely on the right hand carb to look at what might happen, and the bike idled okay. the color tune showed that the plug was firing, but there was no color visible through the mica-- no ignition of the charge.

so i put the 30 back in and took the right hand carb apart again. there's no question about the pilot circuit being clear, as on a mikuni you can see right through from the venturi through the pilot into the float bowl. i also used my little 78 drill bits to make sure the idle fuel passages in the venturi were clear, than buttoned everything back up.

still no rich mixture available, even with the mixture screws bottomed out.

odd.

still messing with it

Last edited by kevin; 10/21/15 10:33 pm.

"Are bikes so nice as all that?" asked the mole, shyly...

"Nice? They're the only thing," said the Water Rat Solemnly, as he leaned forward on the handlebars. "Believe me, my young friend, there is nothing -- absolutely nothing -- half so much worth doing as simply messing about with bikes."


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