Needle Jet Number?

[natemoen]

Ok so all this talk about Mikuni only replacement stuff got me wondering if that might be the cause of some of my issues. I was pulling my carbs out anyways so I went and was looking throught the little bag of carb crap that I was given when I bought the bike, noticed there was K&L tag burried in the bag....hmmmm. Also in the bag was old air jets, pilot jets, main jets, and slide needles.

Looking at the old jets and the "new" jets all the marking are the same so how can I tell Mikuni from other brands?

My main question though for this thread is, What needles are the correct needles for a 1978? Currently the carbs have Y265 needles, but I also have a set of 5GZ6, so which are correct?

[bikerphil]

According to the FSM, 5GZ6 jet needle and X-2 needle jet is what was used.

[cywelchjr]

Nate, the wrong needle alone could account for your problems. You may have special needles in there which have the wrong taper and would be too rich in the mid range and cause drivability issues, so just putting the right needles in there might make it run right. IIRC special needles are the only ones that are available as a replacement part anymore, so for standards it's either used or dynajet, and dynajet kits for the old style carbs is a tough find.

[crazy steve]

I used the 'new' needles out of my Georgefix carb kits on my SG, and those were a big part of the problem I had getting the carbs right. Once I installed OEM, they adjusted out no problem....

[natemoen]

Anyone know how to tell the difference between genuine mikuni and other makers?

[motoman]

Nate, Mikuni uses what looks like a tiny square with a curly-que looking thing inside those outer lines. K&L and RD are proud of theirs and use large readable letters opposite side of numbering on the mains anyways.

[natemoen]

Thanks! So most of my jets are mikuni. Some have no markings though, just numbers.....

[XSessiveForce79]

Nate I have a 79F and it has the big bore kit,Pods,and stock exhaust w/o baffles.I believe I am running lean...lots of popping slowing down from highway speed. I asked the previous owner,who built the engine.and he says it has stock jets. my understanding is that 79 std jets are 137s and 42s. where did you get your 142s and 45s. 5k rpm seems like a wall to me, just starves.I want to clean and re-jet. This thing is 32 yrs old and I'm digging it, just want to get it right. I didn't want to start a thread over this,just noticed your jet sizes and figured I'd ask.

[natemoen]

45 is the stick pilot jet on the '78s only. I bumped up the mains cause of the k&n, drilled airbox and the 4-1 supertrapp. You can get the jets lots of places online or local but like has been said make sure the are genuine mikuni.

[ManagerMike]

I've got a 78 as well and stock pilots are 45 and mains are 137.5. The 79 shipped with 42.5 pilots to the best of my recollection (emissions). I bumped my mains to 145 to allow for pod filters and custom exhaust. While the performance was noticeably improved, it never had a "wall" at any given rpm range before the jetting change, I'm thinking you may have issues with clogged or restricted emulsifier tubes given the 5000 rpm falloff you refer to, just a thought.

[MPittma100]

Speaking of Carburetors............. I most recently purchased a 1979 Special. Am doing some work to the bike (overhauling cylinder head). Decided to rebuild carburetors as well. I have the service manual that states that the main jet is supposed to be 137.5 and the needle clip is to be set in the (standard) number three slot on the jet needle. The kits that I purchased do have the correct jets in them per the Yamaha specs.

When I dissaembled the units, they had 122.5 jets with the needle clip set to number 2 slot (one lean).

Does anyone have a clue as to what may have happened? Why were these smaller (leaner) jets/needle position in these carburetors? It is obvious by the screw driver marks on the jets that they have been removed/installed at some point in the past. I looked at all the carb. specs. from 78-81 and did not see a 122.5 main jet anywhere.

MP

[bikerphil]

Maybe somebody (PO) put in leaner mains to make up for the wrong style rich pilot jets. Just sayin', that happened to me before with my 79 Special. Correct stock pilot jet should a 42.5 BS30/96 type, mains 137.5 large round type. Needle clip center position.

[ManagerMike]

Speaking of Carburetors............. I most recently purchased a 1979 Special. Am doing some work to the bike (overhauling cylinder head). Decided to rebuild carburetors as well. I have the service manual that states that the main jet is supposed to be 137.5 and the needle clip is to be set in the (standard) number three slot on the jet needle. The kits that I purchased do have the correct jets in them per the Yamaha specs.

When I dissaembled the units, they had 122.5 jets with the needle clip set to number 2 slot (one lean).

Does anyone have a clue as to what may have happened? Why were these smaller (leaner) jets/needle position in these carburetors? It is obvious by the screw driver marks on the jets that they have been removed/installed at some point in the past. I looked at all the carb. specs. from 78-81 and did not see a 122.5 main jet anywhere.

MP

It is more likely that you have a set of 80-81 model year carbs on that bike, those came stock with 110 mains. At 122.5 that is 5 sizes up from stock and more realistic than someone going 6 sizes down from stock.

[cywelchjr]

It is more likely that you have a set of 80-81 model year carbs on that bike, those came stock with 110 mains. At 122.5 that is 5 sizes up from stock and more realistic than someone going 6 sizes down from stock.

The only problem with that idea is that the late carbs don't have slots and a clip on the needle. Single position only unless fitted with a dynojet kit.

[ManagerMike]

The only problem with that idea is that the late carbs don't have slots and a clip on the needle. Single position only unless fitted with a dynojet kit.

Possibly further evidence that yet another unsuspecting victim has been PO'd.

Serious investigation seems in order by MPittma100.

Just trying to come up with any reason someone would consider 6 jets smaller than stock, unless of course this 'someone' lived in say, Denver. But even then, 6 jets smaller? And as you alluded, dynojet is a possibility as they aren't available for the early carbs.

Having never dealt with Dynojet kits, what would our probable PO victim look for to determine exactly what he is dealing with? I have only my 78 to draw experience from in this regard.