What's up with the mixed jets?

[XSPastor]

Ok - Yamaha put 110's on 1 and 4 and 120's on 2 and 3. This is supposed to even out the temps accross the cylinder banks because the inside cylinders run hotter because of their location. I get all that.

What I don't get is mixing 110 and 120 main jets. I mean, isn't that a HUGE difference? Wouldn't the 120 jets that are 4 sizes (!) bigger than the 110's make cylinders 2 and 3 run A LOT richer? With that big of a difference, why do all my plugs look the same?

The reason I ask is I'm about to go from stock 4 to 2 to Jardine 4 to 1 exhaust. I will probably need to rejet. I have heard that guys run the 81's with 110's all across with no problem in a stock setup.

So, when I re-jet, should I up-size from the 110's or the 120's? It seems like putting 115's all the way across would be about right but that's actually down-sizing two of the jets. Maybe I should go 112.5 in 1 and 4 and 122.5 in 2 and 3? It would be nice to put in all the same size mains like all the other years. What's so unique about the 81's that they had to mix the jets? I'd much rather learn from someone else's mistakes than my own! Any help out there?

[thewiz]

Well, Reverend .... If it were me ...

I would first try it the way it is. It may turn out that it will run just fine .... however, if it doesn't pull strong enough on the top end, try the 120s first. If you just happen to have a spare set of carbs with factory jetting laying around, you may already have the ones you need for an initial trial.

Assume you've the factory airbox ?? ..but .... If I were gonna' buy jets I think I'd buy two 110s, four 115s and two 120s for starters ... maybe get luck right away.

Don't know if this really helps but .... that's how I'd approach it.

BTW, good score on the pipe too

[TopCatGr58]

Hey there XSPastor,

When I got my 81SH back in 83 and got it back to the States from JAPAN, I found a set of 4-1's from JC WHitney and installed them, and never rejetted!!! Ignorant YOUTH!! Ran that way for 9 years, several long distance interstate runs, nice warm weather here in Va., rode it for 1.5 years every day to work and back, 35 miles one way, etc., never had any problems, easily revved to 8.5k rpm, etc.!!

Parked it for 9 years, and then when I went to rebuild it, pulled the carbs and found the 110's straight across!! My bike was made for the US market, was just shipped to Japan for the Navy Exchange system, and then shipped back to the USA in my household affects!!!

Only after my rebuild, with big bore, and Indy filters...same pipes, did I go to 117.5's, have great color and performance. So...if you have stock airbox...you may NOT need to rejet, just watch your plugs!
T.C.

[XSPastor]

I do have the stock airbox but the PO drilled four small (10mm or so) holes in the backside of it. I suppose I could seal those up with some rubber plugs... I digress. I have a K&N air filter. I have no problem with trying a setup or two to get it right. I just don't want to turn my brand new (expensive) Jardine pipes bright blue before I get the jetting right.

From what I've read, if I miss it by too much either way the exhaust gas temps go up. I also know with these engines it's better to be a little rich than a little lean. It just seems like taking too big a swing at it to go to all 120's. That's going up 4 sizes in two of the carbs!

With the jetting recommendations chart, 2 jet sizes up for a 4 to 1 exhaust, 1 jet size up for the K&N filter, 1 size for the drilled air box. Add it all up and minus 1 gives me three jet sizes. The chart then says for every three sizes up, I also need to change the pilot jet up by one size. I'm ok with all this but it's hard to figure three sizes up from what? 110? 120? That's a huge difference...

Ok, now I'm obsessing.

[XSPastor]

BTW, good score on the pipe too

Thanks, Wiz. I bought them from Karla Jones. Bob Jones' widow. Nice lady.

[TopCatGr58]

Hey Pastor,

Depends on if you want to save $ or not. When I did my Dyno run a couple of years ago, I was told that I could probably go up another size on the mains due to a dip in the transition zone. I'm still running the stock 42.5 pilots due to being severely stuck after those 9years of sitting OUTSIDE! I've got the pilot screws turned out about 2 turns, and no stumbles!!

SO...I'd also say get the another pair of 120's and put them in across all 4.

Hate to say it but your pretty pipes WILL change color no matter what your jetting, cause they are single walled, so you'll either get GOLD=lean, or Blue=Rich. They have Blue Away to remove the blue.

OR....you could spend a little more $$ and have the pipes Jet Hot Coated, then they WON'T change color with a nice ceramic type coating!!
T.C.

PS, I can't help you by telling you my pipe color, they are BLACK!

[Rhumba]

Huh?

Blue = Rich? I thought Blue = Very lean.

Most steels, when heated, turn gold first and then blue at higher temperatures, especially stainless steel.

Correct me if I'm wrong.

[thewiz]

"..want to turn my brand new (expensive) Jardine pipes bright blue before ...".

might be easier sometimes to paint the bike so it will eventually MATCH the pipes when they turn .... aftermarket quality ain't all that great these days. Like TC says, very few don't blue.

Oddly enough, I have a SuperTrapp on my G and it is a nice color still. It runs a bit rich at idle but I hate to fix it .......that just may be why the pipe ain't blueing

[fredintoon]

the hotter the bluer

About single wall pipes going blue, it's the temperature of the exhaust gases that does it. From running Britbikes where you took it back to the dealer whining about how blue the pipe (Yeah, pipe, singular, who needs more than one cylinder?) had gone and the dealer says; it's supposed to do that. Sir.
If the bike is running just right you'll get purple just at the head flange, fades to blue around the first bend, fades to dark straw through pale straw to clear 2/3rds down the first straight. If you chance to see an old Indian V-twin that's had it's pipes re-chromed you'll notice the classic blueing from the heads and another blue zone at the front & rear pipe joint, must be where the exhaust backs up or re-burns to give a local hotspot.

[tiredoffixing]

I have an 81 and i run jardine 4-1 and pod filters. I have 120's and it runs very good

[XSPastor]

Thanks tiredoffixing! Even though I have a stock airbox, it has been drilled and has a K&N so I'll probably start out trying the 120's. I've got a set of 115's to try if runs too rich. I appreciate your reply.