rejetting opinion

[skippy344]

Ok, so I am tweaking my '80 special with the Mikuni carbs fitted for the '80 model.

Original main jets in order were 1- 110mm, 2-120mm, 3-120mm, 110mm, per the factory original setup. I assume we are heavier on 2 and 3 to cool them down a bit. The size increase for these carbs appears to be 5mm increments.

Ok, so the formula most agree on when jetting up is, add one unit for every performance mod, take that total ,subtract one, and that result is the number of steps up in increase in the jet size.

So, I added a K&N filter, Accel Super Coils and a Mac 4-ito-two exhaust. 1+1+1=3-1=2. So I should rejet two sizes up in each carb, right?

With what I had around the shop, I moved 1 to 120mm, 2 to 125mm, 3 to 125mm and 4 to 120mm. I didn't have any 130mms handy.

Packmule runs great, with very little popping on decelleration.

I very respected and expert mechanic suggested that I even all mains to the same jet value.

He also thought about increasing the pilot jets from 42.5mm to a higher value.

What has been your experiences with the '80 Special carbs and running main jets to all the same size? Performance? mileage?

Thanks,

greg

[pggg]

Im don't know much about 80' carbs but my 78' bike had the same thing, 13 previous owners, uneven jet sizes all thru, float tangs bent every which way, even different slide needles and broken off idle screws, still idled and ran OK but not perfect of course

[mro]

Packmule runs great

That sorta says it all

Would check plug "color" at different RPM's.
Lean = Hot = Shorter engine life
Would not think new coils would reqire new jetting.
If center cylinders are not running rich would leave em.


mro

[TADracer]

I think you are right about the bigger jets on the 2 & 3 carbs. The 2 & 3 cylinders do not get as much cooling (so they say) since they are inboard. My 79 special has the same main jet sizes across the board but the emulsion tubes are different on the 2-3 carbs to accomplish the same thing. I am not sure it is really that big a factor.

As far as jetting up, you are on the right track but the guidelines you state are only a starting point. Trial and error are what will get you the correct jet sizes for your setup. I have similar mods to you and I am up 3 sizes on the mains and 1 on the pilot jet. My stock sizes are 137.5 and 42.5. I am currently running 145 mains and 45 pilot.

If you change jets I seriously recommend you stick with real Mikuni jets. They should be available in 2.5 CC increments. The numbers on Mikuni jets represent CCs of fuel per minute under a certain flow pressure. Aftermarket jets are sometimes just not as accurate in their measurement as Mikuni and can cause you some tuning headaches. That is why many people say to stay away from K&L jets.

My opinion.

[TomB]

The numbers on Mikuni jets represent CCs of fuel per minute under a certain flow pressure

I know the old British Amal carb jets are sized by this method.

I always assumed the Mikuni carb jets were sized in MM, eg. a #132 jet is 1.32mm diameter. And a #140 jet is 1.40mm diameter.

I know they are sized like that from experience drilling my own jets, is it just coincidence that they are the same or calculated out that way?

[trbig]

Hey Greg,

It's been said on here to not mess with the pilots unless you go up 3 sizes on the mains. Like was said... I'd just keep an eye on your plugs. Get somewhere where you can run it up over 4k rpm for a few minutes, pull in the clutch and kill it, rolling to a stop. Take out the plugs and take a look at them... making sure to burn the %$#* out of your hand in the process! I've heard of several people on here just going with the same size jets across the board with no problems, but those little white coated guys seemed to think things through pretty well when they were making this stuff.

I know it just kills you engineering types not to be messing with something... but if it runs good, don't fix it!

Tod

[pggg]

Originally posted by TomB


I know the old British Amal carb jets are sized by this method.

I always assumed the Mikuni carb jets were sized in MM, eg. a #132 jet is 1.32mm diameter. And a #140 jet is 1.40mm diameter.

I know they are sized like that from experience drilling my own jets, is it just coincidence that they are the same or calculated out that way?

http://www.ds650.net/mikunivsdynojet.htm

[TomB]

Wow Thanks PGGG

From the site

This is not true for Mikuni or most other OEM equivalent jets. This size Mikuni jet, (N102.221 Super BN), is marked according to its' flow rate, ie: 150 = a rating of 150 cc of fuel per minute.

I've been using old standard jets and drilling them out for years, there will always be a good supply of old standard jets around, just ask around, most people jet up not down so theres loads of old standards in peoples garages. I found that the new jets I've bought have ALL been exactly the drill size they have as a number, so now I drill jets bigger one size each time until getting the right fueling then buy new jets the size I've drilled out to, the engine always runs the same with the new jets as with my drilled ones so it must be a pretty good method of sizing jets.

And it saves a fortune on jets when trying to tune an engine/carbs.

Here is another good site that may help folk on here with Jetting Click

[TADracer]

Originally posted by trbig

Take out the plugs and take a look at them... making sure to burn the %$#* out of your hand in the process!:

Here is a handy tip. Go to the housewares store and get one of those kevlar type kitchen gloves for taking hot things out of the oven. Fits right in your back pocket and you can handle a red hot plug with ease right on the side of the road. Just remember to clean it before you put it back in the kitchen drawer or your SWMBO will want to kill you next time she takes the roast out of the oven.

[skippy344]

Great Advice all!

Thanks guys. I appreciate all the good advice. If it ever stops raining, I'll ride the bike and check the plugs!

greg

[CruisingRam]

I don't know if you guys know this- but yamaha dealers sell jets that screw right into place- they just have a smaller head, same orifice diameter- 120 etc- but just with a smaller screw head- I have been buying new jets right from the yami dealer locally for my mikuni carbs.

[skippy344]

Plugs look fine!

I ran Packmule this morning up the highway for about 5 minutes and pulled the plugs on #1 and #2 to compare them.

The Tips are clean, the insulators are white and there is NO soot anywhere! So, I guess I am dialed in for right now.

You are right Ram. While I was at my local Yami dealer getting Packmule inspected, I picked up some 130's to have on hand when I pull the carbs again. They are distributed by a subsidiary of EBC, called ED.

They look just like mine, and have trays for all the different sizes.

Bragging time...

I had to get Packmule inspected, and the mechanic asked me what work I had done to the motor. I said that the motor is stock, with just the K&N filter, Accel coils, Barnett Carbon Fiber clutch and the Mac 4-into-2 exhaust.

He said it was one of the nicest running engines he has seen in a long time! He said it is good and strong, and that they just don't make 'em like that anymore!

He also said that Packmule was nice looking for her age.

Its always kinda cool when a mechanic says stuff like that.

Well, gonna go burn some dino remains,

greg