Carburetor Jetting

[natemoen]

The air bleed jet for the pilot circuit in the spare carbs is a 210.

Thanks

Take that Greg!

[MPittma100]

Jetting

Well, the carbs are back together after another cleaning. I added the fuel filters this time. Carbs idled out and synced perfectly. Seem to have a little stumble right around 2000-2500 but runs fine up to there and beyond that point. The gas was a couple months old. Put in some new and rode it just a little. May clear up. Has one new coil (1&4) and new iridium plugs. Vacuum advance is working. I ohmed out the pickup coils and checked for broken wires. All that seemed fine.

I did notice that I hear a ticking sound on either 3 or 4 cylinder plug wire near the plug. What causes that? Do not see an arc jumping, just hear it.

The air bleed jet for the pilot circuit in the spare carbs is a 210.

MP

[natemoen]

Yup '78's had 45's for the pilot.

Just out of curiosity, on your spare carbs with the '45 pilots look at the airbox side of the carbs and there is another removable jet in one of those hole there, see what number that jet is, probably either 210 or 180.

[BA80]

I think Nate said his E came stock with 45 pilots. I'm not sure about that but some have swapped out the 42.5s for those and it seems to help on the lower end. I might try it myself next time I have to get in the carbs.

The holes in the sides are air bleed holes, more holes = more air less fuel-less holes = less air more fuel. These engines will have a stumble with the 8 hole pilots BTDT. I don't know what they would do with less.

[MPittma100]

Jetting

When I first took carbs apart (bought bike used), the main jets were 122.5? Pilots were correct at 42.5. Metering rods were set to one lean rather than standard (middle X-2) position. I put back 137.5 mains and adjusted metering rod e-clips to correct position.

I assume that someone decided to try to get better fuel mileage by leaning this engine out. 15 jet sizes is a BIG move. Not to mention fried exhaust valves and HIGH engine temps.

Bought a set of used carbs on Ebay for spares. They had correct mains, but #45 pilots-not sure where those came from.

I noticed a post that mentioned pilots with 6 and 8 holes stating that the 6 hole jets were correct and the 8 hole jets would not let the engine run properly. What is the diference in the number of holes doing that adversely affects driveability?


MP

[natemoen]

If they are the proper early carbs then the jets should be no smaller than 137.5 for the mains and 42.5 for the pilots. Could be larger but not smaller.

[MPittma100]

Jetting

Thanks for the info. These carbs from your description must be early. They have both sets of tees, the metering rods are adjustable, and the idle mix screws are up out of the carbs. I believe the jets to be proper for these carbs.

MP

[BA80]

If it has adjustable metering rods it's either an early (78-79) carb or someone has installed a dynojet kit. The 79 takes 137.5 mains and 42.5 pilots stock.

A couple of ways to tell what year your carbs are is the early carbs have the idle mixture screw sticking up in the front of the engine sie and the later carbs the mix screws are recessed. Also, the early carbs have what looks like 2 sets of fuel inlet tees. The uppers being bowl vents, the later carbs there is an extra hole on the airbox sideof the intake bell for this.

When you get jets, particularly pilots, be sure and get genuine Mikuni of the proper styl or it will never run right.

[DGXSER]

THIS THREAD will help you identify the model of carb you have.

[MPittma100]

Proper year carbs?

Not sure if these are the proper year carbs. Is there a way to tell?
I did put the proper kits with the right jets, etc. for the 1979 per the Yamaha service manual. The jets that I took out were way smaller (10 numbers) than even the later XS11's where they leaned out the two inner cylinders.

[DGXSER]

It is common to find the carbs have been swapped out on these bikes. Someone broke a lfoat post, or a jet got stuck and just would nto come out, so another set of carbs gets swapped in. Like Nate is suggesting, a PO could have gotten a set of 80-81 carbs to put in instead of the 79 model carbs. The newer carbs, as your documentation will show, had much leaner mains.

[natemoen]

Are they actually 79 carbs?

[MPittma100]

I have documentation on all of the different year model factory carburetor specs. When I went through my carbs shortly after I bought my 1979 XS11 Special, I found considerably smaller main jets and the metering rods were set to the 1 lean position instead of the standard. I also found number 2 exhaust valve burned and all others were white (lean). Has anyone run into this before?

Just wondering if Yamaha did this or someone put on some carbs off of a smaller bike.

I installed all of the correct (factory specs.) parts back in these carbs and had the cylinder head reworked.

MP