Runs too rich, poorly at low RPM's

[hyaall]

Five months into ground-up rebuild of flood salvaged 79 XS11 and it snowed today (life is grand in Canada). Started her up for the first time two weeks ago - lo - she runs! But: Too rich, plugs foul very quickly on cyls 1 and 2 followed by 4. 3 initially ran dry but now runs lean after exchanging needle/plunger/diagphram.

With clean plugs, the engine seems to run nicely 3000 RPM + and roughly (stumbling, missing) under that. Running rough I've got idle set at about 1500. After she's taken for a run she wants to idle at about 3000.

Running on the highway at about 4000 the bike will occasionally surge.

Carbs have been rebuilt by my local Yamaha shop - jetted to 78 specs, which I don't suppose should mind. I am running 4 into 1 with aftermarked muffler. Main Jet needles were foolishly cleaned (by me) with emory cloth.

Advice? Help? Time is running out!

[Mike Hart]

Did the shop work on the carbs without the bike around? Seems to me if they had the bike, they should make it work.

What kind of jets did they put in there? Mikunis, or some kit jets? Makes a big difference. Carbs are obviously still not right, I've heard of shops breaking off idle screw tips, etc etc. Get that idle circuit fully inspected (preferably by your self) and make sure it has Mikuni pilot jets in it.

Oh, and the carbs were synchronized? Float levels?

I doubt you messed up the needles with emory cloth - very fine emory cloth, yes?

[hyaall]

Thanks for the reply MH. Ya, fine emery cloth. Carbs were taken into the shop early on (without bike). Dunno whose jets they installed. I'll ask. Do Mikuni's have an MK stamp or something like this to identify their manufacturer? Carbs are not synchronized. Thought I should get the bike running a little better first. Float levels were adjusted in shop. How to tell if they are OK?

[Mike Hart]

Synch can be approximated by Pre-Synchronizing Carburetors, that will get you real close, if you don't have synch tool.

Also see Setting Carburetor Float Height. It explains how to measure.

(Lots of good stuff in the tech forums!)

The Mikuni logo is on the jets, it is a square within a square, it is visible in this pic, to the left of the '47':

[drdwb]

You are probably aware that the four into one exhaust may require a change in jettin, I ran into this when I took off the orginal air box on my 79XS750 sf and installed K&N air filters, the main jet mow running too lean so had to go one size bigger jets, I'm not sure if the 4 into one exhaust would make it run leaner or richer but it will make a diff
Dave

[kenkrieger]

Great Picture Mike,,interesting though the pilots on my 81 Special have staggered holes in the sides,,,yet when I ordered them new from Bike Bandit the ones I recieved looked like this one in the picture. I never installed them, just wanted a backup set. So did I get the wrong jets ,or do I have the wrong jets in the carbs now ??? Hmmmmmmm Can't find anything in the manual. Inquiring minds want to know. Anybody know??

[Mike Hart]

Ken, I hope somebody has a definitive answer, cuz I sure don't

Those are 47.5 pilots out of my XJ that I bought with 0 miles on it from a dealer, so I feel safe in assuming they are stock XJ pilots.

Other styles have different nr of holes, not sure which go with models, or even if it makes a difference...

How about you pop those into your 81 carbs and see if it runs the same?

[woodstoc]

Don't know if this helps the discussion, but my 79 XS11 SF had 42.5 mikunis with the staggered holes in the pilots (bought the bike second hand from original owner who didn't ever have it worked on).

A set of 80 SG carbs I picked up from Florida also had the 42.5 mikunis with staggered holes, but I don't know the history of these.

[kenkrieger]

Ok Mike, I will give it a try when I do the clean.

[nashville_bill]

I just got my new Mikuni "45.0" jets from Bike Bandit, only took them a month, and they are just like my originals that have the holes in the sides.