Off idle stumble

[MPittma100]

Bike is a 79 XS11 Special (XS1100SF). I have cleaned carbs recently and added fuel filters. The carbs have the correct air and fuel jets as per the Yamaha service publication. Also has new Dyna Coils, spark plugs, and wires. I have ohmed out the pick up coil wires at the ignitor and looked for breaks in the p/u coil wires at the side cover. Compression is good in all holes.

The engine is somewhat cold natured. I take that as a given. What it has is what I can best describe as an "off-idle stumble". It idles fine. It RPMs fine after that, but just has that annoying little stumble. There is not a nice enough "roll" off idle like I think it is suppose to have. If I were to guess an RPM range for the stumble, it would be just off idle to maybe 1700-2000. It is idling at about 1050-1100. If you go WOT from idle it will RPM quickly. The engine does have plenty of power everywhere else.

So, is this normal, or have I overlooked something?

Thanks in advance for your help.

MP

[BA80]

Are you CERTAIN the correct style of pilot jets are in those carbs? That is exactly what I experienced with aftermarket pilot jets with too many air bleed holes in them.

[MPittma100]

Correct Pilot Jets

Yeah, I checked that when they were out. They were the same as the ones that were in there originally. The hole pattern was 2-1-2.

I bought a set of carbs off of Ebay a while back for spares/parts. Those had 45 pilot jet and 200 pilot air jet. Mine have 42.5 and 180. I have had fleeting thoughts to put those in mine?

MP

[BA80]

The 79 takes 42.5 pilots and 137.5 mains and 180 air jets stock.

What are you running as far as exhaust and air filter?

[MPittma100]

Engine Set up

This bike had only 10,200 miles on it when I got it about a year and a half ago. It had been cosmetically restored- repainted and some rechromed items.

So, it has the original exhaust and factory air filter housing (box).

A little history on the motor: When I rode it for the first time I noticed that the engine seemed to have a miss. I checked compression and number 2 hole was zero and had 100% cylinder leakage by the exhaust valve. So I pulled the head and had that fixed.

Shortly thereafter I found what I think may have caused the burned valve (all of the exhaust valves were bright white as in too lean). That led me to then look at the carbs. What I found there was something that I could not then and still have not now figured out. There were 122.5 main jets in all 4 carbs and the needle was set to the one lean (slot) position. Of course I corrected that situation with new kits.

MP

[BA80]

Are the carbs the correct ones? 110/120 mains, 42.5 pilots and 185 air jets are stock for an 80/81.

Make sure you have the correct carbs on there or at least the correct jets for the carbs you have.

You should run fine with stock stuff if your exhaust and air filter are stock.

[MPittma100]

Carbs

I have no way of telling if they are the originals, but I do know that they have the correct jets, floats and the float settings are correct. Just don't know where else to look for this stumble??

MP

[BA80]

MP, do the idle mixture screws stick up in the front or are they recessed?

[LuckyEddie]

Off idle stumble...

MP,

Get a birdseye view of the carbs (from the top), with the bike off, slowly give the throttle a twist and watch the linkages to insure that all four carbs open at the same time. Mine did not and I had similar symptoms.

[MPittma100]

Idle Mixture Screws

They stick up in the front - not recessed.

MP

[MPittma100]

Throttle Movement

I will take a look at that (throttle movement) ASAP (tomorrow). What was causing yours to do that. What was the fix?

MP

[bikerphil]

Have you checked the carb sync?

[LuckyEddie]

linkage

The carbs were weathered and stuck, someone attempted force to free them and loosened the fixed bits on the butterfly shaft. I disassembled, restaked the loose bits and put it back together. It was also impossible to properly synch the carbs in that condition.

[MPittma100]

Carb Sync

Yes, they all dialed in nicely. I used the tool with 4 liquid columns.

MP

[bikerphil]

Bike is a 79 XS11 Special (XS1100SF). I have cleaned carbs recently and added fuel filters. The carbs have the correct air and fuel jets as per the Yamaha service publication. Also has new Dyna Coils, spark plugs, and wires. I have ohmed out the pick up coil wires at the ignitor and looked for breaks in the p/u coil wires at the side cover. Compression is good in all holes.

Have you checked the pick up coil gaps, they should be .7MM (.026") between the P/U coil and the reluctor tab, if they are greater it could cause a stumble, JAT