Pilot jet question

[cabinover]

For all of you that play with jets, I have a question. A friends 78 XS11 runs terrible at idle. From 2000~RPM to redline, you can't blink that fast! However it lopes at idle. It came to me with a K&N and no exhaust. Repaired leads from ignition pickups. Cleaned carbs thoroughly, installed factory air filter to solve K&N problem. Found a no-name 4-1 and threw it at it. Ran great above 3000 but no idle. Floats set right and no broken tips. Runs better at idle. Haven't synced carbs with stick(still waiting for it), but got them close with vacuum gauge. Is it possible to have to richen the pilot without the rest of the system needing to be touched? Anything I'm missing? Thanks for any input, Bob

[dragxs11]

Hey Bob,
have the carbs been rebuilt recently? Do you know if the pilot jets have been changed? The jets in rebuild kits aren't always the same as stock. Also different manufacturers use different numbering scales. A 50 from one may be totally different than a 50 from another. Is it lean or rich? You can change the pilot jets alone, but be aware that you may have to try many different sizes to get it right. Accessing the pilot screws on the tops may be a good start. Find out which way you need to go, then back them out for richer, or in for leaner.

[Jeff]

Where are you currently running the mixture screws? And... even though it raps up quickly sitting still..... how well does it pull through the rpm range when riding it? The 78 used a larger pilot jet than any other year XS.... if your carbs are not original.... it may be too small. Stock pilots should be a 45.0 while later models went to a 42.5.

My 78 loped severely when I first got it. But.... I found that I had one pilot jet completely plugged so it would only idle on 3 cylinders. It would come alive sitting still at about 2000 and at about 3000 when riding.... when the bad cyl kicked in.

I've found mine to be really touchy about mixture at idle. When I first got it.... it had no baffles whatsoever in the mufflers of a 4-2 system and to get it to idle I had to run the mix screws all the way out to 4 1/2 turns to get the idle smoothed down. Once I but new Jardine slip-ons on the system.... I could back them back to 1 1/2 turns to idle smoothly. I've also noticed on mine that having the airbox bottom off.... or even a poor seal when putting it on will cause a rough idle. If the exhaust is leaking any at all... it will raise havoc with your idle as well.

When you cleaned the carbs... did you notice what size the current pilot jets were and did you ensure that the very tiny jet holes in the ends of them...... as well as the small holes along the sides were completely clear and clean? And also that the little hole in the bore the needle jet (emulsion tube) runs through that leads to the pilot jet bore was completely clear? Basically..... when you did your thorough cleaning.... did you actually remove ALL of the jets including the needle jets (emulsion tube)? The holes the pilot jets especially are very small and I've even found that cleaning the carbs once will only dislodge small particles that will plug them up again once back together and ran.

[strom]

I would really like to see a nice list with pictures, what stuff to remove and clean and what to watch out for when cleaning the carbs. I had mine apart at least 5-7 times and I think I didnt get all of the goo out. OK, on the up side, the carbs were emptied before the bike went sitting for 6 years.

LP

[skids]

A no-name exhaust system, hugh? I can tell you this about the 78... The pilot jet is fed from the main jet through a tiny cross bore. That is why it is capped (also because it probably would be above the gas pool it it were uncapped). I found that when exhaust systems are changed, you might find with a colortune tool, that you have to back out the pilot screws too far to get a yellow flame. Alternatively, the next size up in a pilot jet can sometimes cause fouling of the plugs even with the pilot screw turned inward closer than normal. I found that raising the main jet size also helps the pilot circuit so that the pilot screw position can be set nearer to stock position (1.5 turns out from a light seat). Most of the articles about CV carb tuning say to start with the mains with high rpm loading, then adjust the midrange, then adjust the pilot jets, BUT with the 78 and 79 carbs and their dependance on the main jets for rpms below 3000, I say try to stay with clean stock 42.5 pilots and get the mains jets right and try to leave the needle clip positions alone. THEN check to see if the bike pulls hard above (maybe way above) 90 mph.

[skids]

One more thing about pipes: Some pipes have little ability to break-up the echo pulses because of their crappy, or absent, baffles, or otherwise mismatched design. This results in a "dead" zone where "back pressure" actually can fire into the carbs when the valves are both instantaneously open. You can see this with mercury stick as you slowly open the throttle...the vacuum disappears in a given range. Pipes like that can run VERY well in other (usually higher) ranges because of the timing of the echo (exhaust scavenging). I personally believe that if a set of pipes "scavenge" in a particular range, they can losses in torque in other rpms. THAT is my science fiction story of the day my friends! (sorry, I am a bit windy)

[cabinover]

Issues

Look at all the info here!! Thanks for your time and I'll try to answer some of your questions. The exhaust was an e-bay deal and I still can't find any name on it. It's a 4-1 megaphone that's about all I can tell you. I had the carbs completely apart last fall with all the jets out, dipped, cleaned, poked and prodded. I don't remember the sizes other than the inner 2 emulsion tubes were a larger size (more holes)than outers. I replaced the K&N with a stock filter so that I could get the bottom cover to seat properly(a big help) and at least take that variable out of the equation. I guess I will have to pull the rack again and go through it, writing down jet sizes. Other than short blasts around the parking lot, I have not ridden this bike down the road but it does act just like my GS850 with a closed pilot jet. Just yesterday I pulled a bowl on that to clean a pilot. Viola, #3 cyl is just as warm at idle now as the rest!! I'll keep all posted on my progress and thanks again. Bob