Carb issues

[Patrick1980]

So I’m getting to the end of my custom build on my 1980 xs1100 special. I have taken off the muffled and I have put in small baffles into the headers which are 2 into 1 on each side to make it a little less obnoxious. I also have real K&N pod filters. I just wen through the carbs and measured each float to 23mm, all 8 of them. I thought this was going to solve my issue but it did not. The problem I have is that my bike with fire up fast, the choke(enrichment circuit) works. I have a colortune tool that I used and I let the bike warm up then replaced the colortune for one of the plugs. The confusion I’m having is in Order to get the blue color on the colortune which is supposed to be the goal I have to turn my air/fuel mixture screw all the way in. This makes no sense to me because I have the pods and a way more open exhaust. For reference I currently have the stock 42.5 pilot jets in there with 120 mains to make up for the extra air. Why in the world would I be turn my mixture score so far in , cutting off more fuel than stock if I’m getting more air into the carb. I have bought 45 pilots thinking I might need them but it appears that I almost need to go down a size and I’m not understanding why that is.

On top of this when I open the throttle in a quick motion to anywhere, wether it’s 1/8 throttle or wot it sputters/backfires and struggles to get past about 2000-2500. If I go smooth however I get it all the way to redline without much issue.

I know that the main jets only come into play with higher throttle positions but is there anyway that fuel goes through the main jet at idle? Are my 120s allowing fuel into the intake at idle. I’m thinking of putting my 117.5 mains in and seeing if that changes anything but any help is greatly appreciated.

[DiverRay]

I always start at stock, no matter what you have done. Then change one thing at a time, mains, pilots, or tweaks to the idle mixture, just ONE thing at a time. It does sound like the pilots are too big.
The air/fuel mixture flow is determined by the engine. this engine only flows so much, no matter what you do. The ONLY way to make it flow more is via turbo or super charger.

[DEEBS11]

You are running rich in the low rpm range. Check your slide needles. Might not be seated correctly. A needle that sits too high will show the same symptoms as a too-large pilot jet. They are possible riding high and letting in too much fuel at low rpms. You could also have Chinese jets and that would also cause your issue.

[gtem]

I have a somewhat similar situation with my '78 but running a stock airbox, '80 carbs, and a 4-1 RC Engineering exhaust. Stock jet sizes except I'm running 110s for mains. My issue ended up being (mostly) rooted in incorrectly shimmed slide needles (thanks to DEEBS11 for setting me on the right path in my troubleshooting), but even now I'm running the idle mixture screw at only .75 turns out (!!), going to 1.0 turns out makes it obviously rich. So no idea, could be my 4-1 exhaust flows worse than a stock 4-2 or something to do with the /80 carbs being installed on a '78 with the slightly smaller intake valve size(?). Who knows, but at this point I'm happy with how the bike runs.

[Patrick1980]

Ok I went down to 117.5 mains and it is a way better throttle response I still have tuned it perfectly yet with colortune and vacuum guages but I’m going to go on my firs ride on it in about 7 months to test out where I’m lacking power. I haven’t messed with the needle shims yet but if this doesn’t feel right I’ll change the height and see how that goes. I’ll give an update soon.

[DEEBS11]

Hi gtem, that last little bit of richness is caused by the wear on the sides of your needles that I showed in the photos a while back but you have effectively compensated for that issue with the mixture screws.

[Patrick1980]

So I took it for a ride and it went pretty well for no tuning done I was able to rev up easy but letting off the throttle I got some backfire couldn’t tell which side so I think I’ll just use the colortune and try and get rid of that with the mixture screws. I did get some gas leaking out of the njmber 4 carb and I’m not sure why because I triple checked float heights and the little rubber needles are clean. I’m hoping it was just a random Hangup that will sort itself out next time.

[bikerphil]

I believe the light orange color is what you want on the colortune when adjusting the idle mixture, many people who used it have said that. The bunsen blue will be too lean for our engines.

You know you can use the sync gauges, looking at them closely while turning the mixture screw in/out to achieve the highest vacuum. That would be real close to the sweet spot.

[Patrick1980]

So I keep getting gas coming out of the carbs and it’s quiet annoying. I’m looking in there and it looks like gas is coming up through the emulsion tubs past the needle with the bike off. I’m thinking this is also why I am running so rich at idle. Does this mean I need to lower the needle in the slide so it sits further down the tube? Or do I need new needles? I know the floats are set to the right height as well.

[LAB3]

To me gas flowing out of the carbs says either float height or float needle not closing off the fuel supply.

[DiverRay]

Have you replaced the "O" rings in the seats for the floats? If they are leaking, there is no flow control! I always "bench test" carbs before putting them back on the bike. Put the rack upside down, bowls removed, in a vise, and keep it as level as possible. Hook up lines and run water or very thin oil into the carbs, NO GAS! If it's leaking, the gas could go everyplace and be a fire hazzard. Elevate the tank you are using to supply the carbs about 2' above them. Watch for two minutes, there should be NOTHING on the inside of the carbs.

[Patrick1980]

Yea I always thought that too but I just checked float heights yesterday all within half a millimeter of 23mm. I’m doing all this on the stock center stand and my forks are 2 inches lower. Would this angle change be enough that the carbs would leak with that float height? That’s the only thing I can really think of.

[DiverRay]

put a 2X4 under the front wheel while your working on it! That will get you much closer to "normal" and take one less thing out of the equation.

[cajun31]

I believe the light orange color is what you want on the colortune when adjusting the idle mixture, many people who used it have said that. The bunsen blue will be too lean for our engines.

You know you can use the sync gauges, looking at them closely while turning the mixture screw in/out to achieve the highest vacuum. That would be real close to the sweet spot.

Ditto on the slight orange color with the colortune. Blue recommended is too lean.

Also, the OP stated popping on decel. That is generally caused by exhaust gasket leak.

[Crusty Snippets]

I have only just recently removed the carbs from my 79 Special, and a newer Standard, and a newer MNS. My 79 had always run fine, but I ran it out of gas, and got crud inside the carbs. Back on, clean and running, no air filter, just using the rubber tubes. 79 has the brass floats. They can stick when filling from empty. I will study the needles a bit closer next time. A tap with a hammer fixes the sticking. 79 has 140 main jets, 4 into 1 pipes. Usually runs sweet. Newer carbs have 110 main jets and the plastic floats. One set is clean, the other very gungy. Will do more cleaning of the 79, and fire the newer MNS in a few weeks.