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Won't start after carb clean

Discussion in 'The Garage' started by dannyTheBoat, Jul 17, 2019.

  1. dannyTheBoat

    dannyTheBoat XS400 Enthusiast

    I finally got back into the carbs. My last problem was from the choke. I noticed when I took the carbs off that it was 1/4 open. Last time, I tightened up the linkage because it wouldn't stay open when I wanted. It seems that it was getting stuck 1/4 open. Great example of creating a medium sized problem while trying to fix small one. :doh:

    While I was at it I finally got the bad pilot screw out and brushed/cleaned the threads. I also checked my float height. It turns out that I was measuring the wrong way and my floats where at 26.7 mm with the carb upside-down. This corresponds to many threads here. I was measuring with the floats hanging down away from the flange last time and I got 32 mm which is what the Haynes manual says. Haynes doesn't really specify how to measure, so I wonder if this is the source of discrepancy between the two numbers.

    I synced the carbs with feeler gauges this time. I know vacuum syncing is better but I couldn't find the tool easily and I wanted to get some type of sync done during this job. A friend did offer to lend me the tool recently so that's coming soon. As a stop gap I took these steps after the feeler gauge sync:

    1) tune pilot screws for an easy start and good idle at operating temp.
    2) run the bike on the street at high load for 10 min or so
    3) pull the plugs to check mix, observing if one is leaner/richer than the other
    4) adjust sync accordingly
    5) clean plugs
    6) repeat until plugs both come out clean after running at load.

    I came up with this after observing that one plug looked perfect while the other was lean and I wanted to see if this process gave any results. After doing this twice I was able to get both to come out looking perfect after the 10 min ride. However, I did a long ride yesterday and they no longer looked quite as perfect, probably due to the different load conditions and different air temperature. Does this sound dumb or is my logic sound?

    At any rate, the bike is running well and starts easily again. I't so nice to be able to tune the pilot circuit correctly. I took her on a 5 hr, 150 mile ride yesterday and she ran great! not perfect yet but running strong.

    Much appreciation for the feedback and help I have received here. :cheers:
  2. xschris

    xschris XS400 Guru Top Contributor

    With black foam floats and the carbs upside down without the bowl gasket they should be set to 22mm +-1mm at the tallest point. Brass floats 26mm+- 1mm. Trust me!! I have done dozens of carbs for these bikes.
    buztabuzt likes this.
  3. dannyTheBoat

    dannyTheBoat XS400 Enthusiast

    Thanks man! I trust your experience. I'm at 26.7 to the flange surface with brass floats, carbs upside-down. :rock:

    I think it was one of your posts on another thread that clued me in to this spec.
    buztabuzt likes this.
  4. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    How was the bike set up on the 150 mile ride? Keep in mind that if the idle screw port was damaged, you may have to set the carbs differently to get an equal mix. Using a manometer is essential for a nice even idle, but using the feelers will get you close and when youre on throttle it it will wont really matter as much. I wouldnt feel confident about the ride personally.

    Once you get you hands on a manometer, I would probably double check the valves and make sure everything is equal.

    I would probably go longer than 10 minutes to get the bike warmed up. 45 minutes should do it and mix up your riding.

    I am too paranoid about ripping the threads out of the soft aluminum head to do a proper plug chop, even though I use anti seize. One of the plug holes on my bike feels like someone started a cross thread and backed it out early enough. This is why I hand tighten plugs first and that means the bike has to be cool. If you have a nice bright white mini LED youd be able to get a good look down the plugs. You should be able to tell just by riding it and how the engine sounds. When it feels right, run it. Just make sure you arent too lean lol. Modern fuels will leave the plugs looking really clean for a while too. My plugs usually look slightly ashy/grey.
  5. dannyTheBoat

    dannyTheBoat XS400 Enthusiast

    To be honest I wasn't super confident about that long ride but she held up. It had been planned for a while so I didn't want to drop out. I did that work around carb sync the day before the ride and got a 30 mile tester ride in. The LHS idol screw was 3 turns out and the RHS 3 1/2 turns out, needle 3rd notch from the top. You can see the intake and exhaust in earlier posts. The ride started in nice cool coastal air and she was feeling great. Eventually we turned inland and it got hot. I could tell the poor this was struggling a bit around mile 100. She had a hard time breaking 7k rpm at full throttle. After the ride the LHS plug was that nice grey color while the RHS was black. I'm gonna turn in the RHS idle screw 1/2 turn tomorrow and tweek the sync screw 1/4 turn in favor of the LHS before riding to work and see how the plugs look.

    I'm also running fuel stabilizer if that matters.

    So if I understand correctly, the sync matters more at idle and low throttle. I didn't realize that, but it makes sense. My idol is better but still a bit choppy at times. This info makes realize that a perfect carb sync will help me fine tune the idle screws. I have also been playing with the main jet sizes and it seems I have to choose between a powerful low end and high end. Maybe the pilot jet could use a size up?

    I hear you on the spark plugs. one of mine wobbles a bit when I thread it out. I probably won't insert them hot any more. Checking valves will be my next mini project now that the carburetors are stable.
  6. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Any sort of fuel additive would skew your readings. An engine that runs well when its cool, and performance degrades as it reaches operating temps tells me that the carbs are set too rich. With CV carbs, you tune them starting from full throttle performance and work your way down to the other settings. The bike needs to be fully warmed up to get accurate results. This means that the bike may not have a nice even idle or have great performance when you first start off and its cold. My bike will barely hold an idle for the first 5-10 minutes... Thats just the way it is with carbs sometimes. If you can get your bike to start when its cold and not have to use the enricher/choke, your pilot is too big or set too rich. Anything more than 3-3.5 turns out is borderline to move up in pilot jet. I havent had to change my pilot jets but I run XS360 carbs and they run rich.

    With the idle circuit, you turn the mix screws in and make sure the bike stalls, if it doesnt stall, pilots are too big. Screw them in until bike starts to stall, stop. Screw them out until the bike starts to run like crap, then set the mix screw right in the middle of both those positions and then you can adjust idle speed if needed.

    You are tuning the bike to run well at operating temps. Youre probably too rich. Leave the other settings stock while you find what main jet you need. Heres a rough guide for you to follow, I used it when I first started out on my XS and I still occasionally use it for reference: http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

    Its a real pain to change only one thing at a time with carbs, but its the best way until you really know what youre doing. It took me all summer to get my bike dialed in. You should be able to narrow it down to 2 jet sizes that will be acceptable.
  7. dannyTheBoat

    dannyTheBoat XS400 Enthusiast

    Wow that's very detailed, exactly what I needed. Thanks a bunch. Next free day I have I'll run through this whole process and see how it turns out.

    One question comes to mind though. If I do all this tuning at 70F and it gets really hot for a week, say 90F, does all that tuning work have to be repeated? Hotter air has less O2 right?

    Also, how the hell do you reach the LHS idle screw with the gas tank on? My petcock is in the way so I have to reach through the other side. My hand can barely fit with a glove on and I have to use a tiny screw tip. It sucks. Any tricks?
    Last edited: Aug 19, 2019
  8. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Average riding temps for me around her are 70-90F riding at sea level, which is what I tuned to. Ive ridden in 35F to over 100F on the highway on the same tune. Yea the bike gets a little funky at high temps when youre in stop and go, I avoid when I can. I go light on the throttle when its very hot. I run 20w50 4T Castrol Go conventional oil and it does well for me. Humidity makes the bike idle a little funky too.

    Cant help you on the tricks, idle mix screws on 360 carbs are located on the side of the carb bodies. I usually carry a little 2 in 1 pen screwdriver with me for quick adjustments at a light. I can practically do it without even looking..

    You might be able to leave the gastank unsecured and get a longer fuel line so you can lift the tank up and work a small flat head in there.

    I have a special tool called a ColorTune. Its not needed at all, but it will help you make sure that both sides have a proper idle mix.
  9. dannyTheBoat

    dannyTheBoat XS400 Enthusiast

    Well I spent some time tuning the carbs. The bike was running really well. Last weekend I went on a ride around town and my LHS cylinder started burning oil at very low throttle. It looks like what was a minor intermittent valve seal leak has gotten more serious. I contemplated doing a quick valve seal job, but I'm tired of doing these small repairs just to have something else go wrong. CA rainy season is coming up so I think I'm just going to get started on a major project. An electronics overhaul and a top end re-build are in order.

    I'll start a new thread for the top end re-build. Thanks for everyone's help on this thread with my fuel problems. I have certainly learned a lot.
  10. kebabs

    kebabs XS400 Addict

    hi .did you ever get your bike running good ?

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