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Might need back pressure??

Discussion in 'The Garage' started by Jasonsblue81, Jul 18, 2023.

  1. Jasonsblue81

    Jasonsblue81 colorado Kid

    Hey guys. this is a bit tricky.. So i cleaned my carbs. everything went well on that. original jets back in. but i found out i basically had no foam filter on the screens of air filter boxes. So i rebuilt the filters. went good. look great with the green uni filter stuff. So.... I got everything back together. decided to cap both ports on my new intake boots. for now. just using a feul shut off in the line and using the prime setting on petcock. So me and kermit[ my mechanic ]. Did a air leak test with carb cleaner. all good no air leaks...Ok.. heres problem. everything is good at stop. Bike fires up. Idles . I can rev it up, and it comes back down to idle. no issue.....But when i take it out on the road.. the needles in carb wont close. gets stuck between 3 and 4 000 rpm. If its not hot i can manually bring it down by putting it in gear and use the brakes to bring the rpm down.[[[[that may not make sense.. ]]]]sorry. So here is my theory......
     
  2. Jasonsblue81

    Jasonsblue81 colorado Kid

    Since the exhaust is straight flow. [ no baffles in those little mikes xs pipes] . I think i created a suction situation with the rebuilt air filters
    air is restricted on the back side and free flow on the pipes side. so thats why i can manually bring the idle down.. what do you think.. Thats my theory guys..... need back pressure in the exhaust pipes. then needles will shut down and bike will come back down to idle. Hows that for a theory.!!!! ps ...I would have kept the original exhaust on the it was cracked and broke and eventually fell off.... thanks..
     
  3. tstidham

    tstidham XS400 Addict Top Contributor

    Inspect the rubber diaphragms carefully, there is probably a small tear or hole in one of them leaking vacuum. It could also be a really lean idle mixture or a problem with the "choke", which is just a way to bypass some vacuum to the spring side of the diaphragms to lift them and enrichen the fuel mixture.
     
    CaptChrome likes this.
  4. gpounce32768

    gpounce32768 XS400 Enthusiast

    I'd also vote for the choke/enrichener leaking fuel. Don't know what carbs your bike has but on the oem '82 carbs its a moderately fancy arrangement which bypasses fuel from the bowl into the airstream behind the butterflies.

    Decreased backpressure tends to somewhat lean out the engine but not do weird stuff like this.
     
    tstidham likes this.
  5. Jasonsblue81

    Jasonsblue81 colorado Kid

    thanks guys. somebody told me they were mikuni 34 mark 3 carbs or something like that, but anyway yea . the plugs are actually looking darker now after i did the air filters. less air coming in. The only issue i had putting the carbs back together was i had trouble getting one of diaghrams to stretch enough to sit in that little groove on top of carb. i really had to tug at it, . I think diaghrams good place to start. but i would like to put some thing in those pipes to give me some backpressure. Those little pipes are super loud. I like it but the fun of the loudness is wearing off. Any ideas on that?? think it would be good for motor too.
     
  6. tstidham

    tstidham XS400 Addict Top Contributor

    My dunstall mufflers have a removable perforated core pipe. When I got mine running it was super loud. A trip to my attic for some fiberglass insulation was all I needed to wrap the cores and quite it right down.
     
    gpounce32768 likes this.
  7. Jasonsblue81

    Jasonsblue81 colorado Kid

    ok. yea , insulation. took a long ride in hot weather today. bike is running good. pulled plugs, right side is a little lean. gonna go out half turn on the mixture screw on rt side.
     
  8. Jasonsblue81

    Jasonsblue81 colorado Kid

    if i drive it normal and down shift like i always it comes back to idle like it should and stays there,... if i pull the clutch going down the road at 60 mph it will sit at about 3000 rpm and stay there until i throw it back in gear but i dont notice it if i slow it down with the tranny like i normally drive it. I am sure there is baffle material i can get for pipes. .....but will that give me some back pressure??
     
  9. gpounce32768

    gpounce32768 XS400 Enthusiast

    Sounds like the sliders aren't closing properly, vacuum leak or maybe issues with springs or sliders binding/sticking.
     

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