Starving For Fuel ?

Discussion in 'Engine, Fuel and Exhaust' started by KingKahuuna, Jun 16, 2012.

  1. KingKahuuna

    KingKahuuna New Member

    Hey folks,
    I picked up a 88 softail classic. I'm the third owner so it's been a bit of exploring to see what the other two fellas did. Here is the basic bike, and the problem I have encountered.

    The engine is a 1340, It has a Mikuni Carb, and has solid lifters, and an Andrew 27 cam Approx mileage since rebuild is 30,000 miles.

    The bike has been timed correctly, the carb has the mixture screw turned out one and three quarters turns, and the idle is set up about 900 RPM.

    Upon acceleration it feels as though the fuel just isn't there in a large enough quantity. The weird part is this comes and goes. Sometimes I seem to lose the top end of the power band particularly the top of fourth and fifth. Halfway through the power band the engine is pounding hard and furious, but as I approach three quarters of the way the bike starts to bog down losing it's strength, but I do have enough going that I can maintain my speed barely. Sometimes when at a stop, upon accelerating, the bike will hammer through the 5 gears with all the horses I could ask from it, but as I said this thing comes and goes fairly fast, and it might flip flop, and run either way.

    My intentions are to check the screen in the pitcock, (fuel shut off valve). I have heard just about all the possibilities that one could imagine, but I believe it is time to ask you guys for your thoughts. Should this turn out to not be a fuel issue, could it be an air leak, and how does one check that out. ?
    Thanks Heaps for any thoughts here, and I seriously consider what I read.

    Keep the dirty side down,



    Hey Folks,

    Sorry for the double posting. One last thing that is a good question to ask is this. If I adjust the idle screw up higher, I do seem to see the top end running a little bit better when it is set higher, (as it does also affect mixture), I will get the run on upon engine shutdown. Is there a way to open the idle mixture, but keep the idle down on a Mikuni carb.?

    Last edited by a moderator: Jun 16, 2012
  2. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    The first thing that worries me is the run on after shut down i thought that modern fuels had pretty much eliminated that problem
    run on is usually caused by a build up of carbon on the piston heads as the bike gets hot the carbon glows any fuel that is in the cylinder can be ignited by the glowing carbon
    This can cause the fuel to ignite at the wrong time and the engine will run very poorly
    In the old days stripping the engine to do a decoke was at a 20k interval however Jack has posted once or twice that a good result can be had by running the engine and and spraying a fine mist of water into the carb as the water boils in the cylinder it strips a lot of the carbon off this should be a very small puff of spray
    It is also a good idea to clean the fuel tap screen and perhaps check the run and condition of the fuel line
    A rebuild of the carb may also be a good idea as modern fuels can after a while erode the effectiveness of the rubber seals and o rings and as mikuni have their carbs well supported a rebuild kit should be easy to find

  3. kdaddy

    kdaddy Active Member

    I had a 96 fatboy with the same setup. Mikuni 42 and Andrews 27. It was the best running bike I ever had. I had a K&N air filter, Screamin Eagle ignition, Hooker longshot headers and always used Champion plugs. It seemed the perfect setup. What you did not mention is if you have a free flow air filter and open exhaust. These would be a must running a factory stock Mikuni 42 on an Evo. If you do have the other Stage 2 setup components then I would think you need a Carb tuning and sounds like you might need the accelerator pump adjusted. Mikuni has complete carb tuning instructions on their website as a PDF and you can print it off. I always ran my idle jet about 2 to 2-1/2 turns out.
    My first suggestion, take it on a long ride down the interstate with a couple tanks of Techron additive. I ran mine from Nashville to Memphis down I-40 (250 miles) and give it a chance to burn off the carbon. Makes a huge difference.
  4. CK103

    CK103 Member

    I have a bit of experience with the Mikuni 42.
    First check for intake manifold leaks. You can do this by sparaying some WD40 right were each intake meets the head while the motor is running. If theres a fluctuation in RPM or it runs differently then you need to change the manifold gaskets.
    Now, on the Mikuni, there are a lot of possibilities, wrong jets or cloged jets, worn out needle or dirty or in wrong position and bad accelarator pump.
    Hope this helps. Heres a diagram of all the parts.
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