Evan Fell Motorcycle Works

My name is Evan Fell and my hobby is motorcycle restoration, customization, and riding. This is a blog about my bikes.

1986 Yamaha TT350

Posted on February 23, 2008 | Posted by | 6 Comments

These old Yamahas are built like a tank. They are very similar to Honda XR400’s in both size, durability, and longevity. As long as you keep them in good condition they will run forever and never let you down. A single cylinder air cooled bike is in my opinion the ultimate all around off road machine.

What interests me about these bikes is their use of dual carburetors. Dual carburetors were added to single cylinder bikes during a transitional period when 4 valve cylinders began being used. Engineers needed to find a way to better control and atomize the fuel going in through the dual intake ports. At the time they had difficulty accomplishing this task using existing single carburetor technology.

What surprises me is that these motors did not use the YICS technology that Yamaha incorporated into it’s streebikes of the early 80’s. The Yamaha Induction Control System was essentially a tube which internally connected all of the cylinder intake ports to balance vacuum pressure and eliminate symptoms of slightly unsynchronized carburetors and even out combustion across 4 cylinder engines.

It seems to me that the best way to approach intake development of a single cylinder motor is to do the same thing. Think of each intake as its own cylinder and balance intake pressures internally prior to combustion. Today’s modern 4 stroke singles do this.

The funny thing is, YICS technology is no longer used on multi cylinder engines. Advancements in carburetor technology as well as fuel injection systems (obviously) counter the need for internal intake balancing. The YICS system also made carburetor adjustment difficult. However, I’ve owned lots of YICS motors and have tuned them to perfection despite the difficulty.

Comments

6 Responses to “1986 Yamaha TT350”

  1. Martin Elliott
    October 22nd, 2008 @ 4:58 am

    Interesting Blog. I have a 96 TT350 and I haven’t been able to get it running well from the day I bought it. It bogs down as soon as it gets warm and I am pretty sure it is overheating. I have tried everything and replaced nearly every carby part. It is running a Staintune muffler which I am pretty sure affects the vaccum in the secondry carby. Jets: 42 pilot, 122main, 125main on secondry.
    I have spent a small fortune on this bike in the past two years trying to get it to run well. Today I even drilled out the main jet on the primary carby to 1.5mm which actually worked but will need some fine tuning.
    I realise you aren’t in the advice business but I have scoured the net, all the forums I can find and still I need a push in the right direction.
    Regards
    Martin

    [Reply]

  2. Evan Fell
    October 23rd, 2008 @ 2:14 pm

    Hi Martin,

    “It bogs down as soon as it gets warm and I am pretty sure it is overheating.”

    There are a few things that can cause this, but it is a tell tale sign of low compression. How many PSI’s is that piston pushing cold? How about warm?

    Those wacky 1.5 carb systems can be a little difficult to get setup properly but it sounds like you’ve definitely put the effort in there.

    If you compression is good and your motor is overheating at idle — a TT350 should never overheat at idle unless you’re in the Sahara — then you are running lean and/or have tight exhaust valves.

    I also assume that you’ve checked:
    - Valve clearances
    - Air leaks (from carb manifolds)
    - Gasket leaks (head in particular)
    - Carb diaphragms (I can’t recall if the TT350 carbs have rubber diaphragms or not, I think they do)
    - Vacuum lines for cracks

    I would have advised against drilling your jets. Even if you have a digital bridgeport in your garage I’d still say it’s much safer to spend the $4 and get the correct sizes from a supplier/dealer.

    I’m interested to know what your compression is. My money is on worn rings. Will your bike always have good compression when cold? Or will you have to kick it a few times to circulate oil to build up that compression?

    Good luck. Keep me posted.
    -Evan

    [Reply]

  3. andre stevens
    January 4th, 2009 @ 1:31 am

    hi there,
    i have a tt350 1991 bike goes really well but i need some help on the indicators don,t blink and are very hard tosee i was wondreing if i could put a battery in it but i need the wire sort could you sheed any light on it,with any photos of wiring and battery place ment
    cheers andre

    [Reply]

  4. michael Scott
    July 14th, 2009 @ 8:18 pm

    i have a 86 tt350 and have been have a bit of fine tuning the dual carbs on this critter have you heard any thing about replacing them with any kind of different setup or if its even worth the effort any ideas will help thanks in advance

    [Reply]

  5. David Sessions
    April 18th, 2011 @ 3:01 pm

    Would like buy rebuilt carborator for my 1986 yamaha tt350 last shop that work on it damaged it beyond repair.

    [Reply]

    Dennis Reply:
    April 23rd, 2011 at 4:21 pm

    David could you describe damaged beyond repair ?
    There isn’t much that absolutely can’t be fixed .
    http://www.kawasakimotorcycle.org/forum/members/kopcicle.html

    you will have have to register .

    [Reply]

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I have owned nearly every make and model of vintage Japanese motorcycle as well as a number of other Europeans and more modern bikes. I do everything from simple fixes to full restorations. I also travel and ride every chance I get.

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