1986 Yamaha TT350

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.


By ef

Hey, I'm Evan and this is one of my motorcycle sites. You can find more about me on my homepage, or visit me on Google Plus: +Evan Fell


  1. 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.


  2. 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.


  3. 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


  4. 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


  5. Ive had a few problems lately with my 89 model TT 350 which ive now sorted, and what I found might help others.
    The bike would splutter and misfire badly once the engine was put under any sort of load above 1/2 throttle. I cleaned out and adjusted the carby to factory specs, which made absolutely no difference.I was just gonna get a rebuilt carby for it, but I was also having trouble with the indicators not flashing properly and were very dim when they did work. I ended up going through all the wiring on the bike, cleaning every electrical connection, including the main frame earth. I fitted a new battery and indicator relay-(a genuine yamaha indictor relay, I found that the the cheap ones I tried were all rubbish), and I found that the misfire was completely gone when I started and rode it, which was a huge suprise to me, as I was sure that the misfire was carby related, it also fixed the indictator problem, which are now fairly bright(for a 6 volt systyem), and they now flash properly. So if anyone is having problems with misfiring etc, check all the wiring connections etc, as it doesnt take much of a bad connection in the wiring to cause these bikes to run badly.


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