Kz750 twin carb / throttle problems

Hello, So I was riding my Kz750 twin up and down the road testing out the new kenda tires I bought when all of a sudden my bike doesn't want to go. I would be in 2nd or 3rd { any gear really} and it would bog down every time I touched the throttle. It would do this until it shuts off . Then I go to turn it on and when I get it on it revs up at around 4 rpm and stays there until I turn the idle down. I read that the high auto rev could be throttle cables so I am going to see if I can adjust them even though they don't have any sharp bends in them . As for the bogging and it getting touchy and shutting off while I'm doing 40-50mph , I have no clue. Is there a magic spot I have to find on the idle screw to fix these problems? Also I am running 1.5 turns out on the tiny screw on the side ( which is stock ). I also have a shorty muffler which is not to shy from running the pipes straight. Just for additional information, My bike doesn't like to idle for more than 3 minutes unless the idle rpm is at atleast the 2. Anything below and it wont be long before it cuts off.

Schmeck wrote: Hello, So I was riding my Kz750 twin up and down the road testing out the new kenda tires I bought when all of a sudden my bike doesn't want to go. I would be in 2nd or 3rd { any gear really} and it would bog down every time I touched the throttle. It would do this until it shuts off . Then I go to turn it on and when I get it on it revs up at around 4 rpm and stays there until I turn the idle down. I read that the high auto rev could be throttle cables so I am going to see if I can adjust them even though they don't have any sharp bends in them . As for the bogging and it getting touchy and shutting off while I'm doing 40-50mph , I have no clue. Is there a magic spot I have to find on the idle screw to fix these problems? Also I am running 1.5 turns out on the tiny screw on the side ( which is stock ). I also have a shorty muffler which is not to shy from running the pipes straight. Just for additional information, My bike doesn't like to idle for more than 3 minutes unless the idle rpm is at atleast the 2. Anything below and it wont be long before it cuts off.

The bogging down and losing power sounds like you have crud filtering down from the fuel tank blocking amain jet. been there, done that.

The "high RPM hang up" is usually a vacuum leak problem if not a binding throttle cable.

Running straight pipes means it is probably running way too lean.

I have a 2 into 1 with a shorty baffle insert. I probably should be running a bigger muffler but these shorty exhausts always sound nice to me. As for the carbs having crud in them I had the carbs cleaned and soaked, and the tank was cleaned with acid and sealed. I'm also running a filter on my gas line. I was told the low end power loss was from a lack of back pressure in the muffler. I doubt there is anything clogging the jets but I wont rule that out cause it happens. What I don't understand is that it isn't always bogging down. Most of the time it rides great. I know the little screw on the side { mixture I think) is set at 1.5 turns out stock. Are there any changes I should make to my jets/ screws to accommodate the shorty muffler? I am unsure of how many turns and on what screws/jets. Btw I will hit the carb boots with wd-40 tomorrow and check for any leaks , thanks

When you test for carb holder leaks the idle speed must temporarily be set VERY low (less than 1000 rpm). This will ensure the slides are closed (or nearly closed) which in turn raises the vacuum level in the carb holders (because the engine is sucking air but not much can come through the closed slides). The combination of the increased vacuum at the carb holders and the low idle will amplify the affect of a carb holder leak and will cause the carb cleaner to stall the engine or at least affect the idle if a carb holder is leaking.

If at all possible, use carb cleaner rather than WD40. It will definitely reveal carb holder leaks and will not leave an oily mess all over your engine. Ed

Okay will do , thanks Ed. Is there anything wrong with a bike idling at 2000 rpm? In my manual it says 1000 rpm but that is with stock muffler and I am running a shorty .

What did you use to seal the tank with? Yours wouldn't be the first one I've seen that sloughed someof the liner off plugging the petcock and filter. Paul

I washed the tank out and sealed it with alittle motor oil. Filled it up with gas and then put alittle seafoam in the gas just incase. I used muriatic acid (sp) to clean the tank. I wanted to use phosphoric acid but the shops near me didnt carry it unless u were willing to spend money on krud cutter

My main jet is a 125 and the slow jet is a 45. Should I go up in jet size because of my exhaust . My bike lags ever slightly on low end power.

I fixed the throttle problem. I loosened the throttle cable and tightened the other cable that pulls the throttle back if Im not using it. Also ran the cables down under rather than over and around. So no more auto rev from what I can tell. For the low end power loss which screw should I switch out to fix that and what size.

Schmeck wrote: My main jet is a 125 and the slow jet is a 45. Should I go up in jet size because of my exhaust . My bike lags ever slightly on low end power.

My twin runs better with 50 pilot jets. The 125 main is standard, I have no idea if it is sufficient for the pipes you have. You may need to raise the needles a bit if you have poor throttle response at low end (?)

All of that stuff is trial and error. On mine, the needle slots are about .020" apart so a full notch is a big step. I got some small steel washers that fir the needle which are .010" thick for a half slot step.

my throttle response is great and very on point. The only issue is in 1st and 2 nd gear when I accelerate it will lag once or twice speeding up . It isnt smooth like my higher gears. I dont know if that is a fuel problem or not . Its like the bike is getting hung up in the gears but then when i hit 3rd all my problems go away.

What do you mean by raise the needles? Going through the top of my carburetor there is a ring that me nor the mechanic could get to . Is there a special tool for that ring ? My snap ring pliers dont reach it .