The motorcycle starts without issues even when cold.

It functions normally at any position of the choke.

When it is still cold and you try to open the throttle, it bogs like on the video (see YouTube link below).

When it is warmed up after some minutes the problem is getting smaller. I can open the throttle, drive, but it bogs sometimes on the road unexpectedly. At the moment that it bogs on the road, if I manage to open the throttle excessively, and if the rpm manages to get high, then it stays there with no issues for some time, but it can still bog at one point (highway drive).

I noticed the problem for the first time while I was on the highway, where it started to bog at some points, but nothing serious. As time went by, it became undrivable.

Here is a video of the issue that I uploaded on YouTube


I checked and confirmed that the following work as they should:

  • the air flow

  • fuel flow

  • Sparkplugs

If you have other ideas, please let me know.

  • definitely some sort of fuel issue, was the bike being driven before? was it fine? if not start by changing the fuel. Also change fuel filter.
    – DatsunZ1
    May 8, 2020 at 12:41
  • "It functions normally on any position of the choke." Do you mean when cold? If so, that might suggest the choke is not fully deactivated. Don't overlook that the choke control also sets the minimum throttle opening which is what seems to be happening when you operate it in the video. If the engine is still partially choked when hot and riding that might explain it too - mixture too rich. Is the exhaust sooty, are the spark plugs? May 8, 2020 at 16:56

1 Answer 1


There are a few things worth checking in the Mikuni carb on a GS5 that can cause odd problems. First would be the floats and whether they are sticking open at all causing it to over-fuel, especially after its been idling doing nothing and not drawing a lot of fuel or cruising and not using fuel at the rate its filling the float bowls.

You can check this by running a length of hose to the carb overflow bowl setting the petcock to prime and loosening the drain screw.Mikuni GS float height You want the fuel level to get to about the point the bowl joins the carb body. In the picture it is slightly high - if you can make out the level in the blue pipe - its about 5mm above the lowest finger. If the fuel level continues to rise with the carb on prime the fuel is overflowing the float bowl. You can check this without removing anything although access isn't great. If the floats are overflowing they you'll need to pull the carb and investigate why - might just be grit in the valve seat or the float valve itself might be worn out.

Something else worth checking is the slide diaphragms. These can pinhole and tear causing odd conditions. enter image description here After removing the tank you'll be able to access the slides in the carb (you don't need to remove the airbox.) Under the black caps pictured (two screws but look out for the tiny o-ring under the vacuum take off point on each side!) are the slides + diaphragms.

They shouldn't be pinched or have any holes and should still be relatively supple. When you have them out (they just pull out complete with needle) you can also check the condition of the jet needle - if it is worn (shouldn't be) it can cause fuelling problems at different points in the rev range. Also check for cracks in the cap. Note - the top-cap screws on the carb are JIS type and round out easily if you use a phillips type driver. I replaced mine with socket cap screws as they had be rounded out in the past (off the top of my head they are M5 14mm but I'm not certain).

Without removing the airbox you'll be unable to check the action of the slides - they should move up and down without binding and there should be resistance up and a sort of slurping noise when they fall, they shouldn't snap shut - they should slide down in a controlled manner and both sides should move at the same speed. Make sure when you reinstall the caps you aren't pinching the rubbers.

The choke might be sticking on but I think that would be causing problems throughout the rev range. Proper choke function is that the bikes idles at about 5000rpm with the choke fully on. The chokes on these carbs are on their own circuit and they don't alter the throttle position. It'd also stink of fuel from running rich the whole time.

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