Mikuni BS34 x 2 - not same idle speed

[Guest]

I have a Suzuki GS400L which I have been taking apart over the winter making it into a cafe racer. In that proces I have put pod air filters on and added more open pipes. So far, all I have done to the carburetor (Mikuni BS34 CV) is changing the main jets. However, the problem I'm describing below, I also had before making the bike into a cafe racer - I realised it just before taking it apart, but decided not to do anything about it since I was going to rebuild the bike anyway.

So here is the problem:
I'm trying to adjust the carbs - I'll start at idle speed with at hot engine (after it has been warming up). The bike is not running equally on both sides at idle speed. When I use the main idle knob to adjust the idle speed, the idle is either perfect on the right side or on the left side. If it is perfect on the right side (about 1000 rpm), then the left side dies (when I pull the spark plug cable on the right side). If I set the left side to about 1000 rpms (after adjusting the main idle speed to fit the left side), the right side is way too high and takes the rpms to about 3-3500 rpms) - starting out on the left side alone and putting the spark plug cable on the spark plug on the right side. So I guess I have to adjust the carbs using the pilot air screw on each side, right (to begin with to see how it responds)? However, I took the two pilot air screws out (just to check them) and I realized that they are not 100% identical:





To me that seems like that could be a problem - am I right? If so, what should I do - change one of the pilot air screws or both of them?

I also noticed another thing - the throttle valves do not close 100% in closed position. It seems to be the case in both sides, but it seems worse in the right side compared to the left side. I have attached a picture from each side, but I'm not sure you can tell from the pictures. But I tried to put at flash light behind and I could easily see the light comping through at the bottom of the valve - is that a problem?

[Steve]

The difference in pilot screws (they are not "air" screws) only shows that someone has been in there before you. The upper one in the photos appears to be more-correct.

The slight opening in the throttle plates is how your engne manages to keep running at idle. If it were totally closed, the engine would die.

Apparently you are pulling plug wires and comparing the running speed on one cylinder? I guess that's one way to do it, but you do NOT adjust that by turning the pilot screw.
There is a screw in the throttle linkage between the two carbs that will synchronize the two carbs. If you have access to a sync gauge, it's even better, as you can see the vacuum levels in both cylinders at the same time. There is no practical way to use a single gauge and move it between the cylinders to do the job properly, you need to be able to see both at the same time.

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[Mugge]

Thanks, Steve
I was only testing it that way to see if the left side was running at all. I know it's not the correct way, but sometimes you do get desperate and don't think straight ;-)

I'll look for that screw you are referring to and I'll see if I can get access to a sync gauge for proper alignment between left and right side.

[Gorminrider]

Have you swapped those screws side to side? that would answer if they were the reason for difference.

[hjfisk]

The tip isn't broke of on one of them is it? Common problem if someone seated it to hard then backed it out. Look in the hole where the screws go with a light and see if you can see light inside where the throttle plate is

[Mugge]

The tip isn't broke of on one of them is it? Common problem if someone seated it to hard then backed it out. Look in the hole where the screws go with a light and see if you can see light inside where the throttle plate is

I'll check with a light, but the tip looks fine. I think the issue is with the the carb syncronization screw as Steve suggested - I just havent had time to look at it yet. Thanks.

[Steve]

This picture is part of a four-cylinder rack, but still shows the relationship of where the sync screw is.



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[Mugge]

I found the sync screw. Mine does not come with a luck but (as far as I remember), but with a spring.