Dead Spot Just Off Idle

[Guest]

Over the last three days I have put over 750kms on my bike (let's call it a 650) and have come across a slight problem that I had not been aware of before. I guess mainly using the bike in the lower gears around the suburbs has disguised the problem and it only became evident on these longer trips in high speed corners.

The carbs are BS32SS models. The problem is there is a flat spot just off idle, it's not even a stumble, more of a momentary dead spot. It is most apparent when I have backed off for a corner to about 50mph and then open the throttle to power on coming out of the corner. There is a dead spot and slight hesitation until it comes "on the needle" and then has full power again.

As most will know I have done a full rebuild on this motor so the carbs are clean as a whistle. I have removed the carby tops and checked the diaphragms for holes or tears, nothing found.

I was going to try one size larger pilot jet, but then put a colourtune on the motor and tested the combustion flame colour for each cylinder and by adjusting the idle mixture screws on each cylinder was able to go from yellow flame to blue flame, which covers the full range. So I do not think changing the pilot jet will make any difference. What do you think?

Obviously I cannot change the cutaway as these slides do not have a cutaway and there are no alternatives anyway.

I have shimmed the needles by one position using the washer method as suggested by Ed (Nessism) and I have really good power when the motor is operating on the needles. If I dropped the needles down to their original position would that make the transition from pilot to needles more smooth?

Also I have two sets of springs that go in the cavity between the diaphragms and the carb tops, a larger diameter set and a smaller diameter set. Would changing these springs over possibly improve things?

I have tried the highest idle method to set up the mixture screws but do not get any change in idle speed.

I have the mixture screws at 2 1/2 turns out, 3 turns out shows the idle mixture to be too rich on the colortune instrument.

Any other suggestions appreciated.

[Nessism]

Hey Don,

I'd try setting the pilot screws open some more and see how it runs. Color tune may show rich at idle but that isn't all that important.

[tom203]

"The carbs are BS32SS models. The problem is there is a flat spot just off idle, it's not even a stumble, more of a momentary dead spot. It is most apparent when I have backed off for a corner to about 50mph and then open the throttle to power on coming out of the corner. There is a dead spot and slight hesitation until it comes "on the needle" and then has full power again."

Backed off throttle, so piston springs are pushing vacuum pistons down, then you accelerate- maybe those slinky springs are just a tad strong and not " releasing" quick enough.

[Suzuki_Don]

Thanks guys for the replies:

Ed I will try the mixture screws 3 turns out as they were at 2.5 when doing this trip. Do you find that the idle changes much when doing the "highest idle" mixture screw setup. I found the idle speed would drop when the screws were turned in and the revs came back up when the screws were turned out again.

Tom I think the other set of springs that I have are weaker ones and of a smaller diameter than the springs fitted at the present time. I will try it with the mixture screws 3 turns out and then sync the carbs while the tank is off the bike. I did a quick check with my Morgan last night and the metal rods were out a bit, but not that bad – I will try and get them a bit closer to perfect over the next couple of nights. Then if things are still no better I will replace the current springs with the weaker ones. It’s a headache having to remove the tank and set up an auxiliary fuel supply just to set the mixture screws. Don’t have to do this with the VM carbs as the mixture screws are on the sides of the carbs.

Picture below of the present size spring in my CV carbs.



Thanks again for advice given. Any other thoughts, don’t be shy.

[Suzuki_Don]

Spark Plug Colour

This was the condition of one of the spark plugs that I pulled when doing the colortune tests.

Any comments on the condition?



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[Suzuki_Don]

I have checked out the diaphragm springs that I have on hand and have realized that I have put diaphragm springs in my carbs from the GSX750 set of carbs that I have. I now remember putting them in the carbs when I rebuilt them, but this fact had slipped my mind.

So the plan is now to install the original springs into the 650 carbs. In the picture below the large springs are from the GSX750 and the small diameter springs are from the 650G carbs that are on my motor now. These smaller springs might still not be the absolute correct ones as my motor is the equivalent of a 650E and these smaller springs are off a 650G and the carbs settings of these two 650 models are quite different. Different mains, different air jets, different starter jets, and different throttle valves.

Anyway I will try the different springs and the mixture screws at 3 turns out from seated.

Opinions.





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[Tom R]

If it's not too late.... Don't let impatience get the better of you. One change at a time.

[Suzuki_Don]

Yes, that's true.