Pilot jet plugs and slow revving first 1/4 turn of throutle

[Richsuz]

I know this is not a bike that is talked about here, but I have a Generic Carb problem. I am talking about a 1984 GS125. It has a single (of course) Mikuni BS26 carb. I have done a thorough carb clean, replaced the diaphragm which had a small pinhole.

The bike idles well (steady 1500 rpm). Main 105, pilot 40 and another jet under the Diaphragm is 110.

The question is why do I have low rpm response problems, once I hit 3500 rpm it picks up fast.

Also I noticed that the parts manual lists plugs on the jets, the bike does not have them and some mechanics at Suzuki say they are inconsecuencial. Is this true? what are they for?

I am getting to the point to changing the carb and getting rid of the CV which I hear is the reason for the slow response. ANY comments or suggestions welcome please....

 

[Steve]

Actually, the CV carbs were introduced to improve throttle response with larger-bore carbs.

The only ones who would really notice the "lack of response" would be racers, who want it ALL, and they want it NOW.
Most of those of us who only ride on the street (not the track) appreciate the lack of "bogging" when the throttle is suddenly opened, at least when the carb is jetted correctly.

There are three areas that I know of that might have "plugs" in a CV carb:
One is at the bottom, next to the main jet and plugs the access to the pilot fuel jet. This one MUST be installed. If you look just above where the plug should go, you will see a passage between the pilot jet well and the tube above the main jet. At idle, all your gas is coming through the pilot jet, which should be drawing through the main jet, first, because the hole below it is plugged. As throttle openings are increased, you will draw more fuel through the needle circuit, which is also drawn through the main jet. If that rubber plug is not installed over the pilot jet, the needle can also draw fuel through that cross-passage, really upsetting the mixture. As you increase throttle opening even more, the needle is out of the way and you are relying on the main jet to meter the amount of fuel. Since the rubber plug is not there, it can also draw fuel through that passage, too.

A second plug is the one that covers the idle mixture adjustment screw. This one has to be removed to take the screw out to clean the carbs properly and change the o-ring that lives under it, but aren't really needed. They might keep some water off the adjustment screw if you keep the bike outside, but other than that, they are not needed.

A third plug lives in the bottom of the throat, just above the edge of the float bowl. There is a square-section o-ring that goes between it and the bowl gasket that holds it in place. Might be plugging an access port for machining, but I can't really tell. That one must also be installed, and it should have that o-ring/spacer under it, too.

If your mechanic is telling you that you don't need those rubber plugs, you really need a new mechanic.

Why don't you apply for that position? :-k

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[79FiveFifty]

If your problem is just opening the throttle you are looking at fine tuning your pilot circuit.

When you yank the throttle open you're probably leaning out and the bike hesitates, try removing the little cover from the air screw and adjusting that 1/8 turn at a time to the right (tighter) and see if that helps. You will allow less air into the pilot circuit and thus richen the idle / low rpm mixture and should end up with better throttle response if that is the problem.

You may have to clean the carburetor also, there's no saying weather or not any debris made it's way into the fuel passage.

 

[Richsuz]

If your problem is just opening the throttle you are looking at fine tuning your pilot circuit.

When you yank the throttle open you're probably leaning out and the bike hesitates, try removing the little cover from the air screw and adjusting that 1/8 turn at a time to the right (tighter) and see if that helps. You will allow less air into the pilot circuit and thus richen the idle / low rpm mixture and should end up with better throttle response if that is the problem.

You may have to clean the carburetor also, there's no saying weather or not any debris made it's way into the fuel passage.

I figured it is the pilot circuit causing the problem. Do you think I need to change the jet instead if so bigger or smaller? You talk about adjusting the AIR screw, I assume we are talking about the mixture screw? I have played with that. BTW, it has no plug, nor it looks like any plugs anywhere. The other problem I have is that I cannot get any info or x reference for this mikuni carb, anywhere. BS26 is used on a number of bikes, and they all seem a little different.

All the passages were cleaned as well.

 

[Steve]

When you yank the throttle open you're probably leaning out and the bike hesitates, try removing the little cover from the air screw and adjusting that 1/8 turn at a time to the right (tighter) and see if that helps. You will allow less air into the pilot circuit and thus richen the idle / low rpm mixture and should end up with better throttle response if that is the problem.

That might work if he had a VM carb, but the BS carb (sometimes called a "CV" carb) on his bike does not work that way.

The BS carbs have a single "idle mixture adjustment screw", not separate fuel and air screws. This single screw controls how much of a mixture is admitted to the cylinder. To richen the overall mixture, turn that screw OUT, not IN.

A good starting point for that screw adjustment is three full turns out from lightly seated. Warm up the bike then slowly adjust that screw, listening for highest engine speed. If you find a very broad range with no increase in speed, turn the screw IN slowly until the engine speed drops a bit. Back the screw out 1/4 and turn the bike off. Turn the screw IN, counting the turns, until it is lightly seated then put it back where it was. Record this number for future reference.

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[Dave8338]

...and keep in mind that these settings are good estimates however, every bike and carb seem to be a little different so use the suggested settings as a baseling, NOT your final tune. :-\\\

 

[Steve]

...and keep in mind that these settings are good estimates however, every bike and carb seem to be a little different so use the suggested settings as a baseling, NOT your final tune. :-\\\

Yep, that's why I mentioned "starting point", then went on to tell him how to fine-tune.

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[Richsuz]

Great. I will try that later today. One last thing, don't mess with jet sizes at all, right? It is just that, the gn125 was originally the same as the gs125, but for some reason the carb setups and carbs have changed since 1984, I imagine it was for a reason. The gn's seem a lot peppier I can only imagine is a carburation difference.

 

[Richsuz]

So I went out and followed Steve's advise. The iddle did not change even when the mix screw was all the way in. Does this mean wrong size pilot jet?

 

[Steve]

No, it means that someone might have twisted it in a little too tight and broken the tip off.

To find out for sure, you will have to remove the carburetor. When you have it off, feel inside the top of the outlet of the carb to see if you can feel the tip of the needle sticking into the carb throat. If you can, remove the adjustment screw, along with its spring, metal washer and o-ring. I use the barrel of a Phillips screwdriver to roll over the broken tip and usually manage to force it back up. Tip the carb over (watch out for gas in the float bowl, unless you have emptied it), dump out the broken tip. Get a new screw and put it back together.

Second possibility: If you don't feel the tip of a broken screw, the small passages that feed gas to the adjustment screw might be plugged. In that case, the carb needs to be stripped and dipped, then re-assembled with new o-rings.

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[Richsuz]

Thanks for all the advice gents. I was not able to get the carb to work. So I got a hold of a GN125H carb. A MIKUNI BS26SS (I wish I could find a place that explains the differences between al the MIKUNI variations of the same carbs). The bike now idles, and works well. Great for in town errands. Apparently it is a common fix for a design flaw to change the carb.