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B&S V-Twin, Nikki Carbs not adjusting!


4thought's Avatar
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08-11-04, 09:18 AM   #1  
4thought
B&S V-Twin, Nikki Carbs not adjusting!

I have an American Lawn Products 20 hp lawn mower with a B&S twin cyl. engine, model 351700. It has a twin barrel Nikki carb. Recently I replaced the fuel filter and fuel pump for the engine was "cycling". Now it doesn't cycle but still does not run real smooth.

I have a B&S service manual and have attempted to adjust the idle mixture screws in accordance with their instructions. A problem immediately surfaces. When I adjust the screws, nothing seems to happen. All the way down or all the way out, there is no noticable change -- or very little. I expected it to quit running with extreme adjustments as most engines I have dealt with do.

Something is probably clogged but if so, I'm amazed at how well it runs!

 
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08-12-04, 01:39 AM   #2  
Hello 4thought!

I believe your original and current conditions are due to a carburetor in need of cleaning. The adjustments should make noticeable changes, unless you have a limiter cap on the adjustment screw that doesn't allow more than 1/2 of a turn.


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08-12-04, 02:10 AM   #3  
4thought
Posted By: cheese Hello 4thought!

I believe your original and current conditions are due to a carburetor in need of cleaning. The adjustments should make noticeable changes, unless you have a limiter cap on the adjustment screw that doesn't allow more than 1/2 of a turn.
No, it does not have limiter caps.

It would help if I understood carburetion a bit more. What do the idle mixture screws actually do? The B&S Repair Manual I have does not show a "schematic" of the carburetor. If the screws are screwed all the way in, where does the carb get its fuel at idle?

There is one other symptom that should be mentioned: idling is not very stable. The engine tends to vary in speed at idle and is difficult to adjust with the idle speed screw.

 
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08-12-04, 10:23 AM   #4  
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I am not familiar with Nikki carbs but the theory should be the same. Mixture screws basically regulate the amount of fuel allowed into the throat to be mixed with incoming air. Under ideal conditions and no restrictions, the incoming air should be a constant, (subtle changes due to air temp, altitude, humidity yada yada....) thus the mixture screws to change the amount of fuel to achieve the perfect fuel/air ratio.
Most carbs have two main circuits, low speed and high speed. I know some Mikuni and Weber carbs have quite elaborate low speed circuits, the claim is to help throttle response, transition from low to high ..........so on.
The mixture screws when seated all the way, should restrict fuel to the point that the engine will starve.
I agree with Cheese that you are in need of a carb cleaning and unfortunatley not just a can of carb cleaner. My attempts at using NAPA carb cleaner on mikuni carbs has been hit and miss. Most motorcycle shops that do alot of carbs have elaborate setups for cleaning carbs. Steam baths, ultrasonic chemicals........
Other things to look at or check, your mixture screws are basically a needle valve, and shaped or tapered for a specific seat. While its not likely for them to wear from normal use, they can be damaged and alot of high end carb kits will come with new ones.
Also have a look at your butterfly, look in the throat of the carb and actuate the throttle, (make sure you look at the throttle valve and not the choke plate) on one side or the other should be a flat, notch or something similar that will rest over a small hole in the throat of the carb. If the valve is restricted or damaged here it can effect idle.
Well enuff of Carbs .101 , a good cleaning and kit should get ya going tho

 
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