So I'm at a bit of a loss about what the problem could be

A friend of mine has a suzuki s40, 2008 i believe

The bike starts and idles fine but when accelerating it sputters and won't
accelerate beyond 15 miles an hour

We've cleaned the carbs with the exception of the accelerator pump? or maybe it's a decelerator pump, it's got 3 stuck philips head screws holding it in
I didn't have my impact driver on me at the time so that will have to be cleaned within the next few days

all ports and jets have been cleaned

The petcock has been tested, somewhat, the fuel will run from the line when it is in the prime position, it will also run in the on position and reserve (checked by disconnecting the vacuum line from the petcock while idling and it died)

we also pulled the petcock off and verified there wasn't anything blocking it, clean as could be

Nothing is blocking any air ports including the one in the front of the carb, the port that lets air into the upper diaphragm, I know sometimes aftermarket air filter housings can block them

The bowl float moves freely as well as the needle that the floats actuate

The upper diaphragm area has been cleaned and i cannot see any holes or anything in the upper diaphragm

We also checked the spark plug and verified it worked as well as the wire
I also checked the voltage at the battery thinking maybe it could be a charging issue with the battery voltage dropping but that checks out too

I disconnected the decompression cable thinking maybe the valve was stuck open, nada

double checked all the lines from the carbs and tank
the tank vent line is clear
the the vacuum line from the carb to the petcock is working and is pulling fuel into the carb
I verified the fuel flow by opening the carb drain bolt, gas came out so I know that is working
i also tried to run it with the gas cap off in case there was a vacuum issue with it, nothing


I'm not sure what else it could be, it was running fine until it was parked for a few weeks, had a little rain that probably got under the bike cover but it wasn't like soaked

I assume if the rain did something to the wires then there would be a blown fuse or something

I also pulled the exhaust to check the gasket, that was good as well

Could this accelerator pump be related to the issue?
Or is the piece on the side of the carb a decelerator pump?

To me it feels like the carb slide isn't reacting the way it should, almost as if it had a hole in it but i can't see one

The diaphragm behind the 3 screws is the TEV (Tranient Enrichment Valve), it is supposed to provide additional fuel when decelerating to help prevent backfiring noise.

Have you tried riding the bike with the petcock in the prime position?  The petcock diaphragm can become stiff as it ages, and fail to allow fuel flow at speed as the engine vacuum decreases when the throttle is opened.

Have you checked the battery/charging system for voltage?   A weak ignition system can cause these kind of symptoms.

When you had the carb apart....did you take the needle out of the slide....and put the spring and plate back properly?  The spring needs to be on the top of the needle pushing it down, and the plate needs to be installed so the holes in the bottom of the slide are not blocked.

Did you remove the float seat from the carb body and clean the screen on top of the brass seat assembly?

The brass plate also has to be positioned ,with the dimple (right next to the needle) protruding down into the slide-(which should be clean and free of even fingerprints) and you should be able to see light through the two tiny vacuum ports in the bottom of the slide.
     In the bowl, besides what Dave and Youzguyz  said, you should check to see if the washer is on the main jet , it holds the needle jet body tightly up into the carb body and MUST be in place. you should also set the float level last before closing up .

I had the exact same symptoms but it was my fault since I had taken the top off the carb and got the slide spring cocked or something.

It sounds like you had the problem before you touched the carb so it is probably not what happened to me. Just a thought.

Dave wrote on 02/12/19 at 07:07:41:


Thank you for clarifying that, on my yamaha road star it is labeled as a deceleration pump and it does the same thing

Dave wrote on 02/12/19 at 07:07:41:


Yes I have tried riding it in all 3 positions with the same results each time

Dave wrote on 02/12/19 at 07:07:41:

Yes i've checked it while running, twisting the throttle does not effect the voltage

Dave wrote on 02/12/19 at 07:07:41:

When I had the carb apart I didn't end up taking the needle out of the slide, i just removed the diaphragm /slide assembly along with the needle and sprayed the needle and slide body off with carb cleaner, my friend who owns the bike had just cleaned his carbs out a few months prior so when I go check it out tomorrow I will take a look at the needle/slide assembly

Dave wrote on 02/12/19 at 07:07:41:


I didn't end up taking the float needle out i just verified the needle was moving up and down, i didn't realize there was a screen on the top of the seat I just assumed since the bowl was filling/ able to idle that it had sufficient gas flow

How would i go about testing the amount of gas coming out per minute for the on and reserve position of the petcock?

As for testing the flow rate per minute on the carb, I think  would I just leave the petcock on prime, allow the carb bowl to fill up, then after it is filled open the drain screw on the carb and measure how much pours out in ten seconds, then multiply that by 6?
Giving me the flow rate per minute?

I will take a look at the float seat and screen when i go look at it tomorrow



I need to talk him into just getting a gravity feed petcock, seems much easier to deal with

youzguyz wrote on 02/12/19 at 07:59:13:


That sounds like it could be the issue, It hadn't occurred to me that it could simply be getting stuck

What would you recommend for cleaning the top rubber portion?
I had just wiped it off gently with a paper towel but there is a good chance it could be dirty

I will try to clean that and check the needle and slide tomorrow

justin_o_guy2 wrote on 02/12/19 at 11:08:46:


That was my first thought and I had him dump the old gas from the tank and the petcock and get new stuff in there, no luck with it though  

  OMG!     A velocity stack is terrible !!!!!!     ( even a cone filter-which isn't great- would be better) it will never work with a main jet as small as the stock 147.5 and the stock 52.5 Idle jet,  the bike is running dangerously lean !!!!!!  It's no wonder it stalls when the throttle is open! You'll need to get a proper filter , do the spacer mod and go to much larger jets , and I'd hate to ask what you're running for a muffler , (but I hope it's not a straight pipe) you should tell us as that also affects jetting.