Hi guys,

I've hit a bit of a dead-end with troubleshooting a surging issue with my S40. I was wondering if anyone could shed some light or point me in the right direction. I'm working through the basic mods on the site but just can't nail this surging thing. The bike is a couple of years old and has done 4800 km (2990 miles)

I installed a Dyna muffler and experimented with the jets until I settled on 52.5/152.5 and spacer at 50% (2 brass washers). I had a spell of about a week where it ran really well - smooth and strong all the way through the throttle, and impressive top speed. It was a joy to ride.

Then one day I ran the fuel too low and had to switch to reserve (on the stock petcock). I had a raptor on order at the time because my vac line was leaking fuel. When I filled her up and took off, the surging started again. I have since installed the raptor. The bike no longer smells like fuel when stationary but the surging continues at low speeds in low gears.

In desperation, I upped the pilot jet to 55 and reduced the spacer to 25% in an attempt to resolve the problem. The surging continued and the performance dropped at the wide end of the throttle, and I couldn't hit the same top speed. It also felt like it was running rich, especially at open throttle. So I think I've hit the limit of the jets and will return to 52.5/152.5 and 50% spacer.

A couple of things that might be relevant:

1: I am running the stock air filter

2: Whenever I take the bottom of the carb off there is a tiny amount of black "dust" settled in the lowest point of the float bowl.

3: The washers I used for the spacers where originally too big on the outside diameter. I filed them down to fit. They now actually have a smaller OD than the original spacer.

Any ideas on what might be going on?

Lean mixture or a vaccuum leak

Steve H wrote on 02/02/17 at 04:19:32:


Idle fuel mixture screw............

Also, if you fiddled with the plunger that holds the needle (and you had to in order to adjust the needle) you need to make sure that:

a) you put the metal tab back in there properly with the holes lined up and if I recall correctly, the little dimple down. You should be able to see directly through the bottom through the two little holes once it is screwed in.

b) you didn't damage the plunger. If you cleaned it with anything caustic it could be the little debris that you see floating in the bowl, but I doubt it as I am not sure how that kid of debris could get down in the bowl.

c) make sure your plunger didn't get marred on the sides. If you put it in a vise or something like that to hold it wile you worked on it, that is a  bad idea as any little marring will create a catch point and not allow the slide to "slide" smoothly. If it can't slide up properly when you nail the throttle, it isn't letting enough fuel in.

d) as others noted, make sure you don't have crap in your tank or failing fuel line, etc.  If you never get to reserve you may never be getting down to the bottom of the tank and getting any of the debris that may be settled down there and when you hit reserve recently you picked some of it up.

Good luck!

Thanks heaps for the responses guys. There are a couple of avenues in there that sound promising.

Dave, I'll check the air intake. The vac cap is brand new and fits very snug. The fuel line idea is really interesting. The dust in the carb is definitely black. I reckon it could be little bits of black rubber from the fuel line.

Ruttly, I'm pretty sure about the fuel mix in terms of the jets. So that does lean it toward an issue with air.

Dave, I didn't know there was an o-ring on the idle mix screw. I've never had it all the way out before. I'll keep that in mind.

Gary, I've already changed to the raptor. I didn't inspect the fuel thoroughly, but it was fresh fuel and I have a pretty fine filter on the funnel I use when filling the bike. However I'll check the fuel as part of the torubleshooting. When you say "spray the carb", what do you mean? What do I spray it with?

Stew, that's a really good point about the diaphragm tube. I didn't damage it, but due to my inexperience, I didn't clean particularly carefully. I didn't even realise unti now that that tube slides up and down. I'll take care of that. As for the diaphragm itself, I wass very careful in getting that out (and in putting it back in). I also made sure the rubber was seated properly between the top of the carb and the carb body. That piece goes in and out very easily.

Justin, Yeah, I was thinking the same as you. The jets were working well for a week. It went off again after I ran on reserve. I figured it probably wasn't the jets but just wanted to try something (I had been meaning to test higher jets anyway, as part of my learning). As I mentioned I hit the upper level and started to drop the performance. So at least I know where the limit of the jets is now  

Yes, 'surging' is subjective. It comes on in second and third gear in the 1/8 - 1/4 throttle range. Everything else runs great. It feels they way it feels when I'm riding into a moderate swirling headwind. The bike feels like it's being held up ever so slightly in short irregular bursts. Based on all my reading on here, that points to the spacer mod (which I have done, and had working). I think you're correct - it's something else.

As for the washer OD size, I haven't measured them but they are definitely smaller than the spacer, and slightly irregular circumference. I thought maybe that was creating some sideways play in the needle.

IslandRoad wrote on 02/02/17 at 12:53:08:

Gary When you say "spray the carb", what do you mean? What do I spray it with?

Use a standard carb cleaner. In the US we have a product called Gum-Out

I don't know what "white tube" you are talking about?

I have never seen that tube in any carb I have worked on....must be a non-US thingy.

I would most likely mount it at the top of the spring so that it is stationary - if you mounted it at the bottom it would move up/down with the slide.