Hi all, fairly new member here. I’m hoping some experienced members can lend me some guidance on assessing the bike after a muffler upgrade and re-jetting.

First some background. I have a 2014 S40. I bought it with 1500 km on the clock about two months ago. It’s now got near 3000 km on it. It is stock, although a previous owner had drilled a few holes in the exhaust. The holes had been subsequently welded (I assume a dealer traded it and needed to return the bike to original condition to sell it as a learner approved bike). The fuel mix plug had also been removed from the carb. It had been knocked over once, resulting in slightly twisted handle bars and a bent foot peg (I straightened those two things out pretty easily.)
I have no experience with mechanical projects but know my way around power tools etc. I have a decent set of tools; gear wrenches, spanners, screwdrivers, grinder, drill, etc. This bike has become my introduction to the world of motors – specifically bikes.

When I bought the bike, it had the usual little things going on that I’ve read about here on the forum; backfiring on deceleration, surging at about ¼ throttle, and the top of the header pipe starting to turn blue and gold. I adjusted the idle screw to about 1 ¾ turn out, which pretty much eliminated the backfiring. The surging was still there – pulling the choke out one notch pretty much got rid of the surging and made the bike accelerate. Overall it was responsive, had plenty of power, and cruised well on the freeway. I live pretty much at sea level and we are heading into summer, so humidity sits around 80% - 100 % now.

After doing a lot of research here on the forum I bought a pair of HD Dyna mufflers which came off a new bike (the kind with the slotted bolt rail). I used Serowbot’s method of mounting it to the bike and managed to get it installed and mounted with no exhaust leaks. (First time I tried it, it leaked, so I got a muffler shop to make an adapter, but that pushed the mounting-rail out past the bike’s mounting rail – and it still leaked). I revisited Serowbot’s approach (reading the instructions more carefully) and she worked a treat!

I learned that the bike would also need re-jetted, so I ordered some jets online based off the info on this site about stock jets. I found an Australian distributor for Mikuni. The jets were $6 each. I managed to get the float bowl off the carb without removing the carb from the bike. The stock jets were 145/52.5. I stepped them up to 150/55. So, I went two steps on the main and one step on the pilot (I basically just read tons of posts on the forum and then chose this configuration, considering my altitude, humidity etc).

As I said, I’m new to all this so I am trying to put off taking the carb out, unless I have to. Also, postponing the needle washer mod, unless I have to. I will probably have to read the plug but am not confident to get it out on the side of the rode, yet. Going one step at a time.

So, the results. I seem to need the choke on to start it now (I didn’t before). The idle mix screw works best now about a quarter to half a turn CW from where it was before. The engine almost dies at full CW rotation, and is similar at about three turns CCW. The bike still feels responsive. The surging is almost gone. What’s left of the surging probably kicks in a little further along the throttle rotation. If I travel at about 90 km/hr in fourth gear then roll-off the throttle a little, it doesn’t surge. If I pull the choke half a notch on the highway it has no effect. Backfiring is back, worse especially at slow speed in low gears. I try not to come off the throttle hard but still get some backfiring at low speeds – enough that it feels like things aren’t quite right.

My questions are:
Does this sound like I need to step up one jet size on the pilot?

or

Should I bite the bullet and do a plug reading?

And/or

Should I jump in and do the spacer mod?


I’m trying to go little steps at a time, given my inexperience. I’m not sure which step is next – or even if this all sounds pretty good, and I should just stop here.

I would really appreciate some guidance from anyone with experience.

Cheers from Aus

Kieran

Hey Zippi,

Thanks for the reply. I was wondering if I should have just done the main jet and seen how that went. But when I got the float bowl off I was so excited I thought, ah well I'll jump in and do both while I'm here.

Next step might be to step the pilot jet back down ... then the spacer mod.

Are you running the stock pipe?

That's really interesting Eau,

I'm not sure how to proceed from here. I'm probably doing too many things at once. Like I said above, I was happy to get the float bowl off and thought I'd do it all at once. I'm pretty sure something is not quite right. The bike is completely ride-able, just doesn't feel quite right near idling speed. I might need to revert to the original jets, take it for a spin and then work through the carb-tuning guide on the site. This was mostly an experiment to get me going with making mods Time to trouble shoot now

Thanks Zippi,

I'm starting to think the stock pilot jet and one step up from stock on the main jet might be closer. In the morning I'll revert to stock and go for a ride with that in mind

Thanks Dave,

I'll drop the pilot jet back down to stock this morning ... and read up again on the spacer mod. Pretty sure I can tackle it with the carb in place if I remove the fuel tank.

I just need to find some suitable washers. I'm having trouble finding the dimensions for those. I've seen them on here referred to as '#4 machine washers'. I'm not confident my local hardware guys here in Aus will automatically know what I'm asking for ... we'll see.


Dave wrote on 12/28/16 at 03:32:54:

Thanks, nothing beats a visual comparison ey?

ohiomoto wrote on 12/28/16 at 11:42:00:

Are you turning the Idle adjusting screw on the left side of the engine or the Pilot screw on the right side of the engine ?
If you are turning the Pilot screw then at 1 3/4 turns out, is this the spot where you got the highest rpm ?  If not, then you need to turn the screw out some more until you get to the spot where the rpm is highest.
At that point, whether you turn IN or Out the rpm will decrease.
This is the best place for the pilot screw.

You are currently running a 52.5 pilot and 150 main jet... and a white spacer mod as well ?  
The 52.5 should be good for the engine with the dyne muffler, and the 150 is likely ok but I personally like the 152.5 main jet for this type of setup.

Hi Lancer,

I adjusted both screws. The one on the left, just a touch so the engine isn't loping at idle. The one on the right, the pilot screw, is at 1 and 3/4 turns out, which is the spot where I get highest rpm. If I turn it fully clockwise the engine just about stalls. If I keep going CCW from where it is now the rpm drops. 3 turns CCW is not as dramatic a drop as fully dialed in CW.

I haven't done the spacer mod.

As for the jets, I set them back to stock to start working my way through tuning, one step at a time. So now they are at 52.5/145

It sounds like I have the pilot jet right.

If the bike seems to run ok on the stock jets, what would be the rationale for upping the main jet? Is it to improve performance, or would it help in keeping the engine in good shape?



LANCER wrote on 12/28/16 at 14:27:47: