2017 Suzuki GSX150 SF - running rich

[JessMan213]

Hi everyone - firstly apologies in advance - will likely be a long post but want to be as detailed as possible

Back in December I picked up a 2017 Suzuki GSX 150SF with 19000km on the clock. I have put on about 3500km since. The bike originally ran well, but that is becuase I was doing less than 60km/h city driving. Then I took it on a 400km trip where there were a multitude of free ways with a speed limit of 120km/h. Not that the bike could do that, but I was having an issue in 4th and 5th gear where the engine would just cut out and moderate to WOT. The engine just kind of shut off for a second or 2 then would run normally. But there were 2 instances where instead of returnin to life it would just shut off. Would have to come to a complete stop on the side of the road and 2-15 minutes later would then only start again. While trying to start, it would just crank.

Did a full service on the bike, replacing the oil, oil filter, air filter and spark plug, all looked good except spark plug indicated the bike was running rich (otherwise in good condition).

After service the bike ran a lot better, but would still have these instances where it would cut out for a second and then run again. While doing this it would sputter and backfire, but no smoke. What I found helped in the interim is if I tucked in to be a bit more aerodynanic. During this time I also replaced the ignition coil - this did nothing.

To my most recent developments. Checked the new plug and also quite black indicating running rich. So I assumed bike was running too rich and under load fuel would foul the plug momentarily until the plug could get clean enough to fire ok again. The dash and everything stays on when the engine shuts off, so I dont think its electrical.

If i put the bike on its centre stand and just hold the revs arounf 4-5k it does a lot of mild backfiring/ popping

Bike Mods
- Has a full free flow system. Honestly too loud for my liking, but cant see how that would affect the running. Have tried to source an original exhuast with no luck - would have to replace the entire system. No other mods at all. Ive run bikes before with the muffler completely removed and never run in to this kind of issue.

Things I have checked
- Side stand switch - bike doesnt have one
- Clutch switch - was bridged by the previous owner, dont see breaks in any wires
- Spark plug is firing
- Have also played with the fuel mixture screw. This only seems to make the issue worse
- Fuel filter - bike doesnt a fuel filter

In short, not sure what else i can check and try. Problem seems to occur under higher load, localised to 4th and 5th gear.

All signs lead to overfuelling based on spark plug, but not sure how i can test that spark is consistently firing under high load.

[Robby]

This could be one of the rare occasions where it really is a jetting problem.

It's a 2017 150cc bike and you mentioned a mixture screw. I assume you're in somewhere like Thailand or Vietnam, where 150cc bikes exist and could still be using carbs at that point. This is mostly a UK based forum, where 150cc bikes are very rare and fuel injection was fairly standard from 2014.

My best guess is that someone fitted a main jet in the carburettor that is too big, probably when they fitted the exhaust. The main jet is responsible for controlling the fuel-air mixture when the throttle is 3/4 to fully open and the engine is under heavy load - like high speed riding.

The mixture screw only affects the mixture at idle and very small throttle openings. It will have no effect here.

The usual fix is to find out what size your main jet is, find out what size the original factory size was, and pick something in the middle. So if the standard fitment is a 130 and you have a 170, try a 150.

Or better still, find a standard exhaust and put your carb back to factory settings. You will need a workshop manual to get the right sizes and settings for main jet, slow jet, needle position, float height and mixture screw.

[JessMan213]

Thanks a ton

I will get that checked out. I am actually from South Africa and while carb models are quite uncommon on the newer bikes, the 150cc and below still predominantly use Carbs.

I want to upgrade to a bigger bike anyway, just financial situation at the time didnt allow for it.

As a further update. I reset the mixture scew and turned it out quite a bit (about 4 full turns) and the bike was running terribly, anything 80km and above and it would want to spit and backfire a lot. I then turned it back all the way in and about 2 turns back out and now its running MUCH better, the only real issue I am getting is if I close the throttle somewhat at higher speeds (90-110) then it does the thing where it momentairly shuts off the engine and comes on again. It still wants to cut out ever now and again but much less frequently now. So I think you may be right about the main jetting.