Rich or Lean...I can't tell

[Gabriel]

I cannot get this flat spot out of a 200S engine. I've tried everything I know except ordering a jet kit. I'm about to do just that. But do I go richer or leaner?

If you mash the throttle hard, you get the dreaded Wuhhh. So with the breather cover off, I gave a mist of starting fluid half an instant before hitting the gas and it revs up EXACTLY the way it should. That kinda says it wants more gas. BUT, if you remove the plug it is black as asphalt. Heavy layer of soot. It almost seems the main jet is too lean the the secondary jet is too rich.

It doesn't change much when I change the needle position one up or one down.

VERY frustrating.
Thoughts?

[ps2fixer]

One thing worth mentioning, if you floor it super fast, like just slam the throttle as fast as you can from an idle, basically every carb'ed engine will bog slightly. They don't have an accelerator pump to pump the extra fuel needed for the short instant, and is why automotive carbs have it. Under normal riding and flooring it, that's completely different.

I haven't really seen a good write up on carb tuning, but a video I watched on a scooter had a nice method for that exact case. He noted down his stock jetting, and did a performance measurement on it (floor it across flat road and time 0-30mph, back and forth 4 times and average the results. Not exactly so easy with a 3 wheeler, but gives an idea. He went leaner till he saw for sure signs of being lean, then went richer till he saw signs of being too rich. Of course he picked the jet one step richer for the best performance low end and high end, he also increased his MPG over stock by a large margin. I suspect doing the same thing, but at a smaller scale (buy a jet one on each side and try them), then work with the needle adjustments and such and data lot the whole process. Find the two points that issues comes up, and pick the setting in the middle. Winter time generally takes a bit more fuel need for the cold air, so probably about perfect time to tune an engine, and tune it more or less perfect while it's cold and it shouldn't have starting problems etc. If the temp gets much colder in your area, might want to go a notch richer for that case.

I never seen anything say jetting is easy, so probably takes a lot of time unless you really know what you're doing and listening for, or have an exhaust analyzer machine (an O2 sensor in the exhaust might work well for tuning, but you'd have to dedicate a head pipe to the project).

Anyway, good luck with the tuning.

[Dirtcrasher]

Jet kit?? Who sells a "jet kit" for a 200S?? The only factor that screws things up when reading a plug, is burning oil. If SOMETHING was allowing oil into the combustion chamber, you will find a black plug, and your crankcase oil will drop down each ride.

If the plug is black, and it's not oil being burned, you are obviously RICH.

You see, lean is easy, nothing aside from a lean (usually) main jet, and or a leaking intake/other boot leak will allow a lean mixture. If it's white and light, it's lean. And lean tends to rev up quicker and run quite well until it blows up.

You need to separate the issues. You can take a compression test, then add a TSP of oil into the sparky hole and see if the compression bumps up. But that won't tell you if there's an oil ring issue or another way of crankcase oil to get into the sparky hole.

It seems like your afraid to make a jetting move one way or another. If this were mine, after I ruled out a few other possibilities, I'd simply change my main jet down 3 sizes, and or up 3 sizes, and place my needle and air screw in the middle of recommended settings and see what the plug says.

When you squirt that crap in there, you are obviously introducing a LEAN mixture.

Do not let the flat spot dictate which direction you are looking at, flat spots have dozens of possibilities...

[Gabriel]

It’s a new top end.
It’s not oil. You can wipe the plug clean with a rag.

I’ve befriended a guy locally that raced 200cc models back in the day.
He’s been dumping parts on me lately. Tomorrow I’m going to clear out his carburetor parts.
I’ll then pull the carb and try a smaller jet. It’ll work or it won’t but at least I’ll get direction to go.

[yaegerb]

What jets are you running?

[ps2fixer]

By jet kit, I meant a kit that comes with several sizes of jets, kind of like a bulk kit. Individually they are like $5, unless parts stores sell them cheaper, but I doubt that. Of course a kit would probably be even more, even though the price per jet is cheaper. What's a good supplier for the jets?

[ATC King]

Things aren't always what they seem, and certainly all performance issues aren't the carb.

Everything is a process of elimination.

Is the exhaust plugged up? The OE header on those is double walled and the inner wall can collapse and not be visually detectable from the outside. The inner wall can rust/erode and it does happen.

Is the mechanical advance on the cam moving freely or are the springs old and weak? Is the timing set correctly?

Is the carb REALLY clean?, as in it just doesn't dump liquid fuel into the engine, it mixes with air just after the main jet in the emulsion tube, so if the air passages in the carb are plugged, you'll get a slug of liquid fuel instead of fuel atomization. The air for the carb circuits comes through those little holes in the intake side of the carb, not the main venturi.


Are the valves set correctly? Is the choke staying open when open and closed when closed? Is the ignition providing a good spark?



A word of extreme caution on the starting fluid. It doesn't burn, it explodes and can do so before spark ignition. It has a much lower flash point than gasoline (hence ignition by compression alone) and causes and extremely violent rise in cylinder pressure. It's very hard on pistons and rings, even on diesel engines when ambient temps are well above the need for it's use. It's not a diagnostic tool, it's a starting aid for very low temps in diesel engines without glow plugs or precombustion chambers. Cracked piston rings, ring lands, and broken pistons are very likely to happen with it's use as a diagnostic tool.