Carb Air/Fuel Adj Set Point Question By chaslittell (Read 2,418 times)

[chaslittell]

Hi Folks: I don't see this specifically, so I will shout out for assistance: My new 150cc (MC-16K)'s low speed air/fuel adjustment stumbles and runs slow until the screw is just under 2 turns out, then picks up and runs smooth. However, I can get another 10 or 20% rpm's by continuing to run the screw out, another 1 or 2 turns. It seems, in engineering terms (?), to have a "knee" at around 1.75 turns below which adjusting makes a big difference, and after that, continuing to adjust only provides slight but noticable improvement.

I assume that this "knee" around 1.75 turns, or just past it, is where I want to set it. But, am I wrong, should I continue to adjust for the absolute highest rpm? That's a lot of adjustment for a little improvement. I don't want to waste fuel, but I also don't want to burn the poor thing's innards running too lean. Performance is not an issue, it seems to run fine regardless.

I hope this is clear...opinions?

PS: The MC-16K (bought for the wife) is a great running 10"-wheel scoot. I had no trouble riding it home 100 miles on the two-lane roads, it kept up with most traffic fine. The previous owner dumped it with 100 miles because it wouldn't start hot, but that was a simple valve adjustment. What a pain in the rear though to get the plastic off to get at those valves!

[Bashan]

There's a lot of debate on whether the A/F screw that you're referring to has a great deal of influence on the carb. Sprocket, who's certainly no slouch, thinks it has an effect up through the mid-range. Here's a really good article on the CV carb, you'll notice the circuits chart at the beginning that shows the theoretical influence the slow jet/pilot screw (SJ/PS) has on the overall function of the carb. The pilot screw is the same as the air/fuel (A/F) screw. The other article you should read is from Battle Scooter. The standard setting for the A/F screw is about 2 1/2 turns out. If you're getting out around 4 that's too much and in fact the screw could pop out with a bit more CCW turning. Here is one method for setting your A/F screw, and here is the venerable Alleyoop's. If you're having to set at 4 turns out for optimum performance something is wrong such as jets that need cleaning. Rich

[sprocket]

Yup the pilot jet is 'always on' it contributes a percentage of fuel throughout the entire throttle range. However, as the main jet opens up the needle the percentage of fuel contributed to the mix reduces as a percent of all fuel available...

Want some fun some time when you have nothing to do, play with main jet diaphragm tapers...it is amazing at some of the power effects you can get...that is how we tune racing engines, for different applications and race conditions, weather etc...

[chaslittell]

Thanks folks. But: My question remains--adjusting the A/F continues to slightly raise RPM, while the links you provided suggest there should be a setting from which the RPM will fall off no matter which direction you adjust. QUESTION also, where would I obtain a service manual for either of my scooters--though both (MC-16K and MC-75) are 2011 models and likely have identical drive trains. I would like a definitive spec on what the "factory" carb A/F setting is, as I have read 2 turns, 2.5 turns, and others--perhaps different for different carbs? I was given parts manuals and other lit which I think will tell me the carb model, maybe I can google that....Chas

[raford3]

Mine tends to stall on take off with the screw backed off too far.

[Alleyoop]

The manual does not take into consideration the area you live and altitude and the air density in your area. The manual is basically a starting point, also identical engines never run the same. The parts that went to build one engine may have been molded with a different batch of metal and therefore may have inperfections. Also the finishing on the parts may have been different.

When you adjust the Fuel Ratio Mixture obtain the HIGHEST RPMS whichever way it turns out to be Leaning it or Richen it then on the Last 1/4 turn where the MOTOR DOES nothing or Drops rpms you go the other way 1/8 turn. If getting the highest RPMS was Leaning it, then backing off 1/8 will just slightly richen it. If getting the highes RPMS was Richening it, the backing off 1/8 will just slightly lean it.

Every engine requires its own settings, no two are alike.
Alleyoop

[sprocket]

Sorry there is NO such thing as a definitive spec.. this isn't Boeing.. it is fly by your ASSSSSSSS

[Bashan]

One of my points was that if you keep getting higher RPMs as you turn the screw out something is wrong. At some point they should drop off on the idle as the mixture gets hinky. You're probably getting too much air and adding more fuel with the A/F just keeps working. You might have a vac leak at the intake manifold or maybe some venting line isn't plugged in on the airbox side. If the bike runs OK I'd set it for 2 3/4 turns out and leave it for a while and see what shakes out. Roketa doesn't have a lot of manuals out there but your scoots are pretty much generic Chinese. Go to our library manuals board and check out the generic manuals, they should work for service. This is a really good 150cc service manual. Rich

[chaslittell]

Great manual Rich. Makes me want to do $2000 worth of rebuilding on a $650 scooter. I guess I can't count labor cost if I do it myself though. Seriously though, one thing stands out: The manual specifically says to screw the dipstick in to get a good reading; almost all other references I find (including my owner's manual I think) say the opposite. Also the illustrated bike has a manual petcock. And of course it isn't a C.A.R.B. emissions set up. Add all this up and I think the publication is years old. In turn, this makes specs, like carb A/F nominal setting, suspect. I wonder if there is a no-kidding short/medium/long RY6 service manual out there with verified current info? Again--Thanks! Keep up the good work.

[runningslow]

Oct 13, 2011 4:24:59 GMT -6 chaslittell said:

Great manual Rich. Makes me want to do $2000 worth of rebuilding on a $650 scooter. I guess I can't count labor cost if I do it myself though. Seriously though, one thing stands out: The manual specifically says to screw the dipstick in to get a good reading; almost all other references I find (including my owner's manual I think) say the opposite. Also the illustrated bike has a manual petcock. And of course it isn't a C.A.R.B. emissions set up. Add all this up and I think the publication is years old. In turn, this makes specs, like carb A/F nominal setting, suspect. I wonder if there is a no-kidding short/medium/long RY6 service manual out there with verified current info? Again--Thanks! Keep up the good work.

I think you're looking for a manual holy grail that doesn't exist. If it does exist, its probably in Mandarin.

I've never been able to get a good definitive answer on the dipstick. I personally don't screw it on when checking the level and I don't have issues. Well, I have issues, but they're not related to the oil level. Ha.

As for the age of manuals relating to GY6 motors... age is irrelevant. The motor hasn't change. Sure its had a few things added to it recently, like all the emissions equipment. But, at the end of the day, its the same motor hiding under there and every one of them has a personality of its own. The generic manuals just give you a good starting point and you've got to find the sweet spot for scooter.

The GY6 is alot like the 4 cylinder engine in my Ford Ranger. Ford used the same 4 cylinder block from 1983 all the way to 2001. Sure, it started as a carburated motor with 4 spark plugs and ended up as a fuel injected motor with 8 plugs, but its the same block. Heck, that same block actually started life in the Pinto and found its way into turbo charged Mustangs and Thunderbirds.

[sprocket]

chaslittell

This forum is your manual...use the search function. The information here has been gleaned over the years and is more correct than ANY manual in print.

The scooters manuals are notoriously WRONG and full of errors as well... don't use the general torque settings they give... you will strip the bolts in the aluminum long before you ever reach the correct torque for example.

Every company does their own tweaks to the electrical system, the fuel system, the air boxes and the emissions. If you have an alarm get rid of it and the remote starter too. The emissions are often problematic as well on many scooter makes. Probably a good idea to remove it.

These things are made in companies from a few employees to a few thousand. Parts for engines are made in home made aluminum smelters and sand cast or in modern die casting facilities...and everything in between. There are no laws, no copyright nothing...

The dip stick problem is easy.. the oil sump takes 750 ml of oil. Next change measure it and add it to the engine then mark the level on the dip stick...screwed in or out it doesn't matter just measure it the same way every time...

[chaslittell]

Folks: Thanks again for the responses. I think I will search "remove emissions" for one thing. The dip stick question is no problem, since I don't get any foaming with the unscrewed measure, and more is better when it comes to oil, especially for an air-cooled engine methinks. Re. low spd A/F mix: This morning was cool, and I had to crank a good 20 seconds with the engine sputtering before it would idle--obviously I have it set too lean. I'll fix that. Next question, many references say the plug condition is a good indicator of this setting, but how can that be if it only affects the idle and low speed mix? 98% of the time I am mid to max throttle, and it seems like that would determine the plug condition. Just wundrin'. Oh, and speaking of max throttle--lots of folks comment that running max is not good for the engine--really? It feels like it LIKES to be turned loose! Are folks just assuming that max running is bad? Just curious since my commute is about 40% wide open to keep up with traffic. Thanks again--Chas in Dayton

[Bashan]

Oct 14, 2011 8:45:21 GMT -6 chaslittell said:

I think I will search "remove emissions"

Here is my own journey in that regard.

Oct 14, 2011 8:45:21 GMT -6 chaslittell said:

The dip stick question is no problem, since I don't get any foaming with the unscrewed measure, and more is better when it comes to oil, especially for an air-cooled engine methinks

More is definately not better on the GY6 with oil. It plays havoc with many systems and Sprocket yells at me about this.

Oct 14, 2011 8:45:21 GMT -6 chaslittell said:

Re. low spd A/F mix: This morning was cool, and I had to crank a good 20 seconds with the engine sputtering before it would idle--obviously I have it set too lean.

Have you checked your auto-choke?[/url]

Oct 14, 2011 8:45:21 GMT -6 chaslittell said:

Next question, many references say the plug condition is a good indicator of this setting, but how can that be if it (the A/F) only affects the idle and low speed mix?

You set the mixture that affects the plug by moving the jet needle.[/url]

Oct 14, 2011 8:45:21 GMT -6 chaslittell said:

Running max is not good for the engine--really?

Sprocket maintains that the GY6 is designed to run at 80%. I'm like you, it just feels right at 100% but logic dictates WOT isn't good for any engine. Plus he yells at me about that too. Did I already tell you I'm from Toledo Chas? Rich

[chaslittell]

Rich: Wow great post on the emission crud. I don't think I want that big a project unless (like you had) I have a malfunction that it would repair.

I had to strip the plastic on my MC-16K retro to get at the valves when I bought it last week; the original owner sold it for a song after owning it for a month and 100 miles--two reasons, he decided he liked scooters and moved up to a bigger engine to suit his rural location, and: It wouldn't start when hot! I guessed it was the vavlves, and I was right.

This afternoon I set the low A/F mix a bit more rich, and it seems to idle well enough now. The idle is slower until it's good and warmed up--it's in the 50s today--but I think that's probably about right, summer weather will balance things out.

This was purchased last weekend for my wife, after I found myself having so much fun with the MC-75 I bought a month ago. It needed the valves opened up as well, but it ran ok. Once I had them side-by-side, I realized the 75 was missing an exhaust manifold cap nut, much louder than the 16!

But, regarding the 16's plastic--we should sell refit kits to make these plastic bodies accessible!! I couldn't believe the grief, I thought the 75 was bad but whew!! I would like to recheck the valves once they get seated, but $# if I'm going to mess with that plastic again until I darned well HAVE to. Oh, and mine does have the floorboard tank; I assume this means it has some kind of vacuum fuel lift or something, that the gravity feeds wouldn't need. The fuel filler trap door is screwed to the side shells from the inside, maybe if I had gotten the floorboard up first the whole shell assy would have lifted off in one piece? I bet that's why I had so much trouble now that I think about it. Is it typical that the floorboards come out first to free the other panels?

And yes, the "choke" does function, I could tell when it kicked off after a few mnutes.

[sprocket]

Hey... I run engines all the time WOT... but I also rebuild and mag flux them after ever couple of hours of running. Not too practical for a street machine...

While the engines last longer and perform better at 80% there is also a safety consideration...a bit of extra throttle can save your butt.. have seen it often...