Carb tuning on RG-150

[beanz]

Bought a bike, rode it for a week, then thought it was about time to put it all apart...

Part of this was cleaning the carb, from which I extracted every screw, bolt, and anything which looked vaguely removable. Soaked it in carb cleaner, scrubbed every oriface like I'd spent a week in Mt Eden Pen, and reassembled.

Now comes the tuning. Its got to be the worlds most basic carb. 2 external screws, an idle speed screw, which sets how far the slider valve goes down, and another screw which the service manual says is the mixture screw. It says to first adjust the idle speed screw so it idles at 1400rpm. Done. Then adjust the mixture screw until the engine revs the highest (ie, turning it one way will increase the rpm, then any more turning will decrease the rpm). Played with that for a while, and it seems that nothing changes at all when I turn the screw. No rpm change, no more/less smoke, nothing.

Any ideas whats going on? Maybe a clogged hole in the carb... but I swear I gave that carb more attention than my girlfriend when I cleaned it!

The reason I started all this, was cos the bike wasn't running too well. Hard to start, sputtered when cold, and almost no power below 6K. Are these just undocumented features of owning a 150cc 2 stroke lawnmower engine?

Running out of ideas. Checked plug, exhaust valves, rings, cylinder aswell.

Somebody help me, just to shut me up on this board!

[jrandom]

Hard to start, sputtered when cold, and almost no power below 6K. Are these just undocumented features of owning a 150cc 2 stroke lawnmower engine?

I'd say they were pretty well documented mate!

[erik]

It says to first adjust the idle speed screw so it idles at 1400rpm. Done. Then adjust the mixture screw until the engine revs the highest

Does it give a starting position for the idle mixture screw to have it at when you're setting the idle stop screw?
Maybe the throttle is too far open so that most of the fuel is going in through one of the other jets, making the idle mixture screw not do anything. Did you check how many turns out the idle mixture screw was before you pulled the carb apart?

It sounds like it would be fun to try messing around with it to get it to work, but to be honest, I'm only a beginner and may or may not be talking nonsense...

I dunno if it'll be any help, but I found this post on google when I was trying to figure out how to set my carbs and thought it was interesting, because it was pretty much the only bit of information i found that gave a method of telling if the mixture was too rich or lean. One thing I noticed with my bike was that if it was too rich and I'd accelerate away and then cut the throttle, the engine would just die.

[Posh Tourer :P]

I would agree, the main jet needs to be closed for the idle jet to have any effect. I suspect this is your issue. Try lessening the idle speed and see what happens. You will need to adjust this anyway, as once you have done your idle jet, you readjust the mainjet idle speed setting. 1400 is just a number the manufacturer picked out that normally works, you may have to play with it a bit. The other thing to keep in mind is that the response may be a bit slow, and it occasionally helps to give the throttle a bit of a blip to settle the idle occasionally. I know some bikes have a "threshold" level, just above which it idles fine (maybe something to do with momentum???) and just below which it slowly drops to a stall, so it sometimes helps to blip it and see what happens.

[Motu]

If your mixture control screw has no effect it means the mixture is out beyond the ability of the adjustment - I'd check your float level for a start,then check around for air leaks.Best you find out if it's running rich or lean - best way is just to turn the fuel off,if you can.After awhile it will run out of fuel - if it revs up before it dies it's running too rich,if it just dies it's too lean.

[beanz]

Thanks guys!

I think it is running rich, when its cold anyway, because its exhibiting a little bit of "hicupping" after about 7000rpm until it warms up. I didn't really know what this was at first, but experimented with pulling the choke out when it was warm, and it did the same thing... too rich.

I checked the float level, and corrected it to the height that the manual specified. I talked to a mate, and he said sometimes the float can get saturated in petrol over its life, and not "float" anymore. I might take it out again later, and test it overnight in a pot of petrol. Wish I did this when I had it apart.

Great suggestion about rich/lean Motu. But to complicate things a bit, I discovered that the fuel tap doesn't actually work properly. I had it apart for cleaning, and it looked fine, but after trying a suggestion (to run the bike out of petrol at the end of a day by turning off the fuel and letting it run to a stop) it just kept on running... and running... Will pull it off and check it again. Maybe I put it back together wrong.

Maybe the hard starting problem is because the petrols leaking into somewhere overnight, and flooding it. That doesn't quite make sense tho, cos regardless of the fuel tap, the fuel-in needle valve in the carb should be closed... unless the floats arn't floating... AHHHH.

OK, I'll try playing around some more. Thanks heaps for the suggestions. Bit confused what the "main" jet and the "idle" jet's are tho.

[pete376403]

Needle valve and seat in carb does wear, and will cause rich running. Have a close look at the tapered tip of the needle, if you can see any sort of step its worn. You can sometimes get away with lightly reseating it using valve grinding paste (if it is solid brass or steel, this won't work with a rubber tipped needle. Some carbs have replaceable valves ie unscrew the seat and screw in a new one.

[beanz]

Thanks pete. The needle valve looks fine. No wear on the rubber tip or seat.

[Racey Rider]

The RG 150 I had was a bit temperamental at times when cold. After a number of kicks, it would fire up,, just,,and die if you twisted the throttle. Then sometimes it would fire first kick and be fine.

Two strokes! What can you do?!

[Posh Tourer :P]

Thanks guys!

I checked the float level, and corrected it to the height that the manual specified. I talked to a mate, and he said sometimes the float can get saturated in petrol over its life, and not "float" anymore. I might take it out again later, and test it overnight in a pot of petrol. Wish I did this when I had it apart.

Shouldnt do this after 6 years unless they make it out of sponge!!....

Great suggestion about rich/lean Motu. But to complicate things a bit, I discovered that the fuel tap doesn't actually work properly. I had it apart for cleaning, and it looked fine, but after trying a suggestion (to run the bike out of petrol at the end of a day by turning off the fuel and letting it run to a stop) it just kept on running... and running... Will pull it off and check it again. Maybe I put it back together wrong.

These things can run for a while.... The beemer does 1.5km before it runs out of fuel. theres a lot of fuel in a carb, when you use less than a drop for each firing. However, if you have had the tap apart, it is quite possible it isnt sealing very well. Check it by riding off in the morning with the fuel off, and wait till it starts spluttering or dying slightly. If you can then get the fuel on without stopping, it'll fire again as soon as there is fuel (principle of bumpstarting as I believe it is technically called) I do it all the time,as I often forget the fuel taps, although the beemer normally gives me some warning, as one cylinder stops first. The honda and the MZ just used to cut out. Tho the honda had a strange system. I suspect it was bordering on needing a fuel pump, cos when it was running it was fine, but at idle it would stall if it needed reserve. I think it was just not enough fuel pressure unless it was sucking it through a bit.

Maybe the hard starting problem is because the petrols leaking into somewhere overnight, and flooding it. That doesn't quite make sense tho, cos regardless of the fuel tap, the fuel-in needle valve in the carb should be closed... unless the floats arn't floating... AHHHH.

Easy way to check. Kick it over once in the morning to get the fuel in the chamber sloshing about, and pull out the spark plug. If it is wet, its flooded. If you cant see any wetness on it this isnt your issue. Its unlikely to be this anyway. As you say, it doesnt make sense.

OK, I'll try playing around some more. Thanks heaps for the suggestions. Bit confused what the "main" jet and the "idle" jet's are tho.

Ok. Main jet is the hole for the main needle in the carb that is pulled up by the throttle. This hole lets your petrol into the upper part of the carb to mix with the air and go into your combustion chamber. The idle jet is the hole that allows fuel to get through when your main jet is closed. It is this hole that is opened by the choke lever. Thus when your main jet is closed, (ie the main hole, controlled by the throttle cable, which also controls, at the same time, your air intake - the main method of accelerating) thus minimum air is being let in, and you open the choke, there is more fuel coming in. This idle jet also let fuel in when the choke is off and the throttle is closed ie at idle.

Just to clarify that point about air supply, an internal combustion engine will accelerate perpetually (theoretically) if you dont shut off the air supply. Thus at wide open throttle it is at it's most unrestricted, and it is at maximum acceleration. When it is at idle, it is severely restricted in airflow. Also, the main jet should be fully open at about 1/4 throttle (if I recall correctly) and the only control after that is air flow.

This is all my experience from 3 carbed bikes, all three of which have required some carb work... I dont know if it is all accurate to the nth degree, but it is certainly pretty accurate as far as I know

[beanz]

Thanks Posh for all that info. Man, I'm glad I've got a bike with 1 cylinder and 1 carb. Must be a bit of a pain tuning up something with multiple carbs!

[Posh Tourer :P]

Normally they are linked in some ways eg the ZXR with linked throttle, and choke and idle. The CB125 has linked choke,but separate throttle cable inputs and idle adjusters, as does the beemer (throttle cable splitter)