2 stroke tuning issues

[RaWarrior]

So once again I'm balls-deep in the annoying world of "making an old carb 2 stroke run well", this time it's for my FL350 Odyssey.

Little background:
329cc single cylinder, air cooled, reed valve 2 stroke mill
STOCK carb was a 32mm Keihin PE round slide, #142 main #58 pilot
Motor compression is perfect book spec at 130psi

The p/o said the main jet snapped off in the original carb when he took it apart to clean it, so he replaced the whole carb with an "equivalent" Mikuni. It would start and idle and rev up OK, but under load it would fall on it's face and wouldn't rev past 4-4500 or so. He said the jetting was probably wrong and I agreed. The plug was really black and wet, and it seemed like it was just flooding out when you got on the gas. Looking up the conversion table, a #142 Keihin jet is about equal to a 125 Mikuni. There was a 250 in the replacement carb, I swapped in the 125. Better but far from perfect.

Idle is OK, about as good as you can expect from an old 2smoke. Hangs out at about 1000rpm or so. To get it "off idle", you need to "tickle" the gas. Slowly pressing on the flipper results in it bogging out and stalling. Once it's revved up, quick stabs at the gas are snappy and responsive, but if you press it past 3/4 throttle or so, it falls on its face. With the 250, anything past 1/2 throttle would just make it sputter and fart and bog and not really go anywhere (but not kill it...just wouldn't have any power or pick up RPMs)

Out driving it, it seems to run great between 1/4 and maybe 90% throttle. Push it that last tad to WOT and it's as if you hit the kill switch, goes from screaming to dead, back off and it recovers instantly.

Someone wrote "32" in sharpie on the carb bowl, I measured the ID of the motor side and got 34mm(which I guess is a common replacement size). Looking into some other forums, people report their setups for this carb run anywhere from 310(stock) to 450(piped ported) mains.....how can that be if I'm mega-rich with a 125? And with the 250 in there it ran like total crap.

The plug is a darkish gray with maybe a hint of tan in there. It's usually wet, and the exhaust smells rich. Here's the weird thing....pulling the choke (bypass valve like on the vmax) makes it run better. The bogging gets better, and hitting WOT doesn't kill it anymore. Similarly, dropping the needle(to try and lean it out), makes everything worse....best case was with it on the lowest/richest clip.

So I'm kind of stumped here....this doesn't add up. I went through the carb, and I couldn't find anything out of place...float valve didn't leak and was set properly, no jets backed out, nothing plugged, the carb looked almost new.

Anyone good with these 2 smokers? Any ideas?

 

[Barry barker]

Is it possible that the equivlant Mikuni is actually to big? The stock carb is a 32, is the Mikuni a 32. I tried to change carbs on a 125 elsinore many moons ago and it simply would not work. Not much help here, good luck!:ummm:

 

[Fastatv]

So once again I'm balls-deep in the annoying world of "making an old carb 2 stroke run well", this time it's for my FL350 Odyssey.

Little background:
329cc single cylinder, air cooled, reed valve 2 stroke mill
STOCK carb was a 32mm Keihin PE round slide, #142 main #58 pilot

Idle is OK, about as good as you can expect from an old 2smoke. Hangs out at about 1000rpm or so. To get it "off idle", you need to "tickle" the gas. Slowly pressing on the flipper results in it bogging out and stalling. Once it's revved up, quick stabs at the gas are snappy and responsive, but if you press it past 3/4 throttle or so, it falls on its face. With the 250, anything past 1/2 throttle would just make it sputter and fart and bog and not really go anywhere (but not kill it...just wouldn't have any power or pick up RPMs)

Out driving it, it seems to run great between 1/4 and maybe 90% throttle. Push it that last tad to WOT and it's as if you hit the kill switch, goes from screaming to dead, back off and it recovers instantly.

Someone wrote "32" in sharpie on the carb bowl, I measured the ID of the motor side and got 34mm(which I guess is a common replacement size). Looking into some other forums, people report their setups for this carb run anywhere from 310(stock) to 450(piped ported) mains.....how can that be if I'm mega-rich with a 125? And with the 250 in there it ran like total crap.

The plug is a darkish gray with maybe a hint of tan in there. It's usually wet, and the exhaust smells rich. Here's the weird thing....pulling the choke (bypass valve like on the vmax) makes it run better. The bogging gets better, and hitting WOT doesn't kill it anymore. Similarly, dropping the needle(to try and lean it out), makes everything worse....best case was with it on the lowest/richest clip.

So I'm kind of stumped here....this doesn't add up. I went through the carb, and I couldn't find anything out of place...float valve didn't leak and was set properly, no jets backed out, nothing plugged, the carb looked almost new.

Anyone good with these 2 smokers? Any ideas?

My guess is that the very bottom end bog is because your pilot is too small....you saying it runs better with choke proves that.
- General old rule of thumb is to get the main correct first. This protects the engine from damage. From what your saying, the main seems lean. If it were rich, it would "4 stroke as its called...spit, sputter, jerk at WO throttle. Try a 135 main and see what you get. You'll need to do a top end plug chop. Looking of course for top end performance but tan colored plug. Pipe design can also play here. The pipe you have installed may be for bottom and mid, and fall off on top.
- The off idle bog usually means that the pilot is too small but the pilot is only used for the first 1/8 movement of throttle, then 1/8 to 3/4 throttle is primarily the needle, and 3/4 to WO is strictly the main. The bog could be too lean of needle depending on throttle position. Have you tried to adjust the a/f screw?
- So, one must be careful and pay close attention to throttle position when diagnosing carb issues.
Bogging usually means too much air or not enough fuel. For me, its always best to only change one thing at a time or things can get confusing and I always start with the main. Hope this helps

 

[RaWarrior]

Thanks that was helpful....I fiddled around with it this morning and came to the same conclusion.....main is too lean. Before I was just running it in the garage in neutral, this morning I actually took it for a ride to put the motor under load.

The "best" I got was with the air screw closed and the needle in the fattest clip. Off idle didn't bog any more, but it still isn't snappy....better but not perfect. Midrange lacked power....i.e it would rev up strong, but as soon as the clutch engaged(which on this thing is very firm and sudden), it would fall on it's face.....lots of RPMs but no power. Once you got rolling it was more or less OK, but still pushing it past 3/4 throttle just made it die.

I got my EGT set up also, and it was pushing toward 1200 after maybe 10-15 seconds of "as fast as it would rev", which it was showing was about 6000rpm but only maybe 2/3 throttle. I've found that WOT temps should be in the 1100 range for a "safe" everyday riding setup, so that would reflect it's too lean also. They say the piston melts at about 1300, and even a few seconds past 1250 will toast the rings. As soon as it got close to 1200 I backed off.

It has the stock pipe and muffler. From what I heard, the stocker is actually pretty good and the few aftermarket ones are only good for flat out top end. Only mod is the EGT probe, which shouldn't really affect anything.

 

[slobmanrf]

For sure you need to richen it up.Gray is bad,Chocolate to tan is good. mikuni does not have a air screw just a fuel screw,and pretty much for idle to just off. Pilot meters fuel just off idle to 1/4 throttle then needle/emulsion tube up 3/4 then its all up to the main.Now the main also meters fuel for the emusion tube,That is what it screws into. Its always best to start rich and adjust to the lean side.You can burn a two stroke down in a very short time.Watched it happen grass draging snowmobiles,took 500 ft.! So be careful loading it hard with till you get it to a tan plug. And on that point, the plug is your key to what is going on.Run it in the rpm range that you have problems with and shut it off without idiling and pull the plugs. It doesnt take long at idle to change the plug color. Dont be afraid to run the plugs on the dark brown side,fuel makes power.Also make sure you air jets in the inlet bell of the carb are clean,they get over looked often and can make you chase you tail. Now once you get a safe plug color,pay attention to clutching. If you have a CVT trans, a driven clutch that wont back shift will give you a bog also. Get the jetting safe then worry about that. Wish i lived closer,would gladly come over and put the super tune on it! If you want i could talk you through it on the phone. PM me for my number if you need to...........Rob

 

[slobmanrf]

Great,the EGT will make things real easy.

 

[tugla]

think your on the right track, if you can get a 32 i would, thats a huge difference 34 form 32. do you still have the old carb with broken jet? might still be able to get it out and fix it. how are the reeds? take a good look if you have a crack, missing piece, loss of tension so they dont shut from worn out etc, take a good look, might need to be cleaned while you are looking at them.

 

[satariel666]

So once again I'm balls-deep in....

...Anyone good with these 2 smokers? Any ideas?

Usualy when all that chinesse scooter 2T craps(my sis owns one) runs good on idle and bad under load this means NO VACUUM. That was also the rule with my old 2T Simson. Messured compression was high to spec but motor runs like shit. I spent weeks trying to figure out whats going on.

2T engines needs to have good compression but also a good vacuum.
Usualy the "lower end" will worn with the second one.


Webber air flow tool (used on v-max when v-boost was removed) will work here.

Prez

 

[VMax-Mike]

Check and make sure your intake boot dont have cracks in it. Also your best bet is to get the right carb for it. Check for a fourm that can help you find a carb that will work. If you have the oem carb and the jet is broke off take the carb apart and soak it in carb rebuild cleaner. than try to drill and easyout the jet. If you dont have oem carb than find one or go aftermarket. If you know the carb you have is the right fit rebiuld it and make sure the jets are correct size. The first thing i would check is the intake for cracks or gasket leaking. all you need to do is make sure air cleaner is on and spray carb clean systematicly from the engine intake gasket to the carb. one swip at a time until you have covered the whole intake and gasket. dont get any in the air filter area. If you have a leak the engine rpm will go up when you hit it with the carb spray.

 

[06VMAXIMUS]

Your running a 392 snowmobile engine in it, with the oem 350 pipe? Could be some of the problem, as 2 strokes can be fickle with different expansion chambers. My blaster ran like sh&t when I wrecked and dented the pipe.....ran great again when I replaced it with a new one.

 

[RaWarrior]

Stock motor, stock pipe (the "350" Odyssey was cut down to a 329) in a recall, since the original 350's had a penchant for burning up due to a poorly designed exhaust port bridge.

I've looked into the carb setup. Most "experts" seem to agree that the stock 32mm carb was put on to limit power output purposefully, in the interest of making the engine last longer and/or safety. Replacing it with a bigger one is pretty unanimously agreed nets a noticeable power gain, and with proper jetting no adverse affects to engine life. The most commonly talked about one on forums is actually a Mikuni 38mm, which I guess can be set for anything from totally stock to a full on piped/ported big bore. Even heard some people running 41mm flatslides, supposedly running well on "wild" builds.

I don't have the stock carb, p/o said he just chucked it.

I did check for leaks in the intake and didn't get any. If I stick my palm over the airbox "snout" with it running, it sucks down hard, almost collapses the airbox lid and stalls out instantly. Carb spray around joints and the boot didn't net any RPM change.

When I have the carb off again I will pull the reeds and check them. Stock reeds are metal and supposedly very resillent but worth a look. A lot of people on forums run the Boyesen reeds as a good replacement but there's no verdict they actually improve over stock other than people going "well, it seems a bit faster...."

I actually don't know what pilot is in there....couldn't find a tiny enough screwdriver to get it out. Whatever it is I'll get a couple sizes larger, hopefully that will clear up the laggy off-idle.

-Check reeds
-Bigger pilot
-Bigger main

...and tune from there

Thanks for the input....

 

[slobmanrf]

I would'nt buy a pilot jet until cleaned the one that's in it. That 34 MM is just fine. Buy or borrow some main jets,adjust your needle and clean the pilot. Start with the fuel screw at 1 1/2 -2 turns out.Get all the curcuit's cleaned out and then see what you have for temps.Adjust from there.Just listen to it feel it, it'll tell you what it needs.

 

[RaWarrior]

Are you sure that's a fuel screw? It's a stubby, rounded needle, which IME are typically air needles, fuel needles are pointier. You can see it closes off one of the little orifices in the intake side throat.

Plus it ran worse with it opened than closed, since it's running lean that would suggest opening the screw added more air, not more fuel.

 

[slobmanrf]

My bad ,If its on the airbox side of the slide its air. Fuel screws are found between slide and engine. Dont see many kuni's with air screws.Does it have a fuel screw past the slide also?

 

[Barry barker]

I think you are past this or already tried it, but you can block off 1/2 of the air filter with tape to richen the mixture or remove the filter completely to lean it out. i am assumming there is an air box and test without the filter would be prudent for a short period. This might help in the part trottle positions. i have used these tests to fine tune the jetting with some sucess. I find it very hard to determine rich or lean, and it should be easy.

 

[RaWarrior]

Nope, no fuel screw. Just an idle adjust(slide stopper) and the air screw.

I did try running it, then sticking my hand over the airbox snout intake. Covering about 3/4 of it would allow it to rev up quicker, and not die on quick throttle openings. So it's too lean. Going to try a few different fatter mains and see which one gets it closest.

People say these like to be run on the fat side too...free air 2 stroke...needs all the help it can get keeping temps down.

 

[RaWarrior]

Well thanks to Sean for sending me a tackle box full of mains to play with....it was too lean, which was causing the top end bogging. Apparently a 32mm Keihin jets nothing like a 34mm Mikuni. Not even the same ball park. Stock carb took a 142 main, this carb needs a 230.

I put the needle in the middle, the air screw 1.5 turns out, and started trying different mains. A 180 allowed it to rev out, but the plug was a light gray to white, and the top end didn't seem very strong, so deff too lean(some of that "tink tink tink" 2 stroke lean ping as well). Went up to a 200, which was better, little stronger, but the plug still looked grayish white. Tried again, going to a 230. Now it's better....plug was a light mocha/"creamed coffee" color, a light creamy brown. No bogging when you nailed the gas and the top end screamed. Idle tended to get fast when hot, which IME means the crankcase leaks a bit. Turning the air screw in 1/2 turn seems to remedy this. Still kind of boggy off idle though, have to feather the gas to get it to rev up, but then it's fine. It's kind of finicky to start as well, gotta finesse both the gas and the choke to keep it going cold. It's got a #45 pilot in there, I'll bump it to a #50 and see what happens.

I just had to go by plugs, since my Racepak EGT took a dump. Don't know why, but all the EGT channels just wigged out. Was fine two weeks ago...now one says 1700 all the time, one is stuck at 0, and the other two say -99(no probe) all the time. Oh well, easy come easy go I suppose.

Now I have to try and get a 1977 Polaris Colt 250 running. Got spark, got compression, but will only run off ether or gas shot right down the throat.....went through the carbs, all the circuits are clear....but not even a sputter.

 

[Fastatv]

Now I have to try and get a 1977 Polaris Colt 250 running. Got spark, got compression, but will only run off ether or gas shot right down the throat.....went through the carbs, all the circuits are clear....but not even a sputter.[/QUOTE]
Glad your getting the 2 stroke dialed in. Sounds like a larger pilot may resolve the lean bottom. On this other engine, is it two or four stroke and by the way, what is it......the Polaris Colt I mean? Anyway, I'll help you if I can......Rick

 

[RaWarrior]

It's an old snowmobile....250cc twin cylinder(twin carb) two stroke. Every other 250 I've ever seen has been a single.

Going to drop the carbs in the sonic tank at work tomorrow. The only problem I could find is the choke circuit pickup in the bowl is plugged on one of the carbs, and carb spray/air/still wire wouldn't clear it. Might have to gently proceed with a drill, since it's a straight tube.

They're really small Mikuni slide valve....the old school style where the main screws into the bowl drain, then the carb body press-fits into a recess in the bowl to seal onto the main circuit.

 

[Barry barker]

http://www.iwt.com.au/mikunicarb.htm Better late than never. i am now trying to make a stock moto-4 run correctly and i found this link on carburetor tuning. It is the same information previously given just in a concise article. Tuning a carb would be more fun if the things were easier to get to.:bang head: